Treatment of Pharmaceutical-manufacturing Wastewaters by UV Irradiation/Hydrogen Peroxide Process Ahmed Bedoui1, Khaled Elsaid2, Nasr Bensalah*, 1, 2, and Ahmed Abdel-Wahab2 1
Department of Chemistry, Faculty of Sciences of Gabes, University of Gabes, Cite Erriadh, Zrig 6072, Gabes, Tunisia 2 Department of Chemical Engineering, Texas A&M University at Qatar, Education City, PO Box 23874, Doha, Qatar
Abstract: In this study, the treatment of pharmaceutical-manufacturing wastewaters (PMWW) by advanced chemical oxidation using UV irradiation/hydrogen peroxide (H2O2) process has been investigated. Effects of experimental conditions such as H2O2 dose, initial organic matter concentration, temperature and initial pH value on the removal efficiency and kinetics of organic matter were investigated. Results of this study indicated that UV/H2O2 process can be successfully used to completely destroy aromatic compounds, and to remove chemical oxygen demand (COD) and total organic carbon (TOC) with removal efficiencies more than 95% and 90%, respectively. Kinetic experiments have demonstrated that TOC removal rate followed pseudo-second order kinetics. Rate constants of 1.12x10-3 A-1 min-1 and 2x10-5 L mg-1 min-1 were calculated for UV absorbance at 277 nm and TOC decay, respectively. These results indicate that the mechanism of pharmaceuticals degradation involves two main steps: (i) Rapid degradation of aromatic compounds by hydroxylation followed by oxidative opening of benzene rings to form aliphatic derivatives and (ii) subsequent slow fragmentation of aliphatic derivatives into small carboxylic acids which are mineralized into CO2, H2O and other inorganic ions during the final steps of degradation.
Keywords: Pharmaceuticals, photochemistry, pollution, remediation, hydroxyl radicals, degradation. Introduction Pharmaceutical substances are common pollutants detected in many municipal wastewaters, surface water and groundwater (1-6). Residual pharmaceuticals released to the environment poses a serious environmental hazard because these contaminants are extremely bio-resistant and can impose serious toxic and other effects to humans and other organisms (7-11). The main source of pharmaceutical substances in water systems may be the therapeutic use of these substances by humans and animals. It has been demonstrated that pharmaceuticals are not completely absorbed during therapeutic treatment and large amounts of these compounds are emitted into water streams and the environment as a mixture of metabolites or as unchanged substances after administration to humans or animals (12-13). Pharmaceutical industry may be also responsible for the release of pharmaceutical substances to the environment through discharge of highly polluted wastewaters containing a variety of organic and inorganic compounds used in different pharmaceuticalmanufacturing operations (14-17). Several studies indicated that pharmaceutical substances are not completely degraded during conven*Corresponding author; E-mail:
[email protected] ISSN 1203-8407 © 2011 Science & Technology Network, Inc.
tional biological treatment (18-19). This is due to the fact that pharmaceuticals are manufactured to be biologically active, hydrophilic and persistent organics in order to prevent their degradation before they function as therapeutic medicines. Therefore, more effective methods are needed to completely destroy pharmaceuticals and remove them from wastewaters before being discharged into the environment. Advanced oxidation processes (AOPs) appear as the most effective methods for treatment of wastewater containing pharmaceutical compounds (20-22). Effective destruction of these contaminants is due to the generation of highly reactive radical species mainly hydroxyl radicals (HO). These radicals are strong oxidizing agents which are able to react with persistent organics and mineralize them into CO2, H2O and other inorganic ions or at least convert them to easily biodegradable organic compounds. Different combinations of chemical reagents and activating methods were employed for the removal of pharmaceuticals from water including chemical, photochemical and electrochemical technologies (23-30). Homogeneous oxidation process that combines hydrogen peroxide (H2O2) with UV irradiation is a promising AOP for the removal of organic micropollutants (31-33). The UV/H2O2 system generates J. Adv. Oxid. Technol. Vol. 14, No. 2, 2011
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good quality of treated water in comparison with other AOPs such as Fenton, UV/TiO2 and photo-Fenton processes (29-32). This process can be also carried out under ambient conditions and in most cases it leads to complete transformation of organics into friendly environmental products with absence of phase transfer problems and secondary pollutants (sludge). Recently many researchers have investigated treatment of synthetic wastewaters contaminated with various pharmaceutical substances by UV/H2O2 process (2427, 34-36). However, limited information is available on the effectiveness of using UV/H2O2 as practical treatment process for removal of pharmaceuticals industrial wastewaters where high loads of organic and inorganic substances are present (37-38). Therefore, there is a need to investigate the feasibility of UV/H2O2 process for the removal of a mixture of pharmaceuticals in complex wastewaters. The overall goal of this research is to develop UV/H2O2 process into effective method for the treatment of pharmaceutical-manufacturing wastewaters (PMWW) in order to support an eventual development of this process as practical method for treatment of wastewater contaminated with pharmaceuticals. Effects of initial pH, H2O2 dose, organic load, and temperature on the kinetics and the efficiency of the treatment process were investigated. The efficiency of the process was evaluated in terms of absorbance at 277 nm (UV277nm) which corresponds to the maximum UVabsorbance of PMWW, chemical oxygen demand (COD) and total organic carbon (TOC) measurements. Kinetic experiments were performed to determine the reaction rates and identify the rate constants of the reactions. The general pathway of degradation of pharmaceuticals contained in PMWW was also discussed.
Material and Methods PMWW Effluent PMWW used in this study was obtained from pharmaceutical manufacturing industry located in Sousse, Tunisia and it was stored in dark at 4 °C. Different pharmaceutical compounds such as dogmatil (sulpiride), amoxicillin, flucloxaciclin, atenolol, hexamidin, nifuroxazide, novadol, are manufactured in this industrial site. Subsequently, the real wastewater studied here contains a complex mixture of organic and inorganic compounds generated from different manufacturing operations. The main physicochemical characteristics of PMWW are given in Table 1. PMWW is characterized by acidic pH, high amounts of COD and TOC and a very low BOD5 ratio which proves the COD
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Table 1. Physic-chemical characteristics of PMWW.
Parameter pH COD (mg O2 L-1) BOD5 (mg O2 L-1)
Value 3.30 1700 120
TOC (mg C L-1) Chlorides (mg L-1) Sulfates (mg L-1) Nitrates (mg L-1)
460 625 356 15.2
Phosphates (mg L-1)
13.5
high toxicity and bioresitance of pharmaceutical substances contained in PMWW.
Chemicals Hydrogen peroxide was a 30% (w/w) solution (AR grade, Fluka chemical). The other chemicals such as K2Cr2O7, Na 2SO3, H2SO4, NaOH, HgSO4 and Ag2SO4 are of analytical grade and purchased from Aldrich or Merck.
UV/H2O2 Reactor System and Experimental Procedures Photochemical experiments were performed in a batch thermostated Pyrex photo-reactor of 2 L capacity equipped with a 40 W Heraeus Noblelight YNN 15/32 low pressure mercury vapour lamp (Hanau, Germany) located in a quartz sleeve at the centre of the reactor in an axial position and emitting irradiation at 254 nm. The pH of the solution was adjusted to the desired value by addition of sodium hydroxide or sulphuric acid solutions. After the light of the photo-reactor was turned on, precise amount of hydrogen peroxide solution (30%) was mixed with 1000 mL of PMWW by means of magnetic stirrer. A specific H2O2 quantity was adjusted in the beginning of the treatment and no further H2O2 was added during UV irradiation of PMWW. The reaction time for all experiments was in the range of 6-9 hours. Samples of 10 mL were collected each 30 minutes and then 2-3 drops of Na2SO3 saturated solution are added to react with the unconsumed H2O2 till analysis is performed. Then, the samples were analyzed for pH, absorbance at 277 nm, TOC and COD final concentrations.
Analytical Methods The TOC concentration was monitored using a Shimadzu TOC-5050 analyzer. Chemical oxygen demand (COD) was determined using a HACH DR200 analyzer and measured according to colorimetric methods. Chloride, sulphate, nitrate and phosphate were measured by ion chromatography using Dionex
Table 2. Results of the treatment of PMWW by UV irradiation, H2O2 chemical oxidation and UV/H2O2 advanced oxidation process.
1 2 3
H2O2 dose (g L-1) 0.0 3.0 3.0
TOC content (mg C L-1) 460 460 460
T (°C)
UV
28 28 28
irradiation No irradiation Irradiation
ICS2000 system including an AS autosampler, a CD25 conductivity detector, and GP50 Gradient pump. The column used was an IonPac AS19 and the eluent source was EGC-KOH II cartridge contains a fixed potassium hydroxide concentration of 20 mM. Absorbance was measured using a HATCH DR2500 UV-Visible spectrophotometer using a quartz cell with 1 cm optic path length at 277 nm. The pH was measured by Micronal pH-meter (model B474). Hydrogen peroxide was measured according to Eisenberg, 1943 (39).
Results and Discussion PMWW treatment was conducted using homogeneous oxidation process (UV/H2O2) under various operating conditions such as initial hydrogen peroxide dose, initial TOC content, temperature and initial pH in order to determine the optimal treatment conditions. The performance of this photochemical treatment was evaluated by monitoring the same parameters commonly used to monitor the performance of conventional wastewater systems. These parameters are chemical oxygen demand (COD) and total organic carbon (TOC). The TOC removal can be used to evaluate the percentage of mineralization of organic matter contained in PMWW, while COD removal indicates the degradation of organic compounds contained in PMWW. Additionally, spectrophotometric measurements were performed to evaluate aromatics removal. The PMWW sample scanning of wavelengths in a range of UV-Visible shows a maximum absorption band at 277 nm. This band is generally attributed to π π* transition in aromatic compounds (40). The absorbance at 277 nm UV277nm is proportional to the total amount of aromatics contained in PMWW and its decay can be used to measure the aromaticity removal (40). In order to evaluate the performance of the UV/H2O2 treatment process, various experiments have been conducted using UV irradiation alone, H2O2 chemical oxidation alone and a combination of UV and H2O2. The results of these experiments are given in Table 2 and Figure 1. UV irradiation alone and H2O2 chemical oxidation alone showed UV277nm removals of only 14.4% and 38.6%, respectively. However, no significant changes in COD (2.2 and 8.0% respectively) and no removal of TOC were detected in these two cases. On the other hand, the combination of UV
UV277nm 14.4 38.6 96.0
Removal (%) COD 2.2 8.0 76.9
TOC 0.0 0.0 71.0
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Figure 1. UV277nm removal as a function of time during the treatment of PMWW by: () UV- irradiation alone, () H2O2 (3 g L-1) alone, and () UV/H2O2 (3 g L-1). Experimental conditions: TOC0 = 460 mg C L-1, natural pH 3.3, T=28 °C.
irradiation with hydrogen peroxide (UV/H2O2) resulted in 96%, 76.9% and 71% of UV277nm, COD and TOC removals, respectively. These results confirmed that UV/H2O2 can be used as an effective technology for the removal of pharmaceuticals from PMWW. The high removal efficiency using combination of UV/ H2O2 compared to UV irradiation alone could imply that pharmaceutical substances contained in PMWW are light-resistant which makes their degradation using UV light irradiation not feasible in the absence of oxidizing agents. Also, hydrogen peroxide by itself as oxidizing reagent is not strong enough to mineralize refractory organics. However, when UV irradiation is combined with H2O2, hydroxyl radicals are generated from the homolytic photodecomposition of H2O2. These powerful oxidants undergo non-selective and rapid reactions with pharmaceutical substances which significantly enhance the aromaticity removal and mineralization of organic compounds. The performance of UV/H2O2 process for wastewater treatment containing various organic compounds is related to the amount of hydroxyl radicals (HO ●) generated from the photo-decomposition of H2O2. Hydrogen peroxide is the main source of hydroxyl radicals although little amount of HO ● can be formed by water photolysis. Accordingly, H2O2 dose can be considered as the main factor that limits the production J. Adv. Oxid. Technol. Vol. 14, No. 2, 2011
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of hydroxyl radicals during wastewater treatment by UV/H2O2 (42-44). In order to study the effect of H2O2 dose on the efficiency of UV/H2O2 process used to treat PMWW containing 460 mg C L-1 at pH 3.3 and under ambient temperature (28 ºC). H2O2 dose was varied from 1 g L-1 to 5 g L-1. Figure 2 shows the effect of H2O2 dose on UV277nm, COD and TOC removals. Figure 2 indicates that increase of H2O2 dose from 1 g L-1 to 3 g L-1 resulted in increasing the UV277nm removal from 50% to 95%. Higher doses of H2O2 do not lead to any variation of UV277nm kinetics and removal efficiency. Also, the increase of H2O2 dose from 1 g L-1 to 3 g L-1 augments COD and TOC removals from 38% and 35% to 76.9% and 71%, respectively. However, the increase of H2O2 dose above 3 g L-1 has negatively influenced COD and TOC removals and little decrease of the removal efficiency was observed. These results indicate that the optimal dose of H2O2 required to achieve maximum removal efficiency of COD and TOC from PMWW containing 460 mg C L-1 is 3 g L-1. It seems that at low H2O2 doses the amount of hydroxyl radicals generated by photodecomposition of hydrogen peroxide is not sufficient to degrade and mineralize all the organic matter contained in PMWW. Increasing H2O2 dose up to 3 g L-1 leads to generating satisfactory amount of HO radicals and as a result the aromaticity and COD removals and mineralization are highly improved. For H2O2 doses above 3 g L-1, although theoretically higher amounts of HO radicals are supposed to be produced, no significant increase in COD and TOC removals was observed. This can be explained by the fact that when H2O2 dose becomes higher than certain dose, three limiting phenomena are involved in determining the efficacy of UV/H2O2 process. The first phenomenon is related to quantum yield which becomes insufficient to produce more hydroxyl radicals because the lamp power is maintained invariant in all experimental sets. The second phenomenon is the competition between two parallel reactions: the reaction of hydroxyl radicals with pollutants and the reaction of hydroxyl radicals with excess H2O2 added to produce HO2 (equations 1-4). This could consume considerable amount of HO radicals and thus limit the COD and TOC removal potential. Also, HO2 radicals formed during the reaction between H2O2 and HO are less powerful oxidants than HO (31-33, 41), and therefore it does not affect rates of COD and TOC removals. The latter phenomenon that can occur is H2O2 auto-decomposition into O2 and H2O. This reaction consumes H2O2 found in excess and therefore limits the extent of HO radicals production.
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Figure 2. Influence of initial H2O2 dose on (a) UV277nm, (b) COD removal and (c) TOC removals during the treatment of PMWW by UV/H2O2. Experimental conditions: TOC0 = 460 mg C L-1, natural pH = 3.3, T=28 °C.
H2O2 + h 2 HO
(1)
HO + R R-OH OH + H2O2 OH2 + H2O
(2) (3)
2 H2O2 O2 + 2 H2O
(4)
Physico-chemical characteristics of PMWW can vary from day to day which has a direct influence on the efficiency of UV/H2O2 process. The composition and concentration of pharmaceutical substances in
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the competition of H2O2 photodecomposition into hydroxyl radicals with secondary reactions such as auto decomposition of H2O2 to O2 and H2O and then reducing hydroxyl radicals formation. Also, when TOC0 content increases, higher amounts of carboxylic acids can be accumulated during UV/H2O2 treatment. These carboxylic acids are very difficult to mineralize by hydroxyl radicals so that more refractory carbon will be detected at the end of the treatment.
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PMWW cannot be controlled and this makes it difficult to investigate the effect of pharmaceutical composition and concentration on the performance of the treatment process. However, the effect of pharmaceuticals load on the performance of UV/H2O2 treatment can be studied by diluting the real wastewaters received from the industry to keep the nature of pharmaceutical substances invariant and still use initial TOC content (TOC0) as an analytical measurement representing pharmaceuticals in PMWW. This requires that the ratio of hydrogen peroxide dose to H 2O2 mg L1 ) must be maintained TOC0 content ( TOC 0 mg C L1 constant and equal to 6.5 since a H2O2 dose of 3 g L-1 was previously determined as the optimal dose of hydrogen peroxide to treat PMWW containing 460 mg C L-1 TOC0. Figures 3 (a, b, c) present UV277nm, COD and TOC removals with time during UV/H2O2 treatment of PMWW containing different TOC0 contents at natural pH and 28 °C. The influence of TOC0 content on COD and TOC is much higher than that on UV277nm. The increase of TOC0 content from 180 mg C L-1 to 460 mg C L-1 leads to a significant decrease in the COD and TOC removals. However, almost total removals of UV277nm were achieved independently of TOC0 content under the same conditions. Also, nonexpected decrease in UV277nm removal was observed during the first hour of UV/H2O2 treatment (especially when low TOC0 contents were treated) indicating aromaticity persistence at the beginning of the process. Pharmaceuticals contained in PMWW are aromatic organic compounds that can undergo hydroxylation and other reactions through HO attack and be converted to more UV absorbing-intermediates. However, these intermediates can be totally destroyed as time proceeds. In addition, high TOC0 contents reduce the quantum yield used for photo-decomposing hydrogen peroxide into hydroxyl radicals through the absorption of higher amounts of UV radiation by aromatics. At the same time, when high TOC0 contents are present in PMWW, high hydrogen peroxide doses are added to
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Figure 3. Influence of initial TOC on (a) UV277nm, (b) COD and (c) TOC removals during the treatment of PMWW by UV/H2O2.
H O mg L =6.5, natural pH 3.3, 1
Experimental conditions:
TOC0 mg C L1 2
2
T=28 °C.
Temperature is an important variable that could influence the rate and efficiency as well as the cost of the treatment system. To evaluate the influence of temperature on efficiency of PMWW treatment by UV/H2O2 process, some experiments were conducted at various temperatures which ranged between 20 °C J. Adv. Oxid. Technol. Vol. 14, No. 2, 2011
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Table 3. Results of the treatment of PMWW by UV/H2O2 process under different experimental conditions (H2O2 dose, initial TOC content, temperature and initial pH).
Exp. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
H2O2 dose (g L-1) 1.0 2.0 3.0 4.0 5.0 1.2 1.5 2.2 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
TOC content (mg C L-1) 460 460 460 460 460 180 230 345 460 460 460 460 460 460 460 460 460
T (°C)
Initial pH
Final pH
28 28 28 28 28 28 28 28 28 20 28 35 40 28 28 28 28
3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 5.0 7.0 9.0
3.0 2.9 2.9 2.8 2.8 3.2 3.0 2.9 2.9 3.2 2.9 2.5 2.2 2.9 3.7 3.2 2.8
and 40 °C. This temperature range was chosen because it is considered safe for both the UV lamp and the photo reactor. Table 3 presents the results of PMWW treatment by UV/H2O2 at different operating temperatures. Experiments 10-13 have shown that temperature has an important influence on UV 277nm, COD and TOC removals. The increase of temperature from 20 °C to 28 °C leads to increasing UV277nm, COD and TOC removal efficiencies from 86.7%, 68.1% and 61.9% to 96%, 76.9% and 71%, respectively. However increasing temperature more than 28 °C has a negative effect on the efficacy of UV/H2O2 and small decreases in UV277nm, COD and TOC removal efficiencies were obtained. It should be noticed that temperature has more considerable influence on COD and TOC removals than on UV277nm removal. This indicates that temperature influences the final steps of pharmaceuticals degradation by UV/H2O2 process. In fact, an increase in temperature from 20 °C to 28 °C has a direct influence on H2O2 photodecomposition by increasing quantum yield, and consequently more hydroxyl radicals are formed which may explain the improvement of UV/H2O2 efficiency for COD and TOC removals. Increasing temperature above 28 °C accelerates the rate of auto-decomposition secondary reaction of H2O2 into O2 and H2O which consumes a considerable amount of H2O2 and results in producing limited the amount of hydroxyl radicals. To examine the role of initial pH value on the efficiency of PMWW by UV/H2O2 process, a series of experiments were conducted in which initial pH value was varied from the PMWW pH of 3.3 to pH 9 while 231
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UV277nm 54.4 78.6 96.0 91.9 93.4 99.6 96.1 95.2 96.0 86.7 96.0 90.4 89.8 96.0 98.9 95.6 94.6
Removal (%) COD 35.2 43.0 76.9 73.8 69.4 92.7 86.1 80 76.9 61.1 76.9 67.6 78.2 76.9 93.0 85.8 67.7
TOC 30.7 40.0 71.0 70.7 66.1 91.1 82.4 73.8 70.1 68.9 71.0 72.8 71.9 71.0 90.2 78.8 65.8
keeping the other parameters at their optimum values obtained from previous experiments (3 g L-1 H2O2, 28 °C). Experiments 14-17 of Table 3 have shown that initial pH has no significant effect on UV277nm removal and that more than 95% of UV277nm removal was obtained for all initial pH values. However initial pH strongly influenced the COD and TOC removals. An increase of initial pH from 3.3 to 5-7 enhanced COD and TOC removals. However, further increase of pH up to pH 9 resulted in a decrease of the removal efficiencies of COD and TOC. These results indicate that the UV/H2O2 process achieves complete degradation of the aromatics on their aliphatic derivatives regardless of initial pH, but at initial pH 5-7 range, the degradation was faster. Furthermore, changes of pH vs. time during the treatment of PMWW by UV/H2O2 process at different initial pH values are presented in Figure 4. Rapid decrease of pH was observed for initial pH 9, 7 and 5 to pH 3 after reaction time of 90 min, and then pH remains constant with time. However for initial pH 3.3 no important change of pH with time was observed. The evolution of pH at the beginning of the treatment for the different initial pH values can be interpreted by the formation of acidic intermediates during the first steps of pharmaceuticals degradation by UV/H2O2. According to literature, some authors have reported that the effect of initial pH on the efficiency of UV/ H2O2 treatment depends on the nature of effluent studied. Bedoui et al (41) indicated that the efficiency of UV/H2O2 treatment of the olive mill wastewater was independent of initial pH. Catalkay and Karji (42) reported that UV/H2O2 efficiency increases with the
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Figure 4. Evolution with time of pH at different initial pH values during the treatment of PMWW by UV/H2O2. Experimental conditions: 3 g L-1 H2O2, TOC0 = 460 mg C L-1, T= 28 °C.
increase of initial pH during the treatment of pulp and paper mill wastewaters. Jirajor et al. (43) concluded that the UV/H2O2 treatment of effluents containing EDTA achieves the highest percentage of TOC removal at pH 6. In the present work, the highest efficiency of UV/H2O2 for PMWW treatment was achieved at pH values in the range of 5-7. The fact that maximum removal efficiency occurs at pH values near neutral pH could lead to a hypothesis that acidbase reactions might be involved during the degradation process and these reactions might affect the efficiency of the treatment process. At neutral pH conditions, electronic interactions between hydroxyl radicals and pharmaceutical molecules will be minimized which facilitates chemical oxidation of these molecules by HO radicals and decelerates secondary reactions of H2O2 auto-decomposition and proton reduction. The UV/H2O2 process is thought to be a successful treatment step before bioremediation in order to improve the biodegradability of the real pharmaceutical industry wastewaters (PMWW). In order to evaluate the effect of UV/H2O 2 treatment on the biodegradability of PMWW (TOC0 = 460 mg C L-1), COD and BOD5 of selected treated samples were measured where treatment conditions were set at the optimum values (3 g L-1 H2O2, pH 5, T 28 ºC) and the results are presented in Figure 5. Figure 5 shows a rapid decrease of COD with time while an increase of BOD5 with time was observed until it reached a maximum value of 285 mg O2 L-1 at 90 min and then a rapid decrease occurs with time until the end of the reaction. Biodegradability (BOD5/COD) from initial value of 0.07 to a value higher than 0.4 at 90 min of the treatment process. At 90 min, 82% of UV277nm, 53 % of COD
Figure 5. Evolution with time of BOD5 and COD during the treatment of PMWW by UV/H2O2. Experimental conditions: 3 g L-1 H2O2, TOC0 = 460 mg C L-1, pH 5, T= 28 °C.
and 44% of TOC removals were observed. However, after that time, the rates of UV277nm, COD and TOC removals and biodegradability of PMWW considerably decreased. These results indicate that UV/H2O2 is capable to enhance the biodegradability of PMWW and that 90 min reaction time is enough to removes the major part of aromatics and to achieve more than 50% removal of initial COD while attaining the highest values of BOD5 and biodegradability. Consequently, UV/H2O2 can be applied as a pretreatment method to remove pharmaceutical substances from PMWW and be combined with biological treatment for complete reduction of BOD5, COD, and TOC contents. Due to the presence of a complex mixture of organic compounds in this effluent, it is difficult to determine the reaction pathways and to establish kinetic models for degradation of pharmaceuticals during the treatment of PMWW by UV/H2O2 process. However, an approach based on the evolution of the absorbance with time at 277 nm and the rate of TOC removal can be used to define the kinetics of aromaticity removal and mineralization of organics during UV/H2O2 treatment of PMWW under optimal conditions. Figure 6 presents the changes of UV277nm absorbance and TOC with time during the treatment of PMWW (460 mg C L-1, 3 g L-1 H2O2, pH 5, 28 °C) by UV/H2O2 process. As it can be seen, UV277nm absorbance and TOC removal have the same behaviour with time: A rapid decrease in the first 90 min followed by exponential decay was observed. It is also notable that UV277nm absorbance decreases more rapidly than TOC. The continuous decrease of TOC confirms the progressive mineralization of pharmaceutical substances during UV/H2O2 process. The fact that the conversion of TOC into carbon dioxide is slower than the decrease of UV277nm indicates the disappearance of aromaticity J. Adv. Oxid. Technol. Vol. 14, No. 2, 2011
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Figure 6. Evolution with time of UV277nm and TOC during the treatment of PMWW by UV/H2O2. Experimental conditions: 3 g L-1 H2O2, TOC0 = 460 mg C L-1, pH 5, T = 28 °C.
and the formation of different intermediates without complete oxidation of organics into CO 2. In particular, the fast removal of UV277nm absorption and the decrease of pH observed at the beginning of UV/H2O2 treatment confirm the formation of aliphatic carboxylic acids as intermediates. Kinetic analysis indicated that UV277nm and TOC decay fitted well pseudo-second order kinetics with rate constants of 1.12x10-3 A-1 min-1 and 2x10-5 L mg-1 min-1, respectively (with correlation coefficient >0.98) which confirms that aromatics removal is more rapid than mineralization. Based on the experimental results, it can be concluded that the mechanism of pharmaceutical substances degradation involves several steps which involve a sequence of chemical oxidation/reduction reactions. The degradation of pharmaceuticals contained in PMWW by UV/H2O2 process, starts with instantaneous hydroxyl radical react with aromatic molecules to form more hydroxylated aromatic intermediates which undergo a rapid oxidative opening of aromatic rings into aliphatic derivatives. These aliphatic derivatives are slowly fragmentized into small carboxylic acids which could be mineralized during the final steps of UV/H2O2 treatment.
Conclusion UV/H2O2 process can be successfully used for the treatment of real pharmaceutical-manufacturing wastewaters and can achieve almost complete aromaticity removal as well as sufficient COD and TOC removals under optimal conditions (460 mg C L-1, 3 g L-1 H2O2, pH 5, 28 °C). Kinetics and efficiency of this photochemical treatment process depends mainly on H2O2 dose, initial TOC content, temperature and initial pH. According to the results obtained here, a general mechanism can be proposed for the removal of 233
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pharmaceutical substances from PMWW by UV/H2O2 system. During the first stages of pharmaceuticals degradation, instantaneous attack of hydroxyl radicals on aromatic rings occurs and more hydroxylated aromatic intermediates are formed which then undergo to rapid oxidative opening of benzene rings into aliphatic derivatives. These aliphatic derivatives are slowly fragmented into small carboxylic acids which are mineralized during the final stages of treatment. Kinetic analysis indicated that aromaticity removal occurred during the first stages of UV/H2O2 treatment is more rapid than TOC removal (mineralization). During the first stages of PMWW treatment by UV/ H2O2 process, a significant increase in the biodegradability was observed. This indicates that UV/H2O2 process can be used as an effective pretreatment technology that is capable to remove aromatics content and to improve the biodegradability of effluent of pharmaceutical industry.
Acknowledgement The authors acknowledge Texas A&M University at Qatar and Qatar Foundation for providing partial financial support to accomplish this research work.
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