removing toxic compounds from wastewater

Environmental Engineering and Management Journal

September 2014, Vol.13, No. 9, 2153-2158

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

REMOVING TOXIC COMPOUNDS FROM WASTEWATER Cristina Orbeci, Gheorghe Nechifor, Rodica Stănescu University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu Street, 011061, Bucharest, Romania

Abstract The paper presents a hybrid method for the advanced removal of low biodegradable organic compounds from water, by combining the separation selectivity of the membrane with the oxidation efficiency of the photocatalytic process. The aim of this study is to evaluate the performance of the hybrid method to remove toxic compounds from wastewater. The advanced oxidation process, based on a photocatalytic reactor with continuous recirculation, ultraviolet (UV) radiations and a membrane, was used for the removal of 2,4–dichlorphenol (2,4-DCP). The membrane was prepared by immobilization on the carrier material such as fiberglass through the sol-gel specific method, with layer-by-layer deposition. Key words: 2,4–dichlorophenol, membrane, toxic compounds, wastewater Received: March, 2014; Revised final: August, 2014; Accepted: September, 2014

1. Introduction The last decades have shown a revaluation of the issue of environmental pollution, under all aspects, both at regional and at international level. A large number of toxic compounds were detected in industrial and municipal wastewater (Arsene et al., 2013; Pavelescu et al., 2014). The presence of phenol and phenolic derivatives in water induces toxicity, persistence and bioaccumulation in plant and animal organisms and represents a risk factor for the environment and for human health (Ahmed et al., 2010). Chlorophenols are the largest group of phenols used by industry. They are used or produced mainly by chemical, textile, pharmaceutical and metallurgic industries (Oberg, 2004). Chlorophenols could be naturally produced in soil or natural water by the chlorination of mono and polyaromatic compounds existing in the environment (Michałowicz and Duda, 2007). The presence of chlorophenols in the environment is also related to the use and degradation of organic compounds like growth regulators, pesticides and, in particular,

phenoxyherbicides and phenolic biocides (Arsene et al., 2013; Michałowicz and Duda, 2007). The largest part of the population is exposed to very low concentrations of chlorophenols (of the order of magnitude of ppt) through drinking water that has been subject to disinfection with chlorine. But the most exposed persons are those who work directly with these compounds either in their manufacture or their use as pesticides. For example, in timber manufacture a mixture of chlorophenols is used for the preservation of wood; in this case, the major way in which humans are exposed to chlorophenols is through the skin, when the chlorophenols from the treatment of wood come into contact with the skin. Chlorophenols can enter the human body by ingestion along with the food, through dermal contact by skin absorption, or by air. They accumulate mainly in the liver and kidneys, and less in the brain, muscles and fat (White, 1992). Clinical investigations have shown the chlorophenol effects on health in the case of the occupational exposure. According to literature data, the mixture of chlorophenols or sodium salts from these compounds is probably carcinogenic for

 Author to whom all correspondence should be addressed: e-mail: [email protected]; Phone: +40721259875

Orbeci et al./Environmental Engineering and Management Journal 13 (2014), 9, 2153-2158

animals (Stackelberg, 2013). An admissible human daily dose of chlorophenol without inducing carcinogenic changes is 5μg/kg of body weight for 2chlorophenol, and 3μg/kg of body weight for 2,4dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol (EPA, 1982). The health effects of chlorophenols are in direct proportion with their degree of chlorination. An acute exposure to low-phenol chlorination results in muscle spasms, twitching, tremors, weakness, ataxia, convulsions and collapse (White, 1992). In consequence, the identification and monitoring of these compounds detectable in drinking water and surface waters are imperative (Gogate and Pandit, 2004). 2,4-dichlorophenol is used as an intermediate in the manufacture of herbicides, preservatives, antiseptics, disinfectants and other organic compounds such as higher chlorophenols, anthelmintic drugs and polyester films. It has a high toxicity on many different living organisms, but this feature is very important for the purpose of its use as germicide, fungicide and preservative for wood. Since it has the constant acidity value of 7.8, 2,4dichlorophenol can exist both in undissociated and deionized forms in water and in soil, depending on the pH conditions (White, 1992). Many of the organic substances, including the synthetic ones, are degraded in natural processes, such as photolysis (decomposition by means of light energy), hydrolysis (reaction with water) or biodegradation (decay by microorganisms). The degradation rate is measured in terms of the half-life (the time required to reduce the substance concentration by half). Persistence in water or sediments can be measured through specific methods. Advanced oxidation processes (AOPs), in a broad sense, refer to a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials from water by oxidation through reactions with hydroxyl radicals (·OH). Nevertheless, in the wastewater treatment field, this term usually refers more specifically to a sequence of chemical processes that involves ozone (O3), hydrogen peroxide (H2O2) and/or UV light. Generally, AOPs could be divided into three stages: i) formation of OH; ii) initial attacks by ·OH of organic molecules and their breakdown into smaller molecules; iii) subsequent attacks of the small molecules by ·OH until complete mineralization. AOPs have a number of advantages in the wastewater treatment:  high degradation rates of organic compounds from aqueous phase, without transferring pollutants into another phase;  high reactivity of ·OH radicals that react with almost all pollutants from water;  during oxidation processes, heavy metals could precipitate as hydroxides and be removed in a subsequent stage;  ·OH radicals facilitate the disinfection during thewastewater/water treatment simultaneously with organic degradation; 2154

 theoretically, no new organic compounds with higher toxicity are produced. AOPs are used mainly for the treatment of waste waters that contain recalcitrant organics (e.g. pesticides, surfactants, coloring matters, pharmaceuticals). At present, there is a high interest in nanostructured TiO2 materials with a good specific surface area, due to their potential applications in catalytic and photocatalytic processes (Gaya and Abdullah, 2008). Photocatalysis, also called the "green" technology, represents one of the main challenges in the field of treatment and decontamination systems, especially for water and air. Its operating principle is based on the simultaneous action of the light and a catalyst (semiconductor), which allows the pollutant molecules to be degraded without damaging the surrounding environment (Hoffmann et al., 1995). At present, separation membranes have many uses with a growing potential for industrial applications in biotechnology, nanotechnology and membrane based separation and purification processes. The practical applications of the membranes are related to the separation of organic components, industrial wastewater treatment, removal of particulate matter from air and reactive suspension filtration. Available technologies include the advanced oxidation processes (AOPs), based on the formation of hydroxyl radicals with high oxidation potential. The separation and/or removal technologies based on membrane and photocatalytic processes have a great application potential in the field of advanced wastewater treatment (Gaya and Abdullah, 2008). The photodegradation of 2,4dichlorophenol found in water bodies can occur by direct photolysis or by the reaction with oxidizing agents formed by the action of sunlight (atomic oxygen or peroxide radicals). In general, people can be exposed to 2,4-dichlorophenol through the consumption of contaminated water or inhalation of contaminated air. The existence of 2,4dichlorophenol in the soil is affected by many factors, such as solubility in water, soil pH, organic matter content, soil texture, biological degradation, and photodegradation evaporation (Ahmed et al., 2010; EPA, 1992). The persistence of 2,4dichlorophenol in water is low or, in the case of adapted micro flora, able to biodegrade, but this compound is reported to be moderately persistent or very persistent, depending on the environment conditions (Ahmed et al., 2010; Chang et al., 2009; Dilaver and Kargi, 2009; Kargi et al., 2005). Many literature studies have shown that the identification and monitoring of phenolic compounds in water are imperative. This topic gives an overview of how to obtain advanced materials for the removal of toxic compounds from wastewaters. 2. Experimental The materials used in this study are: fiberglass-support for TiO2 deposition (AR-fiber

Removing toxic compounds from wastewater

type, chopped strand) made by Vesta Intracon BV, the Netherlands; ethanol analytical reagent from Merck; sodium hydroxide, titanium tetrabutoxide (Ti(OBu)4) and dimedone (5,5-dimethyl-1,3cyclohexanedione) analytical grade from Fluka. The method for obtaining of the membrane consists in two stages: 1. functionalized membrane; 2. finally membrane preparation. The evaluation of the membrane consists in a preliminary examination and tests to demonstrate its feasibility in removing toxic compounds from wastewater. The functionalized membrane was produced in accordance with the following steps: i) activation of the fiberglass support (cylindrical shape) with sodium hydroxide in ethanol ii) generation of titanium dioxide using titanium tetrabutoxide (Ti(OBu)4) and ammonia 0.1M in alcoholic medium, in the presence of 5,5-dimethyl-1,3cyclohexanedione (dimedone) as titanium alkoxides chelating agent, which plays also the role of the moderator in the sol-gel process; iii) thermal treatment for 1 hour at 105°C. The TiO2-functionalized membrane produced through the above technique was coated with a layer under similar conditions except the thermal treatment step that was carried out for 1 hours at 250°C. Practically, the photocatalytic membrane was produced through the layer-by-layer technique. The membrane was characterized by X-Ray diffraction (XRD) using a PANalytical X’PertPRO MPD Diffractometer with Cu tube, Scanning electron microscopy (SEM) using Inspect S PANalytical model coupled with the energy dispersive X-ray analysis detector, (EDAX). For this purpose, the samples were prepared through grinding, dispersing by sonication in alcohol and passing through a copper sieve covered with an amorphous carbon film. Before being tested, the membrane was washed in distilled water and activated for 2 hours in the photocatalytic reactor with continuous recirculation, in the presence of the hydrogen peroxide and UV radiation. After activation, both cylindrical samples were washed in distilled water. The test substance was 2,4-dichlorophenol (2,4DCP), analytical grade from Aldrich. The solution stock of 30% (w/w) hydrogen peroxide used was an analytical reagent from Fluka. The experiments were conducted in the experimental setup (Fig. 1) designed for testing the photocatalytic behavior of the prepared membrane during the UV irradiation for 2,4-DCP containing synthetic wastewater. In this diagram the most important component is the photocatalytic reactor with continuous recirculation, equipped with UV lamp, in a quartz tube and the membrane. In this case, a high pressure mercury lamp for generating UV radiations(power of 120 W) was used, placed in quartz tube in the middle of the reactor (Fig. 2). The cooling aims to maintain the solution temperature around 30°C. Due to exotermic processes the solution temperature was between 30°C and 33°C. A synthetic solution of 2,4-dichlorophenol

(2,4-DCP) with an initial concentrations of 300 mg O2/L COD was used. The photocatalytic reactor volume was 1.5 L, the total solution volume was 2.0 L and the recirculation flow rate 1 L/min.

Fig. 1. Diagram of photocatalytic oxidation setup (1-reactor; 2-cooling jacket; 3-cylindrical membrane; 4-UV lamp source; 5-recirculation pump; 6- reservoir; 7-quartz tube; 8-UV lamp; 9-thermometer)

Fig. 2. Cross section of the reactor (1-cooling jacket; 2wall; 3-membrane; 7-quartz tube; 8-UV lamp)

All the photocatalytic experiments were performed at an initial pH 3±0.1, H2O2 above stoichiometric requirements for the total oxidation of 2,4-DCP/H2O2 (50% excess) for 2 hours. The initial pH was measured using a Jenway 370 pH-meter. The samples collected at different reaction times were stabilized by MnO2 addition for a quick decomposition of unreacted H2O2. The samples were then filtered and analyzed through standard methods. Chemical Oxygen Demand (COD) was performed using a Digestor DK6. For Total Organic Carbon (TOC) a Multi N/C 2100 (Analytik Jena) TOC analyzer was used. 3. Results and discussions 3.1. Characterizations of the materials In Fig. 3 there is presented the XRD patterns of the fiberglass and the membrane as the final product. The comparative X-ray diffraction analysis of the fiberglass and the membrane (Fig. 3) indicates the formation of a crystalline phase corresponding to the TiO2. The diffraction pattern of the membrane (Fig. 3b) reveals the presence of the TiO2 specific peaks corresponding to 2theta: 25.3, 37.8, 38.6, 48, 54 and 56 degree. 2155


Fig. 3. XRD spectra: a) fiberglass; b) membrane

The images of fiberglass, obtained through scanning electron microscopy (Fig. 4) indicate that the AR fiberglass is a woven fabric. The filaments are coated with alkali-resistant sizing and the woven fabric is extra coated with alkali-resistant waterproof high polymer material. The fiberglass has a very well defined cylindrical shape presenting a smooth external surface. The diameter of the glass fibers has a micrometric size, ranging between 8-9 µm. This type of fiberglass has a high specific surface area, allowing the deposition of large amounts of TiO2. It also presents large free voids allowing a considerable fraction of UV radiation to disperse through the membrane into the whole volume of the photocatalytic reactor.

Fig. 5. SEM image of the membrane

Fig. 6. EDAX analysis of the fiberglass

The result of EDAX analysis of the membrane is presented in Fig.7. The specific chemical elements of the fiberglass support (O, Si, Al, Ca, Mg and Na) are present, and also titanium, certifying TiO2 layer deposition.

Fig. 4. SEM image of fiberglass

The SEM images of the membrane (Fig. 5), obtained by scanning electron microscopy, shows the formation of a relatively uniform TiO2 layer on the external surface of the fiberglass support. The crystallite size is in the range of 120-190 nm. The EDAX analysis of the fiberglass is presented in Fig. 6. The presence of specific chemical elements (O, Si, Al, Ca, Mg and Na) of the fiberglass is to be noted.

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Fig. 7. EDAX analysis of the membrane

Comparing the data acquired with SEM and EDAX techniques for the membrane support (fiberglass) and the prepared membrane, it could be concluded that a TiO2 layer was deposited on the

Removing toxic compounds from wastewater

support. This shows that the thermal treatment and proposed technique used to obtain the membrane have reached the initial purpose. At a quick glance, the TiO2 deposition on the fiberglass was found to be mechanically well developed and cannot be easily removed by rubbing. 3.2. Evaluation of the membrane The degradation of the organic substrate was tested separately in the following conditions: in the absence of the support, in the presence of the fiberglass support and with a photocatalytic membrane. The degradation intermediates and initial organic substrate were overall estimated as COD. Fig. 8 shows the degradation efficiency of the organic test substance as the ratio between COD at time t and initial COD in the photo-oxidation process. For the first two situations the COD decreases as a result of the organic substrate oxidation by H2O2, while in the presence of the TiO2 membrane the organics (as COD) were reduced by 92% after 30 minutes.

Fig. 8. COD/COD0 vs. time

Similarly, the efficiency of the 2,4-DCP and its organic intermediates resulted from photodegradation was estimated as the ratio between the TOC at time t and the initial TOC (Fig. 9). After 30 minutes in the presence of the TiO2 membrane the organics (as TOC) were reduced by 89% after 30 minutes. The experimental data show the catalytic role of the TiO2-functionalized membrane to removetoxic compounds from wastewater. The oxidation is preceded by an adsorption process and by the transfer of the organic compound from the solution to the photocatalytic reaction zone through the functionalized membrane. Titanium dioxide, deposited on the membrane, acts as a photocatalyst in the presence of UV radiations increasing the efficiency of the oxidation process in a relatively short reaction time.

Fig. 9. TOC/TOC0 vs. time

In the case of the membrane, the oxidation process takes place at a higher reaction rate than in the case of the fiberglass support, which confirms the photocatalytic role of the TiO2 deposited on the fiberglass support. The oxidation process of the 2,4DCP may be attributed to the hydroxyl radicals generated by the direct photolysis of the H2O2 but especially to the hydroxyl radicals generated by the photocatalytic mechanism that occurs on the surface layer of the TiO2 in the presence of UV radiation. The breakdown of the 2,4-DCP is complex. During the experiment, the 2,4-DCP was transformed into other organic intermediates. Under the study conditions, it can be seen that a very long time of operation was not required for the degradation of the 2,4-DCP. Therefore, due to a much higher number of hydroxyl radicals generated in the reaction medium through the TiO2 photocatalytic effect, the oxidation process takes place with a relatively high rate, a partial mineralization of the organic substrate being possible in a relatively short time (first 30 minutes of the reaction), in the specified working conditions. After 120 minutes of reaction time, the efficiency to the removal of the 2,4-DCP was about 97%. In real conditions, a membrane has to maintain its activity and mechanical properties for a long time. The behaviour of the TiO2 layer at successive cycles of 120 minutes was tested. The experimental data, obtained through four successive tests using the same membrane are compared and shown in Fig. 10. The evaluation after four successive tests indicates a good stability of the photocatalytic activity of the membrane. The COD/COD0 values, determined at the same reaction time, indicate an insignificant variation for successive tests. Also, the visual examination of the membrane after several successive tests did not indicate any mechanical deterioration of the support or the TiO2 layer. The initial shape and general appearance was similar with the membrane used in only one test.

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Fig. 10. COD/COD0 values profile versus reaction time

4. Conclusions The degradation of an organic compound (2,4DCP) using a functionalized membrane was evaluated. The method consists in a photocatalytic process, which is performed using a reactor equipped with a functionalized membrane obtained through a sol-gel specific method, with layer-by-layer deposition. The advantage of this hybrid method comes from the TiO2 deposition on fiberglass, which combines the advantage of the sol-gel method with the layer-by-layer deposition technique. This procedure for obtaining a membrane (photocatalyst with TiO2) on a fiberglass support is viable. The membrane showed a good mechanical strength and a high photocatalytic activity and stability in the oxidation process of the organic compound. The main factors which determine the characteristics of the membrane (including mechanical strength, chemical stability and the photocatalytic activity) are: the fiberglass type, the method and the thermal treatment for the preparation of the membrane. Acknowledgements Authors recognize financial support from the European Social Fund through POSDRU/89/1.5/S/54785 project: "Postdoctoral Program for Advanced Research in the Field of Nanomaterials” and prof. Ignazio Renato Bellobono, R&D Team Leader of B.I.T. s.r.l., Milan (Italy) who generously offered the testing installation.

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