ABSTRACT In this study, the degradation of polychlorinated biphenyls (PCBs), Aroclor 1260, using the UV/H2O2 system was investigated. The effects of initial H2O2 concentration, initial pH of the solution and initial PCB concentration were studied using a batch tubular reactor equipped with a 254 nm low-pressure Hg lamp. PCB concentration was analyzed using GC-ECD as total aroclor. Results show that there is an optimum H2O2 concentration that maybe used beyond which no significant increase in the degree of degradation was achieved. The highest PCB degradation efficiency of 87.58% after 60 minutes was obtained at a peroxide concentration of 24.71mM and PCB concentration of about 40 ppm. The initial pH of the solution also proved to have an effect on PCB degradation. Highest oxidation efficiency of 87% was obtained at pH 7 after 60 minutes at a peroxide concentration of 24.71 mM. For the effect of initial concentration of PCBs, up to a certain concentration, degradation was found to increase with increasing PCB concentration. Highest degradation efficiency of 68% and 86% after 30 and 60 minutes, respectively, were obtained at a PCB concentration of 40 ppm. A decrease in degradation efficiency was, however, observed at 80 ppm which gave a lower degradation efficiency of 79% after 60 minutes of reaction In all the runs conducted, dechlorination is the main mechanism verified by GC-MS analysis. The use of UV/H2O2 for PCB degradation seems very promising in the treatment of wastewater containing PCBs. The results of this study are highly significant in performing a kinetic analysis on PCB degradation. Current efforts involves improvement of the process
Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 9 - Effect of Process Parameters on the Degradation of Polychlorinated Biphenyls in Water Matrix using UV/H 2 O 2 C. CENTENO *1, 2 , L ABELLA *1 , S. GALLARDO *1 *1 Asian Regional Research Programme on Environmental Technology (ARRPET), De La Salle University, College Of Engineering, 2401 Taft Avenue, Manila, PHIL. E-mail: arrpet@dlsu.edu.ph *2 Faculty of Engineering, University of Santo Tomas, Espaňa, Manila, Phil. 1008 ABSTRACT In this study, the degradation of polychlorinated biphenyls (PCBs), Aroclor 1260, using the UV/H 2 O 2 system was investigated. The effects of initial H 2 O 2 concentration, initial pH of the solution and initial PCB concentration were studied using a batch tubular reactor equipped with a 254 nm low-pressure Hg lamp. PCB concentration was analyzed using GC-ECD as total aroclor. Results show that there is an optimum H 2 O 2 concentration that maybe used beyond which no significant increase in the degree of degradation was achieved. The highest PCB degradation efficiency of 87.58% after 60 minutes was obtained at a peroxide concentration of 24.71mM and PCB concentration of about 40 ppm. The initial pH of the solution also proved to have an effect on PCB degradation. Highest oxidation efficiency of 87% was obtained at pH 7 after 60 minutes at a peroxide concentration of 24.71 mM. For the effect of initial concentration of PCBs, up to a certain concentration, degradation was found to increase with increasing PCB concentration. Highest degradation efficiency of 68% and 86% after 30 and 60 minutes, respectively, were obtained at a PCB concentration of 40 ppm. A decrease in degradation efficiency was, however, observed at 80 ppm which gave a lower degradation efficiency of 79% after 60 minutes of reaction In all the runs conducted, dechlorination is the main mechanism verified by GC-MS analysis. The use of UV/H 2 O 2 for PCB degradation seems very promising in the treatment of wastewater containing PCBs. The results of this study are highly significant in performing a kinetic analysis on PCB degradation. Current efforts involves improvement of the process. KEYWORDS Polychlorinated biphenyls, advanced oxidation processes, hydroxyl radicals, UV/peroxidation INTRODUCTION Polychlorinated biphenyls (PCBs) are mixtures of chlorinated derivatives of biphenyl with different structures depending on the number and position of chlorine atoms on the biphenyl skeleton. They have been widely used as electrical insulation and heat exchange fluid because of their chemical and thermal stability. However, prior to cessation of PCB production, evidences have piled up that the very properties that made these compounds industrially significant also made them into toxic and persistent organic pollutants. PCBs are found at very low concentrations in water matrix with most Aroclor mixtures having solubility values of about 3 ppb to 15 ppm (Erickson, 1997). However, strict standards are being imposed on drinking water and ground water resources relative to PCBs. The maximum PCB level allowable based on USEPA standards is 0.5 ppb and 0.09 ppb for drinking water and groundwater, respectively. In Japan, the standard for drinking water is 0.3 ppb which is even stricter. Thus, an effective treatment technology is required to eliminate PCBs found in water matrix. In the past decade, there has been considerable research and commercial interest in the use of advanced oxidation processes (AOPs) for the treatment of organic contaminants in aqueous, Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 10 - gaseous and solid phases. AOPs are typically characterized by the generation of the highly reactive hydroxyl radicals (•OH) and are the principal agents responsible for the oxidation of numerous pollutants (Jardim et al., 1997). Among the AOPs, the UV/H 2 O 2 system is considered the simplest in terms of mechanism, design, operation, and maintenance. It involves the production of OH• from the cleavage of O-O bond in hydrogen peroxide according to the following reaction: H 2 O 2 + hv → 2OH• The system has been proven effective in the degradation of various organic compounds in aqueous solutions such as 1,3,5-trinitrotriazaxyclohexane (RDX) (Bose et al., 1998), trichlorophenols (Hugul and Demirci, 2000), n-methyl-p-aminophenol (Andreozzi et al., 1999), tetrachloroethylene (Alibegic et al., 2001) and p-chlorophenol (Ghaly et al., 2001). The dosage of H 2 O 2 in the solution has a remarkable effect on the rate of degradation of the target pollutant. When the initial H 2 O 2 concentration is increased beyond the optimum level, the excess H 2 O 2 becomes an OH• radical scavenger consequently decreasing the rate of degradation of organic compounds. This was observed by Crittenden et al. (1999) in their work on the degradation of 1,2- dibromo-3-chloropropane (DBCP). Esplugas et al. (2002) and Ghaly et al. (2001) reported the same observation on the degradation of phenol and p-chlorophenol, respectively. These studies show that there is an optimum H 2 O 2 concentration that can provide the highest rate of pollutant degradation. The pH of the solution being irradiated affects the rate of radical generation in most AOPs. Therefore, appropriate values of this parameter must be chosen. There are, however, conflicting reports on the effect of pH on the degradation of organic compounds for the UV/H 2 O 2 system. A review on chemical oxidation by the photolysis of H 2 O 2 (Liao and Gurol , 1995) reported that the best oxidation efficiency is obtained at acidic pH. Ku et al. (1998) observed higher decomposition rate of EDTA when more alkaline solution was used. A review by Legrini et al. (1993) on photochemical processes for water treatment cited that H 2 O 2 has higher dissociation rate at higher pH. On their study on the degradation of phenolic compounds, Alnaizy and Akgerman (2000) reported that photolysis combined with hydrogen peroxide was found to be more effective in neutral solutions. The initial concentration of the solution being irradiated was also found to have significant effects on the rate of pollutant degradation as well as the kind and concentration of intermediates formed during reaction. Alnaizy and Akgerman (2000) reported that as the initial concentration of phenol is increased, the efficiency of the UV/H 2 O 2 system decreased. They further reported that p- benzoquinone was detected in all of their experimental runs except those that were performed at low phenol concentrations. This indicates that initial pollutant concentration maybe a factor in the mechanism of the reaction. The study of Wang et al. (2000) on the degradation of humic acids cited that, at a fixed concentration of hydrogen peroxide, increasing the humic acid concentration decreases the rate of degradation. Wang et al. (2000) further elucidated that the higher the concentration of humic acid initially present, the greater would be the competition between the H 2 O 2 peroxide and the acid itself for the photons generated during irradiation. Thus, the major mechanism for humic acid degradation in this study was basically photolysis. The objective of this study is to determine the effects of initial H 2 O 2 concentration, initial pH of the solution and initial PCB concentration on the degradation of PCBs in water matrix. Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 11 - MATERIALS AND METHODS Materials, Reagents and Instrumentation Stabilized H 2 O 2 (30% w/w) supplied by Riedel-de Haen was used as oxidizing agent in this study. Pesticide grade hexane, AR grade methanol, and Varian C18 cartridges were used for sample preparation and extraction. For the analysis of total PCB concentration in the samples, a Gas Chromatography Shimadzu GC-14B equipped with an Electron Capture Detector and installed with a SPB-5 capillary column was used. For qualitative analysis of samples, a GC-MS Perkin Elmer Clarus 500 was used. Photoreactor The tubular photoreactor, shown in Fig.1, was fitted with a 10 W Sterilight Hg lamp emitting light at 254nm. The UV lamp was positioned in the center of the reactor and was enclosed in a quartz sleeve. The reactor was equipped with a thermocouple to monitor the temperature of the solution and a recirculation pump that allowed the continuous mixing of the solution. The reactor was also fitted with a sampling port where samples maybe withdrawn at desired time intervals and a gas vent that served as exit for possible gases formed during experimental runs. Experimental Procedures Stock PCB solution was prepared by dissolving 0.1 g of the transformer oil in 10 ml of pesticide- grade methanol. Aqueous PCB solutions were prepared by spiking the required amount of the stock solution into 1 L of distilled water. The spiked solution was kept uncovered and continuously stirred inside a hood overnight to allow the solvent to evaporate. The UV light in the reactor was first turned on for about 10 minutes to allow the UV energy to stabilize. The aqueous PCB solution was then fed batch wise into the reactor through an influent port. Ten ml samples were taken at the desired sampling times. The pH of the samples and the temperature of the solution inside the photoreactor were continuously monitored. The adjustment of the pH of the solution was done using 1.0N NaOH and 1.0N H 2 SO 4 . The effect of the initial pH of the solution was evaluated using pH values of 3, 7, 5.8 and 10. For the effect of initial PCB concentrations, H 2 O 2 concentrations used were based on the optimum amount obtained, maintaining the PCB/hydrogen peroxide ratio in the solution in all the experimental runs. Figure 1. Tubular Photoreactor Recirculation pump UV reactor level indicato r Feed port Thermocouple UV lamp with quartz sleeve Sampling port Gas vent Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 12 - In order to verify the main mechanism involved during the UV/peroxidation process, the best run with an initial PCB concentration of 40 ppm was subjected to GC-MS analysis. The samples taken were prepared for GC-ECD injection based on USEPA method 8080A. Extraction was done using the Solid phase extraction method (EPA 3535). RESULTS Sampling on Contaminated sites PCB-containing transformer oils were collected from Clark Special Economic Zone (CSEZ). The highest concentration measured was 720 g/L of Aroclor 1260 which was used for the preparation of stock solutions (Moradas et al., 2003). Aroclor 1260 is one of the most persistent PCB isomers containing an average of 6.3 chlorine atoms per molecule. It contains about 10% tetrachloro-, 50% pentachloro-, and 40% hexachlorobiphenyl, all highly chlorinated PCB congeners (Ahlert and Peters, 2001). The solubility of Aroclor 1260 in water is reported to be about 3 ppb. Effect of initial H 2 O 2 concentration To evaluate the effects of initial H 2 O 2 concentrations on the degradation of PCBs, different dosages of H 2 O 2 was added to the system. Experimental runs involved the use of several dosages of H 2 O 2 to determine the possible optimum value. No pH adjustment was made with the aqueous solutions used. pH values of the samples at designated sampling times were monitored. H 2 O 2 dosages used for the preliminary trials include 1 ml, 2 ml, 3 ml and 4 ml of the 30%w H 2 O 2 per liter of the aqueous PCB solution. These values correspond to 8.8mM, 17.64 mM, 26.47mM and 35.29mM, respectively. Results of these preliminary trials are shown in Figs. 2 and 3. As shown in Fig. 2, a drastic decrease in pH after only 5 minutes of irradiation was observed for all dosages used. The pH of the solution then becomes constant until about 120 minutes except for the 35.29mM run. This maybe attributed to dechlorination occurring in the early stages of the reaction and/or formation of reaction intermediates as reaction progressed. The drastic increase in the pH observed in the 35.29mM may indicate a very fast dechlorination and/or formation of intermediate products. It is possible that highly chlorinated congeners were converted to lower chlorinated 2 3 4 5 6 7 0 60 120 180 240 300 360 420 time, min pH 8.8mM 17.64mM 22.05mM 24.71mM 26.47mM 35.29 0 10 20 30 40 50 60 70 80 90 100 0 60 120 180 240 300 360 420 time, min PCB Degradation, % 8.8mM 17 . 6 4 mM 22.05mM 24.71mM 26.47mM 35.29mM Figure 2 Change in pH over time [PCB] 0 ≈40 mg/l; pH soln = 5.8 – 6.0) Figure 3 Percentage degradation at various H 2 O 2 concentration ([PCB] 0 ≈40 mg/l; pH soln = 5.8 – 6.0) Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 13 - congeners. The solution may contain the chloride ions removed causing the decrease in pH. This phenomenon may also be explained by the possible formation of acidic intermediates. The GC-MS analysis verified this premise. Increasing the initial H 2 O 2 dosage showed increasing percentage PCB degradation, observed in Fig. 3, at initial H 2 O 2 concentration of 8.8mM and 17.6 mM. When H 2 O 2 concentration was increased to 26.47mM, data at each sampling time showed a decrease in PCB degradation. This was further proven by once again increasing the concentration to 35.29mM. This observation shows that further increase in the concentration of H 2 O 2 from 17.6mM did not significantly increase PCB degradation. In fact, a decrease in percentage PCB degradation was observed. This indicates the possibility of OH• radical scavenging by the excess H 2 O 2 in the solution. At higher peroxide concentration, the H 2 O 2 itself reacts with the hydroxyl radicals and hence, acts as an inhibiting agent for PCB degradation. From the initial data gathered, it maybe inferred that the optimum H 2 O 2 concentration might lie between 17.6mM and 26.47mM. Thus, additional trials using initial peroxide concentrations of 2.5mL (22.05mM) and 2.8mL (24.71mM) were conducted to verify the inference made. Results of these trials are also shown in Figs. 2 and 3. From Fig. 3, an initial peroxide concentration of 24.71mM increased percentage PCB degradation further. Inhibiting effect, however, was already observed at a concentration of 26.47mM. It may also be observed almost the same degradation efficiency was obtained for the 22.05mM and 24.71mM runs during the first 60 minutes of reaction time. This maybe important in the economics of the process and may influence the amount of hydrogen peroxide to be used in the process. Based from the results of this phase of the experiment, the optimum H 2 O 2 is 24.71 mM or 2.8 ml per liter of the aqueous PCB solution containing about 40 ppm of pure PCBs. Percentage PCB degradation at 180 minutes for 8.8mM, 17.6mM, 22.05mM, 24.71mM, 26.47mM and 35.29mM are 75.33%, 81.16%, 85.69%, 87.58%, 83.94% and 75.87%, respectively. Effect of pH The results of the degradation of PCBs at different pH values are shown in Figs. 4 and 5. Even when the initial pH of the solution was adjusted to 10, a drastic decrease in the pH of the solution for all pH values used was observed as shown in Fig. 4. The same behavior was observed until about 45 minutes of photooxidation. This was subsequently followed by slight increase in the pH of the solution which maybe attributed to possible dechlorination and/or formation of acidic species in the solution. This was verified in the latter part of the study. 2 3 4 5 6 7 8 9 10 0 60 120 180 240 300 time, min pH pH 3 pH 5.8 pH 7 pH 10 0 10 20 30 40 50 60 70 80 90 100 0 30 60 90 120 150 180 210 time, min PCB Degradation, % pH 3 pH 5.8 pH 7 pH 10 Figure 4 Change in pH over time ([PCB] o ≈ 40 mg/l; [H 2 O 2 ] 0 = 24 mM) Figure 5 PCB degradation at various pH ([PCB] o ≈ 40 mg/l; [H 2 O 2 ] 0 = 24 mM) Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 14 - Highest PCB degradation was obtained with pH 7 as shown in Fig. 5. This observation coincides with the report of Alnaizy and Akgerman (2000) on the degradation of phenolic compounds. After 60 minutes of irradiation, 76% PCB degradation was obtained with pH 3, 78% with pH 10, 87% degradation was obtained with pH 5.8 while 91% degradation was achieved with pH 7. From these values, it maybe inferred that PCB degradation is best carried out at around neutral pH values as evidenced by higher degradation attained both for pH 5.8 and for pH 7. Degradation efficiencies among the pH values used may not be significantly different within the reaction time employed. However, the considerably high percentage efficiency obtained with the actual pH of the solution (pH = 5.8) is, again, highly significant in the economics of the process again. This means that good PCB degradation may still be obtained even without adjusting the pH of the aqueous PCB solutions. Effect of Initial PCB Concentration The results of the degradation of PCBs at different initial concentrations are shown in Figs. 6 and 7. In all the initial PCB concentrations used, a decrease in the pH of the solution was observed as shown in Fig. 6. This further verified the results presented in Figs. 2 and 4. However, it maybe observed that at higher concentration, there was a greater decrease in pH and this acidic range (about pH= 3.5) was seen for a longer period during the reaction time than those of the lower concentration runs. While there was initially a decrease in pH for the low concentration runs, this was immediately followed by increase in pH. This behavior may again be explained by the possible dechlorination occurring in the solution and that at higher concentration more Cl - ions may detach themselves from the parent compound. Also, greater concentration of acidic species maybe obtained when higher PCB concentration was used. The subsequent increase in pH maybe explained by the possible formation of intermediates or the decrease in Cl - ion concentration in the solution. Since pollutants in the solutions were understood to play the dual roles of UV light photon absorber and OH• scavenger, the higher the initial concentration, the lower is the amount degraded. However, this was not quite consistent with the results presented in Fig. 7. Up to 40 ppm, percentage PCB degradation was observed to increase with increasing concentration. Highest degradation efficiency of 68% and 86% after 30 and 60 minutes, respectively, were obtained at a PCB concentration of 40 ppm. A plausible explanation for this would be the role of the Figure 6 Change in pH over time at different initial PCB concentration [pH soln =5.8-6.2; T reactor =28 – 33 o C; [H 2 O 2 ] based on 24.71 mM at 40 ppm ] 3 3.5 4 4.5 5 5.5 6 6.5 0 50 100 150 200 250 300 time, min pH 1 p p m 5 ppm 10 p p m 20 ppm 40 ppm 80 ppm Figure 7 Concentration Profile at different initial PCB concentration [pH soln =5.8-6.2; T reactor =28 –33 o C [H 2 O 2 ] based on 24.71 mM at 40 ppm ] 0 50 100 150 200 time, min C/Co 1 ppm 5 ppm 10 ppm 40 ppm 80 ppm Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 15 - intermediates in the reaction. It is possible that fast formation of reaction intermediates occur when low initial concentrations were used that the reaction favors the degradation of these rather than the parent compound. While for those runs which employed higher initial concentration, the degradation of the parent compound maybe favored than the degradation of the intermediates. This similar observation was reported by Wang et al. (2000) on the degradation of humic acids. Decrease in degradation efficiency was, however, observed at 80 ppm which gave a lower degradation efficiency of 79% after 60 minutes of reaction. The amount of PCBs in the solution may have inhibited the degradation of the compound and it is possible that competition for UV irradiation occurred between the hydrogen peroxide and the PCBs. In this case, the primary mechanism at 80 ppm may be photolysis, a much slower reaction, than reaction with the hydroxyl radicals. The 40 ppm PCB run was subjected to GC-MS analysis. The results are shown is Fig. 8. Figure 8 Degradation Profile of PCBs as Aroclor 1260 [PCB] initial = 40 ppm] , 12-May-2004 + 18:17:15 3.23 5.23 7.23 9.23 11.23 13. 23 15.23 17. 23 19.23 21.23 23. 23 25.23 27. 23 29.23 31. 23 Time 0 100 % 0 100 % 0 100 % 0 100 % 0 100 % 40 ppm 0min Scan EI+ TI C 4.00e8 21.08 11. 14 8. 00 7. 28 2. 63 10. 53 8. 09 10. 47 9. 49 11. 36 16. 40 13.68 13. 17 15. 89 14. 13 20. 4619. 64 17. 87 23.13 21. 63 22. 46 23.94 26. 03 25. 31 40ppm30min Scan EI+ TI C 4.00e8 11. 13 7. 99 7. 26 7. 16 2. 61 10.52 8. 07 10. 46 9. 48 8. 82 12. 79 11. 36 11. 49 13.68 16. 41 15. 89 14.03 21. 51 20.68 17. 88 19.65 21. 94 23. 15 40ppm60min Scan EI+ TI C 4.00e8 11. 14 8. 00 7. 27 2. 60 10. 54 8. 09 10. 47 9. 50 11. 37 16. 40 13.68 13. 17 15.89 14. 13 21.47 20. 66 17. 87 19.66 22. 32 40ppm120min Scan EI+ TI C 4.00e8 11. 14 8. 01 7. 28 2. 63 10. 53 8. 21 10. 48 9. 50 16. 40 13.68 11. 50 13. 17 15.89 14. 14 21.47 20. 66 17. 87 22. 30 40ppm180min Scan EI+ TI C 4.00e8 11. 14 8. 01 4. 13 2. 63 7. 28 4. 56 10. 54 8. 10 10. 48 9. 50 11. 37 16. 41 13.68 13. 17 15. 89 14. 14 21.47 17. 87 20. 66 22. 31 PCB peaks Long chain alkane peaks PCB peaks Long chain alkane peaks Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 16 - From the GC-MS results, it can be seen that the PCB sample used contains long-chained alkanes like dodecane and eicosane and highly chlorinated PCB congeners. It can also be observed that peaks corresponding to the PCB congeners in the sample decreased dramatically in size in the first 30 minutes of the reaction and almost disappeared after 180 minutes of the reaction. The very fast decrease in the size of the peaks corresponded to the highest percentage degradation as shown in previous experimental runs on the effect of process parameters. From Figs. 3, 5 and 7, the highest degradation efficiency was observed at the first few minutes of the reaction. Fig. 8 also corroborated the discussion presented for the results shown in Figs. 2, 4 and 6 where a drastic decrease in pH during the first few minutes of the reaction was observed. Dechlorination was attributed to be the main mechanism during the first few minutes of the reaction causing the drastic decrease in pH. No organic intermediates, however, were observed in the GC-MS results. This maybe explained by the non-selective attack of OH• radical which might have also attacked the intermediates. Hirvonen et al. (1996) and Hugul et al. (2000) reported that many intermediates formed during degradation of organic compounds are meta-stable and are susceptible to oxidation along with the parent compound. Since OH• radicals are known to be very reactive and attacks organic compounds in the aqueous solutions indiscriminately, the degradation of PCBs and that of organic intermediates may occur almost simultaneously. CONCLUSIONS The use of the UV/H 2 O 2 system for PCB degradation in water matrix gave significant results. The determination of the optimum H 2 O 2 concentration is very important in the application of the system. H 2 O 2 acts as both an initiating and scavenging agent of hydroxyl radicals. Increasing H 2 O 2 dosage beyond the optimum value has been found to have an inhibiting effect on PCB degradation. In this study, effective degradation was obtained at H 2 O 2 concentration of about 2800 ml (of the 30%H 2 O 2 solution) per liter and initial PCB concentration of about 40 ppm. The initial pH of the solution also has a significant effect on the rate of degradation of PCBs. PCB degradation was found to yield better results at around neutral pH values. Up to 40 ppm, degradation efficiency was found to increase with increasing initial PCB concentration. Possible inhibition was, however, observed at 80 ppm as evidenced by the decrease in degradation efficiency. In all the runs conducted, dechlorination takes place rapidly during the first few minutes of the reaction making this process suitable for the preliminary treatment of PCB containing wastewater. Although, the results of the present study are encouraging, the following needs to be carried out in order to improve the process. Identification of reaction intermediates to verify if there was any toxic intermediate formed; Determination of the effects of UV intensity to confirm possibility of obtaining higher PCB degradation at shorter reaction time; Analysis of the kinetics of PCB degradation. ACKNOWLEDGMENTS The study is part of the Asian Regional Research Programme on Environmental Technology implemented by the Chemical Engineering Department of De la Salle University, coordinated by the Asian Institute of Technology and funded by the Swedish International Development & Cooperation Agency. Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 17 - REFERENCES (1) Ahlert, R. and Peters, R. (2001). Treatment of PCB-contaminated soils: I. Evaluation of in situ reductive chlorination of PCBs, Environmental Progress, Vol. 20, No.2, pp. 108-116. (2) Alnaizy, R. and Akgerman, A. (2000). Advanced oxidation of phenolic compounds, Advances in Environmental Research, Vol. 4, 233-244. 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Comparison of different advanced oxidation processes for phenol degradation, Water Research, Vol. 36, 1034-1042. (9) Ghaly, M., Hartel, G., Mayer, R. and Haseneder, R. (2001). Photochemical oxidation of p- chlorophenol by UV/H 2 O 2 and photo-Fenton process: A comparative study, Waste Management, Vol. 21, 41-47. (10) Hugul, M. and Demirci, S. (2000). Modelling of kinetics of UV/hydrogen peroxide oxidation of some mono-, di-, and trichlorophenols, Journal of Hazardous Materials, Vol. B77, 193- 208. (11) Jardim, W. F., Moraes, S.G., and Takiyama, M.M.K. (1997). Photocatalytic degradation of aromatic chlorinated compounds using TiO 2 : Toxicity of intermediates, Water Research, Vol. 31, No. 7, 1728-1732. (12) Ku, Y., Wang, L. and Shen, Y. (1998). Decomposition of EDTA in aqueous solution by UV/H 2 O 2 process, Journal of Hazardous Materials, Vol. 60, 41-45. (13) Legrini O, Oliveros, E. and Braun, A.M. (1993). 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The adjustment of the pH of the solution was done