Plac komuny Paryskiej 5a, 90-007 Åódź ...... E. Kalliala, P. Talvenmaa, Environmental profile of textile wet processing in Finland, J. Clean. Prod. 8 ...... MożliwoÅci, Uniwersytet Marii Curie-SkÅodowskiej w Lublinie WydziaÅ Chemii, 2012: pp.
Decolorization of textile wastewater by Advanced Oxidation Processes (AOPs) under industrial conditions Zastosowanie metod pogłębionego utleniania do odbarwiania ścieków włókienniczych w warunkach przemysłowych
Doctoral thesis MSc. Lucyna Bilińska
Supervisors: Prof. DSc. PhD. Eng. Stanisław Ledakowicz, Faculty of Process & Environmental Engineering, Lodz University of Technology, Poland PhD. Eng. Marta Gmurek, Faculty of Process & Environmental Engineering, Lodz University of Technology, Poland
Lodz 2017
Copyright© 2017 by Textile Company Bilinski, Lodz University of Technology Publisher: Fundacja Rozwoju Polskiej Kolorystyki Plac komuny Paryskiej 5a, 90-007 Łódź
Cover design© Agnieszka Pawlik-Podsiadły Printed by PKITOR Szlaski i Sobczak Sp.j. Tomaszowska 27, 93-231 Łódź
Reviewers: DSc. PhD. Eng. Ewa Felis, Silesian University of Technology, Faculty of Energy and Environmental Engineering, Poland Prof. DSc. PhD. Eng. Andrzej K. Biń, Warsaw University of Technology, Faculty of Chemical and Process Engineering, Poland
ISBN 978-83-944176-0-4
Preface A common mission of the Society of the Polish Chemists and Colourists and the Foundation for the Development of Polish Colouristics is promotion of knowledge concerning the chemical processing of textile materials and the application of modern and environmentally friendly technological solutions in this field. Textile industry, especially the chemical processing of textile materials due to dyeing and finishing operations, consumes significant amounts of water and generates environmentally harmful wastewater containing dyes, surfactants, and mineral salts. The Water Framework Directive regulates environmental and water protection policies in the European Union countries. According to the provisions of this Directive, "water is not a commercial product like any other but rather a heritage that must be protected, defended and treated as such." It can therefore be assumed that the European Union's policy is to ensure that all EU citizens have access to good quality water resources while enabling the industrial sector to use it in a sustainable way, while protecting the environment. These issues are in the scope the presented monograph: "Decolorization of textile wastewater by Advanced Oxidation Processes (AOPs) under industrial conditions" by Lucyna Bilińska. The author, conducted studies on the decolorization of textile wastewater by advanced oxidation processes (AOPs) as a topic of her doctoral dissertation at the Faculty of Process and Environmental Engineering of the Lodz University of Technology under the supervision of Prof. DSc. PhD. Eng. Stanisław Ledakowicz and PhD. Eng. Marta Gmurek. The object of the research was industrial wastewater after dyeing of cellulose fibers with reactive dyes, originating from Textile Company Bilinski (Poland), as well as the model one. The selected methods of advanced oxidation have been verified for the effectiveness of decolorizing the above-mentioned wastewater. Because of the interesting results obtained during this work, the Society of the Polish Chemists and Colourists decided to disseminate the results of the research, expecting that they could be of interest to those involved in the field of chemical processing of textiles and industrial wastewater treatment.
President of the Foundation for the Development of Polish Colouristics Włodzimierz Dominikowski
Od Wydawcy Stowarzyszenie Polskich Chemików Kolorystów wraz z Fundacją Rozwoju Polskiej Kolorystyki prowadzi działania w zakresie upowszechniania wiedzy dotyczącej chemicznej obróbki materiałów włókienniczych oraz stosowania nowoczesnych i przyjaznych dla środowiska rozwiązań technologicznych z tego zakresu. Przemysł włókienniczy, a w szczególności procesy chemicznej obróbki włókna zużywają w procesach aplikacyjnych i wykończalniczych znaczne ilości wody generując niepożądane ze względu na ekologię środowiska ścieki zawierające barwniki, środki powierzchniowo czynne i sole mineralne. W krajach Unii Europejskiej politykę proekologiczną w zakresie gospodarki wodnościekowej i ochrony wód reguluje Ramowa Dyrektywa Wodna. Zgodnie z zapisami tej Dyrektywy przyjmuje się, że „woda nie jest produktem handlowym takim jak każdy inny, ale raczej dziedzictwem, które musi być chronione, bronione i traktowane jako takie”. Można zatem założyć, że polityka Unii Europejskiej dąży do zapewnienia wszystkim mieszkańcom UE dostęp do zasobów wodnych dobrej jakości, jednocześnie umożliwiając sektorom przemysłowym korzystanie z niej w sposób zrównoważony, przy jednoczesnej ochronie środowiska naturalnego. Tym zagadnieniom jest poświęcona prezentowana monografia Lucyny Bilińskiej pt. „Decolorization of textile wastewater by Advanced Oxidation Processes (AOPs) under industrial conditions”. Autorka, w ramach pracy doktorskiej realizowanej na Wydziale Inżynierii Procesowej i Ochrony Środowiska Politechniki Łódzkiej pod kierunkiem prof. dr hab. inż. Stanisława Ledakowicza i dr inż. Marty Gmurek, wykonała badania dotyczące odbarwiania ścieków metodami pogłębionego utleniania (AOPs). W badaniach wykorzystano ścieki po barwieniu włókien celulozowych barwnikami reaktywnymi, pochodzące z Zakładu Włókienniczego Biliński Sp.J., a także ścieki modelowe. W pracy zostały zweryfikowane wybrane metody zaawansowanego utleniania pod kątem skuteczności odbarwiania wyżej wspomnianych ścieków. Ze względu na interesujące wyniki uzyskane w trakcie tej pracy Stowarzyszenie Polskich Chemików Kolorystów postanowiło upowszechnić rezultaty przeprowadzonych badań spodziewając się, że mogą być one interesujące dla osób zajmujących się chemiczną obróbką wyrobów włókienniczych oraz problematyką oczyszczania ścieków przemysłowych. Prezes Zarządu Fundacji Rozwoju Polskiej Kolorystyki Włodzimierz Dominikowski
Acknowledgements The author would like to thank to the supervisors Prof. DSc. PhD. Eng. Stanisław Ledakowicz and PhD. Marta Gmurek for their great help and support in creating this work. The valuable help of the employees and colleagues from Department of Bioprocess Engineering, Faculty of Process & Environmental Engineering, Lodz University of Technology is greatly appreciated. Great acknowledgements to DSc. PhD. Eng. Jadwiga Sójka-Ledakowicz, Assoc. Prof., PhD. Renata Żyłła and the employees of Textile Research Institute for their cooperation. Special thanks to PhD. Kazimierz Blus for the valuable suggestions and discussion. Great acknowledgements to the Foundation for the Development of Polish Colouristics for publishing this monograph. Many thanks to my family for their support and understanding. Lucyna Bilińska
The research was proceeded with cooperation of Textile Company Bilinski, Poland. A part of the research was supported by The National Centre for Research and Development (NCBiR) in Poland [grant number PBS2/A9/22/2013].
Table of Contents Nomenclature Abstract Streszczenie Introduction I. Characteristics of the textile wastewater II. Advanced Oxidation Processes for textile wastewater treatment II.1. Fenton oxidation II.2. H2O2/UV process II.3. Ozonation II.4. Ozone-based AOPs II.5. The overview of the literature III. Objective of the thesis IV. Content of the thesis IV. References 1. Biodegradability assessment of textile dye house wastewater streams Abstract 1.1. Introduction 1.2. Experiment 1.3. Discussion of the results 1.4. The management of the wastewater streams 1.5. Conclusions 1.6. Acknowledgement 1.7. References 2. Application of advanced oxidation technologies for decolorization and mineralization of textile wastewaters Abstract 2.1. Introduction 2.2. Experimental 2.2.1. Materials 2.2.2. Analytical methods 2.3. Results and discussion 2.3.1. Effect of the initial dye concentration 2.3.2. Effect of pH 2.3.3. Effect of reagents dosages 2.3.4. NaCl influence 2.3.5. Surface-active agent influence 2.3.6. Color, TOC and COD removal 2.3.7. Industrial textile wastewater treatment
13 15 21 27 29 30 30 32 33 35 36 39 40 43 51 53 53 54 56 59 61 62 62 65 67 67 68 68 69 70 70 72 73 76 77 78 79
2.4. Conclusions 2.5. Acknowledgement 2.6. References 3. Application of Fenton reagent in the textile wastewater treatment under industrial conditions Abstract 3.1. Introduction 3.2. Experimental 3.2.1. Materials 3.2.2. Application of Fenton reagent 3.2.3. Pulse radiolysis 3.3. Results and discussion 3.3.1. Effect of pH on Fenton process 3.3.2. Effect of ferrous dosage and FeSO4: H2O2 ratio 3.3.3. Effect of NaCl on Fenton process 3.3.4. Effect of Perigen LDR addition on Fenton Process 3.3.5. Decolorization of simulated wastewater and real industrial textile effluents 3.4. Conclusions 3.5. References 4. Comparison between industrial and simulated textile wastewater treatment by AOPs – Biodegradability, toxicity and cost assessment Graphical abstract Abstract 4.1. Introduction 4.2. Experimental 4.2.1. Materials 4.2.2 Analytical methods 4.3. Results and discussion 4.3.1. H2O2 impact on the •OH radicals formation mechanism 4.3.2. Comparison of the oxidation methods 4.3.3. Mineralization and Biodegradability 4.3.5. Toxicity 4.3.6. Costs evaluation 4.4. Conclusions 4.5. Acknowledgements 4.6. References Supplementary materials to chapter 4 5. Textile wastewater treatment by AOPs for brine reuse Graphical abstract Abstract 5.1. Introduction
81 81 81 83 85 85 87 87 88 88 89 89 90 91 95 95 97 97 99 101 101 102 103 103 105 106 106 107 110 114 116 117 118 118 121 127 129 129 130
5.2. Experiment 5.2.1. Materials 5.2.2. Methods 5.3. Results and discussion 5.3.1. Influence of the wastewater matrix 5.3.2. Comparison of the AOPs 5.3.2.1. Dyes treated separately 5.3.2.2. Dyes treated in the mixture 5.3.3. Mineralization 5.3.3.1. Dyes treated separately 5.3.3.2. Dyes treated in the mixture 5.4. Conclusions 5.5. Acknowledgements 5.6. References Supplementary materials to chapter 5 6. Ozonation as a key stage in the textile wastewater treatment process 6.1. Introduction 6.2. Experiment 6.2.1. Experimental set-up 6.2.2. Methods 6.2.3. Wastewater characteristics 6.3. Results and discussion 6.4. Conclusions 6.5. Acknowledgements 6.6. References 7. Modeling of ozonation of C. I. Reactive Black 5 through a kinetic approach Abstract 7.1. Introduction 7.2. Experimental 7.2.1. Materials 7.2.2. Analytical methods 7.3. Results and discussion 7.3.1. The stoichiometry of BR5 reaction with ozone 7.3.2. Kinetic of self-decomposition of ozone 7.3.3 Kinetics of RB5 oxidation with ozone 7.3.4 Modeling of ozonation process 7.3.4.1. Mass balance 7.3.4.2. Estimation of model parameters 7.3.4.3. Evaluation of the model 7.4. Conclusions
132 132 132 134 134 136 137 139 139 139 140 142 143 143 147 151 153 153 153 154 155 155 157 158 158 161 163 163 165 165 165 166 166 168 169 171 172 172 173 176
7.5. Acknowledgements 7.6. References 8. Summary and Conclusion 8.1. Summary 8.2 Brine recycling from ozone treated industrial textile wastewater 8.3 General remarks and overview 8.4. References
177 177 181 183 187 190 191
Nomenclature 𝐴
absorbance
𝐴𝑂𝑆
average oxidation stage
𝐵𝑂𝐷
biochemical oxygen demand, mg/L
𝐶𝑂𝐷
chemical oxygen demand, mg/L
𝐶𝑖
concentration of the i, mg/L, M
𝐶0𝑖
initial concentration of the i, mg/L, M
𝐺 𝐶𝑂3
concentration of ozone in gas phase, mg/L
𝐶𝑂3
concentration of ozone in liquid phase, mg/L
𝐶𝐿∗
equilibrium molar concentration of ozone in liquid phase, mol/m3
𝐶𝐿
molar concentration of ozone in liquid phase, mol/m3
𝐸0
irradiation intensity, eistein L-1 s-1, where eistein is mol of photons
𝐹
fraction of absorbed irradiation
𝑘𝑎𝑝𝑝
apparent kinetic constant, s-1
𝑘𝐷
kinetic constant of ozone decay, s-1
𝑘𝑖
kinetic constant of i decay, M-1 s-1
𝑘𝐿 𝑎
volumetric mass transfer coefficient, s-1
𝑙
length of the light path, cm
𝑁𝑇𝑜𝑡𝑎𝑙
nitrogen total, mg/L
𝑃𝑇𝑜𝑡𝑎𝑙
phosphorous total, mg/L
𝑄𝑖𝑛
gas flow rate, L/h
𝑟𝑈𝑉
direct photolysis rate, M s-1
𝑟𝐷
ozone decay rate, M s-1
𝑟𝑖
rate of i decay, M s-1
𝑡
time, s, min
𝑇𝐶
total carbon, mg/L
𝑇𝑂𝐶
total organic carbon, mg/L
𝑇𝑂𝐷
total ozone dose, g/L
𝑇𝑆𝑆
total suspended solids, mg/L
𝑧𝑖
stochiometric factor of the reaction between ozone and i
13
Greek letters
14
𝛼
spectral absorption coefficient, m-1
𝜀
molar absorption coefficient, M-1 cm-1
𝜆
wavelength, nm
𝜑
quantum yield
Abstract
Water is a priceless resource that must be protected. Due to a rapidly growing human population, the amount of water used in industrial processes and farming persistently increase. One area where water consumption is extremally high is the textile industry. Whilst water management, is a crucial issue in modern textiles factories, production processes are still highly water-consuming. At the same time, huge volume of greatly polluted wastewater from textiles manufacturing are being released into the environment. Dyes are the most troublesome pollutants within textile wastewater management. These high molecular weight complex organic compounds are poorly biodegradable and their presence disturbs living conditions in the biosphere. Across the world, tons these pollutants are released into industrial wastewater every year making dyes a serious environmental problem. Treatment of highly polluted, colored textile wastewater has become an issue of high importance. Research into efficient treatment methods that can be implemented into industrial practice is extremely desirable. One such method is use of advanced oxidation processes (AOPs). Due to the high oxidative potential of ozone or hydroxyl radical, which are the main oxidants among AOPs, many poorly degradable contaminants, including dyes, can be decomposed. The main objective of this work was to investigate the efficiency of selected AOPs in color removal from industrial textile wastewater and the following processes were considered: Fenton oxidation, H2O2/UV process, ozonation and ozone-based AOPs: O3/H2O2, O3/UV and O3/H2O2/UV. The potential for industrial implementation of these processes was one of the most important challenges considered in this thesis. A preliminary study on textile wastewater characteristics was carried out as the first stage of the thesis. It was found that textile wastewater has a very complex matrix with many organic and inorganic compounds, beside dyes, present. Industrial wastewater parameters from various technological operations were investigated, and have been divided into specific streams based upon these results. The least biodegradable stream with very high residual color and salt content was selected to undergo AOPs as the object of this research. The next stage of the research was a general overview on the efficiency of the main types of AOPs: Fenton oxidation, H2O2/UV process and ozonation. The influence of typical process parameters including dye concentration, pH and reagent doses as well as textile wastewater specific parameters, NaCl and surfactant presence, were tested. This allowed determination of the AOP sensitivities to textile wastewater matrices. The Fenton reagent and H2O2/UV process were both
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found to be influenced by NaCl and surfactants occurring in the wastewater. The most promising results were obtained for ozonation. The causes of Fenton reagent sensitivity to the textile wastewater matrix were identified. The NaCl inhibition in the oxidation by Fenton reagent was explained by comparison of the rate constants between the dyes and Cl2•− or HO• . The surfactant effects of were explained by formation of Critical micelle concentration (CMC). The effectiveness of Fenton reagent in the decolorization of dye solutions and wastewater gave information about its poor suitability for implementation into the industry. The H2O2/UV process and ozone-based AOPs were investigated in the next part of the research. The oxidation results for simulated and industrial wastewater containing the most popular textile dye, Reactive Black 5, were compared. In contrast to the H2O2/UV process, ozone-based AOPs were effective in color removal from the wastewater. Whilst the investigations of color removal, mineralization, biodegradability, average oxidation stage (AOS) and toxicity were promising for the ozone-based AOPs, a scavenging effect caused by the wastewater matrix occurred. Due to the scavenging of hydroxyl radicals by H2O2 there was no significant synergy of simultaneous use of O3, H2O2 and UV among the AOPs. These observations in combination with a cost evaluation proved the advantages of ozonation for industrial implementation. The use of AOPs for the most common textile dyes, Reactive Yellow 145, Reactive Red 195 and Reactive Blue 221, were also investigated. These dyes are used as a set in the trichromatic dyeing technique and as a result also occur in the wastewater together. Treating them separately showed that the oxidation pathway is depended on the type of a dye whilst color removal and mineralization were also different for each dye. However, treating them as a mixture showed how the interaction between the dyes can influence the AOP processes. The lack of the synergetic effect among AOPs was confirmed in this study. The lack of wastewater matrix influence on decolorization by ozonation was explained by showing the dose of ozone absorbed during the process. The advantages of ozonation were proved for several types of dyes. Ozonation was proven as an efficient treatment in color removal from the textile wastewater streams of several technological operations. At the same time, treatment caused the partial mineralization of this wastewater and the biological plant was proposed as the further step in its treatment technology.
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Once ozonation has been considered as the best method among the AOP treatments, a mathematical model to describe to the process has been proposed. Based on experimental data, second order rate constants for reaction between dye Reactive Black 5 and ozone were found. Model parameters were estimated by solving a non-linear inverse problem using the experimental data. The model gave a satisfying match between the predicted and experimental data in acidic reaction medium. The re-dying process using the treated wastewater was conducted successfully. Promising color matching parameters DEcmc for samples of cotton fabrics dyed in various shades were obtained. Additionally, analysis of heavy metals and carcinogenic amines content further proved the advantages of ozonation as an effective method for wastewater purification. The results presented in the thesis are intended for use in the textile industry when systems for brine recycling will be implemented.
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Streszczenie
Woda jest bezcennym dziedzictwem, które powinno być chronione. Ze względu na rosnącą populację ludności, coraz większa ilość wody pitnej jest zużywana w przemyśle i rolnictwie. Jedną z gałęzi przemysłu, w której konsumpcja wody jest wyjątkowo wysoka jest przemysł włókienniczy. Chociaż w nowoczesnych fabrykach wyrobów włókienniczych zarządzanie wodą jest kwestią kluczową, procesy produkcyjne wciąż są niezwykle wodo-chłonne. Wiąże się to z emisją do środowiska ogromnej ilości wysoko zanieczyszczonych ścieków towarzyszących produkcji wyrobów włókienniczych. Główne źródło zanieczyszczeń w ściekach przemysłu tekstylnego stanowią barwniki. Te złożone związki organiczne o dużej masie cząsteczkowej trudno ulegają biodegradacji, a ich obecność w wodach powierzchniowych zakłóca warunki życia w biosferze. Ze względu na fakt, iż tony barwników są odprowadzane do ścieków na całym świecie każdego roku, stały się one poważnym problemem środowiskowym. Oczyszczanie silnie zanieczyszczonych, barwnych ścieków przemysłu tekstylnego stało się tematem dużego zainteresowania. Szczególny nacisk kładziony jest na poszukiwania skutecznej metody oczyszczania, którą można wdrożyć w przemyśle. Jedną z takich metod mogą być procesy zaawansowanego utleniania (AOP). Ze względu na wysoki potencjał utleniający ozonu i rodników hydroksylowych, które są głównymi utleniaczami w metodach AOP, wiele trudno ulegających degradacji zanieczyszczeń, włączając barwniki, można rozkładać z wykorzystaniem tych procesów. Głównym celem niniejszej pracy było zbadanie wybranych metod AOP pod kątem skuteczności odbarwiania ścieków przemysłu tekstylnego. Zakres pracy obejmował następujące procesy: utlenianie odczynnikiem Fentona, proces H2O2/UV, ozonowanie oraz metody AOP z zastosowaniem ozonu: O3/H2O2, O3/UV, O3/H2O2/UV. Przy czym najważniejszym kryterium podczas realizacji tej pracy była zdolność do przemysłowej implementacji badanych metod AOP. Pierwszym etapem niniejszej pracy były wstępne badania dotyczące ścieków tekstylnych. Stwierdzono, że ścieki tekstylne mają bardzo złożony charakter pod względem zawartych w nich zanieczyszczeń. Obok barwników występuje w nich wiele związków organicznych i nieorganicznych używanych w procesach produkcji. Rzeczywiste ścieki przemysłowe pobrane z różnych operacji technologicznych zostały zbadane pod kątem charakteryzujących je wskaźników i na tej podstawie zostały podzielone na odpowiednie strumienie. Najmniej podatny na biodegradację strumień o bardzo intensywnym zabarwieniu i wysokim stężeniu soli został wytypowany jako obiekt badań do oczyszczania metodami AOP.
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Kolejnym etapem badań było przekrojowe sprawdzenie skuteczności głównych typów badanych metod AOP: utleniania odczynnikiem Fentona, procesu H2O2/UV oraz ozonowania. Badano wpływ typowych parametrów procesowych, takich jak początkowe stężenie barwnika, pH, dawki odczynników, jak również te specyficzne dla ścieków przemysłu tekstylnego: obecności NaCl i środka powierzchniowo czynnego. W ten sposób określono wrażliwość poszczególnych metod AOP na zanieczyszczenia występujące w ściekach włókienniczych. W przypadku odczynnika Fentona i procesu H2O2/UV stwierdzono znaczącą inhibicję procesów spowodowaną obecnością NaCl i środka powierzchniowo czynnego. Najbardziej obiecujące wyniki uzyskano dla ozonowania. Dalsze badania nad zastosowaniem odczynnika Fentona do oczyszczania ścieków włókienniczych pozwoliły na wyjaśnienie przyczyn ograniczonej wydajności tej metody. Inhibicja spowodowana przez NaCl została wyjaśniona przez porównanie stałych szybkości pomiędzy badanymi barwnikami a rodnikami: Cl2•− i HO• . Natomiast inhibicja, której przyczyną był środek powierzchniowo czynny została umotywowana występowaniem w badanych roztworach krytycznego stężenia micelarnego (CMC). Wyniki otrzymane dla odbarwiania roztworów barwników i ścieków odczynnikiem Fentona potwierdziły niedostateczną przydatność do wdrożenia w przemyśle tej metody. W następnej części pracy zbadano proces H2O2/UV i metody AOP wykorzystujące ozon. Porównano wyniki utleniania otrzymane dla ścieków symulowanych i przemysłowych zawierających najbardziej powszechnie wykorzystywany we włókiennictwie barwnik – Reactive Black 5. Wszystkie badane metody AOP wykorzystujące ozon wykazały dużą skuteczność w odbarwianiu ścieków, w przeciwieństwie do procesu H2O2/UV, który okazał się wysoce nieefektywny. Pomimo, iż dla wszystkich metod AOP wykorzystujących ozon uzyskano pozytywne wyniki odbarwiania, mineralizacji, biodegradowalności, średniego stopnia utlenienia (AOS) i toksyczności, zauważono jednak w ich przypadku występowanie zjawiska zmiatania rodników hydroksylowych. Przyczyną zjawiska „zmiatania” rodników HO• był dodatek H2O2. Nie stwierdzono zatem synergii wynikającej z jednoczesnego stosowania O3, H2O2 i UV pośród badanych metod AOP. Możliwość oczyszczania metodami AOP została zbadana także dla najczęściej stosowanych podczas barwienia trójchromatycznego barwników reaktywnych: Reactive Yellow 145, Reactive Red 195 i Reactive Blue 221. Ponieważ barwniki te używane są w procesach technologicznych jednocześnie, razem występują również w ściekach. Zaobserwowano, iż proces utleniania jest
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zależny od rodzaju barwnika. Stopień odbarwienia i mineralizacja były różne dla każdego barwnika. Utlenianie barwników jako mieszaniny wykazało, wpływ oddziaływań między barwnikami na szybkość dekoloryzacji ścieków. Potwierdzono jednocześnie brak występowania efektu synergii między utleniaczami w badanych metodach AOP. Na podstawie dawki ozonu zaabsorbowanej w trakcie procesu wyjaśniono zjawisko braku wpływu składu ścieków na odbarwianie podczas ozonowania. Skutecznie przeprowadzono ozonowanie dla kilku typów barwników. Skuteczność ozonowania potwierdzona została także w przypadku odbarwiania ścieków, składających się z uśrednionych strumieni pochodzących z kilku operacji technologicznych. Ozonowanie pozwoliło na częściową mineralizację tych ścieków, więc oczyszczanie biologiczne zostało zaproponowane jako kolejny krok w technologii oczyszczania. Ponieważ stwierdzono, iż ozonowanie jest najbardziej efektywną metodą oczyszczania ścieków pośród badanych AOP, zaproponowano model matematyczny opisujący ten proces. Na podstawie danych eksperymentalnych wyznaczono stałe szybkości drugiego rzędu dla reakcji pomiędzy barwnikiem Reactive Black 5 i ozonem. Model poprawnie przybliżył dane eksperymentalne dla reakcji zachodzącej w środowisku kwaśnym. Skutecznie przeprowadzono proces ponownego barwienia z wykorzystaniem oczyszczonych ścieków. Parametry różnic kolorystycznych DEcmc uzyskane dla próbek tkanin bawełnianych wybarwionych w kilku odcieniach były bardzo obiecujące. Ponadto wyniki stwierdzające brak metali ciężkich i amin kancerogennych w barwionych tkaninach pozwalają sądzić, iż możliwe będzie stosowanie ozonowania jako sposobu oczyszczania ścieków. Wyniki badań przedstawione w niniejszej pracy mogą być podstawą do wdrażania systemu recyklingu solanki w przemyśle tekstylnym.
25
26
Introduction
I. Characteristics of the wastewater produced by textile manufacture Textile manufacturing is one of the most water-consuming industries; between 80 and 400 L of water are required to produce 1 kg of textiles [1–3]. At the same time, a great amount of highly polluted wastewater is also produced, thus the treatment of textile wastewater is a topic of a great importance. Table 1 presents the wastewater characteristics that can be found in the literature, describing the general effluents from a dyeing house. It can be seen that most of the parameters are defined by values within a broad range, which is disadvantageous where the selection of an appropriate wastewater treatment is concerned. Moreover, the processes of textiles production are extremely varied and complex, and textile wastewater form different technological operations can be different [3–5]. Table 1. Textile wastewater parameters describing general effluents form a dyeing house [2–4,6,7] Parameter, unit
Value
COD, mg/L
450–3000
BOD5, mg/L
160–1200
TSS, mg/L
330
NTotal., mg/L
5–70
PTotal, mg/L
1–6
Conductivity, mS/cm
4900–12500
Chlorides, mg/L
1000–1750
Sulfates, mg/L Color by Hazen scale, Pt-Co units pH
250–600 1000–3300 8.5–11
Although such textile wastewater is characterized by high variability, most of the research has been conducted using wastewater taken from the general effluent of textile plants, considered as an “averaged” wastewater originating from several random operations [8–16]. In contrast to this approach, EU legislation recommends the division of textile wastewater into different streams based on biodegradability approach. A guiding principle of wastewater treatment strategy is to first treat the most highly contaminated non-biodegradable wastewater streams — which could otherwise cause malfunction to biological treatment — using appropriate techniques, and then to subject this pre-treated effluent to a further biological treatment [17]. The recent study by Paździor et al. [18], shows the results of oxygen uptake rate (OUR) experiments, and the bioreactor batch tests, which proved the positive effect of separating highly loaded dyeing discharge prior to the biological treatment of textile
29
wastewater. Therefore, this wastewater stream separation strategy should be considered in the design of textile wastewater plants.
II. Advanced oxidation processes for textile wastewater treatment Advanced oxidation processes (AOPs) are the group of water and wastewater treatments in which the main oxidative agent is the hydroxyl radical (HO• ). This oxidant is characterized by one of the highest oxidative potentials in nature (Table 2) [19], and therefore many organic compounds may be decomposed by AOPs [20]. The use of Fenton reagent, H2O2/UV, ozonation and ozone-based AOPs, which are effective and relatively easy to perform, seem to be the most suitable for implementation in the textile industry as a method of treating the most highly polluted wastewater streams.
Table 2. Standard redox potentials of some oxidant species [7,19] Oxidizing agent
Electrochemical oxidation potential (EOP), V
Fluorine
3.06
Hydroxyl radical
2.80
Oxygen (atomic)
2.42
Ozone
2.08
Hydrogen peroxide
1.78
Hypochlorite
1.49
Chlorine
1.36
Chlorine dioxide
1.27
Oxygen (molecular)
1.23
II.1. Fenton oxidation Oxidation by a Fenton reagent (H2 O2 /Fe) occurs mainly due to the in situ generation of •
HO . The reactants in a classical Fenton oxidation are hydrogen peroxide and ferrous ions (Fe2+ ), and the HO• radicals are produced by the decomposition of hydrogen peroxide according to equation (1) [21]: 𝐻2 𝑂2 + 𝐹𝑒 2+ → 𝐹𝑒 3+ + 𝐻𝑂− + 𝐻𝑂•
(1)
Due to their high oxidative potential (Table 2), HO• can easily react with organic pollutants (RH) in a non-selective manner, by addition to unsaturated bonds such as C=C or abstraction of a hydrogen atom as in equations (2) – (4) [21]:
30
𝑅𝐻 + 𝐻𝑂• → 𝑅 • + 𝐻2 𝑂
(2)
𝑅 • + 𝐹𝑒 3+ → 𝑅 + + 𝐹𝑒 2+
(3)
+
𝑅 + 𝐻2 𝑂 → 𝑅𝑂𝐻 + 𝐻
+
(4)
The overall mechanism of Fenton oxidation is very complex, involving numerous competitive reactions between ferrous ions, hydrogen peroxide and various by-products as in equations (5) – (8) [21,22]: 𝐻2 𝑂2 + 𝐹𝑒 3+ ↔ 𝐻 + + 𝐹𝑒𝑂𝑂𝐻 2+ 𝐹𝑒𝑂𝑂𝐻 𝐹𝑒
2+
2+
+
+
𝐻𝑂2•
𝐻𝑂2• ↔
+
↔ 𝐻 + 𝐹𝑒 𝐻𝑂2−
(5)
2+
(6)
+ 𝐹𝑒
3+
(7)
𝐹𝑒 3+ + 𝐻𝑂2• ↔ 𝑂2 + 𝐹𝑒 2+ + 𝐻 +
(8)
At the same time, the scavenging of HO• can occur according to equations (9) – (11) [21,22]: 𝐻𝑂• + 𝐹𝑒 2+ → 𝐻𝑂− + 𝐹𝑒 3+ •
𝐻𝑂 + 𝐻2 𝑂2 → 𝐻2 𝑂 + •
(9)
𝐻𝑂2•
(10)
•
𝐻𝑂 + 𝐻𝑂 → 𝐻2 𝑂2
(11)
The use of this process has been widely studied since its initial discovery by J. H. Fenton in 1884 [23]. Application of classical Fenton oxidation (homogenous process with ferrous ions) has been demonstrated to be an effective treatment for color removal form textile dye solutions [22,24–36] and from wastewater as well [11–13,16,37–39]. The efficiency of Fenton oxidation can be improved by UV irradiation and this kind of process is known as photo-Fenton (H2 O2 /Fe/UV). The effectiveness of this process can be enhanced by photo-chemical reactions (12) and (13): 𝑈𝑉 + 𝐻2 𝑂2 → 𝐻𝑂• + 𝐻𝑂• 𝐹𝑒
3+
•
+ 𝐻2 𝑂 + ℎ𝑣 → 𝐻𝑂 + 𝐹𝑒
(12) 2+
+ 𝐻
+
(13)
The satisfying results of the H2 O2 /Fe/UV process have been reported in numerous studies [26,30–32,35,38,39].
31
More recent studies have investigated heterogeneous systems in the Fenton process where the source of ferrous ions was mineral- or polymer-based [15,40–43]. Although the presented literature reports showed satisfying results for the use of Fenton reagent in this regard, there are some disadvantages to this treatment. A few authors have reported the scavenging effect by Cl− [24,44–48], and NaCl which is a source of these ions is commonly used in the textile industry. However, the effect of Cl− on dye decolorization and the influence of other textile auxiliaries have not been discussed so far.
II.2. H2O2/UV process The simultaneous use of H2O2 and UV-C irradiation leads to the production of HO• , which are the oxidative agent in this AOP. The absorption of UV irradiation by hydrogen peroxide results in its decomposition (homolysis), which can be described by equation (14) [49]: 𝐻2 𝑂2 + ℎ𝑣 → 2 𝐻𝑂•
(14)
The efficiency of H2O2 photolysis depends on the molar absorption coefficient of H2O2, ε, and quantum yield, φ, and it can be described by equation (15) [19,50]: 𝑟𝑈𝑉 = 𝜑𝐸0 𝐹(1 − exp(−2.303 𝑙 𝛴𝜀𝑖 𝐶𝑖 ))
(15)
Where: 𝑟𝑈𝑉 is the rate of direct photolysis, φ is the quantum yield, 𝐸0 is the irradiation intensity (eistein L−1 s−1), 𝐹 = 𝜀𝐶/𝛴𝜀𝑖 𝐶𝑖 is the fraction of absorbed irradiation, ε is the molar absorption coefficient (M−1 cm−1), l is length of the light path through the solution (cm) and C (M) is the concentration of absorbing substance. The hydroxyl radicals produced in reaction (14) can undergo further reaction with hydrogen peroxide, as it was shown in the previous section for the Fenton reagent in reaction (10) [49]. The scavenging of HO• radicals can be enhanced in an alkaline reaction medium, where hydrogen peroxide would be present in the dissociated form (pKaH2O2 11.6) [19] as it was in • reaction (16). The resulting presence of HO− 2 ions can promote the HO scavenging in
accordance with reaction (17). 𝐻2 𝑂2 + 𝐻2 𝑂 → 𝐻3 𝑂+ + 𝐻𝑂2− •
𝐻𝑂 +
32
𝐻𝑂2−
→ 𝐻2 𝑂 +
𝑂2•−
(16) (17)
Due to the fact that the main oxidative agent in this treatment is the HO• radical, the organic pollutants (RH) are decomposed in a non-selective manner by the addition of HO• to unsaturated bonds or the abstraction of a hydrogen atom [51]. Numerous literature reports can be found where the H2O2/UV process was used for dye removal from solution [48–58] or textile wastewater treatment [10,54,59–62]. Although the H2O2 dose was investigated in those works in order to avoid the scavenging effect described in equations (10), (11) and (16), (17), the possibility of the adverse effects caused by textile auxiliaries have not been widely investigated. Only Riga et al. [48] and Elmorsi et al. [58] mentioned these inhibitory effects caused by the scavenging of hydroxyl radicals by salts and alkalinity.
II.3. Ozonation Ozone (O3) is a strong oxidant (Table 2), which can decompose many persistent pollutants that are otherwise hardly degradable. Ozonation is a relatively easy non-discharge, treatment procedure to perform, and correspondingly it has been wildly used in industrial wastewater treatment [63]. However, there are some issues that should be discussed when considering the use of ozone. Firstly, the O3 molecule is unstable in the gas phase as well as in the liquid phase. Secondly, ozonation is usually carried out in the liquid phase, but ozone is applied in this process in its gaseous form. Therefore, two complications arise: ozone self-decomposition and its absorption from the gaseous into the liquid phase [19,64]. The first mechanism for the self-decomposition of O3 was presented by Wiess in 1934 [19]. Since then, many other mechanisms have been proposed, the most popular of which is the SBH (Staehelin, Buhler and Hoigne) mechanism. The O3 decomposition is given as a chain of the reactions in this mechanism, according to equations (18)–(27) [19]: Initiation: 𝑂3 + 𝑂𝐻 − → 𝐻𝑂2• + 𝑂2−
(18)
𝐻𝑂2• → 𝐻 + + 𝑂2•−
(19)
Propagation:
𝑂3 +
𝑂2•−
→
𝑂3•−
+ 𝑂2
(20)
𝑂3•− + 𝐻 + → 𝐻𝑂3•
(21)
𝐻𝑂3•
+ 𝐻+
(22)
→ 𝐻𝑂 + 𝑂2
(23)
𝐻𝑂 + 𝑂3 →
𝐻𝑂4•
(24)
𝐻𝑂4•
+ 𝑂2
(25)
𝐻𝑂3•
→
𝑂3•− •
•
→
𝐻𝑂2•
33
Termination: 𝐻𝑂4• + 𝐻𝑂4• → 𝐻2 𝑂2 + 2𝑂3
(26)
𝐻𝑂4•
(27)
+
𝐻𝑂3•
→ 𝐻2 𝑂2 + 𝑂3 + 𝑂2
The second most popular pathway of ozone decomposition is the TFG (Tomiyasu, Fukutomi and Gordon) mechanism, where the decomposition can be initiated either by OH − ions, as in the SBH mechanism, or by OH2− ions. Many authors base their work on these two mechanisms [65]. However, the situation can also be described by an empirical equation, as in the work of Biń [64]. In this case, the decomposition mechanism is not considered and instead the equation describes the overall decay of ozone. This kind of empirical equation was proposed by Qiu in 1999 for a very wide pH range from 2 to 13.5, as in equation (28) [66].
𝑟𝐷 = −
𝑑𝐶𝑂3 𝑑𝑡
= 𝑘𝐷 𝐶𝑂3
(28)
Where: 𝑘𝐷 is a kinetic constant, which depends on the concentration of HO− ions according to equation (29): 𝑘𝐷 = 20(𝐶𝑂𝐻− )0.5 + 900 (𝐶𝑂𝐻− ), 𝑠 −1
(29)
The second crucial issue in the ozonation process is the determination of O3 concentration in the liquid phase. When a bubble column is used for ozonation, the mass balance of ozone in the liquid phase can be described by equation (30) [64]: 𝑑 𝐶𝐿 𝑑𝑡
= (𝑘𝐿 𝑎)(𝐶𝐿∗ − 𝐶𝐿 ) − 𝑟𝐷
(30)
Where: 𝑘𝐿 𝑎 is the volumetric mass transfer coefficient in the liquid phase (s−1), 𝐶𝐿∗ is the equilibrium molar concentration of ozone in the liquid phase (mol m−3), 𝐶𝐿 is the molar concentration of ozone in the liquid phase (mol m−3), and 𝑟𝐷 is the ozone decomposition rate (M s−1).
According to the SBH model and other models, the decomposition of O3 results in the formation of free radicals. This phenomenon is enhanced in alkaline pH. Therefore, two separate pathways of pollutants decomposition can be observed during ozonation: direct
34
oxidation by the O3 molecule and indirect reaction with the HO• radical. In contrast to the previously described oxidation by the HO• , the reaction with O3 is selective. O3 can react with molecules at locations where a high electron concentration is present, by either cycloaddition, electrophilic substitution or nucleophilic reactions [19]. Due to the presence of numerous double bonds in the structure of dye molecules, they can therefore likely be decomposed by O3 molecules. There are many reports in the literature where O3 was used to decompose dyes in aqueous solutions [52,56,57,67–87] or in textile wastewater [10,14,60,62,88–97]. However, few works have been carried out which reflect the real conditions of industrial textile wastewater. Muthukumar & Selvakumar [98] noticed an inhibitory effect caused by the presence of NaCl and Na2SO4. On the other hand, Colindres et al. [88] demonstrated a significant acceleration in the ozonation of Reactive Black 5 caused by the presence of salt and alkalis. Oguz et al. [83] showed that the presence of HCO− 3 ions had a negligible effect. Bamperng et al. [67] detected only a small inhibition of the ozonation process by NaCl, and the addition of Na2CO3 resulted in a significant inhibition; similar results were obtained by Perez et al. [82]. Some of the effects observed in the presented studies might have originated from the experimental conditions. Therefore, the role of the textile wastewater matrix during ozonation should be investigated more deeply, especially the effect of surfactants, which has not been discussed so far.
II.4. Ozone-based AOPs The idea of ozone-based AOPs is to produce HO• radicals in systems where O3 is the present. The combination of ozonation with H2O2 or UV irradiation can enhance the oxidative potential of the O3/H2O2 and O3/H2O2/UV systems. The simultaneous use of O3 and UV irradiation capitalizes on the decomposition of O3, resulting in the production of HO• radicals, as described in equations (31) – (34):
𝑂3 + ℎ𝑣 → 𝑂 + 𝑂2
(31)
𝑂 + 𝐻2 𝑂 → 𝐻2 𝑂2
(32)
𝐻2 𝑂2 + ℎ𝑣 → 2𝐻𝑂
•
(33) •
𝐻2 𝑂2 + 2𝑂3 → 2𝐻𝑂 + 3 𝑂2
(34)
The atomic oxygen, which is the product of ozonolysis in reaction (31), is converted into hydrogen peroxide (32). Then a molecule of H2O2 can be decomposed by UV irradiation, in
35
reaction (33), or by O3 in reaction (34); both cases result in the production of HO• radicals. While reactions (31) – (34) are the basis of the O3/H2O2/UV process, reaction (34) is simultaneously the basis of the O3/H2O2 process. It should be noted that similarly for all AOPs where H2O2 is used, the scavenging effect can occur as has been described in previous sections in equations (10) and (16). Therefore, to avoid the adverse effects of H2O2 its concentration should be optimized. Although the treatment of textile wastewater by ozone-based AOPs seems to be well covered in the literature, there are only a few publications focused on a comparison of several ozone-based AOPs used with real wastewater (Chung & Kim [90] – O3/H2O2, O3/UV, O3/H2O2/UV; Azbar et al. [10] – O3/H2O2, O3/UV, O3/H2O2/UV, H2O2/UV, Fenton and coagulation). Moreover, there are only a few more works concerning the treatment of dye solutions by AOPs [85,99–101]. Only one among these works considered the influence of salts — which are a part of the textile wastewater matrix — on the AOPs [101]. However, the authors reported mixed trends with respect to the effects of salts, prompting the need for further research on this topic. Works in which the subject wastewaters were based on industrial compositions did not focus on the influence of the wastewater matrix on AOPs [10,90]. Although Arslan et al. [59] and Arslan Alaton et al. [62] used O3-based AOPs to purify a mixture of dyes and textile auxiliaries, which simulated industrial wastewater after dyeing, these works did not focus on the influence of textile auxiliaries.
II.5. Overview of the literature There are numerous published works concerning the treatment of dye solutions and textile wastewater by ozonation and AOPs. Table 3 presents the more recent studies on this topic that can be found in the literature, in which the references are organized by subject and treatment type.
Table 3. The literature overview of the treatment of dye solutions and textile wastewater by ozonation and AOPs The object of the treatment Dye solutions
Topic of the research Color, COD, TOC or BOD removal
Kinetic study, degradation mechanism
36
Type of the AOP
References
Fenton, photo-Fenton H2O2/UV Ozonation Ozone-based AOPs Fenton, photo-Fenton H2O2/UV ozonation Ozone-based AOPs
[15,21,24,26,27,29,32–36,40– 43,46,48,58,85,102–107] [48,54,55,57,58] [57,67,68,77,83–87,108–110] [85,99–101] [21,22,25–30,33,46,58,102,103,105] [49–58] [52,56,57,67–82,111,112] [99]
Salt or alkaline influence
Toxicity
Simulated wastewater
Cost evaluation Color, COD, TOC or BOD removal
Salt or alkaline influence Kinetic study
Recycling Cost evaluation Toxicity
Mixture of a few dyes Industrial wastewater
Color, COD, TOC or BOD removal
Toxicity
Recycling
Cost evaluation
Reviews
Dyes and textile wastewater
Fenton, photo-Fenton H2O2/UV Ozonation Ozone-based AOPs Fenton, photo-Fenton H2O2/UV Ozonation H2O2/UV, ozonation Fenton, photo-Fenton H2O2/UV Ozonation Ozone-based AOPs Fenton, photo-Fenton Ozonation Fenton, photo-Fenton H2O2/UV Ozonation Ozone-based AOPs Ozonation H2O2/UV, ozonation, ozone-based AOPs H2O2/UV Ozonation Ozone-based AOPs H2O2/UV Ozonation Ozone-based AOPs Fenton, photo-Fenton H2O2/UV Ozonation Ozone-based AOPs Fenton, photo-Fenton Ozonation Ozone-based AOPs Fenton, photo-Fenton H2O2/UV Ozonation Fenton, photo-Fenton H2O2/UV Ozonation Ozone-based AOPs Fenton processes AOPs AOPs + biological treatment Chemical, physical and biological processes
[24,44–48,58] [48,58] [67,82,83] [101] [104] [52] [52,109,110] [62] [8,39,57,59,61,113–115] [59–62] [60,62,88–91,116] [60,61,90,91] [113] [88] [59] [59,60] [60,89,90,116] [60,90] [88] [60] [60] [60,91,117] [60,91] [60] [60,90,116] [60,90] [8–16,37,38] [10,92] [10,14,92–97,118,119] [10] [14] [14,93,118,120,121] [38] [122] [97,123] [10,14] [10] [10,14,96] [10] [23] [7] [124] [2,3,63,125,126]
Although the literature seems to be saturated with reports concerning the ozonation and AOPs of dye solutions and textile wastewater, there are still certain areas that need to be covered. Based on the literature review presented in Table 3, the following main topics for further research can be identified:
37
• Characterization of textile wastewater matrix, • Influence of textile wastewater matrix on efficiency of AOPs, • Detailed study on the treatment of dye mixtures and the interactions between them during AOPs, • Detailed study on the comparison of several AOPs operating under industrial conditions, • Toxicity assessment of textile wastewater treated by AOPs, • Cost evaluation based on industrial equipment (installation).
38
III. Objective of the thesis The main objective of this thesis was to find the most suitable and effective treatment for highly polluted salty textile wastewater, for the production of a recyclable brine that could then be used in the next dyeing operation. The following goals were established within the scope of this research: •
Characterization of the textile wastewater matrix,
•
Examination of the application of AOPs under industrial conditions,
•
Selecting the most effective treatment and validating it by recycling trials.
The research was carried out in close cooperation with an industrial partner, and the results are planned for implementation in industrial practice.
39
IV. Content of the thesis This thesis is structured in eight main chapters as described below.
Chapter 1: Biodegradability assessment of wastewater streams from textile dye house (Julita Wrębiak, Lucyna Bilińska, Katarzyna Paździor, Stanisław Ledakowicz Przegląd Włókienniczy, 5, 2014, 45–49) The preliminary study was focused on textile wastewater matrix investigation. The industrial textile wastewaters from several technological operations were tested. The assessed parameters of the wastewaters revealed huge differences between them. Based on the results, the wastewaters were divided into the streams in terms of their biodegradability. The most polluted, highly colored and salty stream was selected to undergo AOPs. Moreover, the most popular pollutants occurring in this stream were identified and selected for further research.
Chapter 2: Application of advanced oxidation technologies for decolorization and mineralization of textile wastewaters (Lucyna Bilińska, Marta Gmurek, Stanisław Ledakowicz, Journal of Advanced Oxidation Technologies, 18 (2), 2015, 185–194) In this section the main types of AOPs were investigated. The Fenton oxidation, H2O2/UV process and ozonation were compared in terms of color removal and mineralization for selected dye solutions and wastewater. The conditions occurring in industrial textile wastewater, being intense color, alkaline pH, high salt concentration and the presence of surfactants, were determinants of the AOPs efficiencies. Extremely high differences in efficiency were found between the treatments. The Fenton process was found to be the most sensitive to all of the tested conditions. Some additional phenomena, such as the precipitation of ferric ions and elution of chloride ion-radicals during Fenton oxidation were considered as a cause of the very low efficiency observed in decolorization. The sensitivity to intense color was shown to be disadvantageous for the H2O2/UV process. The most promising results were obtained for ozonation of dye solutions in addition to the wastewater.
Chapter 3: Application of Fenton reagent in the textile wastewater treatment under industrial conditions (Stanisław Ledakowicz, Lucyna Bilińska, Renata Żyłła, Ecological Chemistry and Engineering S, 19 (2), 2012, 163–174) The study on the Fenton process gave an explanation for its low efficiency in textile wastewater treatment. The influence of NaCl and the surfactant on the decolorization efficiency was investigated in detail and the mechanism of hydroxyl radical scavenging by chloride ions 40
was studied. The rate constants obtained by pulse radiolysis for the dye decomposition with Cl•− 2 and HO• proved the inhibitory effect caused by NaCl. The negative influence of surfactant on the Fenton process was explained by the formation of a Critical Micelle Concentration (CMC), which impeded the access of hydroxyl radicals to the dye molecules. The argument that Fenton oxidation cannot be used to efficiently treat textile wastewater was confirmed and therefore it was not considered for further investigation.
Chapter 4: Comparison between industrial and simulated textile wastewater treatment by AOPs – Biodegradability, toxicity and cost assessment (Lucyna Bilińska, Marta Gmurek, Stanisław Ledakowicz, Chemical Engineering Journal, 306, 2016, 550–559) The purpose of the research conducted in this section was to compare the H2O2/UV process and the ozone-based AOPs in terms of practical industrial application. The most commonly used dye in the textile industry, Reactive Black 5, was the object of this part of the research. In contrast to the H2O2/UV process, the ozone-based AOPs resulted in a high color removal in both simulated and industrial wastewater. The H2O2/UV process was found to be ineffective in the case of textile wastewater treatment. The increasing degree of mineralization, biodegradability, and average oxidation stage (AOS) as well as a decrease in toxicity were advantages exhibited by the treatments with ozone-based AOPs. The H2O2 used in AOPs was found to be a HO• radical scavenger. There was no additive effect of the simultaneous usage of ozone and H2O2, due to the high pH of the textile wastewater. At the same time, the effect of UV irradiation was extremely poor. These observations as well as the cost evaluation of the AOPs proved that ozonation or ozonation with very low concentrations of H2O2 (0.005 M) are the best treatments for textile wastewater. Chapter 5: Textile wastewater treatment by AOPs for brine reuse (Lucyna Bilińska, Marta Gmurek, Stanisław Ledakowicz, Process Safety and Environmental Protection, 109, 2017, 420–428) The scavenging effect of H2O2 among the AOPs and the lack of synergy between O3, H2O2 and UV irradiation were confirmed by further investigation, which was carried out for Reactive Yellow 145, Reactive Red 195 and Reactive Blue 221. These three dyes are extremely important in the textile industry, being the most commonly used set of dyes, and which when used together in a trichromatic dyeing technique can give a full range of shades, except black. Due to their usage in trichromatic dyeing, they typically occur together in wastewater. Investigating them as a mixture of dyes gave information about their interactions with each 41
other during the AOPs. Firstly, it was found that due to their interactions, the color of the dye mixture does not correlate with the colors of the individual components, and therefore it should be evaluated with the use of a general color indicator (i.e. it should not be quantified by absorbance at λmax of individual dyes). Secondly, it was noticed that the oxidation pathway is specific for each dye, which resulted in differences in the color removal and mineralization. When these indicators were considered for the mixture of dyes, the interactions between dyes were noted. The positive results of color removal, mineralization, biodegradability and AOS proved the advantages of ozonation among the AOPs for the treatment of the selected wastewater stream and dyeing discharge.
Chapter 6: Ozonation as a stage in the technology of the textile wastewater treatment (Lucyna Bilińska, Julita Wrębiak, Stanisław Ledakowicz, Chemical Engineering and Equipment, 54 (4), 2015, 146–147) In this section ozonation was investigated in terms of color removal from the textile wastewater stream, which contained discharges equalized from several technological operations. The mineralization of the wastewater was investigated in this study. A biological plant was proposed as a further step for the effective overall treatment of this wastewater stream.
Chapter 7: Modeling of ozonation of C.I. Reactive Black 5 through a kinetic approach (Lucyna Bilińska, Renata Żyłła, Krzysztof Smółka, Marta Gmurek, Stanisław Ledakowicz, Fibers and Textiles in Eastern Europe 25, 5 (125), 2017, 54 – 60) Based on the previous results, ozonation has been proposed as the best treatment option for textile wastewater. Therefore, in this section a kinetic study of Reactive Black 5 ozonation was carried out. The ozonation was performed in a liquid–liquid system and the second order rate constants for reaction between Reactive Black 5 and ozone were found over the pH range of 1.88 – 6.1. A mathematical model was established to describe the Reactive Black 5 decay on the basis of the experimental data. The parameters of the model were determined by solving a non-linear inverse problem. The model was found to be reliable in acidic reaction media.
Chapter 8: Summary and Conclusion An overall summary of the results was presented in this chapter. The AOPs were compared in terms of their effectiveness in color removal from dye solutions and wastewater, and their suitability for industrial implementation was discussed. The results of mineralization, biodegradability, toxicity and cost evaluation were taken under consideration. A general 42
conclusion on the ability of AOPs for textile wastewater treatment and the recycling of purified brine was given as a source of information to assist in the application of this technology in the textile industry.
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50
Chapter 1 Biodegradability assessment of textile dye house wastewater streams
Julita Wrębiak, Lucyna Bilińska, Katarzyna Paździor, Stanisław Ledakowicz Przegląd Włókienniczy 5, 2014, 45 – 49 ISSN-1731-8645 (original language: Polish)
Abstract An analysis of industrial wastewater taken from a textile factory in the Lodz district was performed. Specific conductivity, pH, toxicity, COD, BOD5, TC, TOC, total phosphorus, total nitrogen and chloride content of samples were determined. Based on the conducted survey, a wide range of bath parameters from the dyeing process was found. Biodegradability of the tested baths were determined according to BOD5/COD, N/P, BOD5/N/P and toxicity values. The main objective of this paper was to propose a way of dividing dye-house industrial wastewater into separate streams, and to suggest methods of their treatment.
1.1. Introduction The use of advanced technologies is a crucial issue within the textile industry and the operations of textiles processing are still characterized by the production of huge amounts wastewater that is released into environment. Based on literature data it can be estimated that in case of dark shade dyeing, the pollution load released from dye baths can be: 80 g of dyes, 190 g of organic and inorganic auxiliaries and up to 1500 g salts (NaCl, Na2SO4), per 1kg of textiles [1–5]. Consequently, textile wastewater is characterized by intense color, high salinity, high levels of dissolved chemical substances, varied pH values, and adverse BOD5/COD ratios. This leads to wastewater that is poorly biodegradable and toxic to aquatic organisms [6]. Industrial wastewater, including textile wastewater, requires a special approach to treatment. As a result, finding efficient and relatively inexpensive methods for the purification of textile wastewater has become a topic of interest recently. Biological treatment methods seem to be the most economically justified in this case, with activated sludge being the most commonly used. Additionally, reports of biological sprinkled bed or wetland purification methods can also be found in the literature. However, in many instances where biological methods were used for the treatment of textile wastewater, the results obtained were not satisfactory - especially with respect to COD levels and color removal, which are often too low [5,7,8]. Other popular methods for the decolorization and purification of textile wastewater include: adsorption, ion exchange (eg. ion exchange resin), oxidation (using ionizing radiation, advanced oxidation techniques - AOPs) and separation (ultrafiltration, nanofiltration, reverse osmosis). Results obtained using these methods were satisfactory: color removal of up to 99%, COD removal rates of more than 90% and increased ratio of BOD5/COD [3,9–16]. However, despite these results, the treatment of wastewater by these methods is significantly more expensive compared to biological treatment processes [7]. 53
The issue of on-site textile wastewater treatment is becoming increasingly significant due to the rising costs of their discharge into the municipal sewage system and the extremely prohibitive discharge limits from both government and EU legislation. According to the Polish Business Register (code: PKD 13.30.Z), there are 1,869 textile dye-houses in Poland, with more than 500 located in the Lodz region. At the same time, a large number of knitting-houses have expanded their activities by providing services such as: washing, bleaching, dyeing and printing fabrics. The largest of these plants can generate up to 5200 m3 of wastewater per day, which has led to more and more textile factories establishing their own modern wastewater treatment plants based on mechanical, chemical or biological processes [17]. Whilst the issue of textile wastewater treatment is highly complex, it has become the topic of great interest due to extremely high wastewater discharge costs. For the treatment of textile wastewater, biological treatment is the most cost-effective method. However when treatment effectiveness is the most important factor, physical and chemical purification methods should be used. A reasonable solution seems to be a combination of the different treatments mentioned, so that a satisfactory degree of pollutant removal can be achieved in an economically feasible manner. In this paper, a parameter analysis of wastewater samples taken from several different technological operations was conducted. Based on the results of this analysis, the wastewater streams most susceptible to biological treatment have been selected and purification methods proposed.
1.2. Experiment An analysis of wastewater parameters was performed for industrial process baths taken from the dyeing operation of cotton with reactive dyes. The dyeing process was conducted at 60°C. The liquor ratio was 1:7 (w/w textile to water) and the industrial batch weight was 160 kg of textile material. The dyeing recipe was set on “steel graphite” shade. The wastewater samples were taken directly from the industrial dyeing machine, just before the bath draining. Dyes used for the dyeing process are characterized in Table 1.1. The chlorides (Cl-), total phosphorus (Ptotal) and chemical oxygen demand (COD) were measured spectrophotometrically using Hach Lange tests with a DR 3800 spectrophotometer. The total carbon (TC), total organic carbon (TOC) and total nitrogen (Ntotal) were determined using the TOC-TN analyzer type IL550. Conductivity was measured using an Elmetron CX461 meter with temperature compensation. Assessment of apparent color (without filtration) was carried out by absorbance measurements at three wavelengths, using a DR 3800 Hach Lange spectrophotometer in accordance with PN-EN ISO 7887:2002 standard. Absorbance at 54
a wavelength corresponding to their maximum values and the color in Hazen scale was also identified (DR 3800). Biochemical oxygen demand (BOD) was determined over 120 hours, by standard dilutions method (samples diluted from 10 to 200 times) [18]. Toxicity of samples were tested using a ToxTrak test by Hach Lange, performing a minimum of 5 repetitions for each test and preparing additional dilution for the selected dye baths.
Table 1.1. Characteristics of the dyes used in the dyeing process Trade name
Structure
Characteristic C.I. Reactive Yellow 145 CAS No. 93050-80-7
Synozol
Reactive, azo, bifunctional type
Yellow
Molecular weight 1026 g/mol
KHL
λmax 420 nm Application: trichromatic dyeing of cellulosic fibers, 60oC C.I. Reactive Red 195 CAS No. 93050-79-4
Synozol
Reactive, azo type
Red
Molecular weight 1136 g/mol
K-3BS150
λmax 543 nm Application: trichromatic dyeing of cellulosic fibers, 60oC C.I. Reactive Blue 221 CAS No. 93051-41-3
Synozol Blue KBR
Reactive based on complex Molecular weight 890 g/mol λmax 604 nm Application: trichromatic dyeing of cellulosic fibers, 60oC
Density measurements have not been performed as previous studies in similar trials have shown density to be nearly equal to the density of water, or slightly higher for samples with a high salt content (i.e. baths 4 and 5). Dry matter, organic dry matter and suspension results are not included in the work, due to the low values of these parameters (according to measurements made previously for analogous samples) [6].
55
1.3. Discussion of the results The bath parameter results from the reactive dyeing process are shown in Table 1.2. Table 1.3 presents results of the spectrophotometric analysis of apparent color. The measures used for color analysis were: absorbance values, absorption coefficients (at four specific wavelengths), and Hazen scale values. Table 1.4 shows the values of BOD5/COD, nitrogen to phosphate N/P and BOD5/N/P parameters. Table 1.2. Parameters of various wastewater streams generated during dyeing of cotton fabric with reactive dyes Clmg/L
COD mgO2/L
BOD5 mgO2/L
TC mg/L
TOC mg/L
Ntotal mg/L
Ptotal mg/L
4920
705
2890
-
1197
1065
77.9
8.01
2400
79.9
2180
-
749
749
37.6
9.36
4.52
1462
65.5
1000
-
370
370
16.8
6.91
4. dyeing
11.5
61597
23275
1200
76
1033
423
24.5
8.52
5. rinsing
10.6
23508
7375
490
35
438
170
10.1
3.00
6. acidification
3.97
11312
3206
852
280
525
525
6.8
4.01
7. washing after dyeing
5.5
5449
1456
1175
400
466
466
10.5
0.90
8. rinsing
6.83
2630
311
536
314
239
217
5.3
0.43
9. rinsing
7.94
1347
211
188
128
129
79
2.2
0.28
10. rinsing & neutralization
5.4
1071
104
215
200
119
119
1.2
0.21
11. mixed
9.85
12576
3567
956
240
533
407
18.7
4.54
pH
Conductivity µS/cm
1. washing with bleaching
9.97
2. acidification
4.38
3. rinsing
No. of the bath
Table 1.3. Color characteristics of individual wastewater streams Absorbance & spectra absorption coefficient at wavelengths: No. of the bath
56
436 nm
525 nm
λ max.
620 nm
Hazen scale Pt-Co
A
α, m-1
A
α, m-1
A
α, m-1
λ max, nm
A
α, m-1
1. washing with bleaching
0.354
35.4
0.24
24.0
0.174
17.4
380
0.515
51.5
2. acidification
0.226
22.6
0.146
14.6
0.101
10.1
374
0.35
35.0
179
3. rinsing
0.09
9.0
0.059
5.9
0.043
4.3
401
0.111
11.1
70.9
4. dyeing
1.053
105.3
1.323
132.3
0.905
90.5
551
1.391
139.1
838
5. rinsing
0.617
61.7
0.773
77.3
0.608
60.8
554
0.846
84.6
492
6. acidification
0.395
39.5
0.539
53.9
0.437
43.7
551
0.599
59.9
314
7. washing after dyeing
0.82
82.0
0.831
83.1
0.952
95.2
559
0.972
97.2
647
8. rinsing
0.404
40.4
0.414
41.4
0.486
48.6
606
0.503
50.3
315
9. rinsing
0.167
16.7
0.163
16.3
0.147
14.7
612
0.204
20.4
135
10. rinsing & neutralization
0.08
8.0
0.072
7.2
0.082
8.2
603
0.084
8.4
64.9
11. mixed
0.412
41.2
0.449
44.9
0.389
38.9
553
0.49
49.0
327
290
Based on the obtained results, it can be concluded that the textile wastewater samples were not highly loaded with organic matter. They are characterized by low biodegradability due to the low content of nitrogen and phosphorus and by variable pH, temperature and salinity. Among the studied wastewater streams, solutions 1, 2 and 4 are most polluted. They are characterized by high TC values and the highest COD levels. It can be concluded that baths 4, 5 and 6 contain large amount of chlorides as confirmed by conductivity measurements, which were caused by NaCl addition to dye bath 4. High chloride content persisting in three consecutive baths can be explained by a low liquor ratio (1:7) that has been set for the industrial dyeing process. Biochemical oxygen demand (BOD5) is the key parameter when degree of pollution and susceptibility to biodegradation are concerned. Unfortunately, the presence of even small amounts of hydrogen peroxide (H2O2) makes the BOD5 results inaccurate by providing a source of oxygen. At the same time, the higher concentrations of H2O2 cause a strong biocidal effect. Therefore, the BOD results of the first three baths could be incorrect and were not presented in Table 1.2. It should also be noted that the results of biochemical and chemical oxygen demand may originate in the molecular structure of the dye or in the characteristics of the wastewater (high pH and salinity). The neutralization of alkaline wastewater, optimization of aeration or flow, and the acclimatization of microflora can be helpful to the biological treatment of dye industry wastewater streams. However, the effectiveness of such treatment methods may still be insufficient. Therefore, it can be concluded that exclusion of wastewater streams characterized by high salt content, and an adverse ratio of BOD5/COD, will have a positive impact on the operating of a biological treatment plant. At the same time, the results shown in Table 1.4, indicate the adverse ratio of nitrogen to phosphorus for biological processes in case of baths 4 and 5.
Table 1.4. Baths biodegradability indicators (baths after dyeing with reactive dyes) No. of the bath
1
2
3
4
5
6
7
8
9
10
11
BOD5/COD
-
-
-
0.06
0.07
0.33
0.34
0.59
0.68
0.93
0.25
N/P
9.73
4.01
2.43
2.88
3.37
1.70
11.62
12.17
7.89
5.51
4.12
BOD5: N :P
-:10:1
-:4:1
-:2:1
9:3:1
12:3:1
70:2:1
442:12:1
725:12:1
459:8:1
935:6:1
53:4:1
57
Figure 1.1 shows the results of toxicity assays of the baths and the histogram shows the average results of five measurements, where error bars are based on the standard deviation. The toxicity assay was based on the use of the resazurin dye, which changes color from blue to pink as an indicator of microorganism respiration. The results of inhibition (%) were obtained by ∆A
using the formula %I= [𝟏− ∆Asmple ] ∙ 100, where A is the absorbance at a wavelength of 603 water
nm. Values outside of ± 10% indicate toxicity of the test sample. The substances present in textile wastewater, especially acidic substances and those that increase the osmotic pressure, strongly inhibit microorganism growth. However, there are some factors that can accelerate a change in resazurin color, resulting in negative inhibition [%] and these results are also considered as toxic. Therefore the values obtained by ToxTrak method do not directly indicate environmental harm, but are a reliable source of information on the toxicity and biodegradability. Extremely high inhibition values (%) have been found for samples 1 and 2, which can be a result of very high hydrogen peroxide content arising from the cotton fabric bleaching process. Hydrogen peroxide at a concentration of a few percent has a strong bactericidal effect, and a 5% solution destroys even the spores of bacteria [19]. A high standard deviation value of sample 3, is due to the gradual decomposition of H2O2, which is also present in this sample (at a lower concentration than in samples 1 and 2). The low pH value of 3.97 in sample 6 is probably influenced by a faster rate of reduction of resazurin. Toxicity of the mixture of all baths from the dyeing processes (sample 11) was influenced by the toxicity of samples 1 and 2. Figure 1.2 shows that sample 11 diluted 10 times was nontoxic. However, the same dilution of samples 1 and 4 did not completely eliminate the adverse effect of the reduction of resazurin, and the samples were still toxic. Detailed analysis of the individual baths taken from dyeing process, allows the evaluation of biological treatment methods for selected textile wastewater streams. BOD5 values are the main indicator of biodegradability, as well as toxicity assay using ToxTrak method which can be used to truly assess the impact of wastewater composition on activated sludge microflora. However, the toxicity results of samples 4 and 7 could be slightly higher because of the sample own color. These samples indicated higher absorbance values at wavelength 603 nm (samples 4 and 7), however it is not necessary to dilute the samples in the ToxTrak method except for those with a very intense color. Based on the results it can be concluded that sufficient biodegradability can be expected in the case of baths 6 to 10. Additionally, it can be concluded that prewash baths without bleaching
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can also be directed into the biological treatment plant. The acidic pH of samples 2, 3, 6, 7 and 10, due to the presence of formic or acetic acid, will not disrupt the purification by biological treatment methods. These acids in dilution can act as a substrate for the microorganisms. Due to the very low load, baths 9 and 10 can be re-used for rinsing without prior purification.
Figure 1.1. The toxicity assay, obtained by ToxTrak method
Figure 1.2. The toxicity results of 10-fold diluted baths (1 - prewash and bleaching, 4 – dyeing, and 11 - a mixture of all the bath of the whole process) obtained by ToxTrak method
1.4. The management of the wastewater streams Based on the results of the research, the characteristics of each bath of the reactive dyeing process were established and the baths were divided into more and less biodegradable groups. In the case of the baths containing hydrogen peroxide 1, 2, 3, a high toxicity for activated sludge was noted. Similar observations were made in the case of dyeing bath 4, and rinsing after dyeing baths 5, 6. These baths are characterized by a low ratio of BOD5/COD, high salinity and intense 59
color. Separation of baths 4, 5, 6 from the other streams, will allow for the effective biological treatment of those streams e.g. sludge method. Figure 1.3, proposes methods for purification of each bath from the reactive dyeing process. The main idea of the bath management system is to avoid directing any wastewater into municipal sewage system. Following the positive literature reports [20–22] and many years of experience among research team members, a combination of chemical and biological (e.g. activated sludge method) treatment methods was suggested. Taking into account the results of the research, bath 1 (due to the high concentration of H2O2 – 0.5 g/L) and bath 4 were selected to undergo the decolorization by advanced oxidation processes (AOPs). This way, ready to reuse brine could be obtained. To avoid dilution of the brine (obtained from bath 4), bath 1 can be a part of the next stream as well. Baths 2, 3, 5, 6 and 7 were selected to undergo biological treatment, preceded by preliminary purification by AOPs. This procedure enables increased biodegradability (by using AOPs pretreatment) of potentially poorly biodegradable wastewater. After pretreatment by this method, the wastewater can be successfully biologically purified. Other baths can also be purified by a biological method and their final parameters improved by AOPs in the next step of the treatment. Directing the wastewater streams that contain low concentration of hydrogen peroxide to the biological reactors to provide an additional source of oxygen for the microorganisms is also possible, but only in case of stable treatment plant process conditions. The baths from the reactive dyeing process are characterized by a great diversity of their parameters. The proposed method of dividing these baths into separate streams, and subsequently merging them into appropriate groups for separate purification, allows for their rational management. Furthermore, there is the possibility of reusing purified water (brine) directly in the production process, which is closing the water cycle. Combination of a number of different purification methods such as the less efficient and cheaper biological option, together with more effective methods e. g. AOPs, allows an overall reduction in the total cost of wastewater treatment.
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Figure 1.3. Diagram of the management of the wastewater streams from the dyeing process in terms of treatment method
1.5. Conclusions Due to the huge amount of wastewater generated by the textile industry, its purification has become a topic of great interest. The on-site treatment plant can be an effective method of cost reduction. Biological wastewater treatment is recommended as the least energy intensive and most ecological method. It can be successfully used for all baths, except these with a high content of hydrogen peroxide or sodium chloride. Additionally, inclusion of laundry and urban wastewater into the textile wastewater plant can further improve its effectiveness. Textile plants have varied production profile and this is the main problem in the development of treatment plants. An additional difficulty is the wide range of chemicals used, which is due to the variety of different textile materials. Due to these very large variations in
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the textile industry, an individual detailed analysis of wastewater is necessary in each plant that plans to implement wastewater purification systems.
1.6. Acknowledgement The research results presented were obtained during the project ICBTOS No. PBS2/A/22/2013 funded by the National Centre for Research and Development
1.7. References
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M. Wawrzkiewicz, Z. Hubicki, Equilibrium and kinetic studies on the adsorption of acidic dye by the gel anion exchanger, J. Hazard. Mater. 172 (2009) 868–874. doi:10.1016/j.jhazmat.2009.07.069.
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J. Perkowski, L. Kos, S. Ledakowicz, Oczyszczanie ścieków włókienniczych nadtlenkiem wodoru w połączeniu z jonami żelazawymi, Przegląd Włókienniczy + Tech. Włókienniczy. 1 (1998) 26 – 29.
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L. Kos, J. Perkowski, S. Ledakowicz, Pogłębione utlenianie ścieków włókienniczych z procesu barwienia bawełny, Przegląd Włókienniczy + Tech. Włókienniczy. 1 (2000) 33 – 35.
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J. Sójka-Ledadowicz, L. Kos, R. Żyłła, K. Michalska, L. Kos, R. Żyłła, Utlenianie chemiczne jako etap wysokoefektywnych technologii oczyszczania ścieków włókienniczych umożliwiających powtórne wykorzystanie wody, Przegląd Włókienniczy Włókno Odzież Skóra. 6–7 (2009) 61 – 64.
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R. Żyłła, J. Sójka-Ledakowicz, E. Stelmach, S. Ledakowicz, Coupling of membrane filtration with biological methods for textile wastewater treatment, Desalination. 198 (2006) 316–325. doi:http://dx.doi.org/10.1016/j.desal.2006.02.008.
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S. Ledakowicz, L. Bilińska, R. Żyłła, Application of Fenton’s reagent in the textile wastewater treatment
under industrial conditions, Ecol. Chem. Eng. S. 19 (2012). doi:10.2478/v10216-011-0013-z. [17]
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W. Hermanowicz, J.R. Dojlido, Fizyczno-chemiczne badanie wody i ścieków, “Akardy,” 1999.
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L.S. P. Olesiak, Skuteczność wybranych związków dezynfekcyjnych wobec przetrwalników Bacillus, Inżynieria I Ochr. Środowiska. 15 (2012) 41–50.
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R. Zarzycki, Zaawansowane techniki utleniania w ochronie środowiska, Polska Akademia Nauk, Oddział w Łódzi, 2002.
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S. Ledakowicz, M. Solecka, R. Żyłła, Biodegradation, decolourisation and detoxification of textile wastewater enhanced by advanced oxidation processes, J. Biotechnol. 89 (2001) 175–184. doi:10.1016/S0168-1656(01)00296-6.
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Chapter 2
Application of advanced oxidation technologies for decolorization and mineralization of textile wastewater
Lucyna Bilińska, Marta Gmurek, Stanisław Ledakowicz Journal of Advanced Oxidation Technologies 18 (2), 2015, 185 – 194 ISSN 1203-8407
Abstract Reactive dyes are the most abundantly used in textile industry due to their high color fastness, wide color spectrum as well as low energy consumption. The presence of these dyes in effluent released into receiving waters has become a serious environmental problem not only related to their color but mainly because of the hazardous byproducts. An environmentally sustainable development policy in textile industry requires development of new technologies to reduce water consumption as well as negative environmental impact of discharged wastewater. Advanced oxidation processes (AOPs) are the most promising technology for decolorization and mineralization of wastewater contamination. This paper presents the results of ozonation, Fenton oxidation and H2O2/UV treatment of simulated as well as industrial textile wastewater containing Reactive Black 5. The AOPs were carried out under varied process parameters such as: dye concentration, pH, oxidant and detergent doses and wide range concentration of NaCl. The decolorization was followed by absorbance while oxidation and mineralization progress by COD and TOC measurements respectively. The almost completely inhibition of decolorization in the presence of NaCl in concentration usually used in industry (60 – 80 g/L) was observed for Fenton reagent. The slight inhibition was observed for H2O2/UV system while for ozonation there was no influence of NaCl on the decolorization. The similar relationship of the influence of anionic detergent concentration was observed. The experiments indicated that ozonation is the best method from TOC and COD removal point of view. For the decolorization of real textile effluents two streams were used 1) containing C.I. Reactive Yellow 145, Red 195, Blue 221 and 2) Reactive Black 5. As a result of the treatment, almost complete decolorization of the wastewater was obtained. Considering both the experimental results and technological problems, it can be presumed that advanced oxidation with an application of ozone or H2O2/UV are a very promising techniques for potential industrial implementation, however from economic point of view the more reasonable is the ozonation process.
2.1. Introduction Reactive dyes are the most frequently used in the textile industry in comparison to other, due to their exceptionally good properties – good cloth application and color fastness. Their usage is very wide. Reactive dyes are successfully used in dyeing processes of cellulose, wool and polyamide. Their usage in textile printing operations is also popular. The annual production of reactive dyes amounts 140 000 tones [1]. Reactive Black 5 is the most commonly used in dyeing processes among them. The use of reactive dyes in textile industry operations cause 67
serious environmental pollution. In case of reactive dyeing process special cloth pretreatment is required, what inserts multitude of operations (washing, bleaching, rising, etc.). Moreover, the use of appropriate auxiliaries to the dyeing bath is necessary. 60 – 80 g/L NaCl or Na2SO4, the dyeing assistant, usually active surface agents (SAA) in amount from 0.5 to 2.0 g/L must be used and alkaline pH is also required (e.g. NaOH in concentration of 1.5 g/L or higher must be used). The relatively low fixation of reactive dyes is additional problem causing pollution. Residues of dyes, detergents, organic and inorganic auxiliaries cause high color, pH and salinity of textile wastewater. It is also characterized by a high COD and a disproportionately low BOD, which means that it is not very susceptible to biological degradation. The average sized dyehouse, which works 24 h/day can generate daily between 2400 – 5200 m3 of wastewater. Therefore, it is important to develop effective textile wastewater treatment methods. Techniques that use high electrochemical oxidation potential (EOP) of hydroxyl radicals (2.8V) or ozone (2.07 V) may be an effective treatment or pre-treatment of the textile wastewater. There are many literature reports concerning the treatment of dye solutions and textile wastewater by applying the AOP and ozone [2]. However, only few works are focused on the effects of salts or SAAs on dyes degradation process [3–8]. In this work an ability of ozonation, H2O2/UV process and Fenton oxidation application for reactive dye solutions and industrial textile wastewater treatment was tested. The initial concentration of the dye, pH and reagents doses as well as the effect of NaCl and SAA presence were considered. TOC and COD reduction was also determined and the best method of textile wastewater treatment was chosen.
2.2. Experimental
2.2.1 Materials C.I. Reactive Black 5 (RB5) as purified reagent was obtained from Boruta-Zachem (Poland). Setazol Black DPT (industrial product based on RB5) was purchased from SetasKiyma (Turkey) and it was present in wastewater samples. C.I. Reactive Yellow 145 as Synozol Yellow KHL (RY145), C.I. Reactive Red 195 as Synozol Red K-3BS 150% (RR195), C.I. Reactive Blue 221 as Synozol Blue KBR (RB221) industrial dyestuffs were purchased from Kisco Co. (Turkey). Table 2.1 contains chemical structures and characteristic of these dyes. Ferrous sulfate heptahydrate (FeSO4×7H2O), NaCl, H2SO4, were purchased from POCH (Poland) all as A.R. NaOH A.R. was product of Stanlab (Poland). The buffer basis: Na2HPO4 and KH2PO4 were purchased from Chempur (Poland) as well as Na2SO4 and hydrogen peroxide 68
solution (30%, w/w) all were analytical reagents. Industrial dyeing assistant – Perigen LDR (SAA – naphthalenesulfonic acid and carboxylates mixture) was obtained from Textilchemie Dr. Petry Co. (Germany).
Table 2.1. Characteristic of the dyestuff Name
Synozol Yellow KHL (RY145)
Chemical structure
Characteristic
C.I. Reactive Yellow 145 CAS No. 93050-80-7 Molec. mass 1026 g/mol λmax 420 nm
C.I. Reactive Red 195 Synozol Red K-3BS150 (RR195)
CAS No. 93050-79-4 Molec. mass 1136 g/mol λmax 543 nm
C.I. Reactive Blue 221 Synozol Blue KBR (RB221)
Setazol Black DPT (RB5)
CAS No. 93051-41-3 Molec. mass 890 g/mol λmax 604 nm
C.I. Reactive Black 5 CAS No. 12225-25-1 Molec. mass 991.8 g/mol λmax 596 nm
2.2.2 Analytical methods Ozonation of the wastewater and dyes solutions was carried out in semibatch glass reactor with capacity of 1 L. The pH and temperature of tested samples was measured by using Elmetron meter (Poland). Mixing was kept by using a magnetic stirrer (Wigo type ES 21). Ozone was produced by using Ozonek ozone generator (Poland). The maximal ozone production provide by this device was 65 mgO3/L at a flow rate of 20 L/h (minimum flow) and 69
the supply current 8.5 kV. The oxygen used to ozone production was supplied from a gas cylinder. Ozone concentration was measured with ozone meter BMT type 963 Vent at the inlet and outlet of the reactor. During the ozonation process samples were collected and reaction progress was stopped by 0.01 M Na2SO3 addition. The H2O2/UV study was conducted in a merry-go-round device with quartz test tubes (volume: 10 cm3, average optical path length: 0.85 cm) placed between two exposure panels (distance between the panels during the study was 30 cm). Each panel consisted of three lowpressure mercury lamps (G8T5 Hg USHIO), which emitted the light with main band at 253.7 nm (88.6%). During experiment test tubes were collected at appropriate intervals, and tubes with pure water (“blank”) were inserted in their place, in order to ensure uniform irradiation of tested solutions. Fenton oxidation experiment was carried out in batch reactor covered with aluminum foil to minimize the exposure of samples to light. Mixing speed was set at 200 rpm. The pH and temperature of tested solutions were measured by using Elmetron meter (Poland). Fenton reaction progress was stopped by pH change (phosphate buffer pH 12). The temperature for all experiments was kept at 23oC (± 0.5). All tested samples collected at certain time intervals were measured by spectrophotometer (Helios Thermo). Calibration plots based on Lambert-Beer’s law for each dye were used to determine the dye concentration of the samples. Total organic carbon (TOC) was measured on HACH IL 550TOC-TN apparatus. chemical oxygen demand (COD) measurement was carried out using standard method with dichromate (VI), at HACH-LANGE apparatus (DR 3800) accordance with the procedure specified by the manufacturer.
2.3. Results and discussion
2.3.1 Effect of the initial dye concentration Various decolorization rates for different initial concentrations of RB5 could be observed for the tested methods (Figure 2.1). In a case of H2O2/UV process, investigated initial concentrations of RB5 were: 50, 104, 125, 163, 214, 346 mg/L. The reaction solutions were irradiated with 6 UV lamps. The experiment was conducted in deionized water (Mill-Q) at pH 6.28. In Figure 2.1 B was shown that degradation RB5 during H2O2/UV process is strongly dependent on initial dye concentration. The degradation efficiency increases with increasing initial concentration of this dye. It rises to some limit RB5 concentration, and then it starts to decrease. This phenomenon occurs in a relatively narrow range of dye concentrations. It can be 70
explained by a fact that that increasing dye concentration increases the fraction of radiation absorbed by this compound, thus the amount of radiation absorbed by the hydrogen peroxide decreases. Moreover, high concentration of the dye makes access of the light to the deeper layers of the solution impossible because of the intensive color. The maximum concentration in the case of RB5 decolorization by H2O2/UV seems to be 125mg/L. The initial dye concentration influence on decolorization rate presents differently to ozone and a Fenton reagent treatment. In case of ozonation the initial concentrations of RB5 were in the range of 125 – 1000 mg/L. The dye was dissolved in distilled water. The reaction was conducted at pH 12 (phosphate buffer) to keep the radical mechanism. The highest decolorization rate was reported at RB5 initial concentration of 500 mg/L. It can be associated with a continuous supply of a certain concentration of O3 in a time unit (a semi-continuous reactor). That allowed the oxidation of sufficiently large number of dye molecules. A significant decrease in the rate could be observed for the initial dye concentration of 1000 mg/L, due to insufficient amount of the hydroxyl radicals in relation to the number of the dye molecules and the degradation products involved in oxidation process with HO• . Slowdown effect for ozone was not as significant as in case of H2O2/UV process (no UV radiation dependence). The Fenton reaction was investigated for initial dye concentrations in the range of 50 to 2000 mg/L. The experiment was carried out in a batch reactor at pH 3 (H2SO4) by dissolving the reactants in distilled water. The highest decolorization rate was observed for RB5 initial concentration of 1000 mg/L at a mass ratio of the reactants equal 0.05 (C0Fe2+ 5 mg/L). But not very high final color reduction was obtained at the same time. It may be induced by an extra coagulation phenomenon caused by Fe3+ ions (which is indicated by experimental data points distribution on the Figure 2.1 C – acceleration in the initial process phase and the subsequent slowdown). Precipitation and sludge production was observed also by other authors [9]. During Fenton oxidation the process slowdown was observed for initial RB5 concentrations higher than 1000 mg/L. Analyzing the decolorization process by Fenton reagent one can recognize two stages: the first one with very high decolorization rate and the second one proceeds with much slower variation. These observations support the hypothesis that Fenton process could be divided into two stages [3,10].
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Figure 2.1. Effect of the initial dye concentration on the decolorization of RB5 solution by: A) ozonation (C0RB5 G 125, 200, 500, 1000 mg/L, CO3 20 mg/L, Q in 20 L/h, pH 12), B) H2O2/UV process (C0RB5 50, 104,125, 214, 346 mg/L, C0H2O2 6.8×103 mg/L, pH 6.28) and C) Fenton oxidation (C0RB5 50, 125, 500, 1000, 2000 mg/L, C0Fe2+ 5 mg/L, C0H2O2 100 mg/L, pH 3)
2.3.2 Effect of pH RB5 decomposition by H2O2/UV process, ozonation and Fenton’s reagent was examined in a wide pH range from 2 to 12. In case of H2O2/UV process and ozonation, the phosphate buffers were used to set pH value and during the Fenton oxidation H2SO4 and NaOH were used. Figure 2.2 shows the effect of pH on RB5 decomposition. A strong pH dependence of the RB5 rate decomposition for all tested methods can be observed. The decay efficiency of the RB5 using H2O2/UV is almost the same for pH 5 and 7 and decreases with increasing pH value. This may be explained by the reaction of dissociation of hydrogen peroxide (pKaH2O2 11.6) [11]. Ions formed during the reaction, react with hydrogen peroxide as well as with hydroxyl radicals. Thus this phenomenon of decreasing the rate of RB5 degradation with increasing pH can be seen. In the case of Fenton reagent the highest RB5 decomposition rate was obtained at pH 3 to 4 (Figure 2.2). Decomposition of hydrogen peroxide is catalyzed by Fe2+ ions in that pH values most efficiently, which is consistent with the results obtained by other authors [9,12]. At higher pH values partial precipitation of iron ions to hydroxide is much more probable. It can be the reason of reduction efficiency in dye decolorization by Fenton reagent. The highest RB5 decomposition rate using ozone was observed at pH 12. Ozone decomposing in alkaline pH generates HO• radicals. Ozonation rate is greater in alkaline pH because HO• radicals (2.8V)
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redox potential is higher than ozone (2.07V), which was confirmed in other studies as well [11,13]. Ozonization was carried out also for reaction mixture without buffer. In these case decrease of pH value from 5.8 to 3 was observed. These phenomenon can be caused by acidic degradation products appearance.
G Figure 2.2. Effect of pH on the decolorization of RB5 solution by: A) ozonation (C0RB5 125 mg/L, CO3 20 mg/L, Q in 20 L/h, pH 12) , B) H2O2/UV process (C0RB5 125 mg/L, C0H2O2 6.8×103 mg/L, pH 6.28) and C) Fenton oxidation (C0RB5 125 mg/L, C0Fe2+ 5 mg/L, C0H2O2 25 mg/L, pH 3)
2.3.3 Effect of reagents dosages The pseudo-first order constants vs oxidative doses have been presented to showed the effects of reagents dosages in Figure 2.3. To determine the effect of O3 dose on the RB5 solution decolorization the ozonation process was carried out in the O3 concentration range of 5 to 42.3 mg/L. Additionally, different values of flow gas rate was used (Qin 20, 30 and 40 L/h). The reaction was conducted at pH 12 (phosphate buffer) and RB5 initial concentration was set at 125 mg/L. A strong dependence of the decomposition pseudo-first order constants on the gas flow rate was observed (Figure 2.3 A). This suggests that in the reactor used in the experiment, the mass transfer limitation is significant. Simultaneously, the growth of RB5 degradation pseudo-first order constants was achieved with increasing doses of O3. For higher concentrations of O3 more of its molecules may have been absorbed in the liquid phase and react with the dye.
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Figure 2.3. Effect of reagents dosages on the decolorization rate of RB5 solution by: A) ozonation (C0RB5 125 G mg/L, CO3 20 mg/L, Q in 20, 30, 40 L/h, pH 12) , B) H2O2/UV process (C0RB5 125 mg/L, C0H2O2 0.34×103, 1.02×103, 1.70×103, 3.40×103, 6.80×103, 10.20×103, 12.59×103 mg/L, pH 6.28) and C) Fenton oxidation (C0RB5 125 mg/L, C0Fe2+ 5 mg/L, C0H2O2 25, 50, 100, 250 mg/L, pH 3)
Additionally, in Figure 2.4 ozonation efficiency for O3 concentration range of 5 to 42.3 mg/L, flow gas rate Qin 40 L/h, pH 12 and RB5 initial concentration 125 mg/L was shown. Figure 4 B shows that increasing O3 fed to the reactor cause faster saturation of the reaction mixture. The efficiency of the ozonation process shows (Figure 3.4 A) that the highest consumption of the ozone can be observed at the beginning of the RB5 decomposition and that with increasing O3 initial concentration more rapid RB5 decomposition can be observed. Moreover, it should also be noted that the reaction solution becomes saturated with ozone only when the RB5 is almost completely removed from the reaction solution. Figure 3.4 C shows that increasing O3 concentration at the reactor inlet results in an decrease of CRB5removed/CO3applied ratio. It can be seen that in order to reach the higher CRB5removed/CO3applied ratio is not necessary to apply higher O3 concentration, because the applied dose of ozone is already in excess.
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Figure 2.4. Effect of the ozone dosage on A) ozonation efficiency, B) ozone concentration on reactor outlet, C) G CRB5removed/CO3applied ratio; (C0RB5 125 mg/L, CO3 5 mg/L, 10 mg/L, 20 mg/L, 30 mg/L, 42.3 mg/L, Q in 40 L/h, pH 12)
In case of H2O2/UV process tested RB5 solutions contained from 0.34×103 to 12×103 mg/L of hydrogen peroxide. 125 mg/L RB5 concentration was dissolved in Milli-Q water (pH 6.28). The results obtained during the irradiation of tested solutions are illustrated in Figure 2.3 B. RB5 decomposition pseudo-first order constants increases with increasing hydrogen peroxide concentration up to a point and then begins to decrease (due to scavenging effect). This decrease is caused by the hydrogen peroxide reaction with hydroxyl radicals. The optimum concentration of hydrogen peroxide for H2O2/UV process is equal 6.80×103 mg/L. For RB5 solution discoloration with Fenton reagent experiments were carried out at a constant Fe(II) ions concentration of 5 mg/L (FeSO4×7H2O was used), and a variable amounts of H2O2 from 25 to 500 mg/L (30% H2O2 – reagent). Reagents ratio examined in the experiment were: 5:25, 5:50, 5:100 and 5:500 for C0Fe2+ : C0H2O2 , respectively. Very high RB5 initial decomposition pseudo-first order constants values were observed even when low concentration of hydrogen peroxide was used (Figure 2.3 C). Therefore, it is reasonable to use lower concentrations of the reactants at a ratio of 1:5. No scavenging effect by increasing H2O2 concentration was noted even up to 500 mg/L. Fenton reaction slowdown effected by the reaction of hydrogen peroxide with hydroxyl radicals, as in the H2O2/UV process, was not observed as well as in the Bahmani et al. work [14]. Although this phenomenon occurred in others works [15]. Moreover, the obtained values of pseudo-first order decolorization rate
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constants for the Fenton reaction were higher than that obtained for the H2O2/UV process and O3 oxidation. That might be caused by an extra ferrous ions coagulation phenomenon already mentioned in section 3.1. Moreover, the two stages of Fenton reaction could be clearly observed. The first – very fast might be caused by combined effects of flocculation with iron compounds and oxidation with the hydroxyl radical. The second – much slower stage, when Fe2+ ion concentration could be not sufficient to produce significant number of hydroxyl radicals and other oxidants like O2•- or 1O2 could appear then. This hypothesis suggests that Fenton oxidation reaction mechanism is much more complicated than in case of H2O2/UV process or ozonation and requires further research.
2.3.4 NaCl influence Effect of NaCl on the RB5 ozonation process was tested at the O3 concentration of 20 mg/L, and the gas flow rate Qin = 20 L/h. RB5 was dissolved in distilled water to achieve 125 mg/L and a pH was adjusted to 12 with phosphate buffer. NaCl concentration was examined in the range of 0 to 65 g/L. The lack of NaCl effect of on the ozonation process (Figure 2.5 A) was noted. No NaCl effect on the ozonation was also found at initial pH approx. 6 (RB5 solution without buffer). Results have not been included in the work. RB5 decomposition during H2O2/UV process using various concentrations of NaCl in Milli-Q water (pH 6.28) was tested. RB5 125 mg/L solution with the addition of 6.8×103 mg/L H2O2 was used. The effect of NaCl concentration from 0 to 80 g/L was studied. The solutions were irradiated with 6 lamps. The results of the reaction are shown on Figure 2.5 B. Experiment demonstrate that there is visible effect of NaCl on the H2O2/UV process. It was noted that with increasing concentration of chloride ions the stronger inhibition of the RB5 degradation takes place. However, for each tested NaCl concentration, almost complete color reduction of RB5 after 90 minutes was achieved. The inhibition in this case can be explained by the formation of chloride ion-radicals Cl•-, which reactivity is much lower than hydroxyl radicals or dye aggregation caused by NaCl [8]. In Fenton oxidation a solution of RB5 (125 mg/L) at pH 3 (adjusted by H2SO4) was examined. The NaCl concentration range used in this study was changed from 0 to 80 g/L. The reagents ratio C0Fe2+ : C0H2O2 was set at 0.05 (C0Fe2+ 5 mg/L, and C0H2O2 100 mg/L). The results are shown in Figure 2.5 C. A strong dependence of the decomposition rate of the RB5 on NaCl concentration was observed. The process inhibition is very significant. Decolorization efficiency of the RB5 solutions containing more than 5 g/L NaCl has been never greater than 20% after 60 min. The conclusion is that chloride ion-radicals formation or dye aggregation 76
takes place. In addition, formation of a precipitate was observed in the reaction mixtures containing NaCl (not shown in the work). Thus precipitation of iron ions can also influence the process. These observations were also presented by other authors [4,6,7].
G Figure 2.5. NaCl influence effect on the decolorization of RB5 solution by: A) ozonation (C0RB5 125 mg/L, CO3 20 mg/L, Q in 20 L/h, pH 12) , B) H2O2/UV process (C0RB5 125 mg/L, C0H2O2 6.80×103 mg/L, pH 6.28) and C) Fenton oxidation (C0RB5 200 mg/L, C0Fe2+ 5 mg/L, C0H2O2 100 mg/L, pH 3)
2.3.5 Surface-active agent influence The ozonation process of the RB5 in the presence of Perigen LDR (surfactant commonly used in dyehouses as dyeing assistant) was performed with a dye solution with a concentration of 125 mg/L. The dye was dissolved in distilled water. pH equal 12 was obtained by using a phosphate buffer. The O3 concentration used in the experiment was equal 20 mg/L, and the gas flow rate was fixed at 20 L/h. The tested Perigen LDR concentration range was from 0 g/L to 1 g/L. The results obtained in the experiment are shown in Figure 2.6 A. There was no effect of surfactant presence on the RB5 decomposition by ozonation. During RB5 degradation with the H2O2/UV method various concentrations of surfactant – Preigen LDR (0.11, 0.3, 0.76, 1.0 g/L) diluted in Milli-Q water (pH 6.28) were used. RB5 solutions were studied at the dye concentration of 125 mg/L and H2O2 concentration was 6.8×103 mg/L. Samples were irradiated with a 6 lamps. The results of the test are shown on Figure 2.6 B). The SAA effect on H2O2/UV process was significant. RB5 degradation rate decreases with increasing concentration of surfactant.
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The similar effect was observed during the decomposition of RB5 solutions with Fenton reagent (Figure 2.6 C). In this case, the tested dye concentration was 125 mg/L. pH value was adjusted to a 3 with H2SO4. The reagents ratio was set at 0.05 (C0Fe2+ 5 mg/L, and C0H2O2 100 mg/L). For concentrations of surfactant equal and greater than 0.3 g/L almost no progress in Fenton reaction was observed.
Figure 2.6. Surface-active agent influence on the decolorization of RB5 solution by ozonation (C0RB5 125 mg/L, G CO3 20 mg/L, Q in 20 L/h, pH 12) , H2O2/UV process (C0RB5 125 mg/L, C0H2O2 6.80×103 mg/L, pH 6.28) and Fenton’s oxidation (C0RB5 200 mg/L, C0Fe2+ 5 mg/L, C0H2O2 100 mg/L, pH 3)
2.3.6 Color, TOC and COD removal Two treatment methods were employed for further tests: ozonation and H2O2/UV process. Fenton reagent experiments was abandoned because of the high inhibitory effect of NaCl and SAA on it (NaCl and SAA are commonly used in the industry). Color, TOC and COD removal was examined for purified RB5 dye as well as for commercial industrial dyes: Synozol Yellow KHL (RY145), Synozol Red K-3BS 150% (RR195), Synozol Blue KBR (RB221). Decolorization of dyes: RB5, RY145, RR195, RB221 was carried out in aqueous solutions for each dye individually. The study was conducted under the same conditions for each tested dye. In case of degradation with H2O2/UV, the dye initial concentration of 125 mg/L, and hydrogen peroxide concentration of 6.8×103 mg/L was used. The solutions were prepared in Milli-Q water (pH 6.28). While, in case of ozone treatment solutions with initial concentration of 125 mg/L were prepared by dissolving tested dyes in distilled water. The pH was set at 12
78
using phosphate buffer. In that examination of the ozonation, an O3 concentration of 42.3 mg O3/L, and the gas flow rate Qin 40 L/h was used. Both studies were carried out during 600 s. Figure 2.7 shows results of ozonation A) and H2O2/UV degradation B). All tested dyes revealed similar characteristics of the decomposition process (similar decay curves could be observed – not shown in the work). For all tested dyes very high degree of discoloration was achieved. More than 90% of color reduction after 10 min could be noted (in both cases). For RR195 and RB221 very similar process efficiency were obtained as well as for RB5. Slightly lower decolorization efficiency was obtained for RY145 (for ozone lower efficiency also for RR195 was recorded). The analysis of TOC and COD shows that mineralization process of RB221 is the biggest (in both cases - ozone and H2O2/UV). Generally, mineralization of tested dyes due to ozone are greater than those due to H2O2/UV. (Although the TOC assessment in case of ozonation was troublesome because of a short time of analysis – significant effects of mineralization were evident after 20 min. – not shown in the work).
Figure 2.7. Color, TOC and COD removal for the decolorization of RB5, YKHL, RK3BS, BKBR solutions by: G A) ozonation (C0dye 125 mg/L, CO3 42.3 mg/L, Q in 40 L/h, pH 12) and B) H2O2/UV process (C0dye 125 mg/L, C0H2O2 6.80×103 mg/L, pH 6.28).
2.3.7 Industrial textile wastewater treatment Real textile wastewater was taken from the Textile Company Bilinski (Poland). Wastewater samples after dyeing of cellulosic cloth (bath after dyeing) were tested. This
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wastewater stream is characterized by the highest: color, salinity and pH value of all wastewater generated by dye-house. Examined wastewater contained: Perigen LDR approx. 0.76 g/L, NaCl 65 g/L, Setazol Black DPT (industrial RB5) approx. 5.68 g/L, Na2CO3 1.4 g/L, NaOH (50% a.s.) 1.95 g/L, and was characterized by: COD equal 3940 mgO2/L, TC equal 1033 mg/L, TOC equal 434.8 mg/L, pH 11.5 and conductivity equal 61.60 mS/cm. The wastewater decomposition by H2O2/UV experiments were carried out using hydrogen peroxide (C0H2O2 6.8×103 mg/L) and 6 UV lamps. During the ozone treatment, the O3 concentration of 42.3 mg/L, and the gas flow rate Qin 40 L/h were used. Both studies were carried out in the natural pH of the effluent (pH 11.5). Experimental results was shown in Figure 2.8. Approximately 50% color reduction after 75 min of H2O2/UV wastewater treatment was achieved. Complete color reduction of treated wastewater did not occur even after time 150 min. Analysis of the TOC and COD reduction during the H2O2/UV process revealed only a slight mineralization of the tested wastewater (only approx. 10% reduction in TOC and COD was observed – Figure 2.8 B). Ozone treatment results are presented on Figure 2.8 A. During wastewater treatment with ozone almost complete color decay at time 60 min was observed. TOC and COD reduction achieved by ozonation was twice higher than that achieved by H2O2/UV wastewater treatment and amounted over 20%.
G Figure 2.8. Color, TOC and COD removal for real textile wastewater treatment by ozonation (CO3 42.3 mg/L, Q in 40 L/h, pH natural) and H2O2/UV process (C0H2O2 6.80×103 mg/L, pH natural)
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2.4. Conclusions On the basis of results obtained the ozonation process can be proposed as the most effective decolorization method for highly loaded textile wastewater. This treatment method is efficient even in case of solutions (or wastewater) with a high concentration of colored substances. While the phenomenon of inhibiting the UV rays access into the solution and light absorption by the dye molecules can be observed during H2O2/UV process (resulting in lower decolorization efficiency). Ozone treatment is particularly suitable for textile wastewater as textile wastewater is usually alkaline. Inhibitory effect on ozonation caused by textile auxiliaries agents was not observed. Moreover, the best color reduction and the highest mineralization degree was found for ozone treatment. However, TOC and COD reduction in case of the tested method was not sufficient to meet the regulation limits. To reach higher mineralization degree it seems reasonable to combine AOP method with biological treatment.
2.5. Acknowledgement The authors thank the National Centre for Research and Development for financial support of this research project ICBTOS under contract no. PBS2/A9/22/2013. Special thanks to Textile Company Bilinski, Konstantynow Lodzki, Poland for the cooperation. Marta Gmurek acknowledges the support from Foundation for Polish Science within the START scholarship.
2.6. References [1]
M. Constapel, M. Schellenträger, J.M. Marzinkowski, S. Gäb, Degradation of reactive dyes in wastewater from the textile industry by ozone: Analysis of the products by accurate masses, Water Res. 43 (2009) 733–743. doi:10.1016/j.watres.2008.11.006.
[2]
A. Al-Kdasi, A. Idris, K. Saed, C.T. Guan, Treatment of Textile Wastewater By Advanced Oxidation Processes – a Review, Glob. Nest Int. J. 6 (2004) 222–230.
[3]
P.K. Malik, S.. Saha, Oxidation of direct dyes with hydrogen peroxide using ferrous ion as catalyst, Sep. Purif. Technol. 31 (2003) 241–250. doi:10.1016/S1383-5866(02)00200-9.
[4]
S.S. Ashraf, M.A. Rauf, S. Alhadrami, Degradation of Methyl Red using Fenton’s reagent and the effect of various salts, Dye. Pigment. 69 (2006) 74–78. doi:10.1016/j.dyepig.2005.02.009.
[5]
I. Arslan-Alaton, G. Tureli, T. Olmez-Hanci, Treatment of azo dye production wastewaters using PhotoFenton-like advanced oxidation processes: Optimization by response surface methodology, J. Photochem. Photobiol. A Chem. 202 (2009) 142–153. doi:10.1016/j.jphotochem.2008.11.019.
[6]
M.M. Alnuaimi, M.A. Rauf, S.S. Ashraf, A comparative study of Neutral Red decoloration by photoFenton and photocatalytic processes, Dye. Pigment. 76 (2008) 332–337. doi:10.1016/j.dyepig.2006.08.051.
[7]
A. Riga, K. Soutsas, K. Ntampegliotis, V. Karayannis, G. Papapolymerou, Effect of system parameters and of inorganic salts on the decolorization and degradation of Procion H-exl dyes. Comparison of
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H2O2/UV, Fenton, UV/Fenton, TiO2/UV and TiO2/UV/H2O2 processes, Desalination. 211 (2007) 72–86. doi:10.1016/j.desal.2006.04.082.
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[8]
Y. Dong, J. Chen, C. Li, H. Zhu, Decoloration of three azo dyes in water by photocatalysis of Fe (III)oxalate complexes/H2O2 in the presence of inorganic salts, Dye. Pigment. 73 (2007) 261–268. doi:10.1016/j.dyepig.2005.12.007.
[9]
R. Liu, H.M. Chiu, C.S. Shiau, R.Y.L. Yeh, Y.T. Hung, Degradation and sludge production of textile dyes by Fenton and photo-Fenton processes, Dye. Pigment. 73 (2007) 1–6. doi:10.1016/j.dyepig.2005.10.002.
[10]
M.S. Lucas, J.A. Peres, Decolorization of the azo dye Reactive Black 5 by Fenton and photo-Fenton oxidation, Dye. Pigment. 71 (2006) 236–244. doi:10.1016/j.dyepig.2005.07.007.
[11]
M. Muthukumar, D. Sargunamani, N. Selvakumar, Statistical analysis of the effect of aromatic, azo and sulphonic acid groups on decolouration of acid dye effluents using advanced oxidation processes, Dye. Pigment. 65 (2005) 151–158. doi:10.1016/j.dyepig.2004.07.012.
[12]
M.A. Behnajady, N. Modirshahla, F. Ghanbary, A kinetic model for the decolorization of C.I. Acid Yellow 23 by Fenton process, J. Hazard. Mater. 148 (2007) 98–102. doi:10.1016/j.jhazmat.2007.02.003.
[13]
K. Sarayu, K. Swaminathan, S. Sandhya, Assessment of degradation of eight commercial reactive azo dyes individually and in mixture in aqueous solution by ozonation, Dye. Pigment. 75 (2007) 362–368. doi:10.1016/j.dyepig.2006.06.011.
[14]
P. Bahmani, A. Maleki, E. Ghahramani, A. Rashidi, Decolorization of the dye reactive black 5 using Fenton oxidation, African J. Biotechnol. 12 (2013) 4115–4122. doi:10.5897/AJB12.1226.
[15]
C.L. Hsueh, Y.H. Huang, C.C. Wang, C.Y. Chen, Degradation of azo dyes using low iron concentration of Fenton and Fenton-like system, Chemosphere. 58 (2005) 1409–1414. doi:10.1016/j.chemosphere.2004.09.091.
Chapter 3 Application of Fenton reagent in the textile wastewater treatment under industrial conditions
Stanisław Ledakowicz, Lucyna Bilińska, Renata Żyłła Ecological Chemistry and Engineering S 19 (2), 2012, 163 – 174 DOI: 10.2478/v10216-011-0013-z
Abstract Application of reactive dyes is very popular in textile industry as these dyestuffs are characterized by good fastness properties. Constapel et al. [1] estimated the production of this type dyes for over 140,000 tones/year. The reactive dyes are mostly (50%) employed for coloration of cellulosic fibers, however they can be also applied on wool and nylon. Unfortunately, they possess a low degree of fixation (50 – 90%), since the functional groups also bond to water, creating hydrolysis and the excess of dyes applied cause a colored pollution of aqueous environment. Moreover, dyeing process requires the use of: electrolytes in the form of aqueous solutions of NaCl or Na2SO4 in the concentration up to 100 g/L, alkaline environment (pH > 10) and textile auxiliary agents (among others detergents). Therefore, the wastewater generated during the reactive dyeing processes are characterized by high salinity, pH value and color, and due to low value of the ratio BOD5/COD are non-biodegradable. The successful methods of the textile wastewater treatment could be an Advanced Oxidation Processes (AOPs), amongst which the Fenton reagent seems to be the most perspective as it is the cheapest and easy in use. Based on the newest literature survey it was found that many successful tests with Fenton reaction were performed mainly in decolorization. However, not enough attention was devoted to decolorization of real industrial wastewater containing dyes, detergents and salts NaCl, or Na2SO4. The experiments carried out in lab scale were focused on the impact of NaCl and textile auxiliary agent (liquid dispersing and sequestering agent) on an inhibition of decolorization process by Fenton reagent. The objects of the investigation were the synthetic mixtures simulating the composition of real textile wastewater as well as the real industrial wastewater generated in the reactive dyeing. The inhibition of the Fenton decolorization in the presence of NaCl and liquid dispersing and sequestering agent was demonstrated. Additional experiments using pulse radiolysis were carried out in order to confirm the inhibition of chloride in the decolorization process.
Keywords: Fenton reagent, Textile wastewater, Decolorization, Inhibition
3.1. Introduction A full range of bright colors, stable covalence bond to the fiber, providing a good fastness of dyeing and relatively easy synthesis – these properties make reactive dyes very popular in textile industry [2]. Constapel et al. [1] estimated the annual production of these type of dyestuff for over 140 000 tones, while Ahmed and El-Shishtway [3] announce, that 50% of all cellulose 85
fibers is dyed with reactive dyes, and only 17% with vat dyes, 16% with direct dyes, 7% with sulfur dyes, also 7% with indigo dyes and 3% with glacial dyes. Reactive dyes are mostly used to dye cellulose fibers, however there are also groups of reactive dyes dedicated for wool and polyamide. Unfortunately, despite these advantages and common use, reactive dyes are marked by low degree of fixation, between 50 and 90%, depending on the assortment. It means, that the excess of dye used in industrial dyeing process does not bound to the fibers and in the hydrolyzed form is expelled to the sewage, causing higher contamination of aqueous environment. The fact, that the process of reactive dyeing requires special technology, also has to be taken into the consideration. A fixation of reactive dye onto fiber is possible, when an electrolyte NaCl or Na2SO4 is used in dye bath, strong alkaline pH is produced and auxiliary surface active agents (SAA) are applied. The consumption of dyestuff in process of reactive textile dyeing amounts on average 0.5 – 80 g per 1 kg of textiles and up to 30 g of organic auxiliary agents, 30 – 250 g of inorganic auxiliary agents, 90 – 1500 g of electrolyte per 1 kg of textiles. Therefore, the wastewater generated in reactive dyeing process is marked by high salinity and coloration, high pH value and SAA presence. Moreover, textile wastewater is characterized by high COD value and low BOD value, what makes it non-biodegradable. A way of purification of this kind of wastewater could be AOPs methods (Advanced Oxidation Processes), amongst which the Fenton reagent is worthy of consideration, as it is not expensive and easy in use. Fenton reagent is a method of generating hydroxyl radicals (HO• ) by using reaction of hydrogen peroxide decomposition catalyzed by ferrous ions Fe2+. The application of oxidizer with high redox potential (for HO• 2.81 V) generated under in situ conditions, with Fenton reaction, enables decomposition of low biodegradable substances, including dyestuff used in textile industry. According to the newest literature report a lot of successful tests were performed not only of reactive dyes, but mostly in decolorization of dye solution used in textile industry. Xu et al. [4], Papić et al. [5], Kusić et al. [6], Tantak and Chaudhari [7] and Arslan-Alaton et al. [8] worked on decolorization of reactive dye solution with use of Fenton reagent. Wang et al. [9], Gulkaya et al. [10], Bianco et al. [11] worked on decolorization of real industrial wastewater. These studies were focused on optimization of the method regarding color reduction, reduction of TOC and comparison of working effects of Fe2+/H2O2 and Fe2+/H2O2/UV. The objective of the present publication is to show the inhibition impact of NaCl and SAA, present in real industrial wastewater, on their decolorization. Similar experiments have already been performed by Alnuaimi et al. [12], Riga et al. [13] and Arslan-Alaton et al. [14] and reveled the inhibition impact of salt on Fenton reaction. Those tests included the impact of different 86
kinds of salts on decolorization process of dye solution with Fenton reagent, however the impact of SAA has not yet been examined. Moreover, the highest salt concentration (NaCl) used in these tests was 15 g/L, while in industrial practice of dyeing process it can reach up to 100 g/L. Therefore, conducting further study on this matter, including higher salt concentration and SAA presence, concentrated on examination of both synthetic mixtures simulating the composition of real textile wastewater and real wastewater generated in reactive dyeing process, seems to be justified.
3.2. Experimental
3.2.1. Materials Tree azo dyes, including two reactive and one acid dyes, were used in this paper (Table 3.1). C.I. Reactive Black 5 (RB5) is known in trade form as Setazol Black DPT supplied by Setas Kimya. Molecular weight of RB5 is 991.82 g/mol and the maximum absorbance for this dye was observed for 596 nm wavelength Acid Red 27 (AR27) and Reactive Blue 81 (RB81) dyes (Boruta Color, Poland) in purified form, without fillers, were used in pulse radiolysis studies.
Table 3.1. List of dyes tested in the experiment C.I.
Type of dyes
Reactive Black 5
reactive, azo, based on H-acid
Dye structure +
O
Na O-
O
O
S
S
N
N
O
O
-
S +
Na
O
OH
NH2
N
O
O S
O
acid, azo, based on R-salt
O
S O
NaO3S
O S
O
O
OH
Acid Red 27
+
Na
O
N O
-
+
O
Na
SO3Na
N=N
SO3Na
Cl
Reactive Blue 81
N
N
acid, azo, based on H-acid
OH NH SO3Na
NH
N
Cl
N=N NaO3S
SO3Na
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3.2.2 Application of Fenton reagent FeSO4×7 H2O, H2O2 conc. 30%, H2SO4 conc. 94% - aqueous solution 1:10, NaOH, NaCl, Perigen LDR supplied by Textilchemie Dr Petry GmBH – liquid dispersing and sequestering agent based on naphthalene sulphonic acid condensate and carboxylates have been used. The aqueous solutions of dyes and auxiliary agents have been prepared by Dosorama machine provided by Technorama. The dyestuff in a hydrolyzed form has been used, since it is present in this form in textile wastewater. The hydrolysis of dye solution has been performed in water bath at temperature 80°C during 2 hours, at pH 12. Initial dye concentration in the simulated industrial wastewater mixtures has been fixed, basing on earlier experiments, on 200 mg/L. The color of the dye water solution has been measured by defining the absorbance with maximum absorbance of the dye. The kinetics of decolorization of water dye solution with Fenton reagent has been examined by measuring the absorbance with Helios transient absorption spectrophotometer, delivered by Thermo company, with constant, automatic tests collection and flow cell. Decolorization has been carried out for the three objects: aqueous solutions of RB5 dye only, simulated lab mixtures imaging the composition of real wastewater (RB5 dye solution with NaCl and Perigen LDR) and for real wastewater after dyeing process. Tests have been taken under conditions of normal alternative climate according to ISO 139:2005, point 3.2 standards in textile testing laboratory.
3.2.3. Pulse radiolysis Kinetic reaction of Selected dyes with hydroxyl radicals and ion-radicals Cl•− 2 was determined by pulse radiolysis. The aim of this study was to examine the extent to which chloride ion-radicals Cl•− 2 may effect on the efficiency of Fenton process. Chloride ion-radicals can be followed spectrophotometrically in the wavelength range 300 – 400 nm. The spectrum has a maximum at a wavelength 340 nm [15]. In order to obtain the chloride ion-radicals, dye solutions were acidified with HCl, the HCl concentration in solution was 0.1 M. To the kinetic study by pulse radiolysis metod a linear electron accelerator ELU-6E was used, in which pulses with a duration from 2 to 17 ns were available. The energy of accelerated electrons was Emax 8 meV and the maximum current in the pulse reached a value of 15 A. Direct observations of formation and decay of radiolysis products were conducted on the basis of spectrophotometry. Measurements were made with time scale from nanoseconds to seconds in the wavelength range from 300 to 750 nm. The kinetics of relative changes of absorbance were recorded every 15 nm. Both in the case of hydroxyl radicals and chloride ion-radicals, solution was saturated with
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N2O before the measurement session for 30 minutes. During the kinetic measurements N2O was a gas surrounding the system, preventing access of oxygen. The solution was placed in a flow system, which was equipped with an automatically controlled valve from accelerator control room. The valve opened up after each shot of electron beam in the sample (1 mL), which allowed to introduce of a fresh portion of solution before next shot. The obtained kinetic runs for each wavelength in a measurement series were normalized to a predetermined dose and light intensity. These calculations were performed automatically by the computer program. Based on obtained results the spectrum of the reaction products was made for selected time intervals.
3.3. Results and discussion
3.3.1. Effect of pH on Fenton process Natural pH value after process of reactive dyeing of textiles reaches 11, or even 12. However, the decomposition reaction of H2O2 is catalyzed most efficiently by Fe2+ ions in water solutions with pH value between 2 and 3. In order to verify this the decolorization of RB5 dyestuff was proceeded for pH values equal 2 and 3. Comparatively also for two other reactive azo dyestuff (YKHL – Synozol Yellow KHL and YHB – Synozol Yellow HB) the same test was proceeded, what has been presented in Figure 3.1. The decolorization with Fenton reagent for all examined dyestuff solutions proceeded the most rapidly with pH value equal 3. Therefore, further tests have been performed with pH value equal 3. In all tests the pH value of examined samples was set with H2SO4 1:10 solution.
Figure 3.1. Decolorization of aqueous solutions of dyestuffs: YKHL, YHB, RB5 with dyestuff concentration 200mg/L and 75 mg FeSO4/750 mg H2O2 at pH value equal 2 and 3
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3.3.2. Effect of ferrous dosage and FeSO4:H2O2 ratio In order to find an optimal dosage and reagent ratio assuring effective decolorization process tests for aqueous solution of RB5 have been performed. The concentrations of FeSO4 within the range 25 – 100 mg/L and reagent ratio FeSO4:H2O2 1:5 and 1:10 have been applied. The results gained for the solutions have been presented in Figure 3.2. In each case higher degree of color reduction has been noticed for the reagents ratio 1:10, which has been chosen for the further tests of RB5 decolorization with FeSO4 concentration amounting 75mg/L. The degree of color reduction of aqueous solution of RB5 exceeded 99%.
Figure 3.2. Dependence of decolorization extent of dyestuff RB5 aqueous solutions on FeSO4 dose
It was found that irrespective of the value of Fe SO4:H2O2 mass ration reduction degree of pollutions expressed in Cod and TOC after oxidation process usually does not exceed 30% for COD and 20% for TOC (Figure 3.3). Most of the pollution (60%) was removed in the precipitation process of iron ions. After the precipitation process treated wastewater typically contained about 10% of pollutants in the untreated wastewater. Low degree of COD and TOC reduction with total decolorization of wastewater is quite often observed for the chemical oxidation methods. In many research work the authors report that the application of advanced oxidation methods cause a COD reduction of textile wastewater up to 50% [16].
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Figure 3.3. The COD and TOC removal during the oxidation and coagulation processes of the model textile wastewater for different H2O2 concentration; concentration of FeSO4×7H2O 3.0×103 mg/L
3.3.3. Effect of NaCl on Fenton process
Figure 3.4. Dependence of decolorization extent of dyestuff RB5 aqueous solutions on NaCl concentration
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The same conditions of Fenton reaction as for aqueous solution of RB5 (75 mg/L FeSO4 and reagent ratio FeSO4:H2O2 1:10) have been used for decolorization of dyestuff solution RB5 containing 1, 5, 10, 20, 40, 80 g/L NaCl. As it can be observed in Figure 3.4 only for 1 g/L NaCl no significant decrease of decolorization efficiency appeared compared with experiments without NaCl. However, the color removal for sample containing 10 g/L NaCl amounted 63%, and for the one containing 80 g/L NaCl – 40% after 45 min. The NaCl impact on decolorization efficiency can be explained as follows. In the presence of Cl− ions excess due to reaction of HO• radicals with Cl− ions the ion-radicals Cl•− 2 are produced which possess much lower oxidative potential than HO• radicals [17]. The mechanism of reaction of HO• radical „scavenging” by chloride ions can be summarized as follows: 𝑘1
𝐻𝑂• + 𝐶𝑙 − ↔ 𝐻𝑂𝐶𝑙 •−
k1 3 × 109 (M-1 s-1)
𝐻𝑂𝐶𝑙 •− + 𝐻 + → 𝐻2 𝑂 + 𝐶𝑙 • 𝑘2
𝐶𝑙 • + 𝐶𝑙 − → 𝐶𝑙2•−
(3.1) (3.2)
k 2 2 × 1010 (M-1 s-1)
(3.3)
Rate constants of ion-radicals HOCl•− and Cl•− 2 formation in reaction (1) and (3) are known and their values are typical for radical reactions, reaction constant k1 3 × 109 M-1 s-1 (at pH 2 – 3); reaction constant k 2 2 × 1010 M-1 s-1 [15]. Using pulse radiolysis method the measurement series of reaction rate of dye molecules with ion-radicals Cl•− 2 were performed (Figure 3.5 B and 3.5 C). Based on the curve changes of absorbance for different wavelength the spectrum of aqueous solution of dye for different reaction times was obtained. By tracking spectrum changes up to 2 µs it may be noticed a rapid absorbance increase at a wavelength between 340 and 350 nm. As it is known from the literature, chloride ion-radicals absorb light at the wavelength range between 250 and 400 nm, while at λ 340 nm the maximum of absorbance can be observed [17]. Therefore, it can be concluded that the appearance of the peak at λ 340 nm results from the formation of chloride ion-radicals. During decay of absorbance in visible range the decrease of absorbance at a wavelength λ 340 nm was reported.
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Figure 3.5. Spectra changes of selected azo dyes AR27 and RB81 in the pulse radiolysis with chloride ion-radicals for time interval 40 µs; C0dye 25 mg/L
Figure 3.6. Time dependent decolouration of dye during reaction with hydroxyl radicals and chloride ion-radicals with pulse radiolysis of: A) AR27; B) RB81; C0dye 25 mg/L
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Figure 3.6 shows the kinetic curves of colour decay by pulse radiolysis with chloride ionradicals and hydroxyl radicals. It follows from Figure 3.6 that the dye decolouration process occurs much more slowly with the participation of chloride ion-radicals, regardless of the tested dyes. Based on experimental data obtained in the 2 µs time interval, reaction rate constants of dye molecules with chloride ion-radicals were determined. Based on the dependence of rate constant of pseudo-first order reaction from the initial dye concentration the value of appropriate reaction rate constant was determined (Table 3.2). Comparing the rate constants of decolourization of dye in the reaction with hydroxyl radicals and chloride ion-radicals it can be concluded that re action involving Cl2•− is much slower. Reaction rate of hydroxyl radicals with chloride ions Cl− (k1 3.06×109 M-1s-1) is comparable to the rate of reaction of these radicals with the dye RB81 molecules (k2 1.98×109 M-1s-1). It can be assumed that the mechanism of decolouration process can proceed either by HO• radicals and Cl•− 2 ion-radicals. The competitiveness of these reactions depends on the initial concentration of reactants. With a large excess of chloride ions relative to the dye molecules, the decolouration process will be conducted mainly through chloride ion-radicals. In this paper the concentration of chloride ions was in large excess to the concentration of dye molecules. Table 3.2. Reaction rate constants of dyes with hydroxyl radicals and ion-radicals Cl•− 2 Type of dye
Rate constant with HO• (M-1s-1)
-1 -1 Rate constant with Cl•− 2 (M s )
RB81
1.98×109
8.98×108
AR27
9.17×109
6.51×108
As it is shown in Table 3.2, the reaction rate constants of dye with Cl•− 2 ion radicals are one order of magnitude lower than the rate constant of reaction with hydroxyl radicals. However, it seems that so significant inhibition of decolouration process is not due only to slower reaction kinetic through Cl2•− ion-radicals. Observing the kinetic curves in long interval of time (40 µs) (Figure 3.6 A and 3.6 B) it can be noticed that the final degree of conversion of dye is much lower than in the case of reaction with hydroxyl radicals. So significant inhibition may indicate the presence of chloride ion-radicals competitive reactions which are faster than the reaction of dye with Cl2•− ion-radicals, as rate constant for the former one is an order of magnitude higher. Chloride ion-radicals can react with each other, or create a free chlorine molecules.
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The obtained results make possible to explain the cause of the significant inhibition of Fenton process, where high concentration of Cl− ions in the reaction medium is observed. Due to the fact that the Fenton process proceeds mainly by hydroxyl radicals, it can be assumed with − high probability that these radicals change into the Cl•− 2 from presence of Cl ions.
The adverse effect of chloride ions also contributes to the fact that as a source of iron ions FeSO4 is preferred to FeCl2. Table 3.3 gives rate constants of reaction of iron ions Fe2+ with H2O2 molecules [15]. The reaction rate constants for FeSO4 are similar to these published by other authors [18]. However, rate constants for the reaction with FeCl2 are lower by half than the constants obtained in the reaction with FeSO4. Table 3.3. Reaction rate constants of dye decolourization during Fenton process (M-1s-1) Dye
FeSO4×7H2O
FeCl2×4H2O
AR27
106
56
RB81
74
44
3.3.4. Effect of Perigen LDR addition on Fenton Process After increasing the FeSO4 dose up to 200 mg/L and keeping the ratio FeSO4:H2O2 1:10 the test has been repeated for samples of RB5 solution containing 0.5, 1.0, 2.0 g/L of auxiliary agent – Perigen LDR. In case of samples containing 0.5 and 1.0 g/L Perigen LDR the Fenton reaction enabled 99% color reduction, however for the samples containing 2.0 g/L LDR the degree of color reduction decreased to 75% after 45 min, what was shown in Figure 3.7 The presence of surfactants in the concentration above the Critical Micelle Concentration (CMC) causes emulsification of dyestuff molecule shielding it against the attack of radicals, which in consequence decreased the efficiency of decolorization.
3.3.5. Decolorization of simulated wastewater and real industrial textile effluents In the next experiment an aqueous solution with concentration of 200 mg/L RB5 containing 80 g/L NaCl and 0.5 g/L Perigen LDR, simulating the composition of real textile wastewater has been subjected to decolorization with Fenton reagent. The doses of reagent amounting 250, 350, 500 mg/L FeSO4 with the ratio FeSO4:H2O2 1:10 were used. As it is seen in Figure 3.8 satisfactory degree of color reduction above 97% both for FeSO4 dose 350 mg/L and 500 mg/L was achieved.
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Figure 3.7. Time dependence of decolorization of RB5 in the presence of surfactant Perigen LDR
Figure 3.8. Comparison of decolorization of simulated textile wastewater with real industrial one of the same reagents concentration
Final experiment corresponding to a real industrial wastewater after dyeing process was performed with concentration of FeSO4 equal 350, 500 mg/L and reagent ratio FeSO4:H2O2 1:10, pH 3. Unexpectedly, the results of decolorization were incomparable with that obtained for the simulated wastewater. In case when 350 mg/L FeSO4 was used almost none color
96
reduction was observed, however for 500 mg/L FeSO4 70% of color was reduced during 45 min. It is difficult to explain these results of decolorization of real industrial wastewater, what needs further investigation. One can conclude that simulated textile wastewater is not the same as real wastewater generated during reactive dying in industrial scale.
3.4. Conclusions Fenton reagent appeared to be very effective method for the degradation of aqueous solution of many dyestuffs. Inhibition effect of NaCl presence in textile wastewater on decolorization has been found: the higher content of NaCl the poorer is decolorization degree. The rate constants of decolourization of selected dyes obtained by pulse radiolysis have shown that the reaction of chloride ion-radicals, which may be formed in the presence of high concentration of Cl-, is slower than the reaction of hydroxyl radicals. The emulsification effect of surfactants present in textile wastewater in the concentration above Critical Micelle Concentration causes a decrease of decolorization rate. Simulated textile wastewater are not the same with respect to decolorization by Fenton reagent as real wastewater generated during reactive dying in industrial scale. It was proved, that nearly 5 times higher reagent dose had to be used to decolorize a mixture simulating the composition of real textile wastewater than in the case of dyestuff solution without any additional substances. Moreover, to decolorize real wastewater generated in industrial reactive dyeing process 500 mg FeSO4/L:5000 mg H2O2/L reagent dosage, i.e. 7 times higher dose has to be applied than in case of dyestuff solution. It is justified to carry out further studies on the decolorization of model wastewater including dyestuffs, NaCl, auxiliary agents and the real wastewater, with Fenton reagent as well as with other advanced oxidation techniques.
3.5. References [1]
M. Constapel, M. Schellenträger, J.M. Marzinkowski, S. Gäb, Degradation of reactive dyes in wastewater from the textile industry by ozone: Analysis of the products by accurate masses, Water Res. 43 (2009) 733–743. doi:10.1016/j.watres.2008.11.006.
[2]
W. Czajkowski, Nowoczesne barwniki dla włókiennictwa, Wydawnictwo Politechniki Łódzkiej, 2006.
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N.S.E. Ahmed, R.M. El-Shishtawy, The use of new technologies in coloration of textile fibers, J. Mater. Sci. 45 (2010) 1143–1153. doi:10.1007/s10853-009-4111-6.
[4]
H. Xu, D. Zhang, W. Xu, Monitoring of decolorization kinetics of Reactive Brilliant Blue X-BR by online spectrophotometric method in Fenton oxidation process, J. Hazard. Mater. 158 (2008) 445–453. doi:10.1016/j.jhazmat.2008.01.109.
[5]
S. Papić, D. Vujević, N. Koprivanac, D. Šinko, Decolourization and mineralization of commercial reactive dyes by using homogeneous and heterogeneous Fenton and UV/Fenton processes, J. Hazard. Mater. 164
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(2009) 1137–1145. doi:10.1016/j.jhazmat.2008.09.008.
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[6]
H. Kušić, A. Lončarić Božić, N. Koprivanac, S. Papić, Fenton type processes for minimization of organic content in coloured wastewaters. Part II: Combination with zeolites, Dye. Pigment. 74 (2007) 388–395. doi:10.1016/j.dyepig.2006.01.050.
[7]
N.P. Tantak, S. Chaudhari, Degradation of azo dyes by sequential Fenton’s oxidation and aerobic biological treatment, J. Hazard. Mater. 136 (2006) 698–705. doi:10.1016/j.jhazmat.2005.12.049.
[8]
I. Arslan-Alaton, B.H. Gursoy, J.E. Schmidt, Advanced oxidation of acid and reactive dyes: Effect of Fenton treatment on aerobic, anoxic and anaerobic processes, Dye. Pigment. 78 (2008) 117–130. doi:10.1016/j.dyepig.2007.11.001.
[9]
S. Wang, A Comparative study of Fenton and Fenton-like reaction kinetics in decolourisation of wastewater, Dye. Pigment. 76 (2008) 714–720. doi:10.1016/j.dyepig.2007.01.012.
[10]
I. Gulkaya, G.A. Surucu, F.B. Dilek, Importance of H2O2/Fe2+ ratio in Fenton’s treatment of a carpet dyeing wastewater, J. Hazard. Mater. 136 (2006) 763–769. doi:10.1016/j.jhazmat.2006.01.006.
[11]
B. Bianco, I. De Michelis, F. Vegliò, Fenton treatment of complex industrial wastewater: Optimization of process conditions by surface response method, J. Hazard. Mater. 186 (2011) 1733–1738. doi:10.1016/j.jhazmat.2010.12.054.
[12]
M.M. Alnuaimi, M.A. Rauf, S.S. Ashraf, A comparative study of Neutral Red decoloration by photoFenton and photocatalytic processes, Dye. Pigment. 76 (2008) 332–337. doi:10.1016/j.dyepig.2006.08.051.
[13]
A. Riga, K. Soutsas, K. Ntampegliotis, V. Karayannis, G. Papapolymerou, Effect of system parameters and of inorganic salts on the decolorization and degradation of Procion H-exl dyes. Comparison of H2O2/UV, Fenton, UV/Fenton, TiO2/UV and TiO2/UV/H2O2 processes, Desalination. 211 (2007) 72–86. doi:10.1016/j.desal.2006.04.082.
[14]
I. Arslan-Alaton, G. Tureli, T. Olmez-Hanci, Treatment of azo dye production wastewaters using PhotoFenton-like advanced oxidation processes: Optimization by response surface methodology, J. Photochem. Photobiol. A Chem. 202 (2009) 142–153. doi:10.1016/j.jphotochem.2008.11.019.
[15]
S. Ledakowicz, R. Maciejewska, L. Gebicka, J. Perkowski, Kinetics of the Decolorization by Fenton’s Reagent, Ozone Sci. Eng. 22 (2000) 195–205. doi:10.1080/01919510008547220.
[16]
A. Tehrani-Bagha, N. Mahmoodi, F. Menger, Degradation of a persistent organic dye from colored textile wastewater by ozonation, Desalination. 260 (2010) 34–38. doi:10.1016/J.DESAL.2010.05.004.
[17]
G.G. Jayson, B.J. Parsons, A.J. Swallow, Some simple, highly reactive, inorganic chlorine derivatives in aqueous solution. Their formation using pulses of radiation and their role in the mechanism of the Fricke dosimeter, J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Phases. 69 (1973) 1597. doi:10.1039/f19736901597.
[18]
C. Walling, Fenton’s reagent revisited, Acc. Chem. Res. 8 (1975) 125–131. doi:10.1021/ar50088a003.
Chapter 4 Comparison between industrial and simulated textile wastewater treatment by AOPs – Biodegradability, toxicity and cost assessment
Lucyna Bilińska, Marta Gmurek, Stanisław Ledakowicz Chemical Engineering Journal 306, 2016, 550 – 559 DOI: 10.1016/j.cej.2016.07.100
Graphical abstract
Abstract Despite the fact that there are many literature reports concerning textile wastewater treatment by advanced oxidation processes (AOPs), mostly they are not oriented on the industrial applications. A practical approach to the enhancement of water reuse by applying ozone-based AOPs to industrial textile wastewater has been investigated. The research focused on Reactive Black 5 (RB5) and its industrial form (Setazol Black DPT), which, despite being hazardous, is still one of the most commonly used dyestuffs in industrial practice. Several ozone-based AOPs (e.g., O3, UV/O3, O3/H2O2, and O3/UV/H2O2) and H2O2/UV processes have been compared in terms of their effectiveness in removing RB5 (purified and commercial form) from simulated and industrial textile wastewater. The color, COD, TOC and BOD reduction were considered. Almost completely color reduction was achieved for the simulated as well as industrial wastewater, but only in case of ozone-based AOPs. The H2O2/UV processes were found highly not effective for industrial application. For industrial wastewater the COD and TOC decrease were not very high (10% of COD and 20% of TOC) in contrast to simulated one (90% and 50%, respectively). However, the mineralization, biodegradability and Average Oxidation State (AOS) assessment indicated that the application of AOPs resulted in more oxidized by-products. The toxicity assessment based on V. fischeri bacteria proved extremely high toxicity of BR5 (EC50 3.86±0.32 mg/L). All of the tested ozone-based AOPs increased biodegradability and decreased toxicity, proving that oxidation should be performed before biological treatment. The cost analysis and the research showed that O3 and O3/H2O2 (0.005 M) can be effectively used in the industry.
Keywords: Ozone-based AOPs; Industrial textile wastewater; Reactive Black 5; Toxicity; Biodegradability; Cost assessment 101
4.1. Introduction The textile industry is one of the most significant sectors of the global economy, especially in countries such as China, India, Pakistan, Bangladesh and Malaysia. In 2014 in Poland, employment in this industry reached 114 thousand people, and the profit was near 3.85 billion euro (statistics for companies employing more than 9 employees, clothing and textile sector) [1]. Wastewater from the textile industry should be considered a serious environmental problem. Despite the use of high-tech equipment and modern technologies, the textile industry is among the highest water-consuming industries and produces a huge amount of wastewater. Water consumption and wastewater generation during the dyeing and finishing of textiles can reach 150 - 350 L per kg of product [2,3]. Residues of detergents, dyes and auxiliary agents present in discharge result in high pH, intensive color and salinity. Textile wastewater is often characterized by a low BOD5/COD ratio, which can have an adverse effect on the biological treatment. The BOD5/COD values are between 0.06 and 0.35 [4–7]. Considering that a BOD5/COD ratio less than 0.4 indicates low biodegradability, the textile wastewater biodegradation is unsatisfactory. Advanced oxidation processes (AOPs) may be effective purification methods of textile industrial wastewater; due to the high oxidative potential of ozone and HO• radicals generated by these methods, many organic substances can be decomposed [8]. There are several literature reports on the degradation of textile dyestuff in both simulated mixtures [9–22] and real industrial wastewater [7,14,23–29] using ozone-based AOPs. However, few of these reports reflect the impact of conditions characteristic for textile wastewater on the oxidation process [16–19]. Some authors studied the influence of auxiliary agents presence in the reaction mixture on dye degradation using ozone-based AOPs. Muthukumar & Selvakumar [16] showed an inhibiting effect of various salts on the ozonation process, but Colindres et al. [17] noticed that salts (Na2SO4) and alkaline (NaOH, CaCO3) conditions accelerated the ozonation. Bamperng et al. [18] indicated a negligible effect of NaCl on ozonation due only to differences in the solubility of ozone in various media. Bilinska et al. [19] showed that there is no visible effect of NaCl and surface active agents (SAA) on the ozonation process. Additionally, neither the mechanism of HO• radicals formation nor the possibility of a scavenging effect caused by textile wastewater conditions was discussed. All of the works found in the literature that applied AOPs for real textile wastewater cleaning were related to global wastewater. Most of them were general wastewater taken from the textile plant, some were equalized wastewater streams taken from several random finishing 102
operations [7,14,23–29]. In most cases, the authors tested only ozonation [7,23,24] or were focused on coupling ozonation with biological treatment [14,25–27]. Few publications focused on a comparison of several ozone-based AOPs used for real wastewater (Chung & Kim [28] – O3/H2O2, O3/UV, O3/ H2O2/UV; Azbar et al. [29] – O3/H2O2, O3/UV, O3/ H2O2/UV, H2O2/UV, Fenton and coagulation). Additionally, Konsowa et al. [20] claimed that their work concerned industrial wastewater, but the research was performed for a synthetic mixture – aqueous dye solutions. Similarly, Bampering et al. [18] suggested treatment of dye wastewater, whereas synthetic mixtures were studied. None of these studies were focused on the purification of the textile wastewater stream most polluted with dyestuff, which is the bath after dyeing operation. This textile wastewater stream is characterized by the most intensive color, the highest pH value and the highest concentration of salt. Only Colindres et al. [17], Arslan et al. [21] and Arslan Alaton et al. [22] used ozone-based AOPs to clean a mixture that simulated real wastewater after dying. However, they did not investigate this type of wastewater after a real industrial dyeing process. Moreover, Arslan Alaton et al., [22] diluted their simulated mixture 15 times before treatment. Than Arslan et al. [21] used a 40 times dilution of the simulated mixture, which does not reflect the conditions of real dyeing wastewater. In this study, several ozone-based AOP methods (O3, UV/O3, O3/H2O2, O3/UV/H2O2) and an H2O2/UV process for simulated mixtures and industrial wastewater after dyeing with RB5 (purified and commercial form, respectively) were applied for the first time. All tests were conducted in similar, comparable research conditions. The color, COD, TOC, and BOD reduction were considered. A toxicity assessment and cost analysis for the AOPs were performed. The mechanism of hydroxyl radical formation in the alkaline reaction medium and the characteristics of the industrial wastewater after dyeing operation were also investigated.
4.2. Experimental
4.2.1 Materials Reactive Black 5 (RB5) was obtained from Boruta-Zachem (Poland) as a purified reagent. The chemical structure and characteristics of this dye are presented in Figure 4.1. The buffer components, NaOH (AR) and Na2HPO4 (AR), were purchased from Stanlab (Poland) and Chempur (Poland), respectively. Na2SO3 and hydrogen peroxide solution (30%, w/w) (both A.R grade) were purchased from Chempur (Poland). Catalase was purchased from SigmaAldrich (USA). 103
Figure 4.1. Chemical structure and UV-Vis spectrum of Reactive Black 5 (RB5)
The substances present in industrial textile wastewater were Setazol Black DPT (industrial product based on RB5, Setas-Kiyma (Turkey)), industrial dyeing assistant – Perigen LDR (SAA – naphthalenesulfonic acid and carboxylates mixture, Textilchemie Dr. Petry Co. (Germany)) as well as NaCl, NaOH, and Na2CO3 (technical products). The exact composition of the dye bath (given by the textile company) and the industrial wastewater indicators (wastewater after the specific dyeing process) are given in Table 4.1. The initial dye concentration of the dyeing bath was 5.68 g/L. Due to dye liquor exhaustion of the textile material, the final dye concentration in the wastewater was 730 mg/L. The wastewater was diluted to achieve 125 mg/L of industrial dye to compare the wastewater and purified dye aqueous solution. Table 4.1. Dyeing bath composition A), dyeing process conditions B), industrial wastewater characteristics C). A) Dyeing bath content Setazol Black DPT (industrial RB5) Perigen LDR (SAA) NaCl Na2CO3 NaOH 50% (a.s.) B) Dyeing process conditions Batch type Weight Dyeing ratio Bath volume Machine type C) Industrial wastewater indicators pH conductivity COD TOC Approx. conc. of the dye
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5.68 g/L 0.76 g/L 65 g/L 1.4 g/L 1.95 g/L 95%Viscose5%Elastane 580 kg 1:13.2 7 656 L Thies TRD 11.23 78.41 mS/cm 2677 mgO2/L 290 mg/L 730 mg/L
4.2.2 Analytical methods Ozonation of the industrial textile wastewater and dye solutions was performed in a semibatch glass reactor (heterogeneous gas-liquid system) that had a capacity of 1 L and was equipped with a porous plate to deliver ozone gas into the reaction solution. Mixing was performed with a magnetic stirrer (Wigo type ES 21). Ozone was produced by an Ozonek Ozone Generator (Poland). The oxygen used for ozone production was supplied from a compressed gas cylinder (O2 purity 99.5%). The ozone concentration was measured at the inlet and outlet of the reactor using a BMT 963 Vent ozone analyzer. For the UV/O3 and O3/UV/H2O2 processes, a UVC low-pressure mercury lamp (Herraus 15W) was added to the experimental set-up. The lamp was placed inside quartz tube and immersed inside the thermostated reactor. For the O3/H2O2 and O3/UV/H2O2 processes, hydrogen peroxide was added to the reaction mixture before the ozone was applied. The reaction progress for all ozone-based AOPs was stopped by the addition of 0.01 M Na2SO3 to the collected samples. The H2O2/UV process was conducted in a merry-go-round device with quartz test tubes placed between two exposure panels with UV lamps. The details are presented elsewhere [19]. All samples collected at specified time intervals were measured by a spectrophotometer (Helios Thermo). A calibration plot based on Lambert-Beer law, ( equation (4.1)), was used to determine the concentration of the samples. 𝐴= 𝜀∙𝑙∙𝐶
(4.1)
Where: A – absorbance, ε – molar absorption coefficient (M-1 cm-1), l – the length of the light path (cm), C – concentration of the sample (M).
The pH and temperature of the samples were measured using an Elmetron meter (Poland). The total organic carbon (TOC) was measured using a HACH IL 550TOC-TN apparatus. Chemical oxygen demand (COD) and biochemical oxygen demand (BOD) measurements were obtained using the a HACH-LANGE apparatus (DR 3800) according to standard methods given by the producer. The COD analysis was done with the dichromate (VI) LCK 514 and 314 tests. The BOD was determinate using low-volume LCK 555 test, which is based on dilution method and the pyrocatechol derivative, which gives red color in the presence of Fe2+, was the spectrophotometric factor in it. Before the BOD measurement a catalase addition was used to remove the residual H2O2. The acute toxicity bioassay was conducted using a Microtox Model 500 analyzer (Modern Water, USA) with the marine bacterium Vibrio fischeri as a 105
bioluminescent indicator. The Microtox® 81.9% Screening Test and the 81.9% Basic Test protocols were used for the toxicity assessment of the samples [30,31]. The pH of the samples was adjusted to 7, and catalase was used to remove residual H2O2 from the reaction solution. Calculations were performed using the Origin 9.1 version Pro software. Each experiment was repeated three times.
4.3. Results and discussion 4.3.1. H2O2 impact on the 𝐇𝐎• radicals formation mechanism Hydroxyl radical formation is the main characteristic of the oxidation processes discussed in this work. Therefore, the impact of hydrogen peroxide addition to the mechanism of hydroxyl radical formation was studied for the wastewater treatment process in which a combination of ozone, H2O2 and UV radiation was used. The impact of the hydrogen peroxide concentration on the RB5 decomposition rate was studied for the O3/H2O2 and O3/UV/H2O2 processes in the H2O2 concentration range of 0 – 0.1 M (the reaction rate constant had been changing significantly in this H2O2 concentration range). The experiment was conducted at pH 12 (which corresponds to the pH of the textile reactive dyeing discharge), as provided by the phosphate buffer. The O3 concentration of the inlet gas mixture was 42.3 mg/L, and the gas flow rate was set at Qin 40 L/h (1.69 gO3/L). RB5 was dissolved in distilled water at 125 mg/L. In this study, pseudo-first-order constants were calculated to describe the RB5 decomposition in the presence of different H2O2 concentrations. The overall reaction process, for AOPs combining UV, H2O2 and ozone, consisted of three contributions: direct photolysis by UV, direct oxidation by ozone and oxidation by hydroxyl radical HO• . Because the investigation was conducted in alkaline solution (pH 12), the main pathway of the degradation should be followed by a reaction of RB5 with the hydroxyl radical [8]. However, the direct oxidation via O3 at this pH occurred and thus cannot be excluded (Figure A 4.1 in supplementary materials). The occurrence of direct photolysis is neglected (Figure A 4.2 in supplementary materials). Therefore, as calculated using equation (4.2), the kapp constant is the general constant of RB5 decay. 𝐶
𝑙𝑛 𝐶 = 𝑘𝑎𝑝𝑝 ∙ 𝑡 0
106
(4.2)
The pseudo-first-order constant of the RB5 decomposition show a strong dependence on the hydrogen peroxide concentration for O3/H2O2 (Figure 4.2 A) and for the O3/UV/H2O2 process (Figure 4.2 B). In both cases, with an increasing H2O2 concentration, a decrease in the pseudo-first-order constant was noted. Based on the studies performed by Azbar et. al [30] and Chung and Kim [28], a synergic effect should occur; however, this effect was not observed in our investigation. This phenomenon can be explained by the scavenging effect of hydroxyl radicals by hydrogen peroxide, as shown in reactions (4.3) and (4.4). 𝐻𝑂• + 𝐻2 𝑂2 → 𝐻2 𝑂 + 𝐻𝑂2•
(4.3)
𝐻𝑂• + 𝐻𝑂2• → 𝐻2 𝑂 + 𝑂2
(4.4)
The scavenging effect was further enhanced in the alkaline reaction medium. In this medium, a significant part of ozone decomposed with HO• radicals (a smaller share of the direct ozone reaction), which could then be scavenged. Furthermore, in the alkaline reaction medium, hydrogen peroxide was present in the dissociated form (pKaH2O2 11.6) [32] , as it was in reaction (4.5), which promoted the HO• scavenging in accordance with reaction (4.6).
𝐻2 𝑂2 + 𝐻2 𝑂 → 𝐻3 𝑂+ + 𝐻𝑂2−
(4.5)
𝐻𝑂• + 𝐻𝑂2− → 𝐻2 𝑂 + 𝑂2•−
(4.6)
The slowdown effect was slightly smaller for the O3/UV/H2O2 process because part of the hydrogen peroxide was converted directly into HO• radicals via UV irradiation, as shown in Figure 4.2 B.
4.3.2. Comparison of the oxidation methods The applied oxidation methods were compared for purified RB5 aqueous solutions (Figure 4.3 A) and both the diluted and undiluted industrial textile wastewater from the dyeing process that used Setazol Black DPT (Figure 4.3 B and Figure 4.3 C, respectively). The RB5 solutions were tested at a concentration of C0RB5 125 mg/L and pH 12 (phosphate buffer) and pH 6.28 (MQ water) for ozone-based methods and H2O2/UV oxidation, respectively. This research conditions were selected based on the experiments, published in previous work [19].
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Figure 4.2. H2O2 effect on RB5 decomposition pseudo-first order constant in the case of the O3/H2O2 and G O3/UV/H2O2 processes; O3 concentration in the inlet gas mixture CO3 42.3 mg/L; the gas flow Qin 40 L/h.C0H2O2 0 – 0.01 M, pH 12, UVC lamp (15 W)
The diluted industrial wastewater was characterized by the concentration of the dye at approximately 125 mg/L. This way results obtained for simulated and industrial wastewater could be compered more easily. The dye concentration of the industrial wastewater without dilution was 730 mg/L (the parameters of the wastewater are shown in Table 4.1). Diluted and undiluted wastewater at its natural pH were treated. In the case of the ozone-based oxidation processes, the O3 concentration in inlet gas mixture was 42.3 mg/L, and the gas flow rate was Qin 40 L/h. An H2O2 concentration of 0.005 M (H2O2 concentration, which gave the highest reaction rate constant) and a 15 W UVC lamp (used optionally) were applied. In the case of
108
H2O2/UV oxidation, the hydrogen peroxide concentration was 0.2 M and 6 UVC lamps (7.2 W each) were used (reagent dosage explained in previous work [19]). As shown in Figure 4.3 A, for all of the tested methods, a high discoloration of the RB5 solutions, close to 100%, was achieved in a relatively short time (20 minutes). The best effect was obtained for ozonation. No synergy effect of various oxidizing together was noted. In the case of the simultaneous use of O3 and H2O2, a decrease was observed in the degradation rate of RB5 aqueous solution, which could be explained by the scavenging effect of hydroxyl radicals caused by hydrogen peroxide. For the diluted industrial textile wastewater containing RB5 (dyebath with Setazol Black DPT - Kyma Setas), the decolorization effect was very close to that achieved for RB5 aqueous solution (Figure 4.3. B). The exception was the UV/H2O2 process. Based on this observation, it can be concluded that the presence of textile auxiliary agents in wastewater – NaCl and SAA inhibited the decolorization with the UV/H2O2 cleaning method. The inhibition of decolorization (Figure A 4.3) and both TOC (Figure A 4.4) and COD decay (Figure A 4.5) is shown in the supplementary materials. The ozone-based AOPs were not influenced by these factors, as shown in the supplementary materials (Figure A 4.1). The influence of textile auxiliaries had been discussed wider in our previous work [19]. For the wastewater without dilution, complete discoloration was not achieved, even after 60 minutes, with any of the tested methods. Approximately 90% color reduction was obtained for ozonation and the O3/H2O2, O3/UV and O3/UV/H2O2 processes after this time. Because of the fact that in the O3/UV system a slightly greater amount of hydroxyl radicals could have been produced, in this case the efficiency of decolorization was a bit higher than in the processes without the light, but still the ozone concentration was too low relative to the amount of dye. The UV/H2O2 process was not effective. Slightly more than 30% color reduction was noted after 1 hour. The influence of textile auxiliary agents was significant in this case. What is more the molecules of dyes have high UV absorption capacity. When the dye concentration is high, the extent of UV absorbed by it can be high and affected the process, as it could be observed in our experiment. At the same time the direct photolysis was not observed (Figure A 4.2).
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Figure 4.3. O3, O3/H2O2, O3/UV/H2O2 and UV/H2O2 processes for: A) RB5 solution 125 mg/L, B) diluted industrial wastewater (Setazol Black DPT approx. 125 mg/L), C) textile wastewater without dilution (Setazol G Black DPT approx. 730 mg/L); ozone concentration CO3 42.3 mg/L, gas flow Qin 40 L/h, C0H2O2 0.005 M, pH 12, UVC lamp (15 W)
4.3.3. Mineralization and Biodegradability For the tested wastewater samples and RB5 solutions with prior O3, O3/H2O2, O3/UV, O3/UV/H2O2, or UV/H2O2 treatment, the degree of mineralization was assessed (Figure 4.4). The reduction of organic compounds, as expressed by COD and the mineralization extent, was satisfactory for RB5 aqueous solutions. Between 80 and 90% COD reduction and 50% TOC reduction was observed after 60 min of treatment. For the UV/H2O2 treatment, the COD reduction was 25% (Figure 4.4 A). In the cases of diluted and undiluted industrial wastewater, the reduction of organic compounds obtained after treatment was much lower than those obtained for the oxidation of RB5 aqueous solutions (in the case of all tested methods). Approximately 50% COD and 30% TOC reduction were achieved for diluted industrial wastewater (Figure 4.4 B). In spite of the fact that RB5 concentration in simulated and diluted industrial wastewater was approximately the same, COD and TOD reductions were not comparable for these two samples. Therefore, in case of research focused on industrial application real research conditions, should be taken under consideration. For undiluted industrial wastewater, there was only a 10% COD and 20% TOC reduction (Figure 4.4 C) due to the greater initial load of the industrial wastewater (compared to the dye aqueous solution).
110
In some cases, the COD reduction was smaller than the TOC reduction due to the occurrence of compounds, which are not oxidized by the dichromate method (as used in the LCK 514 COD test).
Figure 4.4. TOC and COD removal for the decolorization of RB5 aqueous solutions A) and textile wastewater containing Setazol Black DPT: diluted one B) and not diluted one C) by ozonation, UV/H2O2, O3/H2O2, O3 UV/O3/H2O2 (C0RB5 125 mg/L, Cin 42.3 mg/L, Qin 40 L/h, pH 12, UVC lamp 15 W, C0H2O2 0.005 and 0.01 M) and H2O2/UV process (for dye aqueous solution: C0RB5 125 mg/L, C0H2O2 0.2 M, pH 6.28; for wastewater C0H2O2 0.2 M, pH 11.23)
The biodegradability study was performed for the simulated mixtures containing 125 mg/L of RB5 and for the diluted industrial wastewater corresponding to the concentration of the industrial dye (approximately 125 mg/L). Although the transformation products were not identified, the degradation of RB5 into other by-products was monitored by the change in the
111
degree of oxidation, which is an indicator of the oxidation degree of complex solutions and provides indirect information on their probability of biodegradation. Therefore, based on the COD/TOC, BOD5/COD values and average oxidation state (AOS), biodegradability was evaluated. The AOS value was calculated following Reyes et al. [33] by using equation (4.7), where TOC and COD are expressed in mM of C and O2, respectively.
𝐴𝑂𝑆 =
4 (𝑇𝑂𝐶−𝐶𝑂𝐷) 𝑇𝑂𝐶
(4.7)
The results are shown in Figure 4.5 (A, B, C: RB5 aqueous solution, D, E, F: diluted industrial wastewater). The COD/TOC reduction results obtained for the dye solution were satisfying and confirmed that fewer complexed by-products were present after AOP treatment, which resulted in the mineralization of RB5 (Figure 4.5 A). The COD/TOC reduction for wastewater was not as high but was still significant (Figure 4.5 D). For the BOD5/COD values of RB5 solutions after AOP treatments, a visible increase was observed from the value close to zero (0 min) up to 0.8 (60 min). The best results were obtained for the O3/0.005 M H2O2 process and O3 at 0.8 and 0.75, respectively. For the diluted wastewater, BOD5/COD increased after using AOPs. However, the increase of these values was not significantly high. The largest enhancement in biodegradability was found for the ozonation of the wastewater (Figure 4.5 F). Moreover, when using a higher concentration of H2O2, a positive effect on biodegradability was achieved. The AOS values are presented in Figure 4.5 B and Figure 4.5 E (for the dye solution and the wastewater, respectively). The AOS is a valuable parameter that can be used as a general oxidation measure of complex mixtures (where various oxidation products are present). Therefore, AOS was considered. AOS can take values between +4 CO2 (the most oxidized state of C) and −4 for CH4 (the most reduced state of C). All of the tested AOPs resulted in significant increases in the AOS values for the dye solution and lower increases for wastewater. A higher general oxidation degree (covering the main compound and its oxidation products) was achieved in the case of AOPs when H2O2 was used in addition to ozone. With respect to COD/TOC, BOD5/COD and AOS, the O3 and O3/0.005 M H2O2 treatments resulted in more oxidized by-products that could be assimilated by microorganisms in both cases: RB5 aqueous solution and wastewater containing RB5.
112
Figure 4.5. COD/TOC A), AOS B), BOD5/COD values obtained for the decomposition of RB5 aqueous solutions and COD/TOC D) AOS E), BOD5/COD F) values obtained for the decomposition of diluted wastewater by G ozonation, O3/H2O2, UV/O3/H2O2 (C0RB5 125 mg/L, CO3 42.3 mg/L, Qin 40 L/h, pH 12, UVC lamp 15 W, C0H2O2 0.005 and 0.01 M)
113
4.3.5. Toxicity Although high AOS values are frequently associated with biocompatibility of the solutions, it does not necessarily must be related to the formation of by-products characterized by low toxicity. The toxicity assessment was conducted by a screening test and an EC50 test for the decomposition of RB5 aqueous solutions and diluted wastewater containing industrial dye. The results of the screening test for ozonation, O3/H2O2 and UV/O3/H2O2 of RB5 aqueous solutions are shown in Figure 4.6. Using oxidation methods with higher concentrations of H2O2 (0.01 M) and coupling O3, H2O2 and UV resulted in nontoxic mixtures. However, the use of O3 and O3/0.005 M H2O2 resulted in a less significant decrease in toxicity (Table 4.2 shows the EC50 values). Moreover, the tested dyestuff was characterized by very high toxicity. The EC50 value was 3.86±0.32 mg/L, which is in in agreement with the values obtained by other authors for the sodium dithionite form of RB5 [34]. This form of the dye has also been identified by us using mass spectroscopy.
Figure 4.6. Toxicity assessment for the oxidation of RB5 aqueous solutions by ozonation, O3/H2O2, UV/O3/H2O2 G (C0RB5 125 mg/L, CO3 42.3 mg/L, Qin 40 L/h, pH 12, UVC lamp 15 W, C0H2O2 0.005 and 0.01 M)
The toxicity of the RB5 aqueous solution during ozonation was also investigated. A short ozonation time increased the toxicity only slightly, i.e., from EC50 2.81% at the beginning to 2.55% after 5 minutes (very low EC50 value), which suggests the appearance of some toxic byproducts during the initial ozonation phase. Then, after 60 minutes of treatment and the
114
application of 1.68 g/L ozone, the toxicity decreased more than five times, and the EC50 was 19.62%.
Table 4.2. RB5 aqueous solution toxicity assessment, EC50 test Method O3
Time (min)
EC50 (% v/v)
0
3.31
60
19.72
0
3.15
60
21.61
O3/H2O2 (0.005 M)
The toxicity assessment was also conducted for the diluted industrial wastewater. The results of the screening test and the EC50 test are shown in Table 4.3. Each AOP treatment resulted in decreased toxicity. However, the wastewater was not as toxic as it was before the treatment as for the RB5 aqueous solution. According to Gottlieb et. al. [34], the parent RB5 (EC50 27.5 mg/L) and its hydrolyzed form (EC50 11.4 mg/L) are characterized by a lower toxicity than the sodium dithionite form of RB5. During the industrial dyeing process, the sodium salt of the dye is used (technical product with four Na atoms), and hydrolyzation of the dye occurs due to the high temperature dyeing conditions and alkaline pH, which explains the differences between the toxicity of RB5 in aqueous solution and the toxicity of the wastewater. Table 4.3. Diluted wastewater toxicity assessment, screening test and EC50 test Method
Time (min)
Luminescence inhibition after exposure to effluent for 15 min (%)
EC50 (95% CI in brackets) (% v/v)
0
67.51
46.63 (40.77 -53.33)
60
1.59
Not calc.
0
90.77
40.13 (32.19 - 66.38)
60
Not calc.
Not calc.
O3
O3/H2O2 (0.005 M) 0
70.16
46.30 (34.57 - 62.02)
60
2.70
Not calc.
0
53.70
76.05 (52.21 - 84.71)
60
Not calc.
Not calc.
0
58.02
59.77 (48.59 - 79.01)
60
12.87
Not calc.
O3/UV
O3/UV/H2O2 (0.005 M)
UV/H2O2 (0.2 M)
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4.3.6 Costs evaluation The efficiency factor of several ozone-based AOP methods has been discussed from a practical point of view; the cost factor is also important. In industrial practice, efficiency related to costs is an indicator for choosing the best treatment method. In previous studies the electrical energy per order (EE/O) indicator has been used for this purpose. In most cases, this evaluation is related to laboratory-scale experiments [22,29,35]. A cost evaluation performed in this way can be far different from the industrial-scale process. In this study, the costs of the tested treatment methods were estimated on the basis of real industrial conditions. The industrial-scale AOP plant implemented in one of the Polish dye houses (Bilinski Co.) that used an APP Thies ozone apparatus was the model for the cost evaluation. The operating and investment costs were calculated for all of the tested ozone-based AOPs. The input data are shown in table 4.4. The operating costs were related to the ozone dose used in the experiment (1.69 g/L). The industrial process time was calculated following Chung & Kim [28] using equation (4.8).
𝑇𝑜𝑡𝑎𝑙𝑂𝑧𝑜𝑛𝑒𝐷𝑜𝑠𝑒 =
𝑂3 𝑑𝑜𝑠𝑒∙𝐺𝑎𝑠 𝑓𝑙𝑜𝑤−𝑟𝑎𝑡𝑒∙𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 𝑅𝑒𝑎𝑐𝑡𝑜𝑟 𝑣𝑜𝑙𝑢𝑚𝑒
(𝑔/𝐿)
(4.8)
Table 4.4. Cost evaluation input data Parameter
Value
Ozone dose in gas phase
250 g/Nm3
Gas flow-rate
10 Nm3/h
Reaction time
0.67 h
Reactor volume
1 m3
UV lamp energy
16 kW
H2O2 0.005 M
0.51 L(H2O2 30% a.s.)/m3
H2O2 0.01 M
1.02 L(H2O2 30% a.s.)/m3
In figure 4.7, the investment and operating costs of the several tested ozone-based AOPs are shown. The investment costs (Figure 4.7 A) of the ozonation plant are high. In the case of O3/H2O2, the cost of the H2O2 dose is negligible compared to that of ozonation. In contrast, the UV lamp equipment significantly increases the investment costs. The operating costs (Figure 4.7 B) of the ozonation and O3/H2O2 processes are comparable and are approximately 50% the cost of fresh water and discharge industrial wastewater to the city sewage. Therefore, using the O3 and O3/H2O2 process can be economically reasonable, especially when the possibility of environmental penalty is considered (in textile wastewater, the main problem is an excessive chloride concentration). The cost of using O3/UV and O3/UV/H2O2 is 80% that of fresh water
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and discharge industrial wastewater to the city sewage, industrial implementation is not recommended. These differences are due to the UV lamp application. The investment costs and the efficiency of these methods do not provide incentives.
Figure 4.7. The investment A) and operating B) costs of ozone-based AOPs
The total ozone dose (TOD 1.69 g/L) can be far too high when an industrial ozone treatment plant is considered. Due to mass transfer, the limitation ozone was overdosed during the laboratory-scale experiment. The industrial ozone reactor (APP by Thies) is equipped with an ozone injecting system, and mass transfer between the gas (O3 and O2 mixture) and liquid phases (wastewater) is enhanced. Therefore, industrial research should be conducted, the exact industrial TOD should be estimated and the reaction time could be much shorter.
4.4. Conclusions In this study, the wastewater stream after the reactive dyeing process was taken into consideration. Due to high salinity, high pH and intense color, it is the most problematic textile wastewater type from an environmental perspective. This research had led some conclusions to this field. It was indicated that, using O3 or O3 combined with a moderate concentration of H2O2 (below 0.005 M) results in good color reduction and an increase in mineralization,
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biodegradability and a reduction in the toxicity of hazard compound RB5. In turn, for the AOPs in which higher concentrations of H2O2 and/or UV irradiation were used, lower values of BOD5/COD indicator were found. Treating after-dyeing discharge by ozone-based AOPs can be a first stage in the textile wastewater purification process, e.g., prior to biological treatment, or it can be the only operation when the purified after-dyeing discharge is used as a source of concentrated brine (ready to use). Moreover, treating the selected textile wastewater stream, in this case, after-dyeing discharge, by using a customized method can be economically reasonable for industry. It should be noticed that conducting the research focused only on the oxidation of aqueous dye solutions without considering the conditions in real textile wastewater can give distorted results; reliance on these results can be confusing when designing an industrial process. Based on experimental results and costs evaluation, the use of ozone or ozone with very low concentration of H2O2 (0.005 M) can be recommended to textile industry. In contrast to the O3/UV process, the investment and operating costs of the O3/H2O2 process are not much higher than ozonation itself. When greater mineralization is considered, O3/H2O2 is a good option, but global ozonation is the best option within AOPs.
4.5. Acknowledgements The authors thank the Polish Ministry of Science Higher Education for financially supporting this research under contract no. PBS2/A9/22/2013. Special thanks to Textile Company Biliński, Konstantynów Łódzki, Poland for their cooperation. Marta Gmurek acknowledges the support from the Foundation for Polish Science within the START scholarship.
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Supplementary materials to Chapter 4
Figure A 4.1. RB5 decomposition by ozonation (CO3 0.4 g/h) in aqueous solution, aqueous solution with t-But and simulated wastewater; wastewater compounds influence on process (simulated wastewater composition according to industrial recipe: C0RB5 125 mg/L; C0NaCl 65 g/L, C0SAA 0.76 g/L; C0Na2CO3 1.4 g/L; C0NaOH 1.95 g/L)
Figure A 4.2. RB5 decomposition by direct photolysis, „dark” reaction (C0H2O2 0.2 M), H2O2/UV process and H2O2/UV process with t-But (C0H2O2 0.2M; C0t−But 0.1 M and 1.0 M)
123
Figure A 4.3. RB5 decomposition by H2O2/UV process (C0H2O2 0.2 M) in aqueous solution and simulated wastewater – wastewater compounds influence on process
Figure A 4.4. TOC/TOC0 during RB5 decomposition by H2O2/UV process (C0H2O2 0.2 M) in aqueous solution and simulated wastewater – wastewater compounds influence on TOC decay
124
Figure A 4.5. COD/COD0 during RB5 decomposition by H 2O2/UV process (C0H2O2 0.2 M) in aqueous solution and simulated wastewater – wastewater compounds influence on COD decay
125
126
Chapter 5 Textile wastewater treatment by AOPs for brine reuse
Lucyna Bilińska, Marta Gmurek, Stanisław Ledakowicz Process Safety and Environmental Protection 109, 2017, 420 – 428 DOI: 10.1016/j.psep.2017.04.019
Graphical abstract
Abstract The most contaminated textile wastewater stream, dyeing discharge with a high residual salt content, was selected to undergo dedicated treatment by advanced oxidation processes (AOPs). A simulated mixture, based on an industrial recipe and containing Reactive Yellow 145 (RY145), Reactive Red 195 (RR195), and Reactive Blue 221 (RB221), was investigated. These dyes are used together in the trichromatic technique in industrial dyeing, and they occur together in wastewater. In this study, for the first time, several ozone-based AOPs (O3, O3/H2O2, O3/UV and O3/UV/H2O2) were tested under comparable conditions and assessed. Moreover, the roles of hydrogen peroxide and UV during the AOPs were determined. The influence of textile auxiliaries on the AOPs was also investigated. Due to the usage of multiple dyes in the same mixture, colour was evaluated based on whole UV-Vis spectra by recalculating them as integrals. Extremely fast decolorization of the textile wastewater was observed during ozonation. Only 10% colour remained after 10 minutes of treatment, and satisfactory mineralization was also achieved. During the AOP experiments, it was found that ozonation is the best treatment method for implementation in the industry. Textile wastewater treated by AOPs could be reused as a source of “ready to use” brine for the next textile dyeing operation.
Keywords: Ozone-based AOPs; Simulated textile wastewater; Mixture of the dyes; Scavenging effect; Brine recycling;
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5.1. Introduction Currently, in industrial production processes, sustainable development is a major issue. In the modern textile industry, water management is particularly important. The amount of generated textile wastewater can reach more than 300 L per kg of product [1]. The IPPC Directive suggests implementation of BAT (Best Available Techniques) as a preferable solution for textile wastewater treatment [2]. One of the BAT given by “Reference Document on BAT for the Textiles Industry” is “wastewater stream management”. According to the main points of the issue [3], the separation of highly contaminated wastewater, which can cause the malfunction of biological treatments, is recommended. Then, non-biodegradable waste streams are to be treated using appropriate techniques before, or instead of, the biological treatment. This idea was the basis of this study. The most contaminated textile wastewater stream, dyeing discharge, was selected for dedicated treatment by advanced oxidation processes (AOPs). This type of wastewater is very problematic from an environmental point of view. High concentrations of residual NaCl remain in this wastewater stream after dyeing operations. Biological treatment, which can be quite effective, does not ensure salt removal [4]. Moreover, the recent study of Paździor et al. [5] indicates that the separation of salty dyeing discharge can yield positive results in terms of the biological treatment of textile wastewater. In contrast to biological treatment, membrane technologies, such as reverse osmosis (RO), are very effective at salt removal but are extremely expensive and result in the production of highly polluted concentrate [4]. Because of difficulties in finding an appropriate and relatively cheap treatment method, the high salt load continues to be emitted to the environment. Therefore, cleaning this wastewater via AOPs and reusing it as the source of ‘ready to use’ brine is the key result of the present work. This method could be a good option for minimizing environmental damage by recycling salt in the textile industry. Due to the high oxidative potential of hydroxyl radicals, which appear in AOPs, many organic compounds, including dyes, can be decomposed [6]. Although many studies concerning the oxidation of textile dyes by AOPs can be found, few of them have focused on real conditions that occur in industrial wastewater. In the case of textile wastewater after dyeing operations, the most important characteristic is the presence of NaCl and textile auxiliaries, such as alkalines, surface active agents (SAA) and several dyes in the same mixture. Only a few authors have investigated the influence of salts or alkalines on the treatment [7–9]. The influence of SAA on AOPs was shown for the first time in our previous works [10–12]. Issues with dye mixtures have not been very widely discussed in the literature to date. Kim et al. [13], EmamiMeibodi et al. [14] and Dong et al. [15] worked with mixtures of some dyes. They attempted to 130
simulate textile wastewater; however, the solutions were nothing more than aqueous mixtures of few dyes that were chosen randomly without any substantiation. Additionally, no salt or textile auxiliaries were added to it. When real industrial wastewater was considered, the authors dealt with the global parameters of the wastewater. The influence of textile auxiliaries was not discussed in those works [16–24]. Although Arslan et al. [25] and Arslan Alaton et al. (2002) [26] used O3-based AOPs to clean the mixture of dyes and textile auxiliaries, which simulated industrial wastewater after dyeing, these works did not focus on the influence of textile auxiliaries or reusing cleaned wastewater as a source of brine. Moreover, the simulated mixtures were diluted before the treatment; thus, the research did not reflect the real dyeing discharge conditions. In addition, the colour evaluation was performed based on the absorbance measured for only a specific wavelength. Unfortunately, in the case of dye mixtures, the UV-Vis spectra overlap and the characteristic peaks of the dyes are not visible. Therefore, the method used may provide unsatisfactory results. In our study, the AOP treatment was investigated for dyes in a simulated mixture based on an industrial dyeing recipe. For this purpose, the following very popular reactive dyes were selected: Reactive Yellow 145 (RY145), Reactive Red 195 (RR195), and Reactive Blue 221 (RB221). Although these dyes are the basis of one of the most common trichromatic dye groups in industrial use, e.g., Bezactiv S (Bezema), Intracron CDX (Yorkshire), Sunfix S (OhYoung), Drimaren HF (Archroma) or Sumifix Supra (Sumitomo), these dyes are not well covered in the literature. Only a few authors have chosen them as the subject of AOP treatments [27–29]. However, none of these works include all three at the same time. Importantly, these dyes used as a set in the trichromatic technique of dyeing can give a full range of shades, except black. Therefore, RY145, RR195 and RB221 are the most popular dyes for cellulosic textiles in dyeing houses around the world. In our work, the influence of salt and textile auxiliaries, both of which are always present in the dyeing discharge in addition to the dyes, was considered as a very significant factor. Moreover, for the first time, multiple ozone-based AOPs (O3, UV/O3, O3/H2O2, O3/UV/H2O2) were tested under comparable conditions. They were evaluated in terms of decolorization and mineralization. Colour was evaluated based on the whole UV-Vis spectra by recalculating them into integrals. Moreover, the roles of hydrogen peroxide and UV during the AOPs were determined. The presented work is a preliminary study to prepare the system of purification and recycling of the brine from textile wastewater. The system is planned to be implemented at the textile company that cooperated with the authors.
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5.2. Experiment
5.2.1. Materials Synozol Yellow KHL (Reactive Yellow 145), Synozol Red K3BS (Reactive Red 195), and Synozol Blue KBR (Reactive Blue 221) were purchased from KISCO (Turkey) as industrial dyes. The chemical structures and characteristics of these dyes are presented in Figure 5.1. An industrial dyeing assistant – Perigen LDR (an SAA composed of a naphthalenesulfonic acid and carboxylates mixture) was purchased from Textilchemie Dr. Petry Co. (Germany). The buffer components were as follows: NaOH and Na2HPO4 were AR, and the other chemicals included NaCl, Na2CO3 and Na2SO3.
5.2.2. Methods Experimental set-up. A semibatch glass thermostated reactor (heterogeneous gas-liquid system) with a capacity of 1 L was used. Mixing in the reactor was performed with a magnetic stirrer (Wigo type ES 21). Ozone was produced by an Ozonek Ozone Generator (Poland) fed with oxygen from a compressed gas cylinder (O2 purity 99.5%). The O3 concentrations at the inlet and outlet of the reactor were measured with a BMT 963 Vent ozone analyser (this was the basis for absorbed ozone dose calculations). Optionally, a UVC low-pressure mercury lamp (Herraus 15 W) was added to the experimental set. The lamp was placed in the quartz tube and immersed inside the reactor. The temperature and pH were monitored during the experiments by an Elmetron meter, type CX-401 (Poland). Experimental procedure. Four AOPs have been investigated: O3, O3/H2O2, O3/UV and O3/UV/H2O2. In the case of the O3/H2O2 and O3/UV/H2O2 processes, hydrogen peroxide was added to the reaction mixture before the ozone was applied. The reaction progress for all ozonebased AOPs was stopped by the addition of 0.01 M Na2SO3 to the collected samples. The experiment was carried out in three variants: aqueous solutions with a single dye, aqueous solutions with a mixture of the dyes and simulated wastewater based on industrial recipe (Table 5.1). Operating conditions. For all variants of the experiment, the ozone concentration COG3 42.3 mg/L and the gas flow rate Qin 40 L/h were used. The reaction medium was alkaline (pH 12), which corresponds to the pH of the industrial dyeing discharge. For the aqueous solutions with the single dyes and the aqueous solutions of the mixture, the pH was set using a phosphate buffer. For the wastewater, the pH was a result of alkaline dosing (recipe from Table 5.1). The H2O2 concentrations were 0.005 and 0.01 M. These values were chosen from the range of 0.001 132
to 0.1 M. These experimental conditions were based on our previous work [12], and they were found to reliably show the H2O2 influence on the AOPs. Measurements. Colour was determined by spectrophotometer (Helios Thermo). For the single dye aqueous solutions, absorbance at specific points (Figure 5.1) was measured, and calibration plots based on Lambert-Beer law were used to determine the concentrations of the samples. For the dye mixtures and the wastewater samples, the colour was determined based on UV-Vis spectra measurements. The surface areas under the absorbance curves were integrated, and the integrals were the quantitative measure of the colour changes over time. Chemical oxygen demand (COD) was measured with the HACH LCK 514 and 314 tests. Total organic carbon (TOC) was measured in an HACH IL 550TOC-TN apparatus. The samples were diluted before the COD and TOC tests to avoid the influence of the salts. Experimental data were calculated using the Origin® 9.1 version Pro software.
Figure 5.1. Characteristics and Vis spectra of RY145, RR195 and RB221
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Table 5.1. Simulated wastewater content based on an industrial dyeing recipe
Simulated wastewater content RY145 + RR195 + RB221 Perigen LDR (SAA) NaCl Na2CO3 NaOH 50% (a.s.)
125 mg/L 1.5 g/L 38.0 g/L 5.0 g/L 1.0 g/L
5.3. Results and discussion
5.3.1. Influence of the wastewater matrix The wastewater matrix can be a crucial factor when selecting the treatment method. Therefore, it was extremely important to find out how it can influence the AOPs. Because industrial dyeing discharge is a mixture of several dyes and some textile auxiliaries, the impact of these ingredients on AOPs was studied. For this purpose, the dyes mixed together in the same aqueous solution and simulated industrial wastewater (composition according to Table 5.1) underwent ozone treatment. During this process, the initial dye concentration was 125 mg/L, and the pH was 12. Both of these values were set according to industrial conditions.
Figure 5.2. The Vis spectra of the aqueous solution of RY145, RR195 and RB221 mixture treated by ozone at specific times; ozonation conditions: 𝐶0𝑑𝑦𝑒 125 mg/L, applied ozone dose 1.96 g/L, pH 12
It can be noted that, in contrast to the spectra of the single dyes (Figure 5.1), the spectrum of the mixture (Figure 5.2) looks totally different. Due to the interactions between the dyes, the 134
Vis spectra overlap and the characteristic peaks of the dyes are different. Therefore, the colour was determined based on the Vis spectra measurement within 400 – 750 nm. Then, the surface areas under the absorbance curves presented in Figure 5.2 were integrated, and the integrals were the quantitative measure of colour change over time. Figure 5.2 shows how the spectra changed during the ozonation. Every single spectrum is characterized by lower values of absorbance over the treatment time. Based on these results, it can be concluded that the dyes in this mixture were successfully decoloured. In Figure 5.3 A, the quantitative results of decolorization, based on the integrals values, versus time are presented. The study was performed in four variants: 1) for the aqueous solution of dyes only (without a salt addition or the auxiliaries), 2) for the dye mixture with salt (NaCl), 3) for the dye mixture with SAA (industrial levelling agent), and 4) for the simulated wastewater (recipe in Table 5.1). Figure 5.3 A shows that, after 20 minutes of treatment, a colour reduction of close to 100% was achieved for all the variants. This effect corresponds to an absorbed ozone dose close to 0.3 g/L (Figure 5.3 B). In Figure 5.3 C, the absorbed ozone doses for the aqueous solution of the dyes and for the wastewater are presented. It can be noticed that higher pollutant contents (the case of the wastewater) correlate with higher general absorbed ozone dose during the process. However, two stages in the ozonation process can be distinguished. First, the dye decolorization due to chromophore oxidation occurred; then, further decomposition of the byproducts took place. As long as decolorization was the dominant phenomenon, the absorbed ozone dose was almost equal for both the aqueous dye solution and the wastewater. This pattern can be explained by ozone attacking the chromophores of the dyes, regardless of whether the dyes are in a simple aqueous solution, a mixed solution or wastewater. This type of observation has been reported in the literature [8,9]. Although the general absorbed ozone dose was greater for the wastewater than for the aqueous dye solution, in the initial phase of the process, there was not much difference between the different solutions (Figure 5.3 C). These observations may explain why the textile auxiliaries did not have a significant influence on the decolorization stage during ozonation (Figures 5.3 A and 5.3 B). The effect of textile wastewater matrix is negligible in first stage of ozonation, even though certain authors have detected some effects caused by salts or alkalis. Muthukumar & Selvakumar [7] noticed inhibition caused by NaCl and Na2SO4 (at concentrations of 5 and 15 g/L, for both compounds). The authors suggested some scavenging effect caused by salts; however, they did not consider the additional aggregation effect of the acid dyes [30]. On the other hand, Colindres et al. [8] showed significant acceleration in ozonation of Reactive Black 5 caused by salt (10 and 20 g/L Na2SO4) and alkalis (2.5 and 5 g/L NaOH; 10 and 20 g/L Na2CO3). However, no information about using 135
a buffer solution was found in this study. There was no clear information concerning the initial pH of the samples. Therefore, some phenomena shown in this work could be due to an alkaline pH. Bamperng et al. [9] detected only a small inhibition of NaCl (15 g/L), which was explained by its influence on ozone solubility in water. At the same time, the addition of Na2CO3 (15 g/L) resulted in significant inhibition of the process, and the authors tried to justify it based on HO• formation. The presented literature [7–9] does not give a clear explanation of the issue. Some studies might include effects associated with the experimental conditions. In contrast, the findings of our previous studies are in agreement with the results presented in this work [11,12].
Figure 3. The influence of the textile wastewater matrix on the ozonation process: A) relative colour reduction versus time, B) relative colour reduction versus absorbed ozone dose, C) absorbed ozone dose during the process; ozonation conditions: C0dye 125 mg/L, applied ozone dose 1.96 g/L, pH 12
5.3.2. Comparison of the AOPs The additions of H2O2 and UV irradiation are well-known ways of improving AOPs, as reported by many authors [20–23]. Therefore, these methods were checked their effects on textile wastewater. The AOP efficiency was measured as the per cent decolorization after 10 min of the treatment. This length of time ensured approximately 90% colour removal in the most efficient variants. The reaction medium was alkaline (pH 12), and the initial dyes concentration was 125 mg/L, which corresponds with the conditions of an industrial dying discharge. The applied ozone dose was 0.28 g/L. H2O2 concentrations were 0.005 and 0.01 M. A 15 W UV lamp was used optionally during the photo processes. The comparison of the AOPs was performed for the aqueous single dye solutions and the dye mixture. The results are presented in Table 5.2.
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Table 5.2. Decolorization efficiency (%) after 10 minutes for the AOPs Decolorization efficiency (%) AOP RY145
RR 165
RB221
RY+RR+RB
O3
87.5
±0.5
90.7
±0.8
97.6
±0.4
88.6
O3/0.005 M H2O2, O3/0.01 M H2O2,
63.8 41.5
±4.6 ±0.1
68.0 46.5
±0.7 ±0.4
99.4 95.8
±0.6 --
89.5 81.5
O3/UV
82.5
±2.6
76.6
±2.4
89.6
±1.0
83.8
O3/UV/0.005 M H2O2
57.5
±0.9
71.2
±1.4
99.4
±1.7
83.1
O3/UV/0.01 M H2O2
40.9
±1.5
51.4
±0.5
90.1
±1.9
88.6
5.3.2.1. Dyes treated separately In the case of the azo dyes, RY145 and RR195 (Table 5.2), it was observed that the highest decolorization efficiency (near 90%) was achieved for the process using only ozone. When O3, H2O2, and UV were used simultaneously (O3/H2O2 and O3/UV/H2O2), the results were not satisfactory (between 40 and 60% colour removal). In the case of the RB221 dye (Table 5.2), decolorization proceeded rapidly (more than 90% of colour removal for every AOP after 10 min.). Based on the results, it can be concluded that two parallel oxidation mechanisms of RB211 were the cause of the rapid RB221 decolorization process. The data presented in Figure 5.4 shows that the decomposition of RB221 using only H2O2 can also occur. This phenomenon was detected under alkaline pH values (pH 12). Therefore, it can be concluded that the decolorization of RB221 was probably conducted by a dissociated form of hydrogen peroxide (pKaH2O2 11.6). In contrast to the azo dyes, RB221 has a specific metal complex chromophore, which could have been more sensitive to the oxidative conditions. Moreover, it cannot be excluded that the copper, which is a part of the chromophore, may have influenced the oxidative processes. The copper could have acted as a oxidation accelerator by catalysing H2O2 decomposition. It can also be noticed that, in the acidic environment, the absorbance of RB221 was lower than in the neutral environment (Figure 5.4). At pH 2, a partial disintegration of the metal-complex in the structure of formazan could have taken place. The first main conclusion in this section is that the oxidation mechanism depends on the dye structure and is specific to each group of dyes. Another important observation is that there was no synergy or even an additive effect of O3, H2O2 and UV in the AOPs. For higher H2O2 concentrations, smaller colour reductions were noted. This phenomenon is due to the scavenging effect of HO• radicals in the alkaline medium [6], as shown in reactions (5.1) and (5.2). Moreover, the scavenging effect was additionally enhanced by a reaction of the hydroxyl 137
radical with an anionic form of H2O2 (pKaH2O2 11.6) in accordance with reactions (5.3) and (5.4). 𝐻𝑂• + 𝐻2 𝑂2 → 𝐻2 𝑂 + 𝐻𝑂2•
(5.1)
𝐻𝑂• + 𝐻𝑂2• → 𝐻2 𝑂 + 𝑂2
(5.2)
𝐻2 𝑂2 + 𝐻2 𝑂 → 𝐻3 𝑂+ + 𝐻𝑂2−
(5.3)
𝐻𝑂• + 𝐻𝑂2− → 𝐻2 𝑂 + 𝑂2•−
(5.4)
Based on our previous study [12], the scavenging effect could be lower when UV irradiation is present because part of the H2O2 could be directly converted into HO• radicals. However, in the present study, this effect was minimized and occurred only in case of RR195. According to the results presented by Azbar et. Al [20] and Chung & Kim [23], a synergistic effect should have occurred. However, those studies were performed under lower pH conditions (up to 9), in which the scavenging effect of H2O2 was not significant.
Figure 5.4. The RB221 formazan dye oxidation by H2O2 in aqueous solution; C0RB5 125 mg/L, C0H2O2 0.1 M, pH 12, 7, 2 (phosphate buffers)
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5.3.2.2. Dyes treated in the mixture Table 5.2 lists the results of the dye treatment by AOPs for the mixture. A colour evaluation was carried out based on the whole UV-Vis spectra measurements (integrals of surface area under the absorbance curves were the measure of the color). Similar phenomena as those observed during the oxidation of single dyes were noted for the dye mixture. Firstly, the decolorization efficiency was higher for the dye mixture than for RY145 and RR195. This pattern could be caused by the more rapid decolorization of formazan dye (RB 221) in the mixture. Moreover, the copper from RB221 could enhance the processes involving H2O2. Secondly, a scavenging effect appeared, similar to that of the single dye treatments.
5.3.3. Mineralization The mineralization assessment was conducted for the aqueous single dye solutions (RY145, RR195, RB221) treated separately and the mixture. Based on the results presented in section 5.3.1, it can be assumed that the first stage of the treatment of the dyes is decolorization and that the second stage involves further decomposition of the by-products. This hypothesis can be indirectly confirmed by the COD and TOC results measured versus time (Figure A 5.1 A and A 5.1 B, supplementary materials). Although decolorization started immediately after ozone absorption, there was almost no change in COD or TOC. Therefore, mineralization was evaluated after 60 minutes of treatment, when the applied ozone dose was equal to 1.69 g/L.
5.3.3.1. Dyes treated separately The mineralization degrees achieved for RY145, RR195, and RB221 in separate AOPtreated aqueous solutions are presented in Figure 5.5. The decomposition of the organic compounds was expressed using COD/COD0 and TOC/TOC0 values. The highest COD removal was obtained using O3/H2O2 (0.005 M and 0.01 M of H2O2). However, the reduction in TOC was the largest for ozonation (for all dyes), and the COD decrease was satisfactory for this treatment. Therefore, it can be concluded that ozonation yielded the best mineralization results. Interestingly, the use of the O3/UV process for all the dyes produced a COD removal similar to the simultaneous TOC decrease. This result might have been caused by the specific oxidation pathway of the dyes. In this case, the majority of the by-products are organic compounds, which can be further decomposed into CO2.
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Figure 5. TOC and COD removals in the degradation of RY145 A), RR195 B), and RB221 C) by ozonation, UV/H2O2, O3/H2O2, UV/O3/H2O2 (C0dye 125 mg/L, applied ozone dose 1.96 g/L, pH 12, UVC lamp 15 W, C0H2O2 0.005 and 0.01 M)
5.3.3.2. Dyes treated in the mixture Although the single dye treatment by ozonation resulted in the best degree of mineralization, the outcome for the mixture of the dyes was different. Many various byproducts can appear during dye oxidation [31]. A great number of dyes present in a mixture can produce more types of by-products. Some interactions could occur between these by-products. Our attempts to apply chromatography coupled with mass spectrometry (UHPLC/MS) to analyse the intermediates in the reaction mixture did not give satisfactory results because the industrial dyestuffs are contaminated by excipients and many impurities. Therefore, based on the COD and TOC measurements, only a general conclusion can be drawn. Although the
140
scavenging effect might occur during the O3/H2O2 and O3/UV/H2O2 processes, the degree of mineralization after 60 minutes of these AOP treatments (applied ozone dose: 1.69 g/L) was greater than for ozonation alone (Figure 5.6 A).When O3, UV and H2O2 were applied simultaneously, the COD removal was even greater.
Figure 5.6. TOC and COD removal A), COD/TOC values B) AOS values C) for degradation of the RY145, RR195, and RB221 mixture by ozonation, UV/H2O2, O3/H2O2, UV/O3/H2O2 (C0dye 125 mg/L, applied ozone dose 1.96 g/L, pH 12, UVC lamp 15 W, C0H2O2 0.005 and 0.01 M)
The COD/TOC reduction value was also the highest (75%) when the O3/UV/H2O2 process was used (Figure 5.6 B). In the case of ozonation, the reduction was approximately 67%. Economically, ozonation seems to be the better option, especially when mineralization is not the most important factor. Additionally, the average oxidation state (AOS) value is shown in Figure 5.6 C. This indicator can give indirect information about by-products occurring during the treatment. It is useful when these products are not identified. The AOS represents the
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oxidation degree of the whole complex solution. The AOS value was calculated following Reyes et al. [32] using equation (5.5), where TOC and COD are expressed in mM of C and O2, respectively.
𝐴𝑂𝑆 =
4 (𝑇𝑂𝐶−𝐶𝑂𝐷) 𝑇𝑂𝐶
(5.5)
The AOS is a good indicator when the general oxidation measure of a complex mixture is considered (where various oxidation products are present). In our case, in which a mixture of specific dyes and their by-products are the objects of the study, the AOS is the only way to analyse the problem from a practical point of view. The AOS can take values between +4 for CO2 (the most oxidized state of C) and −4 for CH4 (the most reduced state of C). As shown in Figure 5.6 C, all tested AOPs resulted in an increase in the AOS values. Therefore, it can be concluded that the use of AOPs resulted in the occurrence of less complex by-products with higher oxidation states.
5.4. Conclusions Based on the results of this study, it can be concluded that ozonation resulted in fast decolorization followed by further decomposition of by-products. This observation is confirmed by the very high colour reduction after 10 minutes of the treatment (close to 90%) and the slow mineralization process. The COD/COD0 fractions remaining after 60 minutes were between 20 and 30% for the single dyes and 40% for dye mixture. The TOC/TOC 0 fraction remaining after the AOPs was approximately 90% in the worst case. The second general conclusion is that the textile wastewater matrix had no significant influence on the ozone-based decolorization process. Thirdly, there were no synergistic or additive effects among O3, H2O2 and UV in the AOPs. H2O2 was a HO• radicals scavenger when the pH was 12, which is a typical pH of textile dyeing discharge (dyeing with reactive dyestuff). Therefore, the use of ozone or ozone with a very low concentration of H2O2 may be recommended for the textile industry. When greater mineralization is needed, the application of O3/H2O2 is highly recommended (a COD reduction of up to 90% can be achieved when H2O2 is used). According to our previous study [12], the industrial application of O3/UV or O3/UV/H2O2 is not recommended because the investment and operating costs are too high. When the reuse of the dying discharge as a brine is desired and only colour reduction is important, ozonation is the best option among the AOPs. In the treatment of a selected textile wastewater stream, such as dyeing discharge, a
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customized method (that is adequate for the load and the nature of the wastewater) can be economically reasonable for industrial applications. When research focused on industrial implementation is conducted, it is highly recommended to consider the conditions of real textile wastewaters (which include multiple dyes and textiles auxiliaries in the same solution). Otherwise, the results can be confusing.
5.5. Acknowledgements This research was supported by the National Centre of Research and Development (NCBiR) in Poland [grant number PBS2/A9/22/2013]. Special thanks to Textile Company Bilinski, Konstantynow Lodzki, Poland for their cooperation and to Dr Kazimierz Blus for valuable suggestions and discussion.
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G. Ciardelli, N. Ranieri, The treatment and reuse of wastewater in the textile industry by means of ozonation and electroflocculation, Water Res. 35 (2001) 567–572. doi:10.1016/S0043-1354(00)00286-4.
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A. Baban, A. Yediler, D. Lienert, N. Kemerdere, A. Kettrup, Ozonation of high strength segregated effluents from a woollen textile dyeing and finishing plant, Dye. Pigment. 58 (2003) 93–98. doi:10.1016/S0143-7208(03)00047-0.
[20]
N. Azbar, T. Yonar, K. Kestioglu, Comparison of various advanced oxidation processes and chemical treatment methods for COD and color removal from a polyester and acetate fiber dyeing effluent, Chemosphere. 55 (2004) 35–43. doi:10.1016/j.chemosphere.2003.10.046.
[21]
G. Eremektar, H. Selcuk, S. Meric, Investigation of the relation between COD fractions and the toxicity in a textile finishing industry wastewater: Effect of preozonation, Desalination. 211 (2007) 314–320. doi:10.1016/j.desal.2006.02.096.
[22]
C.A. Somensi, E.L. Simionatto, S.L. Bertoli, A. Wisniewski, C.M. Radetski, Use of ozone in a pilot-scale plant for textile wastewater pre-treatment: Physico-chemical efficiency, degradation by-products identification and environmental toxicity of treated wastewater, J. Hazard. Mater. 175 (2010) 235–240. doi:10.1016/j.jhazmat.2009.09.154.
[23]
J. Chung, J.-O. Kim, Application of advanced oxidation processes to remove refractory compounds from dye wastewater, Desalin. Water Treat. 25 (2012) 233–240. doi:10.5004/dwt.2011.1935.
[24]
A. Dulov, N. Dulova, M. Trapido, Combined physicochemical treatment of textile and mixed industrial wastewater, Ozone Sci. Eng. 33 (2011) 285–293. doi:10.1080/01919512.2011.583136.
[25]
I. Arslan, I. Akmehmet Balcioglu, T. Tuhkanen, Advanced oxidation of synthetic dyehouse effluent by O3, H2O2/O3 and H2O2/UV processes, Environ. Technol. 20 (1999) 921–931. doi: 10.1080/09593332008616887.
[26]
I.A. Alaton, I.A. Balcioglu, D.W. Bahnemann, Advanced oxidation of a reactive dyebath effluent: Comparison of O3, H2O2/UV-C and TiO2/UV-A processes, Water Res. 36 (2002) 1143–1154. doi:10.1016/S0043-1354(01)00335-9.
[27]
Ş. Gül, Ö. Özcan, O. Erbatur, Ozonation of C.I. Reactive Red 194 and C.I. Reactive Yellow 145 in aqueous solution in the presence of granular activated carbon, Dye. Pigment. 75 (2007) 426–431. doi:10.1016/j.dyepig.2006.06.018.
[28]
Ş. Gül, Ö. Özcan-Yildirim, Degradation of Reactive Red 194 and Reactive Yellow 145 azo dyes by O3 and H2O2/UV-C processes, Chem. Eng. J. 155 (2009) 684–690. doi:10.1016/j.cej.2009.08.029.
[29]
A. Mangat, I.A. Shaikh, F. Ahmed, S. Munir, M. Baqar, Fenton Oxidation Treatment of Spent Wash-Off Liquor for Reuse in Reactive Dyeing, Tech. Journal, Univ. Eng. Technol. Taxila, Pakistan. 19 (2014) 43 – 47.
[30]
H. Zollinger, Color chemistry : syntheses, properties, and applications of organic dyes and pigments, VCH, 1991.
[31]
M. Constapel, M. Schellenträger, J.M. Marzinkowski, S. Gäb, Degradation of reactive dyes in wastewater from the textile industry by ozone: Analysis of the products by accurate masses, Water Res. 43 (2009) 733–743. doi:10.1016/j.watres.2008.11.006.
[32]
C. Reyes, J. Fernández, J. Freer, M.A. Mondaca, C. Zaror, S. Malato, H.D. Mansilla, Degradation and inactivation of tetracycline by TiO2 photocatalysis, J. Photochem. Photobiol. A Chem. 184 (2006) 141– 146. doi:10.1016/j.jphotochem.2006.04.007.
145
146
Supplementary materials to Chapter 5
Figure A 5.1. Mineralization during ozonation of RY145, RR195 and RB221: A) COD/COD0 and B) TOC/TOC0 (C0dye 125 mg/L, applied ozone dose 1.96 g/L, pH 12)
149
150
Chapter 6
Ozonation as a key stage in the textile wastewater treatment process
Lucyna Bilińska, Julita Wrębiak, Stanisław Ledakowicz Chemical Engineering and Equipment 54 (4), 2015, 146-147 ISSN 0368-0827
6.1. Introduction Biological treatment is one of the most popular and economically justified wastewater purification methods. Numerous literature reports have demonstrated the effective use of this method for the purification of textile wastewater [1–3]. Residual detergents, dyes, salts (mainly NaCl and Na2SO4) and alkalis (NaOH, NaCO3) at varying concentrations are the main problems in the treatment of textile wastewater. Additionally, textile wastewater is also often characterized by unfavorable susceptibility to biodegradability, as determined by BOD5/COD ratio. The use of AOP (Advance Oxidation Processes) and ozonation may be effective ways to support the biological treatment of textile wastewater. Due to the extremely high oxidizing potential of ozone (O3) and hydroxyl radicals (HO• ), many poorly degradable organic substances, i.e. paints, resins, biocides, organic chlorinated, mineral oils, dyes, sulfides, phenols, ethers, amines, among others can likely be decomposed [4]. Many literature reports confirm the successful application of both AOPs and ozonation for the oxidation of dyes and detergents in aqueous solution, as well as for the treatment of industrial textile wastewater [5– 10]. Additionally, Somensi et al. confirmed an increase in the BOD5/COD ratio indicator, after ozonation [11]. The objective of the presented study was the investigation of ozonation with respect to the decolorization and mineralization of a highly loaded wastewater stream from Textile Company Bilinski in Konstantynow Lodzki. The BOD5/COD ratio was determined as the most important factor towards improving the wastewater biodegradability.
6.2. Experiment
6.2.1. Experimental set-up The ozonation of wastewater was conducted in a semi-batch 1 L glass reactor. The experimental set-up is shown in Figure 6.1. The quartz tube with a UVC lamp can be immersed inside the reactor in order to conduct UV-assisted AOPs. To ensure a constant temperature of the reaction medium, the reactor was in an air-conditioned space and immersed in a container with cooling water. Temperature and pH were measured using an external Elmetron CP-411 meter (a thermometer and pH-electrode immersed in the reactor). Mixing was provided by a magnetic stirrer Wigo type ES 21. Ozone was produced from oxygen using an ozone generator made by Ozonek company. The oxygen used for the ozone production was supplied from a gas cylinder. The ozone-oxygen mixture was supplied to the reaction solution by a diffuser, 153
centrally located at the bottom of the reactor. The ozone concentration was measured at both the inlet and outlet of the reactor using a BMT ozone meter, type 963 Vent. Ozonation of wastewater was carried out for 1 hour. The wastewater samples were taken from the reactor at specific time intervals. The ozone concertation in the gas at the reactor inlet G was CO3 42.3 mg/L and the volumetric flow rate was set at Qin equal 40 L/h. The total ozone
dose applied during 1 h of the process was 1.69 g/L.
Figure 6.1. Experimental setup: (1 – pH and temperature meter, 2 – source of oxygen, 3 – oxygen dryer, 4 – thermostat, 5 – sampling cell, 6 – UV lamp placed in a quartz tube, 7 – oxygen-ozone mixture inlet, 8 – reactor outlet, 9 – ozone destructor, 10 – reactor, 11 – magnetic stirrer, 12 – thermostatic tank, 13 – UVC lamp power supply 14 – ozone meter, 15 – ozone generator
6.2.2. Methods Spectrophotometric
analysis.
The
effect
of
color
removal
was
analyzed
spectrophotometrically using a UV-Vis Helios spectrophotometer by Thermo. The measurements were carried out in quartz cuvettes with an optical path of 1 cm. The purpose of this spectrophotometric analysis was to examine the wastewater absorption spectra both before and during the treatment. The spectral absorption coefficient α (DFZ). DFZ measurement is used to determine the color by spectrophotometric measurement in accordance with PN-EN ISO 7887:2002. Measurements were made at standard wavelengths: 436 nm, 525 nm, 620 nm, and at a wavelength of maximum absorbance. Chemical oxygen demand (COD). COD determination was performed using the dichromate standard method (LCK 514 test) with a HACH LANGE DR 3500 spectrophotometer in accordance to the procedure specified by the manufacturer. Five day biochemical oxygen demand (BOD5). BOD5 determination was performed using a dilution method according to the methodology reported by Hermanowicz et al [12]. 154
6.2.3. Wastewater characteristics The study was performed on real industrial wastewater samples from Textile Company Bilinski in Konstantynow Lodzki. The current system for wastewater treatment in this company is based upon separating different wastewater streams and directing them to the most appropriate treatment plants (according to the recommendations of the IPPC Directive). Wastewater generated on the production side is separated into two main streams: •
low-loaded, susceptible to biodegradation, which is directed into a biological
treatment plant combined with chemical treatment. After purification this stream is recycled into production processes; •
high-loaded, colored, salty, characterized by high pH and adverse BOD5/COD
ratio, which is directed into municipal sewer system; this stream, was the main subject of this study. The dominant characteristics of the textile wastewater is the extremely high variability of its composition. Therefore, the wastewater samples have been equalized over 24 hours, in order to get the most representative measurements. The procedure for the wastewater collection was repeated three times at weekly intervals. Table 6.1. shows the characteristics of the wastewater samples. Even with sample equalization, the variability of the composition indicators can still be noted.
Table 6.1. The indicators of the wastewater (raw samples, before treatment) Sample No.
pH
COD, mgO2/L
BOD5, mgO2/L
1.
10.15
1277
373
2.
9.31
960
248
3.
9.46
1250
243
6.3. Results and discussion Figure 6.2 shows the changes in the UV/VIS absorption spectra during the ozonation of high-loaded wastewater. The wastewater sample was characterized by very intense dark brown initial color. Initially, the spectrum was characterized by a very broad peak with high absorbance values across the entire visible range. A gradual decrease in the absorbance values could be observed over successive time steps of the ozonation process. In the initial phase of ozone treatment, a slight increase in absorbance at wavelengths shorter than 550 nm was recorded. This indicate the appearance of new compounds that may be by-products of initial components of the wastewater. After an hour of treatment, decolorization was not fully 155
achieved (absorbance values close to 0.3 in visible region 400 – 500 nm). Table 6.2 shows the quantitive measure of color changes during ozonation, expressed as a DFZ coefficient.
G Figure 6.2. UV/VIS spectra changes during the ozonation process of real wastewater, Q in 40 L/h, CO3 42.3 mg/L
Table 6.2. The color values expressed as the spectral absorption coefficient α (DFZ) and the mean square error MSE with n 3 measurements Time
156
436 nm
525 nm
620 nm
s
α, m
MSE, ±%
α, m
MSE, ±%
α, m
MSE, ±%
0 15 30 45 60 120 180 240 300 600 1200 1800 3600
148.32 149.58 148.74 147.27 145.90 141.91 138.29 134.68 130.84 113.96 87.14 65.55 33.64
0.85 0.92 1.06 0.80 0.75 0.75 0.63 0.59 0.53 0.43 0.30 0.70 0.52
154.72 154.72 153.27 151.30 149.85 144.02 138.93 133.65 128.31 104.87 71.32 48.01 22.02
0.54 0.74 0.69 0.51 0.56 0.44 0.39 0.27 0.28 0.26 0.38 0.68 0.31
134.27 132.37 130.81 128.70 127.12 120.81 115.20 109.25 103.73 79.40 46.77 27.14 12.22
0.46 0.44 0.53 0.32 0.37 0.27 0.29 0.25 0.25 0.27 0.38 0.55 0.42
-1
-1
-1
Figure 6.3. Changes in color, occurring during the ozonation of real high-loaded wastewater
A spectral absorption coefficient (α) value close to zero was not obtained for any of the standard wavelengths, which indicates incomplete wastewater decolorization during the experiment. After a few minutes of the treatment, the characteristic light brown color of the reaction mixture occurred and it remained even after an hour of reaction time as it has been shown in Figure 6.3. Table 6.3 lists COD and BOD5 values of the wastewater samples collected during the ozonation. Whilst the result of color removal was satisfying, the obtained degree of mineralization was low. After 1 hour of ozonation, less than 13% COD and 16% BOD5 reductions were observed. It was also noted that the ozonation slightly impacts the BOD5/COD ratio. It was found that a short ozonation time of c.a. 15 min gives the most efficient results in terms of increased BOD5/COD ratio.
Table 6.3. The values of COD, BOD5, BOD5/COD, and relative standard deviation SD for n 3 measured for ozonation process of the wastewater Time, min
COD average, mgO2/L
SD
BOD5 average, mgO2/L
SD
BOD5/COD average
SD
0
1162.3
175.7
288.0
73.6
0.248
0.049
5
1129.7
155.7
292.4
83.4
0.259
0.060
15
1095.0
156.3
278.7
86.7
0.253
0.058
35
1064.3
157.5
251.0
62.1
0.236
0.041
60
1013.3
146.4
243.0
54.1
0.240
0.041
6.4. Conclusions The experiment confirmed that ozonation can be an effective method for the treatment of textile wastewater. The process of ozonation can be economically justified for the treatment of specific wastewater streams, such as dyeing discharge, because wastewater relatively fast decolorization takes place in the first stage of the process followed by much slower,
157
mineralization. The decolorization results were highly satisfying and no adverse effects on biodegradability were observed. A short ozonation period (up to 15 minutes) improved the BOD5/COD ratio what could increase bioavailability for the activated sludge microorganisms. After 1 hour of ozone treatment, 85% color removal and 13% COD removal was achieved. Through using a combination of the two methods: a short ozonation and biological treatment, satisfactory purification results can be achieved. In contrast to non-integrated wastewater treatment systems, the use of a two-step combined chemical-biological process for the treatment of the high-loaded textile wastewater may be much more promising.
6.5. Acknowledgements The research funded by the National Centre for Research and Development project No. PBS2/A9/22/2013.
6.6. References
158
[1]
E. Kołodziej, Roślinne oczyszczanie ścieków dla przemysłu włókienniczego – uwagi wstępne, Przegląd Włókienniczy + Tech. Włókienniczy. 12 (1997) 34 – 35.
[2]
J. Perkowski, J. Przybiński, S. Ledakowicz, Ekonomiczne aspekty procesów oczyszczania ścieków włókienniczych metodami pogłębionego utleniania, Przegląd Włókienniczy - Włókno, Odzież, Skóra. 4 (1999) 2 – 28.
[3]
M. Mihulka, J. Sójka-Ledakowicz, B. Gajdzicki, W. Machnowski, R. Żyłła, J. Lewartowska, K. Grzywacz, E. Strzelecka-Jastrząb, Charakterystyka technologiczna przemysłu włókienniczego w Unii Europejskiej, (2003).
[4]
R. Zarzycki, Zaawansowane techniki utleniania w ochronie środowiska, Polska Akademia Nauk, Oddział w Łódzi, 2002.
[5]
A. Al-Kdasi, A. Idris, K. Saed, C.T. Guan, Treatment of Textile Wastewater By Advanced Oxidation Processes – a Review, Glob. Nest Int. J. 6 (2004) 222–230.
[6]
G. Ciardelli, N. Ranieri, The treatment and reuse of wastewater in the textile industry by means of ozonation and electroflocculation, Water Res. 35 (2001) 567–572. doi:10.1016/S0043-1354(00)00286-4.
[7]
A. Baban, A. Yediler, D. Lienert, N. Kemerdere, A. Kettrup, Ozonation of high strength segregated effluents from a woollen textile dyeing and finishing plant, Dye. Pigment. 58 (2003) 93–98. doi:10.1016/S0143-7208(03)00047-0.
[8]
N. Azbar, T. Yonar, K. Kestioglu, Comparison of various advanced oxidation processes and chemical treatment methods for COD and color removal from a polyester and acetate fiber dyeing effluent, Chemosphere. 55 (2004) 35–43. doi:10.1016/j.chemosphere.2003.10.046.
[9]
G. Eremektar, H. Selcuk, S. Meric, Investigation of the relation between COD fractions and the toxicity in a textile finishing industry wastewater: Effect of preozonation, Desalination. 211 (2007) 314–320. doi:10.1016/j.desal.2006.02.096.
[10]
M. Constapel, M. Schellenträger, J.M. Marzinkowski, S. Gäb, Degradation of reactive dyes in wastewater from the textile industry by ozone: Analysis of the products by accurate masses, Water Res. 43 (2009) 733–743. doi:10.1016/j.watres.2008.11.006.
[11]
C.A. Somensi, E.L. Simionatto, S.L. Bertoli, A. Wisniewski, C.M. Radetski, Use of ozone in a pilot-scale plant for textile wastewater pre-treatment: Physico-chemical efficiency, degradation by-products identification and environmental toxicity of treated wastewater, J. Hazard. Mater. 175 (2010) 235–240. doi:10.1016/j.jhazmat.2009.09.154.
[12]
W. Hermanowicz, J.R. Dojlido, Fizyczno-chemiczne badanie wody i ścieków, “Akardy,” 1999.
159
160
Chapter 7 Modeling of ozonation of C.I. Reactive Black 5 through a kinetic approach
Lucyna Bilińska, Renata Żyłła, Krzysztof Smółka, Marta Gmurek, Stanisław Ledakowicz Fibers and Textiles in Eastern Europe 25, 5 (125), 2017, 54 – 60 DOI: 10.5604/01.3001.0010.4628
Abstract Reactive Black 5 (RB5) is the most commonly used dye in textile industry. Ozone is a strong oxidant, that can decompose many of hardly degradable pollutants, including dyes. Although, there are many literature reports devoted to the treatment of textile wastewater and dye solutions by ozone, the ozonation mechanism and modeling of the kinetics is still not well covered. In this work a kinetic model of process of RB5 decolorization by ozone has been proposed and validated on the basis of experimental data. The experiments have been carried out in liquid-liquid system, to avoid mass transfer limitation. The model has been established for acid reaction medium. The main RB5 reaction was direct oxidation of the dye with molecular ozone. A self-decomposition of ozone in liquid phase was taken into account and described by empirical equation. The reaction rate constants of RB5 with ozone were estimated from the experimental data from the range (1.88 ± 0.08) × 104 to (2.53 ± 0.10) × 105 M-1s-1 (invariant with initial dye concentration). The empirical equation k ′2 = 1.06 × 108 (COH− )0.31 has been built for the constant, to make it dependent on pH value. A solution of non-linear inverse problem allowed for identification of the kinetic constants on the basis of the obtained experimental data. The model gave good match between the prediction and the experimental data for pH between 1.88 and 4.0.
Keywords: Ozonation; Reactive Black 5; Kinetic model; Second-order kinetic constant with ozone
7.1. Introduction There are many literature reports where dyes were successfully decomposed using ozone and ozone-based Advanced Oxidation Processes (AOPs) in aqueous solutions [1–14], as well as, in textile wastewater [15–22]. However, there are only few publications focused on the determination of kinetic parameters [23–25] or modeling of the kinetic [26–30]. In contrast to research concentrated only on the optimization of the treatment, modeling of the ozonation kinetics can be troublesome. The complexity of the process is the main reason causing difficulty. In most cases ozonation is being proceeded in heterogeneous gas-liquid systems, for example in a bubble column reactor. Then the mass transfer of ozone from gas to liquid phase can be the limiting stage and it can influence on the global reaction rate. This problem can be skipped by using liquid-liquid system during kinetic study, like in Olak-Kucharczyk &
163
Ledakowicz work [31]. In order to avoid this problem presented research has been proceeded in the liquid-liquid system. The next problem that may occur is a high reaction rate. The oxidation of dyes by ozone is a rapid process. Second-order kinetic constants of these reactions can be found within 104 and 106 M-1 s-1 [23,24,28]. Testing of such fast chemical reactions requires specific measurement techniques, like competition kinetics or using stopped-flow method [31]. Moreover, the mass transfer limitation is more possible when very fast reactions occur in gas-liquid ozone system, where the ozone absorption is much slower than the reaction. Finally, ozone characteristics is a very important issue. Ozone is a strong oxidant, that due to high oxidative potential (2.07 V) can decompose many of hardly degradable pollutants [32]. However, one of the main feature of ozone is its instability in gas phase as well as in liquid phase. Many mechanisms of ozone decomposition have been presented since Wiess work in 1934 [33]. The most popular mechanisms are SBH (Staehelin, Buhler and Hoigne) and TFG (Tomiyasu, Fukutomi and Gordon) [33]. Both consider ozone decomposition as a chain of reactions, including initiation, propagation and termination steps. SBH mechanism, where propagation step is the ozone reaction with OH − ion, is the most popular one. However, TFG mechanism, where ozone decomposition can start due to OH − ion, as well as OH2− ion, is recognized as more reliable when pH of the reaction medium is high. The more recent studies of the topic are based mostly on these two mechanisms with only slight modifications [34]. As far as decomposition of ozone is concerned an empirical equation can be a way of description of the issue as well. Then, the decomposition mechanism is not considered and the equation can give information about ozone decay. Based on the experimental data Qiu [35] noted that decomposition of ozone follows the first order kinetic regime and proposed the formula for rate constant which describe ozone decomposition for very wide pH range [35]. Moreover, when ozone decomposes the elution of free radicals takes place, especially in alkaline pH [32]. Therefore, two pathways of pollutants decomposition during ozonation can be mentioned. The first one is direct oxidation by ozone molecule and the second one is a reaction with radical (indirect pathway). Reaction with ozone is a selective one. Ozone can react with molecules structures where high electrons concentration is present, like conjugated double bonds or nucleophilic centers. These kinds of chemical structures are characteristic of dyes molecules. Therefore, dyes can be likely decomposed with ozone. However, free radicals, especially hydroxyl one, are highly non selective and they can easy decompose any organic substance, including dyes. It can be concluded that both, direct (predominant in acidic reaction medium) and indirect (predominant in alkaline reaction medium) mechanisms are possible [36]. 164
Therefore, within the parameters that can normally control a kinetic rate, like temperature or substrate concentration, in case of ozonation pH is the one, which can affect it the most. The objective of this study was to build reliable mathematical model to describe kinetic of RB5 decolorization by ozonation in aqueous solution. The model was tested in acid reaction medium (pH between 1.88 and 6.1). In this pH the main reaction pathways were found as direct ozonolysis. Ozone decomposition was taken under consideration during modeling, as well. It was described with empirical equation based on Qiu work [35]. The second-order reaction rate constants of RB5 with ozone have been estimated from the experimental data. The empirical equation was built for the constant to make it dependent on pH value. The invert problem has been solved to find the parameters of the model.
7.2. Experimental
7.2.1. Materials In kinetic study Reactive Black 5 (RB5) was used. It was obtained from Boruta-Zachem (Poland) as a purified reagent. The characteristics of this dye was presented in Table 7.1. The buffer components were H3PO4, KH2PO4, Na2HPO4 all were A.R grade and purchased from Chempur (Poland).
Table 7.1. Characteristics of RB5 Name
C.I. No.
CAS No.
RB5
Reactive Black 5
Molecular weight (g/mol)
12225-25-1 +
O
O
S
S
N
N
O
O
S +
Na
O
-
azo
Na
OH
NH2
N
O
O S
O
596 +
O
N O
-
Type
991.8
O
Na O-
λmax (nm)
O
S O
O
O S
O
+
O
Na
7.2.2. Analytical methods Kinetic study. In order to avoid mass transfer limitation of ozone the kinetic study was conducted in liquid-liquid homogeneous system. Pre-ozoned buffer solutions were saturated with ozone in bubble column with capacity of 20 L. Ozone was fed into the reactor from the
165
bottom of the reactor with a ceramic diffuser Brandol 60®. Ozone was produced by TOGC8X TROGEN LTD generator, which was equipped with integral compressor and oxygen concentrator. The ozone concentration in the gas phase at inlet and outlet of reactor was measured by ozone analyzer BMT 964 manufactured by BMT MESSTECHNIK GMBH (Germany). Circulation of the liquid phase was forced by a peristaltic pump. The gas effluents from the reactor passed through a scrubber filled with silica gel with indicator in order to remove moisture contained in the gas, and then directed to an ozone destructor. Ozone concentration was measured in liquid phase with Wallance & Tierman z VariSensTM flow cell (Evoqua Water Technologies GmbH, Germany). Ozone solution, immediately after taking it from bubble column, was added into the dye solution placed in spectrophotometer cuvette. The decay of RB5 was measured spectrophotometrically
at
maximum
wavelength
using
UV-VIS
Jasco
V-630
spectrophotometer (JASCO, Japan) with 0.01 s intervals. Ozone decomposition was investigated spectrophotometrically by absorbance measurement (λmax 254 nm) in separate experiment. Each kinetic experiment was repeated from ten to fourteen times. Determination of the stoichiometry. The stoichiometry of RB5 ozonation was investigated in a semibatch glass reactor (heterogeneous gas-liquid system) with a capacity of 1 L. Mixing in the reactor was performed with magnetic stirrer (Wigo type ES 21). Ozone was produced by an Ozonek Ozone Generator (Poland) fed with oxygen from a compressed gas cylinder (O2 purity 99.5%). The O3 concentrations at the inlet and outlet of the reactor were measured with a BMT 963 Vent ozone analyser (this was the basis for absorbed ozone dose calculations). Calculations were performed using the Origin 9.1 version Pro software. Mathematical modeling was conducted with MATLAB software.
7.3. Results and discussion
7.3.1. The stoichiometry of BR5 reaction with ozone The stoichiometric factor for the reaction between the RB5 and ozone was experimentally determined. The pH of the reaction medium was set on 2 with phosphate buffer to avoid secondary reaction of ozone. The initial concentration of RB5 was equal 5.05 × 10−4 M. The reactor has been fed with an ozone-oxygen mixture . The ozone dose which has been transferred into the liquid phase was calculated as the difference between the gas at the inlet and outlet at the specific time. The values of ∆𝑛𝑅𝐵5 and ∆𝑛𝑂3 were monitored during the reaction time and the stoichiometric factor 𝑧1 was calculated as follows: 166
∆𝑛
𝑧1 = ∆𝑛 𝑂3
𝑅𝐵5
(7.1)
In Figure 7.1 the values of z1 have been presented. It can be noticed that the stoichiometric factor 𝑧1 is very high and it is equal 10.6. This result is very close to this obtained by Kusvuran et al. [37].
Figure 7.1. Stoichiometric relationship between ozone and dye RB5
The presented results indicates that as long as the color in the reaction mixture could have been observed c. a. eleven moles of ozone could react with one mole of RB5. The RB5 has a complex molecule and the reaction mechanism may be very complex as well. Because the RB5 samples were contaminated by excipients and many impurities the attempts to apply chromatography coupled with mass spectrometry (UHPLC/MS) to analyze the intermediates in the reaction mixture have been taken, but the results have not been satisfactory and the detailed decomposition mechanism could not be developed. Therefore, only general observation could be done for possible degradation pathway. According to many literature reports, like Colindres et al. [2] and Bamperng et al. [3], decolorization of the dyes supposed to be the a dominant phenomenon in the initial phase of 167
the ozonation process. This observation was reported in authors previous work as well [38]. The easiest explanation of decolorization could be direct attack of ozone on the chromophore of the dye. The RB5 is di-azo dye based on H-acid. The most possible places for ozone attack in chromophore of RB5 are the azo bonds. Next eight molecules of ozone can likely react with C=C bonds in vinylsulfone groups. Therefore, when the decolorization takes place the simultaneous reaction of ozone with the chromophore and vinylsulfone groups of RB5 cannot be excluded. These observations were postulated by Kusvuran et al. [37] as well. Moreover, Lopez-Lopez et al. [25] noted that molecular structure of the dyes was related to ozone consumption and the electrophilic attack of ozone on the olefins groups cannot be excluded during decolorization. It should be noted that all of these observations are true in the initial phase of the process, when the color still can be observed. When the further decomposition of RB5 took place, the stoichiometric factor was even greater (results not shown in the work).
7.3.2. Kinetic of self-decomposition of ozone Ozone molecule is unstable and it decomposes easily with elution of free radicals. An empirical equation can be a way of description of this issue. Based on the experimental data presented in Qiu, 1999 [35], it was noted that decomposition of ozone follows the first order kinetic regime according to the equation (7.2):
𝑟𝐷 = −
𝑑𝐶𝑂3 𝑑𝑡
= 𝑘𝐷 𝐶𝑂3
(7.2)
where 𝑘𝐷 is kinetic constant, which depends on OH − ions concentration according to the equation (7.3): 𝑘𝐷 = 20(𝐶𝑂𝐻− )0.5 + 900 (𝐶𝑂𝐻− ), 𝑠 −1
(7.3)
This empirical formula has been established by Qiu for very wide range of pH values, between 2.0 and 13.5 [35]. The experiment in this study have been carried out within pH equal 1.88 and 6.1. Figure 7.2 shows that the first order rate constants of decomposition of ozone in water, which have been obtained in present work, could be successfully described by the equation (7.3). Therefore, the empirical formula that has been proposed by Qiu was used in this study to build the model of ozonation of BR5.
168
Figure 7.2. First order rete constants of ozone self-decomposition versus pH of the medium (experimental data for 22oC)
7.3.3 Kinetics of RB5 oxidation with ozone During the kinetic study the colour decay of RB5 was observed spectrophotometrically at λmax 596 nm. The RB5 absorbed the irradiation in the wavelength region of ozone detection as well. It was impossible to monitor concentration of ozone during the process. Therefore, to ensure pseudo-first order regime of the reaction with respect to RB5 a high initial concentration of ozone was used (1.04 × 10−4 M). At the same time, respectively lower concentrations of RB5 (between 4.7 × 10−6 and 1.6 × 10−5 M) were applied. Experiment has been carried out in acid medium, where direct ozonolysis takes place. Six pH values between 1.88 and 6.1 were examined. To avoid the mass transfer limitation the kinetic of RB5 decolourisation has been proceeded in homogeneous aqueous system.
169
Figure 7.3. The values of the rate constant of reaction between RB5 and ozone: A) pseudo-first order apparent constants vs initial concentration of RB5, B) second order constants vs pH, C) the constants dependence on pH value by regression method
170
The experimental data was described by exponential decay function (y = y∞ + y0 ∙ exp(−kt)) at the initial reaction phase and the apparent pseudo-first order constants k app , s −1
were determined as the slope of the function like in Turhan et al. [39], Gomes et al. [23], Gomes et al. [24] works. The rate constants of RB5 decay were investigated at each pH and each initial dye concertation. Figure 7.3 A presents that the ozonation of RB5 in the initial reaction phase followed a pseudo-first order regime for various pH values. Moreover, the increase of the constant values with increasing initial BR5 concentration could be observed. Therefore, it can be concluded that ozone was in excess during the initial phase of the reaction [25]. Figure 7.3 B shows the second order constants k 2 (M −1 s−1) vs pH. The values of the second order constants increase with increasing pH values and their values are between 1.88 × 104 and 2.53 × 105 M-1s-1. The values, which have been found in this experiment, are in the agreement
with those in the literature [23,24,27,39]. Based on the plot presented in Figure 7.3 C the empirical equation k ′2 = 1.06 × 108 (COH− )0.31 was built for the constant to make it dependent on pH value (by a regression method). The equation exhibited the best fit for pH values between 1.88 and 4.0.
7.3.4 Modeling of ozonation process As far as ozonation is concerned, the oxidation mechanism of the dyes can be a complex issue. Firstly, as it has been shown in previous section, the RB5 is a complex chemical compound with high molecular weight and numerous double bonds, which can likely react with ozone. Even though, the decolorization had been observed during the experiment a simultaneous decomposition could not be excluded (section 7.3.2). Secondly, the RB5 can react with ozone rapidly and specialist measurement method is needed to monitor the reaction rate (a high-speed spectrophotometer was used during the experiment). Thirdly, ozone is not stable in liquid phase (water) and it decomposes especially in alkaline medium. Therefore, the most general reaction mechanism consists of two contributions: direct oxidation by ozone in acid medium and indirect oxidation by free radicals (HO• mainly) in alkaline medium. The main idea of the work was to develop a simplified mathematical model with a low number of parameters. To describe the complex process of the dye decolorization by a such simple model, only a global approach can be used. Therefore, reaction between ozone and RB5 was the only one considered. In order to eliminate a secondary reaction of ozone and the influence of free radicals on the process, the experiment was carried out in acid reaction medium. Ozone decomposition was taken into account using empirical equation proposed by Qiu [35]. 171
Given all the assumptions, the overall ozonation scheme is given as follows: 𝑘1
𝑂3 + 𝑂𝐻 − → 𝑑𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 𝑘2
𝑧1 𝑅𝐵5 + 𝑧2 𝑂3 → 𝑧3 𝑃 𝑘3
𝑧3 𝑃 + 𝑧4 𝑂3 → 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠
𝑟1 = 𝑘1 𝐶𝑂3
(7.4)
𝑟2 = 𝑘2 𝐶𝑅𝐵5 𝐶𝑂3
(7.5)
𝑟3 = 𝑘3 𝐶𝑃 𝐶𝑂3
(7.6)
7.3.4.1. Mass balance Due to the fact that ozonation took place in liquid-liquid homogeneous system and the volume of the reaction mixture was constant, the mass balance was based only on equations (7.4) – (7.6) and describes the chemical reactions in the system. After taking into account the stoichiometry of the reactions the mass balance can be given by equations (7.7) – (7.9): 𝑑𝐶𝑂3
= −𝑟1 − 𝑧2 𝑟2 − 𝑧4 𝑟3
𝑑𝑡
𝑑𝐶𝑅𝐵5
= −𝑧1 𝑟2
𝑑𝑡 𝑑𝐶𝑃 𝑑𝑡
= 𝑧1 𝑟2 − 𝑧3 𝑟3
(7.7) (7.8) (7.9)
What can be rewritten more precisely as follows: 𝑑𝐶𝑂3 𝑑𝑡
= −𝑘1 𝐶𝑂3 − 𝑧2 𝑘2 𝐶𝑅𝐵5 𝐶𝑂3 − 𝑧3 𝑘3 𝐶𝑃 𝐶𝑂3
𝑑𝐶𝑅𝐵5 𝑑𝑡 𝑑𝐶𝑃 𝑑𝑡
= −𝑧1 𝑘2 𝐶𝑅𝐵5 𝐶𝑂3
= 𝑧1 𝑘2 𝐶𝑅𝐵5 𝐶𝑂3 − 𝑧3 𝑘3 𝐶𝑃 𝐶𝑂3
(10) (11) (12)
7.3.4.2. Estimation of model parameters The system of differential equations (7.10) – (7.12) have been integrated numerically with Rosenbrock method by MATLAB solver. The initial parameters as well as the rate constants obtained for the model have been presented in Table 7.2. The equations of the model were solved for six pH values. The concentrations of OH- anions corresponding to pH and concentrations RB5 between 4.7 × 10−6 and 1.6 × 10−5 M, as well as, concentration of ozone equal 1.04 × 10−4 M were initial values for modeling. The stoichiometry factor was 0.1 for RB5, 1 for P (by-product) and 1 for ozone. The values of the rate constants for the reaction RB5 – ozone were calculated directly from the experimental data
172
k 2 and from correlative equation depended on pH k ′2 . The value of the constant rate between P
and ozone, k 3 , were obtained from model optimization by solving the inverse problem with MATLAB computing script. The optimization method was carried out with use of “fminsearch” MATLAB function. It can be noticed that values of k 2 and k ′2 are different, however their value are corresponding to each other, especially for pH between 1.88 and 4.0. The value of the constant rate between P and ozone, k 3 , was estimated for pH 1.88, where the mechanism of the reaction was typically molecular, and used for pH values up to 4.0.
Table 7.2. The initial parameters and the rate constants obtained for the model Parameter
Values
pH
1.88
2.68
3.46
4.0
5.26
6.1
COH− , M
7.59 × 10-13
4.79 × 10-12
2.88 × 10-11
1.0 × 10-9
1.82 × 10-9
1.26 × 10-9
1.11±0.06×104
2.53±0.10×105
2.06 × 105
3.5 × 105
C0O3 , M
1.04 × 10-4
𝑧1
0.1
𝑧2 , 𝑧3 , 𝑧4
1
𝑘1 , s-1
20(COH− )0.5 + 900 (COH− )
𝑘2 , M-1·s-1
k ′2 , M-1·s-1
1.88±0.08×104
4.75±0.64×104
9.27±0.34×104
k ′2 = 1.06 × 108 (COH− )0.31 1.85 × 104
3.73 × 104
k 3 , M ·s -1
5.19±0.29×104
-1
9.4 × 10
5.73 × 104 3
8.41 × 104
--
7.3.4.3. Evaluation of the model In Figure 7.4 kinetics of the BR5 decolorization by ozone for pH equal: A) 1.88, B) 3.46 and C) 4.0 have been shown. The matching between experimental data point and the model are satisfactory. The RB5 degree of conversion can be quite successfully predicted by a model in all cases. This information can be very useful from practical point of view. At the same time, it can be noticed that with increasing pH the agreement between experimental data point and the model getting worse in the initial phase of the process. It can be explained by appearance of free radicals. When pH is higher than 4.0 the indirect oxidation mechanism is becoming more relevant and this mechanism is not included into the model. Based on this results it can be concluded that highly complex process, like RB5 decolorization, can be successfully described using simplified model with a low number of parameters. It can be a practical and useful approach to predict color decay of the dye in acid pH as long as a direct oxidation with ozone is the main mechanism of the process.
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Figure 7.4. Kinetics of the decolourisation of BR5, measured and calculated from the model: A) pH 1.88, B) pH 3.46, C) pH 4.0 (initial concentration of RB5 1.65 × 10−5 M, initial concentration of ozone 1.04 × 10−4 M)
174
Figure 7.5 shows how the model describes experimental data when initial RB5 concentration changes The initial concentrations of RB5 between 7.7 × 10−6 and 1.6 × 10−5 M were tested. The pH was equal 1.88 for all samples. In all cases the same parameters were used (except initial RB5 concentration). The model successfully described experimental data for various initial RB5 concentrations. Therefore, it can be concluded that obtained values of the rate constants k 2 and k 3 (Table 7.2) were invariant from initial dye concentration.
Figure 7.5. Kinetics of the decolourisation of BR5, measured and calculated from the model for initial concentrations of RB 5 7.7 × 10−6 , 1.1 × 10−5 , 1.3 × 10−5 and 1.6 × 10−5 M (pH 1.88, initial concentration of ozone 1.04 × 10−4 M)
The RB5 concentration was the only variable that could be measured within the process, but the model can give the prediction of ozone and by-product P concentrations in time. Figure 7.6 A) presents RB5 decay. The model gave good match between prediction and experimental data. Figure 7.6 B) shows ozone decay in time. It can be noticed that it was in excess only initial phase of the process. Although, the ozone was used in more than ten times higher concentration than RB5 the high stoichiometry factor and very fast simultaneous reaction with by-product P resulted in its fast consumption. Figure 7.6 C) presents how the model predicted the concentration of the by-product P. This concentration increased rapidly from 0 up to the maximum value and then consequently decreased because of a consecutive reaction of P with
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ozone. It can be concluded that the by-product P appeared when RB5 has been decomposed and it has been oxidised very fast to the next by-products. Both, RB5 and P cannot be decompose entirely due to use of ozone.
Figure 7.6. Kinetics of the decolourisation of BR5, results calculated from the model: A) decay of RB5, B) concentration of ozone, C) concentration of by-product P (initial concentration of RB5 1.6 × 10−5 M, initial concentration of ozone 1.04 × 10−4 M, pH 1.88)
7.4. Conclusions Based on the results of the work it can be concluded that the main objective has been achieved and highly complex process of RB5 ozonation was successfully described by the simplified model with low number of parameters. The model gave good match between prediction and experimental data for pH values from 1.88 to 4.0, where the direct oxidation of RB5 by ozone molecule was the main pathway of dye decomposition. At the same time it was shown in which pH value the ozonation pathway starts to change from direct (oxidation by ozone only) to mixed (simultaneous oxidation by ozone and free radicals). The model predicted the degree of RB5 conversion within the process, what can be useful from the practical point of view. The reaction rate constants of RB5 with ozone have been found in this work. Their values estimated from the experimental data, were between (1.88 ± 0.08) × 104 and (2.53 ± 0.10) × 105 M-1s-1 (for pH 1.88 – 6.1). Moreover, the empirical equation k ′2 = 1.06 × 108 (COH− )0.31 was built for the constant, to make it dependent on pH value. The results
of the work indicated that the rate constants were invariant with initial dye concentration. Although, the results of modeling were satisfying the model can be used only in acid pH up to 4.0. Therefore, the next research should be developed to include the influence of free 176
radicals on the process. Then the determination of rate constant of reaction between RB5 and hydroxyl radical as well as its stoichiometry would be the main challenge.
7.5. Acknowledgements Special thanks to Textile Company Bilinski, Konstantynow Lodzki, Poland for their cooperation.
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Chapter 8 Summary and Conclusion
The main objective of this thesis was to find the most suitable and effective treatment for highly polluted salty textile wastewater, in order to produce a recyclable brine that could be used for the next dyeing operation. In pursuit of this goal, firstly, the textile wastewater matrix was characterized. Secondly, an examination of the application of the AOPs, in terms of industrial conditions, was carried out. Finally, the most appropriate AOP was identified and validated by recycling trials. A discussion of each stage of the research is given in the following sections.
8.1. Summary The results presented in Chapter 1 indicate a large variety of textile wastewaters, with samples taken from several stages in the technological operation of dyeing. The technological operations in textile the industry, such as dyeing, each follow a sequence of steps. One process for coloring a fabric comprises washing, bleaching, rinsing, dyeing, rinsing, washing and rinsing again. In most popular batch dying processes, each operation requires new water; therefore, different kinds of wastewaters are produced within one textile dyeing operation. The wastewaters, which are the subjects of the research in Chapter 1, are the baths from the reactive dyeing of cotton. The analysis of a wide range of parameters including COD, BOD, TOC, TS, Ptotal, Ntotal, Cl-, conductivity, pH and color allow for detailed assessment of the wastewaters. Extreme variations in the wastewater baths parameters were detected. It was found that the most contaminated bath was the dying bath which had the most intense color, high pH value and high salinity. The highest COD and the lowest BOD/COD ratio indicated that the dying bath wastewater had a very low biodegradability. Moreover, the adverse ratio of BOD5/N/P was an additional indicator of the poor biodegradability of this wastewater. The toxicity assay showed that this kind of wastewater could be harmful to microorganisms (e.g. in activated sludge). The others baths were found to be more susceptible to biological treatment. Based on these results, the separation of the wastewaters into streams according to their biodegradability has been proposed. Directing the wastewater streams to the most suitable treatment plant could result in more effective removal of pollutants. It was concluded that combining the less polluted wastewaters and directing them into a biological treatment plant, whilst at the same time excluding the more polluted streams, could be reasonable from an economical point of view; therefore, a scheme of wastewater stream management was proposed. The most contaminated stream, from the dyeing bath, was selected to undergo
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chemical treatment by AOPs. Due to the high content of residual salt in this wastewater, it was intended to be a source of brine after purification.
Extremely high residual color and salt content, an alkaline pH and the presence of a surfactant are the characteristics of the wastewater that were determined in Chapter 1. Due to the high oxidative potential of hydroxyl radicals, the AOPs are thought to be the most suitable treatment for such a demanding matrix. Fenton oxidation, H2O2/UV treatment and ozonation are representative of the main groups of AOPs. In Chapter 2, a comparison of these AOPs was carried out in terms of the characteristic wastewater. Extremely high differences in efficiency between the AOPs were noted. The intense color of the wastewater was a great impediment to the H2O2/UV process. The majority of the UV radiation was absorbed by dye molecules. As a consequence, the radiation could not penetrate deep inside the reaction mixture and the process was extremely ineffective for the solutions with dye concentrations higher than 160 mg/L. It should be noted that the reactive dyes are characterized by low fixation, which means that typical concentrations of the residual dye in industrial wastewaters after dying operations is between 200 and 1000 mg/L (depending of intensity of color). Moreover, the highest efficiencies of the H2O2/UV process were found when the pH was close to neutral. At higher pH, the scavenging of hydroxyl radicals, caused by the hydrogen peroxide in dissociated form, was detected. This presents a great disadvantage because the pH of the wastewater after the reactive dying operation is always extremely high (between 10.5 and 12) due to the necessary alkali dosage in this process. Another disadvantage of the H2O2/UV process was its sensitivity to NaCl and surfactant addition to the reaction mixture. The efficiency of the process decreases dramatically with increasing concentrations of these additives, which are required in textile dyeing processes with reactive dyestuff. The influence of NaCl on the process is explained by the scavenging effect caused by the formation of chloride ion-radicals, Cl•-, which have a reactivity much lower than hydroxyl radicals. Moreover, aggregation of dye molecules, which is highly probable when NaCl content is large, can also contribute to the low efficiencies observed. The adverse effect of surfactants is explained by the formation of a Critical Micelle Concentration (CMC). The molecules of surfactant surround the dye molecules impeding the access of the oxidant. All these observations are confirmed in Chapters 2 and 4 where real
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industrial wastewaters have been treated by the H2O2/UV process. The combined effect of these phenomena could be observed, resulting in a decrease of the process efficiency. When the Fenton oxidation was considered, an even greater decrease in efficiency, caused by the textile wastewater characteristics, was observed. A decrease in efficiency with increasing initial dye concentration has been noted. When dye concentrations are higher, aggregation is more likely to occur, promoting the precipitation of the ferrous ions. Then, instead of the Fenton reagent, ferrous ions are used in competitive process and the oxidation becomes less effective. This observation was confirmed by a specific experimental point arrangement; two phases of the process could be observed. The first, very fast stage, might be caused by the combined effects of flocculation with iron compounds and oxidation by the hydroxyl radical. The second, much slower stage, could occur when the concentration of Fe2+ ions is insufficient to produce a significant number of hydroxyl radicals. Another disadvantage of the Fenton process was the value of the optimal pH of 3. In alkaline pH, Fenton treatment does not work. Therefore, a significant alteration in pH must be performed in order to treat the industrial wastewater by Fenton oxidation. Moreover, a negative influence of NaCl and surfactants was observed, which was even greater than for the aforementioned H2O2/UV process. The causes of these adverse effects were found to be the same as in the case of the H2O2/UV process. These phenomena are discussed in more detail in Chapter 3. The scavenging effect of Cl- ions was proven by detection of Cl2•− via pulse radiolysis. The • rate constants obtained via this method for the dyes’ decomposition with Cl•− 2 and HO proved
the inhibiting effect of NaCl. At the same time it was proven that the reactions resulting in the • formation of Cl•− 2 and HO radicals are competitive and have very similar rates. The
precipitation of ferrous ions, as well as the occurrence of CMC, has been confirmed in the case of dye solutions and industrial wastewaters. Although, there are some works focused on the scavenging effect of Cl- during the Fenton [1–8] and H2O2/UV [6,7,9] treatment processes, the influence of the surfactant has been discussed for the first time in this work. Based on the presented study, the argument that the H2O2/UV process and Fenton oxidation process are both inefficient when treating textile wastewaters has been confirmed; therefore, these treatments were not considered for further investigation. On the other hand, the results shown in Chapter 2 for ozonation indicated a lack of adverse effects caused by textile wastewater characteristics. Ozonation in a bubble column reactor worked efficiently for high dye concentrations (up to 1000 mg/L). The efficiency of the process 185
was high in an acidic reaction medium and even greater in an alkaline pH because of the HO• radicals formed by ozone decomposition. The inhibitory effects of NaCl and surfactant on dye decolorization did not occurred, as further discussed in Chapters 4 and 5.
Once ozonation was found to be the most efficient of the AOPs for textile wastewater treatment, a further study on its development was been carried out. The additions of H2O2 and UV irradiation are well-known methods to improve ozonation (ozone-based AOPs), as reported by many authors [10,11]; therefore, the efficiencies of ozone-based AOPs were investigated. Moreover, the influence of the wastewater matrix on ozonation and ozone-based AOPs was examined. In Chapter 4, the lack of inhibitory effects of NaCl and surfactant on Reactive Black 5 dye decolorization, by ozonation and ozone-based AOPs, was confirmed for dye solutions and the industrial wastewater. The results obtained for Synozol Yellow KHL (Reactive Yellow 145), Synozol Red K3BS (Reactive Red 195), and Synozol Blue KBR (Reactive Blue 221) in Chapter 5 allowed for the detailed explanation of the issue by showing the quantity of ozone absorbed during the process. It was shown that in the initial phase of the ozonation, the absorbed ozone was almost equal for the dye solution and the wastewater. Thus it was proven that the decolorization of dyes is the first stage of their decomposition; the molecules of the oxidant react with the dye chromophore before further oxidation takes place. This is the reason for the lack of an inhibitory effect of NaCl and surfactant on the decolorization process by ozone. Although this phenomenon has been reported before, none of previous works has explained it so clearly [9,12]. Moreover, some other authors detected effects caused by textile auxiliaries (salts, alkaline), these results were inconclusive and they could have been caused by the experimental conditions employed [9,12–16]. The ozone-based AOPs were highly efficient in color removal although the expected synergistic effect of the simultaneous use of O3, H2O2 and UV irradiation appears to have not occurred. The results obtained for the ozone-based AOPs were similar to those obtained by ozonation. The explanation for the lack of a synergistic effect is the scavenging of hydroxyl radicals by H2O2. This phenomenon is directly affected by the pH of the reaction medium. In alkaline pH the hydrogen peroxide occurs in dissociated form (pKaH2O2 11.6) and HO− 2 ions promote the scavenging of hydroxyl radicals.
186
Although some authors have detected a synergistic effect, their experiments were performed under lower pH values (up to pH 9), in which the scavenging effect of H2O2 is insignificant [10,11]. These observations were confirmed for other dyes as well. In Chapter 5, the ozone-based AOPs were tested using Synozol Yellow KHL (Reactive Yellow 145), Synozol Red K3BS (Reactive Red 195), and Synozol Blue KBR (Reactive Blue 221), which were treated both separately and in a mixture. The novelty of the study was indicating the lack of a synergistic effect among the AOPs under alkaline pH, and in showing the influence of the interaction between the dyes when treated as a mixture. It was found that decolorization was dependent on dye structure. When metal atoms formed part of the dye chromophore then it could act as a catalyst to oxidation. The dyes without metal atoms in their chromophores could be decomposed more efficiently when they were in a mixture with metal-containing dyes than when they were treated as single components. The mineralization of the dye mixture was also surprising. It was found that the degree of decomposition, measured as COD, TOC and AOS, was different for the mixture of the dyes and for single components. This is explained by the interactions that could have taken place between the by-products of degradation; however, further study is needed to give a more detailed explanation of this phenomenon. Although there are some works focused on the treatment of the mixture of a few dyes [11,17,18], the interactions between them during AOPs treatments has been discussed for the first time in this work. The mineralization and reduction in toxicity were the advantages of the ozonation and ozone-based AOPs. During the very short initial phase of the ozonation, more toxic by-products occurred; however, generally all treatments resulted in more oxidized and less toxic products.
8.2 Brine recycling from ozone treated industrial textile wastewater To confirm the possibility of ozone treated wastewater reuse the additional study has been carried out. The experiment led to the production of ready to use brine from the wastewater, which has been successfully recycled for use in subsequent textile dyeing processes. Industrial textile wastewater from reactive viscose dyeing with Setazol Black DPT (COD 2677 mgO2/L, pH 11.23, conductivity 78.41 mS/cm and dye concentration ca. 0.5 g/L) underwent ozone treatment. The ozonation was carried out initially at laboratory scale, in a 1 L glass semibatch reactor and then at pilot scale using a 20 L bubble column reactor. Following ozone treatment, cotton dyeing with five types of reactive dyes in various shades was carried 187
out in a LABOMAT BFA-12 system (laboratory dyeing machine made by Mathis AG) using the purified wastewater without diluting it. In both the laboratory and pilot scale processes, applying an ozone dose of around 0.7 g/L resulted in wastewater color removal of ca. 100%. It was confirmed that ozonation works well in an alkaline medium, which is typical to textile wastewater. Moreover, the ozonation process is not influenced by the wastewater complex matrix. Based upon these observations and the results presented in previous chapter of the thesis, it can be concluded that the dye decolorization during ozonation process is not significantly affected by the presence of textile auxiliaries in the wastewater. Therefore among available AOP technologies, ozonation is the most recommended process for the treatment of textile wastewater. The wastewater recycling experiments, performed by dyeing processes using the purified brine, gave very promising results. Very good DE values (color matching parameters measured spectrophotometrically in accordance to ISO 105-J03) were observed between 0.15 and 1.2, with a limiting value of DE 1.5 (Table 8.1). The color fastnesses against washing, sweat and rubbing were satisfactory in accordance with ISO 105-C06, ISO 105-E04, and ISO 105-X12, respectively (Table 8.2). Fabric discharges were free from carcinogenic amines and heavy metals (as determined by GCMS). Therefore, it can be concluded that textiles produced using the recycled brine should be safe for future consumers health. The findings of this experiment indicate that ozonation could be applied in the textile industry as a highly effective method for salty wastewater recycling. However, further research on textiles toxicity is highly recommended.
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Table 8.1. DE values in accordance to ISO 105-J03
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Table 8.2. The color fastnesses against washing, sweat and rubbing in accordance with ISO 105-C06, ISO 105E04, and ISO 105-X12, respectively (5 – the best, 1 – the worst)
Standard
Recycling
Standard
Recycling
Rubbing dry
Recycling
Sweat acidic
Standard
5
Sweat alkaline
Recycling
Washing 400C
Standard
Type of sample
Deph of shade (% w/w)
Fastness against
5
5
5
5
5
5
5
Synozol Yellow KHL C.I. Reactive Yellow 145
1 2
5
5
5
5
5
5
5
5
Synozol Red K3-BS C.I. Reactive Red 195
1
4/5
4/5
4/5
4/5
4/5
4/5
4/5
4/5
2
4/5
4
4/5
4/5
4
4
4
4
Synozol Blue KBR C.I. Reactive Blue 221
1
4/5
4/5
5
5
5
4/5
4/5
4/5
2
4/5
4
5
5
5
4/5
4/5
4
Setazol Black DPT C.I. Reactive Black 5
6
5
5
4/5
4/5
4/5
4/5
4/5
4/5
8
4/5
4/5
4/5
4/5
4/5
4/5
3
4
8.3 General remarks and overview Based on this study, some general conclusions can be drawn: •
Textile wastewater has a very complex matrix and consists of some components that can significantly hinder the treatment,
•
H2O2/UV treatment and Fenton oxidation were both dramatically hindered by certain characteristics of the textile wastewater matrix: high dye concentration, high pH value and the presence of NaCl and surfactant, which are always present in these wastewaters, caused these treatments to be inefficient;
•
H2O2/UV treatment and Fenton oxidation were found to be unsuitable for industrial implementation,
•
Ozonation and ozone-based AOPs were very efficient treatments for color removal, mineralization and toxicity removal of dye solutions and wastewater,
•
The process of decolorization by ozone was not influenced by the textile wastewater matrix,
•
There was a lack of synergistic effect during the simultaneous use of O3, H2O2 and UV irradiation among the ozone-based AOPs, caused by scavenging of hydroxyl radicals by hydrogen peroxide in the extremely high pH of textile wastewater,
190
•
When the treatment of the mixture of multiple dyes was considered, the interactions between them influenced the process.
In conclusion it has been shown that ozonation or ozonation with a very low concentration of H2O2 (0.005 M) were the most suitable methods for the purification of highly contaminated textile wastewater. Moreover, the cost analysis was proven to be advantageous for the ozonation and O3/H2O2 processes. At the same time, the effect of wastewater recycling was positive. The trials of fabric dyeing utilizing the wastewater purified by ozonation gave very promising results (included in section 8.2). The presented mathematical model of the dye ozonation predicted results which showed good correlation with experimental data. The model allowed for quite successful prediction of the degree of conversion, which can be useful in practice. The model was established for reactions in an acidic medium. Development of the model to make it suitable for wider range of pH values presents a challenge for future work.
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A. Riga, K. Soutsas, K. Ntampegliotis, V. Karayannis, G. Papapolymerou, Effect of system parameters and of inorganic salts on the decolorization and degradation of Procion H-exl dyes. Comparison of H2O2/UV, Fenton, UV/Fenton, TiO2/UV and TiO2/UV/H2O2 processes, Desalination. 211 (2007) 72–86. doi:10.1016/j.desal.2006.04.082.
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T.M. Elmorsi, Y.M. Riyad, Z.H. Mohamed, H.M.H. Abd El Bary, Decolorization of Mordant red 73 azo dye in water using H2O2/UV and photo-Fenton treatment, J. Hazard. Mater. 174 (2010) 352–358. doi:10.1016/j.jhazmat.2009.09.057.
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S. Bamperng, T. Suwannachart, S. Atchariyawut, R. Jiraratananon, Ozonation of dye wastewater by membrane contactor using PVDF and PTFE membranes, Sep. Purif. Technol. 72 (2010) 186–193. doi:10.1016/j.seppur.2010.02.006.
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M. Muthukumar, N. Selvakumar, Studies on the effect of inorganic salts on decolouration of acid dye effluents by ozonation, Dye. Pigment. 62 (2004) 221–228. doi:10.1016/j.dyepig.2003.11.002.
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M. Muthukumar, D. Sargunamani, N. Selvakumar, Statistical analysis of the effect of aromatic, azo and sulphonic acid groups on decolouration of acid dye effluents using advanced oxidation processes, Dye. Pigment. 65 (2005) 151–158. doi:10.1016/j.dyepig.2004.07.012.
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A. Pérez, T. Poznyak, I. Chairez, Effect of additives on ozone-based decomposition of Reactive Black 5 and Direct Red 28 dyes, Water Environ. Res. 85 (2013) 291–300. doi:10.2175/106143013X13596524515988.
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E. Oguz, B. Keskinler, C. Çelik, Z. Çelik, Determination of the optimum conditions in the removal of Bomaplex Red CR-L dye from the textile wastewater using O3, H2O2, HCO3- and PAC, J. Hazard. Mater. 131 (2006) 66–72. doi:10.1016/j.jhazmat.2005.09.015.
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