Wet Oxidation of High-Concentration Reactive Dyes - American

The TOC removal is insignificant even when 50% of the stoichiometric amount of H2O2 is used. No color change is observed until the dosage of H2O2 is 1...
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Ind. Eng. Chem. Res. 1999, 38, 1837-1843

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Wet Oxidation of High-Concentration Reactive Dyes Guohua Chen,* Lecheng Lei,† and Po-Lock Yue Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Advanced oxidation methods were used to degrade reactive dyes at high concentrations in aqueous solutions. Wet peroxide oxidation (WPO) was found to be the best method in terms of the removal of color and total organic carbon (TOC). Reactive blue (Basilen Brilliant Blue P-3R) was chosen as a model dye for determining the suitable reaction conditions. The variables studied include reaction temperature, H2O2 dosage, solution pH, dye concentration, and catalyst usage. The removal of TOC and color by wet oxidation is very sensitive to the reaction temperature. At 150 °C, the removal of 77% TOC and 90% color was obtained in less than 30 min. The initial TOC removal rate is proportional to the H2O2 dosage. The TOC removal is insignificant even when 50% of the stoichiometric amount of H2O2 is used. No color change is observed until the dosage of H2O2 is 100% of the stoichiometric amount. The color removal is closely related to TOC removal. When the pH of the solution is adjusted to 3.5, the dye degradation rate increases significantly. The rates of TOC and color removal are enhanced by using a Cu2+ catalyst. Another four reactive dyes, Procion Red PX-4B, Cibacron Yellow P-6GS, Cibacron Brown P-6R, and Procion Black PX-2R, were treated at 150 °C using WPO. More than 80% TOC was removed from the solution in less than 15 min. The process can remove the colors of all these dyes except Procion Black PX-2R. Introduction Wastewaters from fabric and yarn dyeing impose serious environmental problems because of their color and their high chemical oxygen demand (COD). The former blocks light transmission into water, and the latter depletes the dissolved oxygen, thus affecting the aquatic life circle there. Hence, the legislations in many countries prohibit the direct discharge of untreated dyeing wastewater to the sewer. Textile wastewaters have to be treated to meet the legally required standard before discharge. Currently the textile dyeing and printing industry in Hong Kong faces this challenge. Conventional methods of dyeing wastewater treatment include adsorption,1 coagulation,2 electrochemical methods,3 chlorination and ozonation,4,5 advanced oxidation using UV/H2O26 or UV/TiO2,7 and biological oxidation.8,9 Membrane separation, which has found increasing usage over the past 2 decades, may also be applied to the treatment of textile wastewater.10 This method enables the reuse of treated water, thus cutting water consumption costs and discharge fees. Under suitable conditions, the concentrated stream from membrane separation may be recycled and reused; hence, the usage of raw material is reduced. Unfortunately in the case of reactive dyes, the retentate cannot be reused due to the loss of reactivity. Thus, this kind of concentrate from the membrane-separation unit has to be treated. The color of this concentrate poses a more serious environmental problem than that of the wastewater before membrane separation. The concentrate also has a very high COD. Wet oxidation has been proved to be a feasible method for the treatment of wastewaters with high chemical oxygen demand.11 * Corresponding author. Telephone: (852)23587138. Fax: (852)23580054. E-mail. [email protected]. † Current address: Department of Chemical Engineering, Zhejiang University, Hangzhou, China.

Recently, Lei et al. have shown that the desizing wastewaters from the textile bleaching and dyeing industry can be successfully treated by wet oxidation.12,13 The method has the advantage of a fast reaction rate. The oxidant is the dissolved oxygen in the case of wet air oxidation (WAO) and hydrogen peroxide in wet peroxide oxidation (WPO).14 An extensive review of WAO has been presented elsewhere.15 The present paper reports the effectiveness of wet oxidation methods and the determination of the optimal operating conditions for the treatment of high-concentration reactive dye solutions. Experimental Equipment and Conditions A schematic diagram of the wet oxidation system used for the present investigation is shown in Figure 1. The 2-L reactor is equipped with a stirrer for mixing. The reaction temperature was controlled by an electric heating jacket and an internal cooling coil. The system can be used as either a WAO reactor or a WPO reactor depending on the oxidant chosen. The experimental procedures were as follows. First, the reactor was heated to 60-70 °C. The reactor was then purged with pure nitrogen at a total pressure of 1 MPa. One liter of dye solution was fed into the reactor. Thereafter, the reactor was heated to the desired reaction temperature. Once this temperature was reached, pure oxygen (for WAO) or hydrogen peroxide (for WPO) was supplied to the reactor and the reaction was considered to commence. This point is taken as “zero time” in all experiments. It is so defined that the wet oxidation data may be compared on the same basis. Liquid samples were taken from the reactor at selected time intervals. COD, total organic carbon (TOC), pH, and color (in a Lovibond unit) were measured after the samples had been allowed to quench to ambient temperature.

10.1021/ie980617d CCC: $18.00 © 1999 American Chemical Society Published on Web 03/31/1999

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Figure 1. Schematic diagram of the wet oxidation facility. Table 1. Experimental Conditions temp, °C oxygen supply, % COD H2O2 supply, % COD dye concn, C, g/L soln initial pH catalyst

WAO

WPO

150, 200 115 0, 10 50 8.8 Cu2+ (200 mg/L)

110, 130, 150 0 50, 75, 100, 150 12.5, 25, 50 3.5, 7, 8.8 AC-Cu (200 mg/L)

Table 2. Description of the Reactive Dyes Tested color

trade name

color index

COD, mg/g

TOC, mg/g

blue red yellow brown black

Basilen Brilliant Blue P-3R Procion Red PX-B Cibacron Yellow P-6GS Cibacron Brown P-6R Procion Black PX-2R

49 3.1 95 11 mixture

770 1040 730 920 600

175 310 250 280 240

The COD values of the samples were determined by a HACH DR/2000 direct-read spectrophotometer using a HACH COD reactor. The TOC results were determined by a Shimadzu TOC-5000 analyzer. The color was measured with a 25-mm-cell Lovibond tintometer following the Hong Kong Government Chemist standard. The Lovibond tintometer matches the sample color with the composition of the three primary colors: red, yellow, and blue. Each primary color can vary from 0 to 100 units. The summation of the three color units that match the sample color is taken as the color of the sample in the Lovibond unit. The experimental conditions tested for WAO and WPO are listed in Table 1. Wet air oxidation (WAO), catalytic wet air oxidation (CWAO), promoted wet air oxidation (PWAO), and wet peroxide oxidation (WPO) were evaluated with reactive blue (Basilen Brilliant Blue P-3R) used as the model dye pollutant. For CWAO, the catalyst used was Cu2+ ion in nitrate solution. For WPO, the catalyst investigated was Cu impregnated on activated carbon (AC-Cu). The concentrations of the catalysts are measured in terms of their Cu content. The use of copper nitrate as a homogeneous catalyst will lead to a secondary pollution problem and is included for the purpose of identifying an appropriate concentration of Cu to use and for comparison with the effectiveness of the use of Cu as a heterogeneous catalyst. The loss of Cu from the AC-Cu catalyst, if any, is minimal and does not impose a pollution problem. Solutions containing four more dyes were treated using WPO at 150 °C. Their color indices are listed in Table 2. The concentrated dye solutions are made from virgin dye products to simulate the concentrated stream from a membraneseparation process. All these dyes were obtained from local industries in Hong Kong. The hydrogen peroxide used was 30% (w/w) chemical pure grade.

Figure 2. Wet oxidations of reactive blue (Basilen Brilliant Blue P-3R).

Results and Analysis Search for an Effective Wet Oxidation Method. Among the wet oxidation methods, WAO is the simplest. It is a relatively cheap and well-established process for wastewater treatment.15 The oxygen partial pressure (PO2) was so chosen that the supply of oxygen is 15% higher than the stoichiometric amount needed for a complete removal of CODi, the COD of the fresh dye solution. Figure 2 shows that WAO (open circles) at 200 °C is not an efficient method for this particular situation. This inefficiency is due to the high initial COD loading and the low reaction rate at the specified conditions. To enhance the reaction rate, a catalyst was introduced; thus, the process becomes a catalytic wet air oxidation. Cu2+ ion has previously been shown to increase the degradation rate of low-concentration dyeing and printing wastewater.16 Although the COD removal (closed circles in Figure 2) is improved compared with WAO, the overall performance of CWAO is still not sufficient to make the process practicable. After 90 min of reaction, only 41% COD removal was achieved with a residual COD of over 16 000 mg/L. The color removal was insignificant with the Lovibond unit decreased by only 3%. The Lovibond unit after 90 min of reaction was 34.4, which is well above the proposed local discharge standard. An alternative for accelerating the reaction rate is to use a stronger oxidant. It has been reported that WAO can be promoted by adding a small amount of strong oxidant such as H2O2, a process referred to as promoted

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Figure 4. WPO of reactive blue, color change history.

Figure 3. WPO of reactive blue, effect of reaction temperature.

wet air oxidation.17 PWAO experiments were performed by adding H2O2. The amount of H2O2 used in this experiment was 10% of the stoichiometric requirement for the complete removal of COD. A lower temperature of 150 °C was used for PWAO because self-destruction of H2O2 has been observed when the temperature is too high.18 There was no significant difference in the results between PWAO and WAO. The ineffectiveness of PWAO was caused by two opposite effects: enhanced dye degradation due to H2O2 addition but reduced degradation due to the temperature drop from 200 to 150 °C. Thus, WAO, CWAO, and PWAO were found to be unsatisfactory for the present application. WPO, using H2O2 as the oxidant, was then investigated. Almost 80% TOC and COD removal and, most significantly, a 90% color removal were obtained in less than 20 min with the stoichiometric required amount of H2O2. Wet Peroxide Oxidation. The fast reaction rate of WPO as opposed to WAO is due to the following reasons. First, H2O2 is a stronger oxidant. Second, WPO does not require the transfer of oxygen from the gas phase to the liquid phase as required for WAO. The WPO oxidation mechanism may be considered to be similar to that of Fenton’s reaction, with OH•, HO2•, or ROO• radicals being the main oxidant depending on the pH of the solution.19,20 Effect of Temperature. Increasing the reaction temperature has two opposing effects. It can increase the oxidation rate in accordance with the Arrhenius equation, but too high a temperature will result in the destruction of H2O2 to produce O2 and H2O, an undesir-

able side reaction. Hence, a compromise is necessary to achieve an optimal extent and rate of reaction. Because excess Fe3+ resulting from the classical Fenton oxidation gives a dark brownish solution, the present WPO process was therefore conducted without the addition of a ferrous salt. Figure 3 shows the results of COD, TOC, and color removal at three reaction temperatures. The H2O2 used was the stoichiometric requirement for 100% COD removal. As expected, higher initial removal rates for TOC and COD were observed at higher temperatures. The final values of TOC or COD obtained at these temperatures were about 2000 and 5000 mg/ L, respectively. Longer reaction times did not further reduce the TOC and COD levels. This suggests that the remaining TOC contains materials that are recalcitrant to WPO. Since the presence of H2O2 affects the COD of the solution, the analysis of the dye degradation is better measured in terms of TOC removal. Figure 3 shows that 78% TOC was removed as the reaction reached its plateau value. The reaction times required were less than 30, 90, and more than 180 min at the temperatures 150, 130, and 110 °C, respectively. Although the color reduction as measured by the Lovibond unit follows a similar time scale to that of TOC, the rate of color change is quite different from that of TOC. Two regions of color change were observed: an initial phase with little discoloration followed by a rapid reduction of the Lovibond unit to close to zero. Although the Lovibond unit is a very useful indicator of the color of a solution, it should be noted that solutions with the same Lovibond unit may not represent the same color in visual appearance because the unit is the summation of the total color units in terms of the three primary colors. The blue component was seen to disappear at 60 min of reaction time, with strong red and yellow components remaining. This was followed by the disappearance of the red component, leaving only a slightly yellow component, which is also the visual appearance of the color of the treated solution. Effect of the Initial H2O2 Concentration. Figure 5 presents the results of the TOC, COD, and Lovibond unit change with time at four concentrations of H2O2. The volumes of H2O2 used correspond to 50%, 75%, 100%, and 150% of the stoichiometric amount for the complete removal of COD. The reaction temperature was chosen to be 130 °C. At this temperature, the reaction rates are sufficiently fast for industrial application and slow enough for the changing phenomena to be observed. The initial rate of TOC removal was found to increase with the dosage of H2O2. Then the reaction rate gradually levels off depending on the

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Figure 7. WPO of reactive blue, effect of dye concentration. Figure 5. WPO of reactive blue, effect of H2O2 dosage.

Figure 6. WPO of reactive blue, effect of H2O2 concentration on final TOC.

peroxide concentration. When the hydrogen peroxide dosage is 50% of the stoichiometric requirement for the complete removal of COD, the final TOC value is not significantly different from the initial value, although the solution COD has dropped by about 50%. This means that the consumption of the hydrogen peroxide does not immediately mineralize the organics in the dye solution. All of the organic carbons remain in the solution, but the newly formed organics have lower COD per mole. Figure 6 displays the relation between the final TOC value and the H2O2 dosage in terms of the percentage of the stoichiometric requirement of H2O2. It is interesting to note that the removal of TOC reached a plateau at 100% H2O2 dosage. A further increase of H2O2 did not lead to more TOC removal. This behavior

together with that observed for 50% dosage of H2O2 suggests that the oxidation process happened in more than one step. The first step involves the breakdown of the large dye molecules but does not mineralize them. Approximately 50% of the COD equivalent oxidant is consumed during this step. The next step is the degradation of the smaller molecules into carbon dioxide and water. At the end of the reaction, there are residual organics (mainly organic acids) that are very difficult to be oxidized further. Hence, there is a residual TOC that remains not removed. Figures 3 and 5 show that there is no significant change of Lovibond unit until 100% or more of the stoichiometric amount of H2O2 is supplied. Even for situations with a sufficient hydrogen peroxide supply, no significant color change is observed until after a certain percentage of TOC has been removed. This “threshold percentage” is close to the residual TOC value, suggesting that not until all the large molecules have been decomposed and oxidized will the color change significantly. It is not until most of the TOC has been removed before the color can completely disappear. Hence, there exists a threshold TOC removal value associated with the Lovibond unit change. This point will be further discussed in the next section in which the results with a varying initial TOC loading are analyzed. Effect of the Initial Dye Concentration. Figure 7 shows the effect of the initial dye concentration (C) on the removal of TOC and color. The hydrogen peroxide used is the amount equivalent to 100% of the stoichiometric amount for the complete removal of COD of the 50 g/L dye solution. For the comparison on a common basis, the relative change of TOC (TOC/TOCi) was

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Figure 8. Color-TOC removal relation.

Figure 9. WPO of reactive blue, pH effect.

plotted as a function of reaction time in Figure 7a, with TOCi being the TOC value of the original dye solution. For the runs of 12.5 and 25 g/L dye solutions, the removal of TOC and color was very fast. This is because of the excess H2O2 used for these low COD concentration solutions. Although the final TOC values vary from 370 to about 2000 mg/L, the final TOC removed is about 80% for all concentrations. The result reveals that the organic acids formed from the oxidation, with about 20% carbon involved, are very difficult to be further degraded by WPO, not even with four times the stoichiometric amount of H2O2 added. Similar to the results shown in Figure 3, the total TOC removal is also not significantly affected by reaction temperature. The variation of the Lovibond unit with reaction time follows a similar trend as observed in the two previous sections. Figure 8 shows the combined Lovibond-TOC removal data. It can be seen from Figure 8c that there is a definite relationship between Lovibond unit and TOC removal. The variations of Lovibond unit and TOC removal merge into essentially one curve. When the removal of TOC is below 50%, there is no significant change of the Lovibond unit. When more than 50% TOC is removed, there is a sharp drop in the Lovibond unit. When over 70% TOC is removed, the Lovibond unit drops almost to zero. This finding reinforces the twostep oxidation mechanism proposed in the previous section. The Lovibond-TOC relationship is believed to be not caused by the existence of small molecular diluents in the dye because a TOC removal rate decrease was not clearly observed accompanying the color change.

Effect of the Initial Solution pH. The stability of hydrogen peroxide is pH dependent. The least decomposition of H2O2 was found when the initial pH was in the region between 3 and 5.21 Figure 9 shows the results of TOC, COD, and color removal as a function of reaction time at three initial values of solution pH. There was no significant difference among the three solutions in terms of TOC removal until 30 min into the reaction when their respective plateaus were reached for initial values of solution pH of 3.5 and 7. The final TOC removals are 77% and 65% for solution pHs of 3.5 and 7, respectively. The solution with an initial pH of 8.8, however, continues to be degraded by WPO until 1 h later. The final value of the TOC removal for this solution is also 77%, the same as for the initial solution pH of 3.5. In addition to the difference in the initial pH, a solution with pH 8.8 was obtained from the solution of dye powder and deionized water, while the other two solutions underwent a pH adjustment with sodium hydroxide and hydrogen chloride solutions. For the fresh dye solutions so obtained, no visual difference was observed, however. There is a considerable difference between the solutions in which the pH was adjusted (pH of 3.5 and 7) and that in which the solution had no pH adjustment (pH ) 8.8) in the progress of oxidation. For the two cases with adjusted pH, their final values of COD were not as different as were the final TOC values. Similar COD but a higher TOC for an initial pH 7 solution means that the organic compounds formed from WPO are of lower COD. This suggests that there are fewer H

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Figure 10. WPO of reactive blue, catalyst effect.

elements associated with these organic carbons. That is to say that the product of WPO for a solution with an initial pH of 7 has more unsaturated carbon-carbon bonds than for the product from the solution of initial pH of 3.5. Probably for this reason do samples from an initial solution pH of 7 display a strong color at the end of the reaction. The gradual decrease of COD for the solution at pH 8.8 is similar to its TOC removal. The final COD value for this solution was reached after 120 min of reaction. The residual chemicals after WPO may be of the same property as those from solutions of pH ) 3.5 judging from the TOC value and Lovibond unit. It is evident that a solution with an initial pH of 3.5 gives the best results in terms of the reaction rate: 30 min for TOC or COD removal to reach their plateau values and 60 min for satisfactory color removal. In comparison, the same levels of TOC and color removal were achieved in 90 min for the solution without a pH adjustment. Effect of the Presence of a Catalyst. Although the WPO was relatively fast, the removal of 78% TOC and 90% color would take about 90 min at 130 °C without pH adjustment. The use of a catalyst to further enhance the reaction rate was studied. The catalyst used was copper impregnated on activated carbon. Details of the catalyst preparation have been reported elsewhere.22 Figure 10 shows a comparison between the results of WPO without catalyst and those of WPO with the catalyst. It can be seen that there is a considerable increase in the reaction rate by using the catalyst. In the presence of catalyst, it took only about 30 min for

Figure 11. WPO of reactive dyes. Table 3. Final Values of TOC and COD Removal and Reaction Time Required color

trade name

time, min

TOC, %

COD, %

blue red yellow brown black

Basilen Brilliant Blue P-3R Procion Red PX-4B Cibacron Yellow P-6GS Cibacron Brown P-6R Procion Black PX-2R

30 5 10 15 10

77 88 85 90 83

81 95 90 96 84

WPO to reach the limiting TOC level. The final values of TOC and COD were similar to those achieved before. WPO of Other Reactive Dyes. Four more reactive dyes were then treated with wet peroxide oxidation. Since only the performance of WPO was to be evaluated, a higher temperature of 150 °C was selected for the study. The dosages of H2O2 were 100% of the stoichiometric amount required to remove the COD of a 50 g/L dye solution. Figure 11 shows that WPO can satisfactorily treat Procion Red PX-4B, Cibacron Yellow P-6GS, and Cibacron Brown P-6R but not Procion Black PX2R without catalyst addition and pH adjustment. Among the dyes studied, reactive red (Procion Red PX-4B) is the easiest to treat. In less than 5 min, about 90% of TOC was reduced and the color dropped to below a Lovibond unit of 4 in 10 min. Table 3 lists the times that the final reaction plateaus were reached for each dye and the final values of TOC and COD removal. For Procion Black PX-2R, over 80% of COD is removed but there is still a heavy color remaining.

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Conclusions Wet air oxidation, catalytic wet air oxidation, and promoted wet oxidation are not satisfactory in treating the five types of high-concentration reactive dye studied. Wet peroxide oxidation, however, is found to be an efficient process in terms of the removal of total organic carbon and color. The wet peroxide oxidation rate is affected by reaction temperature, solution pH, and amount of hydrogen peroxide used as well as the presence of a catalyst. There exists a limiting value of TOC removal that is independent of the WPO conditions. The color change of the dye solution is closely related to the TOC removal. When less than 50% of TOC is removed, there is an insignificant change of color in the Lovibond unit. A sharp drop of color occurs when a threshold value of TOC has been removed, about 60% for reactive blue (Basilen Brilliant Blue P-3R). Such phenomena suggest that there are two steps involved in WPO: the breakdown of large molecules and the oxidation of the intermediates with a residual level of recalcitrant organics remaining after WPO treatment. WPO can quickly remove TOC from the dye solutions at reaction temperature of 150 °C even without using catalyst. It takes 5 min to remove 88% TOC in reactive red (Procion Red PX-4B) and 10 min to remove 85% and 83% TOC for reactive yellow (Cibacron Yellow P-6GS) and black (Procion Black PX-2R), respectively. For reactive blue (Basilen Brilliant Blue P-3R), 90% removal of TOC is achieved in 15 min, while 77% TOC removal is obtained in less than 30 min. WPO can remove colors from Procion Red PX-4B, Cibacron Yellow P-6GS, Cibacron Brown P-6R, and Basilen Brilliant Blue P-3R solutions but not from Procion Black PX-2R solution. Acknowledgment We acknowledge financial support from Link Dyeing Works Inc. Part of the sample analysis was carried out by Ms. K. Wang, to whom we are grateful. We are also grateful to Prof. Dr. Nikola Getoff of Austria for his valuable comments on the WPO reactions. Literature Cited (1) Bousher, A.; Shen, X. D.; Edyvean, R. G. J. Removal of coloured organic matter by adsorption onto low-cost waste materials. Water Res. 1997, 31 (8), 2084-2092. (2) Kuo, W. G. Decolorizing Dye Wastewater with Fenton’s Reagent. Water Res. 1992, 26 (7), 881-886. (3) Oguetveren, U. B.; Koparal, S. Color removal from textile effluents by electrochemical destruction. J. Environ. Sci., Health Part AsEnviron. Sci., Eng. 1994, 29 (1), 1-16. (4) Namboodri, C. G.; Perkins, W. S.; Walsh, W. K. Decolorizing dyes with chlorine and ozone, Part I, American Dyestuff Reporter. 1994, 83 (2), 17-22.

(5) Strickland, A. F.; Perkins, W. S. Decolorization of continuous dyeing wastewater by ozonation. Textile Chem. Color. 1995, 27 (5), 11-15. (6) Namboodri, C. G.; Walsh, W. K. Ultraviolet light/hydrogen peroxide system for decolorizing spent reactive dye bath wastewater. Am. Dyestuff Rep. 1996, 85 (3), 15-25. (7) Li, X. Z.; Zhang, M. Decolorization and biodegradability of dyeing wastewater treated by a TiO2-sensitized photooxidation process. Water Sci. Technol. 1996, 34 (9), pt 5, 49-55. (8) Banat, I. M.; Nigam, P.; Singh, D.; Marchant, R. Microbial decolorization of textile-dye-containing effluents: A Review. Bioresource Technol. 1996, 58 (3), 217-227. (9) Grady, C. P. L.; Lim, H. C. Biological Wastewater Treatment: Theory and Applications; Marcel Dekker: New York, 1980. (10) Ben Aim, R.; Liu, M. G.; Vigneswaran, S. Recent Development of Membrane Processes for Water and Wastewater Treatment. Water Sci. Technol. 1993, 27 (10), 141-149. (11) Copa, W. M.; Gitchel, W. B. Wet Oxidation. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 1989. (12) Lei, L.; Hu, X.; Chen, G.; Porter, J. F.; Yue, P. L. Wet Air Oxidation of Desizing Wastewater from Textile Industry. Proceedings of the Asia-Pacific Conference on Sustainable Energy an Environmental Technology, Singapore, June, 1996; pp 165-172. (13) Lei, L.; Chen, G.; Porter, J. F.; Yue, P. L. Treatment of PVA-containing Desizing Wastewater Promoted Wet Air Oxidation. The Third International Conference on Advanced Oxidation Technologies for Water and Air Remediation, Cincinnati, Oct 1996. (14) Yue, P. L. Advanced Water and Wastewater Treatment Technologies. Proceedings of the Asia-Pacific Conference on Sustainable Energy an Environmental Technology, Singapore, June 1996; pp 7-16. (15) Mishra, V. S.; Mahajani, V. V.; Joshi J. B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2-48. (16) Hu, X.; Lei, L.; Chu, C. P.; Yue, P. L. A Novel Catalyst for Wastewater Treatment. Asia-Pacific Chemical Reaction Engineering Forum, Beijing, June 1996. (17) Lei, L. Wet Oxidation of High Concentration Wastewater from Textile Industry. Ph.D. Dissertation, Department of Chemical Engineering, Zhejiang University, Hangzhou, China, 1996. (18) Striolo, P.; Debellefontaine, H.; Foussard, J. N.; BesombesVaillie, J. Wet Peroxide Oxidation: Aqueous Organic Wastes Treatment Using Hydrogen Peroxide at High Temperature. The Fourth World Congress of Chemical Engineering, Karlsruhe, Germany, 1991; pp 486-493. (19) Debellefontaine, H. Advanced Methods for Treatment of Organic Aqueous Wastes: Wet Air Oxidation and Wet Peroxide Oxidation. 5th International Symposium on Operating European Hazardous Waste Management Facilities, Odense, Denmark, Sept 1992. (20) Getoff, N. The University of Vienna, personal communication, 1998. (21) Bishop, D. F.; Stern, G.; Fleischman, M.; Marshall, L. S. Hydrogen Peroxide Catalytic Oxidation of Refractory Organics in Municipal Wastewaters. Ind. Eng. Chem. Process Des. Dev. 1968, 7 (1), 110-117. (22) Lei, L.; Hu, X.; Chu, C. P.; Chen, G.; Yue, P. L. Catalytic Wet Air Oxidation of Dyeing and Desizing Wastewater. J. Water Sci. Technol. 1997, 35 (4), 311-319.

Received for review September 28, 1998 Revised manuscript received January 29, 1999 Accepted February 7, 1999 IE980617D