Role of Water in the Photocatalytic Degradation of Trichloroethylene

Environ. Sci. Technol. 1997, 31, 1440-1445

Role of Water in the Photocatalytic Degradation of Trichloroethylene Vapor on TiO2 Films CHUNG-HSUANG HUNG AND BENITO J. MARIN ˜ AS* Department of Civil Engineering, University of Illinois, Urbana, Illinois 61801

The focus of this study was to investigate the role of water vapor in the photocatalytic degradation of trichloroethylene (TCE) on anatase titanium dioxide films immobilized on the surfaces of ring-roughened annular reactors. Experimental variables included TCE concentration (0.7-7 parts per million by volume or ppmv), oxygen concentration (13-120 000 ppmv), residence time (2.779.81 s), relative humidity (0-100%), and reactor length. TCE conversion was not affected by relative humidities up to 20%, but it deteriorated as the gas mixture approached saturation with respect to water vapor. Major intermediates and products from TCE degradation were the same as those previously reported for dry conditions: carbon tetrachloride, chloroform, hexachloroethane, pentachloroethane, and tetrachloroethylene. The formation rates for these compounds increased with increasing water vapor concentrations at relatively low humidities as a result of a stronger deteriorating rate effect of water vapor on atomic oxygen oxidation reactions as compared to that on competing chlorine atom attack reactions. The presence of moderate concentrations of water vapor resulted in greater conversions of chloroform and pentachloroethane as compared to dry conditions due to rate enhancement of hydrogen extraction reactions. In contrast, tetrachloroethylene conversions decreased with increasing humidity primarily because of a deteriorating rate effect of water vapor on chlorine extraction reactions.

Introduction The focus of a previous paper (1) was an investigation of the photocatalytic degradation of trichloroethylene (TCE) vapor on titanium dioxide (TiO2) films in the absence of water vapor. The formation of various chlorinated organic intermediates/ products, primarily carbon tetrachloride, chloroform, hexachloroethane, pentachloroethane, and tetrachloroethylene (PCE), was observed at relatively low oxygen concentrations. The formation of these compounds resulted from chlorine atom attack on trichloromethyl, dichloromethyl, pentachloroethyl, tetrachloroethyl, and trichlorovinyl radicals, respectively. TCE conversion at high oxygen concentrations including that of ambient air resulted in the formation of inorganic compounds, hydrochloric acid, and molecular chlorine as primary chlorine-containing products. The lower formation of chlorinated organic compounds at high oxygen levels was the result of reactions between atomic oxygen species and chlorinated radicals (except trichlorovinyl radical) competing out those involving chlorine atom attack. An * Corresponding author telephone: (217) 333-6961; fax (217) 3336968; e-mail address: [email protected].

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overall scheme including these reactions was proposed to describe predominant TCE decomposition and intermediates/products formation and decomposition pathways (1). Foreseen full-scale applications of heterogeneous photocatalysis include the control of hazardous air pollutants and other organic compounds present in indoor air and emissions from various industrial processes, landfills, hazardous waste sites, and groundwater/soil remediation facilities. A component not considered in the preceding study (1) but expected to be present at various levels in most of these target gas mixtures is water vapor. The effect of water vapor on the photocatalytic degradation efficiency of TCE on TiO2 has been reported by various research groups (2-10). A summary of these studies was presented in a previous paper (1). In general, intermediates and products observed were the same in the presence and absence of water, i.e., carbon dioxide and carbon monoxide, dichloroacetyl chloride (DCAC), hydrochloric acid, phosgene, and molecular chlorine. Unfortunately, most experiments reporting intermediates/ products were performed at relatively high oxygen concentrations, representative of ambient air at which the formation of chlorinated compounds would be very small. The objectives of this second study were to investigate the role of water vapor in the photocatalytic degradation of TCE, study the formation and decomposition of associated chlorinated organic intermediates and products, and ascertain predominant reaction pathways for a wide range of oxygen and water concentrations.

Experimental Section A schematic of the experimental apparatus used for this study is presented in Figure 1. The unit included gas mixture preparation components, photocatalytic reactor, analytical system, and off-gas treatment processes. Most of these components as well as experimental procedures followed, and analytical methods used were described previously (1). The apparatus was slightly modified to include the water vapor feeding system shown in Figure 1. Target water vapor concentrations corresponding to relative humidities in the range of 0-100% were achieved by passing a fraction of the nitrogen gas used through two gas-washing bottles connected in series, each containing approximately 300 mL of distilled water prior to adding the organic compound. The water vapor concentration was varied by trial and error adjustment of a needle valve located at the inlet of a Riteflow rotameter (Manostat, New York, NY). Gas mixtures with nearly 100% relative humidity were prepared by passing the entire nitrogen/oxygen gas mixture flow through the bottles. A psychrometer Model DTH1 (Davis Instruments, Baltimore, MD) was used to monitor relative humidities greater than 20%. Lower water vapor contents were estimated from humidified gas flow to total flow rate ratios assuming that the fraction passed through the gas-washing bottles was nearly saturated with water vapor. TCE degradation experiments were performed at reactor inlet TCE concentrations ranging from 0.7 to 7 ppmv, oxygen content from 13 to 120 000 ppmv, and relative humidities ranging from 0 to 100%. The concentration of water vapor corresponding to saturated gas mixtures ranged from 23 700 to 39 100 ppmv corresponding to the experimental ambient temperature range of 20.4-28.8 °C. Experiments were performed with both long and short ring-roughened annular photocatalytic reactors described previously (1). These two reactors were identical with the only difference that the length of the annular section coated with TiO2 in the short reactor was 40% of that in the long reactor. The overall gas flow rate ranged from 11.5 to 13.1 mL/min corresponding to residence

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FIGURE 1. Schematic of experimental apparatus. times ranging from 6.92 to 7.92 s for the long reactor and from 2.77 to 3.17 for the short reactor. The role of water vapor in the decomposition of major TCE degradation intermediates/products was investigated by performing experiments with the long photocatalytic reactor at a constant oxygen concentration of 68 000-74 000 ppmv. Reactor inlet concentrations investigated were 0.010.1 ppmv for carbon tetrachloride, 0.9 ppmv for chloroform, 0.026-0.031 ppmv for pentachloroethane, and 0.12-0.14 ppmv for PCE. Relative humidities investigated ranged from 0 to 82%. The gas flow rate was kept constant at 9.48 L/min corresponding to a residence time of 9.57 s inside the reactor. An additional experimental set was performed to assess the effect of water vapor on the formation of hydrogen chloride (HCl) and molecular chlorine (Cl2) vapors. Experiments were performed with the small photocatalytic reactor operated at a flow rate of 3.70 L/min corresponding to a residence time of 9.81 s. Gas mixtures tested contained 7.84-8.45 ppmv TCE, and 11 900-131 000 ppmv oxygen. The relative humidities investigated ranged from 0 to 39%. Experimental procedures followed, and analytical methods used were identical to those described previously (1) for experiments performed in the absence of water vapor.

Results Representative experimental results comparing the effect of varying oxygen concentration in the absence of water vapor and at a relative humidity (RH) of 12% are presented in Figures 2 and 3, respectively. These experiments were performed with the short ring-roughened photocatalytic reactor operated at a flow rate of 13.1 L/min corresponding to a residence time of 2.77 s and a reactor inlet TCE concentration of 3.92-4.10 ppmv. Quantifiable intermediates/products formed from TCE degradation were the same in the absence (Figure 2) and presence (Figure 3) of water vapor and included carbon tetrachloride (CCl4), chloroform (CHCl3), hexachloroethane (C2Cl6), pentachloroethane (C2HCl5), and PCE (C2Cl4). TCE conversion efficiencies were comparable for the two experimental sets. On the other hand, some discrepancies were observed in the formation of intermediates/products. Chloroform concentrations were generally lower in the presence of water vapor at 12% RH. In contrast, the concentrations of the other four intermediates/products were generally higher in the presence of water vapor. The most striking difference was that for hexachloroethane, which formed at concentrations of about 50 times greater at 12% RH as compared to dry conditions. Notice that the peaks for maximum formation of fully chlorinated saturated compounds, carbon tetrachloride and hexachloroethane were achieved at somewhat greater oxygen concentrations as compared to those for the three intermediates: chloroform, PCE, and pentachloroethane. These trends were also generally observed for experiments

FIGURE 2. Photocatalytic degradation of TCE vapor and formation of reaction intermediates and products in short ring-roughened reactor as a function of oxygen concentration (reactor inlet TCE concentration, 4.10 ppmv; gas flow rate, 13.1 L/min; gas residence time, 2.77 s; no humidity; temperature, 24.3-25.7 °C). performed in the absence of water vapor (1). The maximum fractions of TCE-Cl converted found in chlorinated organic compounds were 32 and 45% for the dry and humid experimental sets, respectively, both achieved at the relatively low oxygen concentration of about 200-300 ppmv. Representative results corresponding to a set of experiments performed to investigate the effect of increasing relative humidity on TCE degradation and intermediates/products formation at constant oxygen concentration are presented in Figure 4. Experiments were carried out with the short photocatalytic reactor operated at a gas flow rate of 11.5 L/min corresponding to a residence time of 3.17 s and reactor inlet TCE and oxygen concentrations of 1.76 and 1,970 ppmv, respectively. TCE conversion appeared to decrease with increasing water content within the range investigated. This observation was consistent with experimental results reported by others (2, 3, 6). Figure 5 depicts the results for an experimental set performed with the long photocatalytic reactor under approximately the same conditions as those used for the short reactor set of Figure 4. The flow rate of 11.5 L/min corresponded to a residence time of 7.92 s for the longer reactor. Results presented in Figure 4 would correspond to the performance of the long reactor at a distance from the inlet of approximately 40% of its total length. The conversion of TCE in the long reactor was approximately constant at 97-98% for relative humidities up to approximately 20% and deteriorated gradually with increasing greater water vapor contents down to 9% conversion at nearly 100% RH. A comparison of the top plots in Figures 4 and 5 revealed an increase in the overall TCE degradation rate downstream within the reactor. Thus, it appeared that chlorine atom attack, responsible for greater overall TCE degradation

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FIGURE 3. Photocatalytic degradation of TCE vapor and formation of reaction intermediates and products in short ring-roughened reactor as a function of oxygen concentration (reactor inlet TCE concentration, 3.92 ppmv; gas flow rate, 13.1 L/min; gas residence time, 2.77 s; relative humidity, 12%; temperature, 26.5-27.7 °C). downstream within the reactor in the absence of humidity (1), had been taking place also in the presence of water vapor. Results presented in the lower portions of Figures 4 and 5 for reaction intermediates/products revealed that chloroform concentration decreased gradually with increasing water content. Net chloroform formation was comparable for both reactors at relatively humidity levels lower than 20% and greater for the long reactor at relative humidities greater than 20%. In contrast, the concentrations of the other four intermediates/products increased initially with increasing relative humidity, reached maxima at relative humidities of about 6-12% for the short reactor and 20-40% for the long reactor, and decreased at subsequently greater water vapor concentrations. Most of the carbon tetrachloride observed at relative humidity levels up to about 10% appeared to have formed primarily in the upstream portion of the long reactor and mostly in the downstream half at greater water vapor contents. A major fraction of the PCE, pentachloroethane, and hexachloroethane formed in the upstream portion of the long reactor at relative humidities in the range of 0-20% appeared to decompose downstream. PCE and hexachloroethane observed at relative humidities greater than 20% appeared to have formed in the downstream half of the long reactor. Pentachloroethane concentrations at the long reactor outlet were lower than 0.01 ppmv for the entire relative humidity range investigated. Results corresponding to the three separate experimental sets performed to investigate the effect of water vapor on the photocatalytic degradation of intermediates/products are presented in Figure 6. Water vapor appeared to have very different effects on the decomposition of different intermediates/products and corresponding product formation. Carbon tetrachloride did not decompose in the presence or absence

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FIGURE 4. Photocatalytic degradation of TCE vapor and formation of reaction intermediates and products in short ring-roughened reactor as a function of relative humidity (reactor inlet TCE concentration, 1.76 ppmv; oxygen concentration, 1,970 ppmv; gas flow rate, 11.5 L/min; gas residence time, 3.17 s; temperature, 27.327.7 °C). of water vapor within the range of experimental conditions investigated. Chloroform conversion increased by a factor of nearly 5 times at relative humidities of 6-10% when compared to dry conditions. Although conversion decreased at subsequently greater water vapor content, it was approximately double compared to that for dry conditions at the maximum relative humidity investigated of 53%. Carbon tetrachloride was the only chlorinated organic product observed from the decomposition of chloroform. Trends in carbon tetrachloride formation with increasing water vapor content paralleled that for chloroform conversion. It is interesting to notice that the fraction of carbon tetrachloride formed per chloroform converted remained approximately constant for the entire relative humidity range investigated. PCE conversion efficiency decreased with increasing water vapor content, most of the performance deterioration taking place within the relative humidity range of 10-30%. No chlorinated organic products were observed possibly because of the relatively low PCE concentration tested as discussed in a previous paper (1). Pentachloroethane conversion was the least affected by the presence of water vapor. The conversion efficiency of this intermediate compound appeared to increase from 95% in the absence of water to 98% at relative humidities of 10%. No major deterioration in degradation efficiency was observed with subsequent increases in water vapor content until the relative humidity exceeded 60%. Pentachloroethane conversions were somewhat paralleled by the formation of the two chlorinated organic products, carbon tetrachloride and hexachloroethane. Reaction intermediate/product concentrations and corresponding chlorine and hydrogen mass balances obtained

FIGURE 5. Photocatalytic degradation of TCE vapor and formation of reaction intermediates and products in long ring-roughened reactor as a function of relative humidity (reactor inlet TCE concentration, 1.84 ppmv; oxygen concentration, 2,160 ppmv; gas flow rate, 11.5 L/min; gas residence time, 7.92 s; temperature, 23.7-26.4 °C). for four experiments performed with the short reactor at relative humidities ranging from 0 to 31% are presented in Table 1. Reactor inlet TCE and oxygen concentrations were approximately constant at 7.84-8.38 and 11 900-13 000 ppmv, respectively, and the gas flow rate was 3.70 L/min corresponding to a residence time of 9.81 s. Consistent with previous observations, TCE conversion decreased from 93.4 to 52.7% with increasing relative humidity. The fraction of chlorine orginally present in TCE converted recovered in the inorganic and organic intermediates/products ranged from 77 to 89% for relative humidities up to 22%. This range was comparable to that of 76-80% found for experiments performed with varying oxygen concentrations under dry conditions (1). Only 31% of the TCE-Cl converted was recovered at the highest relative humidity of 31%. The corresponding hydrogen mass balances presented in Table 1 were comparable, with hydrogen recoveries ranging from 66 to 94% for relative humidities up to 22%, possibly decreasing with increasing water vapor content within this range. Parallel to the finding for Cl, only 29% of the hydrogen was recovered at the highest relative humidity of 31%, suggesting that the missing chlorine might have resulted from experimental problems such as an increase in stainless steel tubing corrosion rate and corresponding greater HCl consumption in the presence of humidity. Losses of HCl at the higher water vapor concentrations investigated is also consistent with the fact that the [Cl2]/[HCl] ratios corresponding to relative humidities of 22 and 31% and 1.18 and 1.10%, respectively, exceeded the theoretical maximum of unity assuming that all of the chlorine and hydrogen present in intermediates and products originated from TCE. The experimental results presented in Figures 2-6 are generally consistent with the overall scheme of predominant

FIGURE 6. Photocatalytic degradation of chloroform (reactor inlet concentration, 0.886 ppmv), pentachloroethane (reactor inlet concentration, 0.0285 ppmv), and PCE (reactor inlet concentration, 0.129 ppmv) vapors and formation of corresponding reaction intermediates and products in long ring-roughened reactor as a function of relative humidity (oxygen concentration, 67 600-73 700 ppmv; gas flow rate, 9.48 L/min; gas residence time, 9.57 s).

TABLE 1. Reaction Intermediate/Product and Corresponding Chlorine and Hydrogen Concentrations (in ppmv) and TCE Conversions (in %) for Experiments Performed at Varying Relative Humidity with Short Ring-Roughened Reactora relative humidity (%)

CHCl3 C2HCl5 CCl4 C2Cl6 C2Cl4 Cl2 HCl total Cl total H TCE-Cl converted TCE-H converted TCE conversion (%)

0

14

22

31

0.533 0.317 0.009 0.006 0.002 3.96 5.69 16.9 6.54 22.0 7.32 93.4

0.105 0.186 0.015 0.045 0.008 5.69 6.60 19.6 6.89 22.1 7.37 87.9

0.115 0.126 0.016 0.046 0.008 4.48 3.79 14.1 4.03 18.2 6.07 75.1

0.021 0.018 0.001 0.001 0.002 1.33 1.21 4.03 1.25 12.9 4.30 52.7

a Reactor Inlet TCE concentration, 7.84-8.38 ppmv; oxygen concentration, 11 900-13 000 ppmv; gas flow rate, 3.70 L/min; gas residence time, 9.81 s.

reactions proposed in a previous paper (1). However, water vapor appeared to affect the various types of reactions involved in different ways. For instance, the presence of water vapor at relatively low levels appeared to increase the rate of the three major hydrogen extraction reactions part of the overall reaction scheme [Reaction numbers are consistent with those used in a previous paper (1).]:

C2HCl3 f C2Cl3• + H

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(2)

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C2HCl5 f C2Cl5• + H

(27)

CHCl3 f CCl3• + H

(28)

The enhancement of these reactions would be consistent with the greater conversions of chloroform and pentachloroethane observed and the greater production of PCE from higher concentration of trichlorovinyl radical resulting from enhanced reaction 2. The lack of TCE conversion increase in the presence of water vapor could be explained by the fact that reaction 2 was not the predominant pathway for TCE decomposition. Water molecules are known to undergo chemisorption on the photocatalyst reacting with surface species including oxygen and forming chemisorbed hydroxide, which then trap photoinduced holes forming hydroxyl radicals. Though hydrogen extraction reactions are known to proceed both by direct hole and hydroxyl radical attack (11, 12), the higher fraction of holes occupied by hydroxyl radicals at relatively higher water vapor concentrations would be consistent with faster rates of hydrogen extraction by hydroxyl radicals. The deterioration in the degradation rate of chloroform and pentachloroethane at higher water vapor contents could have been the result of hydrogen extraction being inhibited by the formation of a diffusion barrier created by increasing physical adsorption of water vapor molecules as water approaches saturation in the gas phase. The greater levels of chlorinated organic intermediates/ products observed with increasing water vapor concentrations at relatively low humidity levels could be explained by the presence of water having a stronger deteriorating effect on the rates of the reactions between chlorinated methyl and ethyl radicals and atomic oxygen species:

C2HCl4• + O f C2HCl4O•

(6)

CHCl2• + O f COHCl + Cl

(10)

C2Cl5• + O f C2Cl5O•

(20)

CCl3 + O f COCl2 + Cl

(25)

•

as compared to the effect on the rates of competing chlorine atom attack reactions:

C2HCl4• + Cl f C2HCl5

(15)

CHCl2• + Cl f CHCl3

(9)

C2Cl5• + Cl f C2Cl6

(21)

CCl3• + Cl f CCl4

(24)

was not affected much in the presence of water vapor levels at which greater formation of chlorinated organic intermediates/products were observed. Another group of reactions that appeared to have been affected by the presence of water vapor was that involving chlorine extraction. This type of reaction was responsible for the initial production of chlorine atoms from TCE and PCE:

C2HCl3 f C2HCl2• + Cl

(1)

C2Cl4 f C2Cl3• + Cl

(29)

and the formation of chlorinated acetyl chlorides from chlorinated ethoxy radical intermediates:

C2HCl4O• f C2HCl3O + Cl

(7)

C2Cl5O• f C2Cl4O + Cl

(22)

Decreasing rates for reactions 1 and 29 would be consistent with the deterioration observed for the conversions of both TCE and PCE (see Figures 4-5 and 6, respectively) with increasing water vapor concentration. The lesser deteriorating effect on TCE conversion as compared to that for PCE could be due to the fact that a decreasing effect on TCE conversion due to slower chlorine extraction rates (reaction 1) could have been counteracted somewhat by an increasing effect due to higher hydrogen extraction rates (reaction 2). Decreases in DCAC formed per TCE converted with increasing water vapor concentration reported by Jacoby et al. (6) would also be consistent with a decreasing rate for reaction 7. The deteriorating effect of increasing water vapor on the rates of chlorine extraction reactions could be explained by competition between OH and Cl groups to react with sites on the photocatalyst surface (14).

Acknowledgments The authors would like to thank Dr. Loring F. Nies, School of Civil Engineering, Purdue University, for fruitful discussions on halogenation/dehalogenation reactions; Dr. Ko-Ming Wang for assistance during the reactor design phase of this study; and Mr. W. A. Haak, Glass Shop, Physics Department, Purdue University, for reactor construction. Financial support from the Showalter Trust and the Purdue University Physical Plant is gratefully acknowledged.

Literature Cited

The rate of the reactions with atomic oxygen species could have decreased due to water molecules competing with oxygen for photoinduced electrons (12) and the corresponding lower formation of atomic oxygen species or by direct reaction of water with atomic oxygen (13) to form less reactive species on the photocatalyst surface. Another possibility is a reaction between hydroxyl radicals and atomic oxygen species with a corresponding recombination of released holes and electrons (13). Evidence supporting that at least some of the reactions involving chlorine atom attack were not affected much by the presence of water vapor at relatively low levels was that the overall conversion of TCE predominantly due to the reaction

C2HCl3 + Cl f C2HCl4•

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(5)

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(1) Hung, C.-H.; Marin ˜ as, B. J. Environ. Sci. Technol. 1997, 31, 562568. (2) Dibble, L. A.; Raupp, G. B. Catal. Lett. 1990, 4, 345. (3) Dibble, L. A.; Raupp, G. B. Environ. Sci. Technol. 1992, 26, 492. (4) Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. Environ. Sci. Technol. 1993, 27, 732. (5) Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, Eds.; Elsevier Science: Amsterdam, The Netherlands, 1993; pp 387-392. (6) Jacoby, W. A.; Nimlos, M. R.; Blake, D. M.; Noble, R. D.; Koval, C. A. Environ. Sci. Technol. 1994, 28, 1661. (7) Anderson, M. A.; Yamazaki-Nishida, S.; Cervera-March, S. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, Eds.; Elsevier Science: Amsterdam, The Netherlands, 1993; pp 405-420. (8) Yamazaki-Nishida, S.; Nagano, K. J.; Phillips, L. A.; Cervera-March, S.; Anderson, M. A. J. Photochem. Photobiol. A: Chem. 1993, 70, 95. (9) Berman, E.; Dong, J. In The Third International Symposium on Chemical Oxidation: Technology for the Nineties, Vanderbilt University, Nashville, Tennessee; Eckenfelder, W. W., Bowers, A. R., Roth, J. A., Eds.; Technomic Publishing: Lancaster, PA, 1993; pp 183-189.

(10) Lichtin, N. N.; Avudaithai, M. Environ. Sci. Technol. 1996, 30, 2014. (11) Mao, Y.; Scho¨neich, C.; Asmus, K.-D. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, Eds.; Elsevier Science: Amsterdam, The Netherlands, 1993; pp 4966. (12) Schwitzgebel, J.; Ekerdt, J. G.; Gerischer, H.; Heller, A. J. Phys. Chem. 1995, 99, 5633. (13) Bickley, R. I.; Stone, F. S. J. Catal. 1973, 31, 389.

(14) Primet, M.; Basset, J.; Mathieu, M. V.; Prettre, M. J. Phys. Chem. 1970, 74, 2868.

Received for review August 8, 1996. Revised manuscript received December 31, 1996. Accepted January 14, 1997.X ES960685W X

Abstract published in Advance ACS Abstracts, March 15, 1997.

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