Environ. Sci. Technol. 2004, 38, 6685-6693
UV Photolysis of Trichloroethylene: Product Study and Kinetic Modeling K E L I , † M I H A E L A I . S T E F A N , * ,‡ A N D JOHN C. CRITTENDEN† Department of Civil and Environmental Engineering, Arizona State University, Tempe, Arizona 85281, and Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7
Direct UV photolysis of trichloroethylene (TCE) in dilute aqueous solution generated chloride ions as a major end product and several reaction intermediates, such as formic acid, di- and monochloroacetic acids, glyoxylic acid, and, to a lesser extent, mono- and dichloroacetylene, formaldehyde, dichloroacetaldehyde, and oxalic acid. Under prolonged irradiation, these byproducts underwent photolysis, and a high degree of mineralization (∼95%) was achieved. TCE decays through the following major pathways: (1) TCE + hν f ClCHdC•Cl + Cl•; (2) TCE (H2O) + hν f ClCH(OH)sCHCl2; (3) TCE + hν f HCtCCl + Cl2; (4) TCE + hν f ClCtCCl + HCl; (5) TCE + Cl• f Cl2HCsC•Cl2. A kinetic model was developed to simulate the destruction of TCE and the formation and fate of byproducts in aqueous solution under irradiation with polychromatic light. By fitting the experimental data, the quantum yields for the four photolysis steps were predicted as φ(1) ) 0.13, φ(2) ) 0.1, φ(3) ) 0.032, and φ(4) ) 0.092, respectively. The reaction mechanism proposed for the photodegradation of TCE accounts for all intermediates that were detected. The agreement between the computed and experimental patterns of TCE and reaction products is satisfactory given the complexity of the reaction mechanism and the lack of photolytic kinetic parameters that are provided in the literature.
Introduction Chlorinated hydrocarbons, such as trichloroethylene (TCE) and tetrachloroethylene (PCE) are produced in the order of 106 t yearly worldwide given their extensive use as industrial solvents (1). Trichloroethylene (TCE) is the most common and abundant pollutant in the United States (2) as a result of leaks from underground storage tanks and improper disposable practices. This widespread contamination is of concern because TCE has been shown to cause liver damage and kidney failure and is a proven carcinogenic compound to animals (3). TCE is one of the priority pollutants listed by the U.S. Environmental Protection Agency, and the maximum contaminant level for drinking water is 5 µg/L. Conventional treatment technologies such as air stripping and adsorption on activated carbon are effective in removing TCE from contaminated waters, but TCE is transferred to another phase, and the residuals still contain TCE (4, 5). On * Corresponding author present address: Trojan Technologies, Inc., 3020 Gore Rd, London, ON, Canada N5V 4T7; telephone: (519)457-3400; fax: (519)457-3030; e-mail:
[email protected]. † Arizona State University. ‡ The University of Western Ontario. 10.1021/es040304b CCC: $27.50 Published on Web 11/16/2004
2004 American Chemical Society
the other hand, advanced oxidation processes (AOPs) may completely destroy hazardous organics such as TCE and, consequently, can be considered as viable alternatives to the traditional treatment technologies. AOPs produce highly reactive species such as hydroxyl radicals, which react rapidly with electron-rich organics initiating oxidative reactions. Under certain conditions, AOPs could lead to complete mineralization of parent pollutant with mineral acids, carbon dioxide, and water as final products. Some AOPs are lightassisted processes, where either the parent pollutant or the oxidant added to the system (such as hydrogen peroxide) absorbs the UV radiation initiating oxidation processes. Trichloroethylene degrades by both direct UV photolysis and • OH radical-induced oxidation, with different reaction rates and degradation pathways. TCE is a strong absorber of the UV light below 240 nm, and the direct photolysis has proven to be effective in both gas and aqueous phases (6-12). The reactive chlorine atoms (Cl•) generated through the TCE photolysis can promote the degradation of other co-contaminants with lower rates of light absorption (6). Gas-phase photolysis of TCE has been extensively studied (refs 6-10 and 12 and references therein) both in a vacuum UV (∼10-200 nm) (12) and under monochromatic irradiation in the UV-C range at 222 (9), 230 (6-8), and 254 nm (10). The major primary byproduct in the gas phase was determined to be dichloroacetyl chloride, which is more toxic and significantly less degradable than TCE (7). Other byproducts such as phosgene, trichloroacetyl chloride, dichloroacetic acid, trichloroacetyl chloride, chloroform, and methylene chloride were also detected. A primary quantum yield of 0.4 was reported (7). Aqueous-phase photolysis has not received nearly as much attention as gasphase photolysis. The direct UV photolysis of TCE was examined by Sundstrom et al. (4) as a means of determining the increase in efficiency that occurs when H2O2 is present under the same conditions. They found that when TCE was exposed to UV light in the absence of H2O2, only 80% mineralization of a 58 ppm TCE solution occurred within 40 min. Hirvonen et al. (13) studied the removal efficiency of TCE with direct UV photolysis using a low-pressure mercury vapor lamp with 0.8 W/L of radiation at λ ) 254 nm and found it as only 20% of that observed for the UV/H2O2 process. Some studies have focused on the formation of byproducts during TCE photolysis, as some of them could themselves be potentially harmful. In such studies, Hirvonen et al. (14), detected di- and monochloroacetic acids as intermediates generated from aqueous solutions of TCE treated by either direct photolysis and UV/H2O2, although the direct UV photolysis yielded considerably higher concentrations of these acids in comparison to the UV/H2O2 process. The haloacetic acids are also known as disinfection byproducts (DBPs); therefore, they are strictly monitored during the drinking water treatment as their total level should not exceed the regulatory standards set for drinking water quality (e.g., 60 µg/L in the United States). Chu and Choy (15) studied the photodegradation of TCE in surfactant micelles in a merry-go-round photoreactor at 254 nm and observed a 3× enhancement of TCE decay rate due to the addition of surfactant. No chlorinated byproducts were observed as the photolysis occurred through TCE dechlorination. The rate increased at high pH values. In a recent study, Taku and Tanaka (11) investigated the effect of natural organic matter on the photolysis rate of 100 mg/L TCE irradiated in a flow-through system at 254 nm. Distilled water, river water, and groundwater were used in these VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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experiments, and rate constants of 0.090, 0.037, and 0.009 min-1 were measured, respectively. The authors reported an apparent quantum yield of 10.54 for TCE photolysis in distilled water. Most of the studies cited above concern with the process efficiency for TCE removal from various water qualities rather than the mechanistic pathways and degradation byproducts. The objective of this study was to investigate the degradation mechanism and to identify the possible harmful byproducts in the process of TCE photolysis in aqueous solution exposed to a broadband of UV light (200-300 nm) emitted by mediumpressure Hg lamps. Also, based on the product study, a reaction mechanism and a kinetic model were developed that allowed the determination of kinetic parameters such as quantum yields and reaction rate constants. The kinetic model is useful in determining the feasibility of using UV photolysis process for TCE removal from contaminated waters.
Experimental Section Reagents. Trichloroethylene (Fisher Scientific, reagent grade), dichloroacetic acid (Anachemia, 99+%), monochloroacetic acid (Aldrich, 99%), formic acid (Sigma, 99%), glyoxylic acid (Sigma, 98%), oxalic acid (Fisher Scientific, technical grade), and sodium tetraborate (Na2B4O7‚10H2O, Caledon, 99% analytical reagent grade) were used as received. 2,4-Dinitrophenylhydrazine (Eastman, >98%) and O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) (Aldrich, 98+%) were used as received for the derivatization of carbonyl compounds. Apparatus. The UV irradiation experiments were carried out in a stainless steel 1 kW bench-scale Rayox reactor (manufactured by Calgon Carbon Corporation), which was described in detail elsewhere (16). The photon flow entering the reactor from the 1 kW medium-pressure Hg lamp was 2.18 × 10-4 einstein s-1 in the wavelength range 200-300 nm, as determined by potassium persulfate actinometry (17). The fraction of UV light absorbed by TCE and its degradation byproducts within the 200-300 nm range was calculated individually based on the photon flow, water absorption spectrum, and molar absorption coefficients of each compound, accounting for a mean effective reactor path length of 10.4 cm. For additional information, please see Table 1 in the Supporting Information. The total reaction volume was 28 L in all experiments, and measures were taken to ensure that TCE did not volatilize from the reactor. The dissolved oxygen level was monitored during the irradiation with an ATI Orion model 840 DO meter. Analytical Methods. A HP 6890 series gas chromatograph equipped with an autosampler and FID was used to determine the concentrations of TCE and other volatile organic intermediates. The same gas chromatograph equipped with an electron capture detector (ECD) was used for the analysis of carbonyl compounds as derivatized with PHBFA. Monoand dichloroacetylenes were identified with a Varian 3400 gas chromatograph with a model 8230 Finnigan MAT mass spectrometer, using the headspace solid-phase microextraction (SPME) technique. The GC/MS technique was used to identify the pentafluorobenzyloxime (PFBOA) derivatives of carbonyl compounds in hexane extracts. The concentrations of chloride ions and organic acids were determined by ion chromatography (Dionex DX-100 ion chromatograph, equipped with a Dionex anion self-regenerating suppressor). Some aldehydes (formaldehyde, monochloroacetaldehyde, and glyoxal) were also determined as hydrazones using a Waters 510 HPLC system connected to a Waters 486 tunable detector. The reproducibility of the measurement of IC and GC was about 2∼3%. The detection limit is about 1 × 10-6 mol L-1. 6686
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The UV-visible spectrophotometry (HP 8450 diode array spectrophotometer) was used to obtain the absorption spectra of TCE and organic acids at known concentrations, from which the molar absorption coefficients were calculated. Additional information on the analytical methods and TCE and its byproducts characteristics can be found in the Supporting Information.
Results and Discussion Product Study. A typical experiment for the degradation of TCE and its intermediates in dilute aqueous solution through direct UV photolysis is discussed below. The starting concentration of TCE was ∼1.1 mM, which initially absorbs 42.6% of the light (200-300 nm) entering the reactor. The molar absorption coefficients of TCE and organic acids, averaged over 5 nm narrow bands, are given in Table 1 in the Supporting Information. TCE decayed by an order of magnitude in about 10 min of irradiation, indicating large quantum yield for the UV photolysis. Figure 1a-d shows the time profiles of TCE and its degradation products. The examination of TCE decay and growth and decay of its byproducts indicates that mono- and dichloroacetylene (Figure 1a), formaldehyde and mono- and dichloroacetaldehyde (Figure 1b), chloride ions and formic, mono-, and dichloroacetic acids (Figure 1c) are formed at early irradiation stage, whereas the low levels of glyoxal and the other organic acids, such as glyoxylic and oxalic acids (Figure 1d), are mainly generated at longer exposure times. Along with these identified intermediates, some small, undetected levels of 1,2-dichloroethylene and acetic acid can be predicted mechanistically. The dynamics of the irradiated system suggests that chloride ion and formic acid were continuously generated; therefore, they are among the degradation products of the TCE early-stage intermediates. A total organic carbon mass balance was conducted and indicated that the all the major intermediates were identified and properly quantified. After 70 min irradiation time more than 95% of the initial TOC was mineralized. The chlorine balance at the end of irradiation (70 min) indicates approximately 10% less chlorine than the original level (∼3.3 mM, t ) 0 min). This may be accounted for by the molecular chlorine, which should be among the inorganic species formed during the mineralization of chlorinated organic compounds in this system, as predicted by the reaction mechanism. In water, low levels of molecular chlorine dissolve leading to hypochlorous acid (HOCl, pKa ) 7.7). No attempt was made to measure HOCl, but it is expected in the solution from early stages of irradiation. Hypochlorous acid is not a strong absorber of UV light, particularly at low concentrations. Its photolysis generates reactive hydroxyl radicals and chlorine atoms, which may be further involved in oxidative processes. As the organic and hydrochloric acids are formed, the solution pH drops sharply from 6.2 (t ) 0 min) to 2.6 (t ) 10 min), then much slower and levels off to ∼2.4 by the end of irradiation. The mineralization of the organic carbon is associated with the depletion of the dissolved molecular oxygen in solution. The pH and oxygen patterns during the irradiation are given in Figure 2. Reaction Mechanism for TCE Decay and Byproduct Formation. As a result of light absorption, trichloroethylene decays in aqueous solution by four major processes: (i) homolytic cleavage of a C-Cl bond, (ii) photo-stimulated hydrolysis, (iii) loss of molecular chlorine, and (iv) loss of HCl. A radical reaction involving the chlorine atom attack at the CdC bond is an additional path of TCE decay. The homolytic cleavage of C-Cl bond generates a carboncentered radical and chlorine radical:
ClHCdCCl2 + hν f ClHCdC•Cl + Cl•
(1)
FIGURE 1. Time profiles for the UV photolysis of TCE and its intermediates: (a) TCE and chloroacetylenes; (b) aldehydes; (c) chloride ion, MCA, DCA, and formic acid; (d) glyoxal, glyoxylic acid, and oxalic acid. (Solid lines represent the computer modeling patterns.) The high reactivity of C-centered radical toward dissolved molecular oxygen (k ∼ 109 M-1 s-1) leads to peroxyl radicals. In the aqueous phase, the peroxyl radicals generally decay through bimolecular self- or cross-termination reactions to a tetroxide structure, which further decomposes to either a stable compound such as aldehyde, ketone, alcohol, or ester, or oxyl radicals depending on the availability of an R-H and the substitution functional group (18):
2ClHCdC•Cl + 2O2 f 2ClHCdC(Cl)OO• f f
2ClHCdC(Cl)O• + O2 (2)
The oxyl radical generated in reaction 2 forms monochloroacetic acid via an unstable ketene structure (reaction 3) and formic and hydrochloric acids by reaction 4: •
H2O
•
ClHCdC(Cl)O f Cl + ClHCdCdO 98 ClH2CCOOH + H+ + Cl- (3) H2O
ClHCdC(Cl)O• f ClH•CsC(O)Cl f f HCOOH + H+ + Cl- (4)
FIGURE 2. pH and O2 patterns. (Solid lines represent the computer modeling patterns.) for the generation of dichloroacetaldehyde through a swift HCl elimination (18). Reaction 6a is supported by Mertens and von Sonntag’s study (20) on the photolysis of tetrachloroethylene (PCE) at 253.7 nm: H2O
The carbon-centered radical in eq 4 can be also the precursor of glyoxylic acid, which was identified as a byproduct of TCE photolysis [reaction sequence 5 (19)]: -•Cl
O2
ClH•C-C(O)Cl f f •O(ClH)C-C(O)Cl 98 H2O, -HCl
OHC-C(O)Cl 98 OHC-COOH (5) The polar structure of TCE excited state is prone to water addition leading to a chlorohydrin structure that accounts
ClHCdCCl2 + hν 98 ClHC(OH)sCHCl2
(6a)
ClHC(OH)sCHCl2 f Cl2HCsCHO + H+ + Cl- (6b) The chlorinated acetylenes detected in small concentrations in the studied system can be generated through the loss of either chlorine gas or hydrochloric acid from the excited state of TCE. The reaction routes 7 and 8 were also proposed by Mertens and von Sonntag (20) for the UV photolysis of tetrachloroethylene: VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ClHCdCCl2 + hν f HCtCCl + Cl2
(7)
ClHCdCCl2 + hν f HCtCCl + HCl
(8)
The chlorine radicals generated through the photolysis of both TCE and other chlorinated species in the system are strong electrophiles that can add to the double bond of TCE molecule, leading to a C-centered radical which in turn, via reaction with dissolved O2, leads to a peroxyl radical. This reaction sequence 9-11 is shown below, and dichloroacetyl chloride [Cl2HC-C(O)Cl] is postulated as being the major byproduct. Unstable in aqueous media, dichloroacetyl chloride hydrolyzes to dichloroacetic acid and HCl (reaction 11): •
•
ClHCdCCl2 + Cl f Cl2HCsC Cl2 2Cl2HC-C•Cl2 + 2O2 f 2Cl2HC-CCl2O2• f
formation in this study. As mentioned briefly above, the chlorine radicals are formed in a relatively high concentration through the UV photolysis of chlorinated compounds that are produced from TCE. Therefore, along with addition/ substitution reactions involving these compounds, the selfcombination reaction has a high probability, which generates hypochlorous acid (HOCl, pKa ) 7.7). Degradation of Reaction Byproducts. In weak acidic media, mono- and dichloroacetylenes, which are primary products of TCE photolysis, undergo hydrolysis to acetyland chloroacetyl chloride, respectively, which further hydrolyze to acetic and monochloroacetic acid, respectively (reaction sequences 18 and 19): H3O +
HCtCCl 98 CH3C(O)Cl
(9)
H2O
CH3C(O)Cl 98 CH3COOH + HCl
-
H3O +
ClCtCCl 98 ClCH2C(O)Cl
H2O
Cl•
Cl2HC-CCl2O 98 Cl2HC-C(O)Cl 98
H2O
ClCH2C(O)Cl 98 ClCH2COOH + HCl
Cl2HC-COOH + H+ + Cl- (11)
The oxyl radical generated in reaction 10 can also undergo a β-scission process forming phosgene (reaction 12), an unstable compound in aqueous matrixes (half-life 0.026 s) that hydrolyzes to HCl and carbon dioxide (reaction 13):
Cl2HC-CCl2O• f Cl2HC• + OdCCl2
(12)
OdCCl2 + H2O f 2H+ + 2Cl- + CO2
(13)
The C-centered radical formed in reaction 12 reacts with oxygen and, via a peroxyl radical, leads to formyl chloride and the reactive oxychlorine radical ClO• [reaction 14 (21)]. Alternatively, the peroxyl radical may decay through an oxyl radical intermediate to either formyl chloride and chlorine atom (reaction 15) or to phosgene (reaction 16), which further hydrolyzes to CO2 and HCl (reaction 13):
Cl2HC• + O2 f Cl2HCO2• f HC(O)Cl + ClO• (14) Cl2HCO• f HC(O)Cl + Cl•
(15)
HC(O)Cl f H+ + Cl- + CO
O2
(20)
Cl2CH-CHO + hν f Cl2C•H + •CHO
(21)
The dichloromethyl radical leads to HCl, CO, CO2, and chlorine atoms according to reaction sequences 14-17, whereas the formyl radical can dimerize or disproportionate, react with dissolved oxygen [k ) 4.5 × 109 M-1 s-1 (23)], or become hydrated (21) as illustrated below (reactions 2224):
The chemistry of peroxyl radicals in aqueous solutions is very complex, and as indicated by the extensive studies of von Sonntag and Schuchmann (ref 18 and references cited therein), peroxyl radical reactions could lead to various or similar byproducts through different reaction pathways. It is not the purpose of this work to describe all these reactions, but only the major ones that would explain the byproduct 6688
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2•CHO f CO + HCHO f OHC-CHO
(17)
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(19b)
Cl2CH-CHO 98 Cl2HC-COOH
(16)
Formyl chloride is unstable and decomposes primarily into HCl and CO (reaction 17) at a very fast rate [k ) 104 s-1 (22)], and only a small fraction of it hydrolyzes to formic acid and HCl; the latter reaction becomes important only at very high pH (22). The oxychlorine radical ClO• is an oxidant and could initiate degradation reactions by H-abstraction from the organic compounds present in this system; therefore, hypochlorous acid (HOCl) could be produced to a certain extent. However, given the low concentration of ClO•, such reactions are expected to be of minor importance.
(19a)
Given the very low level of monochloroacetylene, the hydrolysis product acetic acid was generated at concentrations below our detection limit. The UV photolysis of acetic acid in aerated aqueous solution leads to formaldehyde, methanol, formic acid, and carbon dioxide. Dichloroacetaldehyde is formed at early stage of irradiation, through reactions 6. It can be further oxidized by molecular oxygen to dichloroacetic acid (reaction 20) and also can undergo direct UV photolysis through a C-C bond scission, leading to two C-centered radicals (reaction 21):
O2
Cl2HCO• f Cl2C•(OH) f f• OdCCl2 -HO2
(18b)
•
2Cl2HC-CCl2O + O2 (10) •
(18a)
•
•
(22a) (22b)
CHO + O2 f CO2 + •OH
(23a)
f CO + HO2•
(23b)
O2
CHO + H2O f •CH(OH)2 98 •O2CH(OH)2 f f
HCOOH + HO2• (24)
Even though low levels of glyoxal (reaction 22b) were measured in the irradiated system, given the low concentration of formyl radicals, the most likely route followed by this radical would be reaction 24, with the last step rate constant k ) 1 × 106 s-1 (23). The UV photolysis of monochloroacetic acid occurs primarily at the C-Cl bond (reaction 25) and is followed by the reaction of C-centered radical with oxygen. The quantum yield of monochloroacetic acid photolysis at 254 nm was reported as 0.34 (24). The expected reaction products are glyoxylic and glycolic acids and formaldehyde, as indicated
by the reaction set 25-30. As mentioned previously, the chemistry of peroxyl radicals is complex and was extensively discussed in the literature (refs 18, 23, and 25 and references therein). The self-termination reaction of acetate peroxyl radical generated in reaction 26 may lead, through a tetroxide structure, to (i) oxygen, glycolic and glyoxylic acids (Russelltype mechanism, reaction 27a); (ii) hydrogen peroxide and glyoxylic acid (Bennett-type mechanism, reaction 27b); and (iii) oxygen and two oxyl radicals (reaction 28). Escaped from the solvent cage, the oxyl radical decomposes with the formation of formaldehyde (reaction 29) or, through a rapid H-shift followed by reaction with dissolved oxygen, generates glyoxylic acid (reaction 30); within the solvent cage, the oxyl radicals can undergo disproportionation reaction forming glyoxylic and glycolic acids: •
ClCH2COOH + hν f CH2COOH + Cl
•
•
CH2COOH + O2 f •O2CH2COOH
(25) (26)
Russell-type mechanism
2•O2CH2COOH f f OHC-COOH + HOCH2-COOH + O2 (27a) •
2 O2CH2COOH
Bennet-type mechanism
f f 2OHC-COOH + H2O2 (27b)
2•O2CH2COOH f [2•OH2C-COOH + O2]solvent cage (28) OCH2COOH f HCHO + CO2•- + H+
•
H-shift
(29)
O2
OCH2COOH 98 •C(OH)HCOOH 9•8 OHCCOOH
•
-HO2
(30) On the basis of the product analysis (see Figure 1, panels b and d), where glyoxylic acid and formaldehyde were detected and quantified, one may conclude that reactions 29 and 30 prevail. The dichloroacetic acid concentration reached a maximum after ∼10 min irradiation, which corresponds to the decay of TCE by 1 order of magnitude. Its photolysis generates a C-centered radical and a chlorine atom. The C-centered radical leads to glyoxylic acid and another chlorine atom, through a peroxyl radical intermediate:
Cl2HCCOOH + hν f •CHClCOOH + Cl• •
CHClCOOH + O2 f •O2CHClCOOH
(31) (32)
-O2
2•O2CHClCOOH 98
2•OHCCl-COOH f 2OHC-COOH + 2Cl• (33)
Other reactions of chlorinated C-centered radicals, such as hydrolysis, were reported in the literature (26). Hydrolysis of C-centered radical generated in reaction 31 would lead to formaldehyde, formoyl radical (•COOH/CO2•-, pKa ) 1.4) and HCl. On the basis of the product yield in the irradiated system, one can conclude that the major degradation pathways of mono- and dichloroacetic acids involve the oxyl radicals. The formoyl radicals (•COOH/CO2•-) react very fast [k ) 2.0 × 109 M-1 s-1 (27)] with dissolved oxygen resulting in CO2 and HO2•. Glyoxylic acid photolysis generates formyl and formoyl radicals, which follow the reactions described above, leading primarily to formic acid, CO2, and CO.
FIGURE 3. Schematic representation of the direct UV photolysis of TCE. Formaldehyde photolysis was extensively studied in the literature. Under UV irradiation in oxygenated dilute aqueous solution, it is oxidized mainly to formic acid (28). The direct UV photolysis of aqueous solutions of formic acid has also been extensively examined (29-31) and leads to CO2, CO, and H2O as end products. Oxidizing species, such as the •OH and hydroperoxyl (HO2•) radicals could also be generated to some extent either directly (e.g., through the photolysis of formic acid) or a result of radical reactions (e.g., reactions 16, 23b, and 24). Their contribution to the degradation of TCE and the intermediate products is insignificant when compared to the light-induced photolysis. The overall reaction mechanism discussed above is shown in Figure 3. Kinetic Modeling of UV Photolysis of TCE and Its Byproducts. The mechanism proposed above was simplified as 31 reactions listed in Table 1, which represent the degradation pathways for TCE and all main byproducts, such as monochloroacetic acid (MCA), dichloroacetic acid (DCA), dichloroacetaldehyde, formaldehyde, glyoxylic, oxalic, and formic acids and two chlorinated acetylenes. Photolysis. Direct photolysis accounts for the degradation of TCE and most of the main byproducts. The photolysis rate for each species (rUV,i) can be calculated by the multiplication of the quantum yield and the volume averaged absorbed photon flow over the irradiation wavelength band as shown in the equation below: 300
rUV,i ) -Φi
∑
{Ep,λ fi,λ(1 - e-Aλ)}
λ)200
Aλ ) 2.303b{(TCE,λCTCE + DCA,λCDCA + MCA,λCMCA + glyoxylic,λCglyoxylic + oxalic,λCoxalic + formic,λCformic)} (49) fi,λ ) 2.303bi,λCi/Aλ where Φ is the quantum yield, which is defined as the ratio of the photolysis rate to the rate of light absorption and is a wavelength-dependent parameter; Ep,λ is the volumeaveraged UV photon flow (einstein L-1 s-1) at wavelength λ delivered by the medium-pressure lamp through the quartz sleeve to the solution and was calculated for each wavelength in the range of 200-300 nm; Aλ is the exponential form of the optical density of the irradiated solution at λ, over the mean effective reactor path length; and fi,λ is the fraction of UV light absorbed by each species i at wavelength λ. The symbol λ defines the molar absorption coefficient of each species at λ. The development of this equation is provided in the Supporting Information. As shown in the reaction mechanism, TCE photolyzes through four possible pathways that lead to different VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Reaction Mechanism Considered in the Modeling of TCE Photolysis reaction 1 2-4 2-3 5
ClHCdCCl2 + hν f ClHCdC•Cl + Cl• ClHCdC•Cl f f HCOOH ClHCdC•Cl f f ClH2CCOOH ClHCdC•Cl f f OHCCOOH
6a 6b 7
ClHCdCCl2 + hν 98 ClHC(OH)sCHCl2 ClHC(OH)-CHCl2 f Cl2HC-CHO + H+ + ClClHCdCCl2 + hν f HCtCCl + Cl2
ΦTCE,4 ) 0.1 1.2 × 10-3 s-1 ΦTCE,2 ) 0.032
18a
HCtCCl 98 CH3C(O)Cl
5 × 10-3 s-1
H 2O
H3O+
H2O
CH3C(O)Cl 98 CH3COOH ClHCdCCl2 + hν f ClCtCCl + HCl
1 × 10-4 s-1 ΦTCE,3 ) 0.092
19a
ClCtCCl 98 ClCH2C(O)Cl
3.8 × 10-3 s-1
20 21-24 9 10 12-17 11 25 26-28 29 30 31 32-33 34
H3O+
H 2O
O2
6 × 10-4 s-1 8.5 × 10-4 s-1 4.88 × 1010 M-1 s-1
Cl2CH-CHO 98 Cl2HC-COOH Cl2CH-CHO + hν f f CO + HCOOH + O2•ClHCdCCl2 + Cl• f Cl2HCsC•Cl2 O2
•
•
5.0 × 200 s-1 130 s-1 ΦMCA ) 0.9
•
5.0 × 103 s-1 2.3 × 103 s-1 800 s-1 ΦDCA ) 0.60
104
O2•
CH2COOH f f OCH2-COOH •OCH COOH f HCHO + CO •- + H+ 2 2 •OCH COOH f f OHCCOOH 2 Cl2HCCOOH + hν f •CHClCOOH + Cl• •
O2
CHClCOOH f f OHCCOOH + Cl•
3.8 × 10-3 s-1
hv, O2
3.0 × 10-3 s-1
HCHO f f CO2 + CO + H2O hv, O2
OHCCOOH 98 HOOCCOOH OHCCOOH 98 HCOOH + CO2
9 × 10-4 s-1
hv
Φglyoxylic acid ) 0.5
hv
Φformic acid ) 0.7
HCOOH f f CO2 + CO + H2O hν, O2
32
s-1
Cl2HC-C Cl2 f f Cl2HC-CCl2O + O2 Cl2HC-CCl2O• f f CO + CO2 + H+ + ClCl2HC-CCl2O• f f Cl2HC-COOH + H+ + Cl- + Cl• ClCH2COOH + hν f •CH2COOH + Cl•
36
38 39 40 41 42 43 44 45 46 47 48
1.4 × 10-3 s-1
ClCH2C(O)Cl 98 ClCH2COOH
35 37
a
ΦTCE,1 ) 0.13 3.15 × 103 s-1 1.4 × 103 s-1 1.35 × 103 s-1
18b 8
19b
refa
rate constant
•-
Φoxalic acid ) 0.3 2.2 × 108 M-1 s-1 5.0 × 108 M-1 s-1 pKa ) 3.75 pKa ) 2.8 pKa ) 1.26 pKa ) 3.30 pKa1 ) 1.46 pKa2 ) 4.40 pKa1 ) 6.3 pKa2 ) 10.3
HOOCCOOH 98 CO2 + H2O + CO2 Cl• + HCO3- f CO3•- + H+ + ClCl• + H2CO3* f CO3•- + 2H+ + ClHCOOH S H+ + HCOOCH2ClCOOH S CH2ClCOO- + H+ CHCl2COOH S CHCl2COO- + H+ OHCCOOH S OHCCOO- + H+ HOOCCOOH S H+ + HOOCCOOHOOCCOO- S H+ + -OOCCOOH2CO3 S H+ + HCO3HCO3- S H+ + CO32-
20 33 33 33 33 33 33 33 33
All kinetic parameters determined in this work, except for those associated with a literature reference.
byproducts at different reaction rates. In the kinetic simulations, the four pathways were considered separately as the reactions 1, 6a, 7, and 8 in Table 1. Each pathway is associated with a quantum yield and the overall quantum yield for TCE photolysis is the summation of the four individual quantum yields. Only the major pathway was considered for the photolysis of each byproduct. Chain Mechanism. Generally, kinetic simulations are simplified versions of a detailed mechanism. During the chain developing process from one stable species A to another species B, there might be many intermediate steps. These steps can be neglected as long as they do not generate stable intermediates or intermediates that would be involved in reactions outside the “A to B cycle”. Since most of the kinetic parameters for this system are not available in the published literature, a generic algorithm was applied to search for the quantum yields and rate constants to fit the experimental data. The quantum yields and rate constants reported in Table 1 were determined by 6690
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fitting the 10 sets of experimental concentration data for TCE, mono- and dichloroacetylenes, formaldehyde, dichloroacetaldehyde, mono- and dichloroacetic acids, glyoxalic acid, oxalic acid, and formic acid. For each data set, the following objective function was used to fit the kinetic model to the data
OFj )
x∑[
]
Nj
Cj_data,i - Cj_model,i
i)1
Cj_data,i
2
(50)
in which, OF is the objective function; j is the index that represents the 10 compounds, i is the index of the sampling times for the compounds varying from 1 to Nj; Nj is the number of the experimental data (concentration at various times) for each compound j, and Cj_data,i and Cj_model,i are the measured and model simulated concentrations for compound j at each sampling time i, respectively. The parameters
were determined by minimizing the 10 objective functions expressed as eq 50. The fitting process was divided into two stages. The first stage was to estimate the range of the rate constants and quantum yields by searching for the minimums of the 10 objective functions. The initial searching range for rate constants was from 1 × 10-10 to 1 × 1011 and that for quantum yields was from 0 to 10. The task in this stage had two parts, first the discovery of bracketing intervals of possible minimums and then the convergence to the minimum within each bracket. The intervals were found by classic downhill algorithm (34), and the best fits were found by bisection search. The searching process started from compounds that involve relatively simple pathways, such as those of TCE, mono- and dichloroacetylenes. Once these objective functions were minimized, the objective functions for the compounds that involve more complex pathways, such as for DCA and glyoxalic acid, were treated in a similar way. On the basis of the results of the first stage, the parameter values were fine-tuned by minimizing the sum of the 10 10 objective functions (i.e., åj)1 OFj) within the narrow range that was determined from the first stage. The fine-tune process was from top to the bottom of the chain (i.e., from TCE to the intermediates and then to the final products). In this stage, values of the parameters for competitive pathways were varied and permuted to find the best fit. For example, the five reactions 1, 6a, 7, 8, and 9 are competitive with each other. The fine-tune process varied the values for each rate constant (or quantum yield) by (10% with an interval of 4% and permuted the varied values. This resulted in a total of 55 possible combinations. The combination that achieves 10 the minimum of the sum the objective functions, åj)1 OFj, was chosen as the best fit. Governing Equations. Modeling eqs 52-71 of the kinetic model are derived based on the governing equation in a complete mixed batch reactor (CMBR) (eq 51):
dCa ) ra dt
Ca|t)0 ) Cao
(51)
where Cao is the influent concentration of species A, Ca is the concentration of A at time t, and ra is the overall kinetic rate expression for species A in the reaction system. Substituting the kinetic rate expressions for all main species in the UV photolysis system for TCE, one obtains the rates of decay for all species accounted for in the model and given in the following set of equations:
d[TCE] ) ruv,TCE,1 + ruv,TCE,2 + ruv,TCE,3 + ruv,TCE,4 dt k9[TCE][Cl•] (52) d[ClHCdC•Cl] ) -ruv,TCE,1 dt (k2-4 + k2-3 + k5)[ClHCdC•Cl] (53) d[Cl•] ) -ruv,TCE,1 + k2-3[ClHCdC•Cl] dt k9[TCE][Cl•] - ruv,MCA - ruv,DCA + k11[Cl2HCCCl2O•] -
k39[Cl•][HCO3-] - k40[Cl•][H2CO3*] (54)
d[HCOOH] ) k2-4[ClHCdC•Cl] + k41[CH3COOH] + dt k34[HCHO] - ruv,OHCCOOH + ruv,HCOOH (55) d[ClCH2COOH] ) k2-3[ClHCdC•Cl] + ruv,MCA + dt k19_b[ClCH2C(O)Cl] (56)
d[•CH2COOH] ) -ruv,MCA - k26-28[•CH2COOH] (57) dt d[OHCCOOH] ) ruv,OHCCOOH - k35[OHCCOOH] + dt k32-33[•CHClCOOH] + k30[•OCH2COOH] (58) d[Cl2HCC•Cl2] ) k9[TCE][Cl•] - k10[Cl2HCC•Cl2] (59) dt d[Cl2HCCCl2O•] ) k10[Cl2HCC•Cl2] dt k12-17[Cl2HCCCl2O•] - k11[Cl2HCCCl2O•] (60) d[Cl2HCCOOH] ) ruv,DCA + k11[Cl2HCCCl2O•] + dt k20[Cl2CHCHO] (61) d[•CHClCOOH] ) -ruv,DCA - k32-33[ClHC•COOH] (62) dt d[HCtCCl] ) -ruv,TCE,2 - k18_a[HCtCCl] dt
(63)
d[CH3C(O)Cl] ) k18_a[HCtCCl] - k18_b[CH3C(O)Cl] dt (64) d[ClCtCCl] ) -ruv,TCE,3 - k19_a[ClCtCCl] dt
(65)
d[ClCH2C(O)Cl] ) k19_a[ClCtCCl] - k19_b[ClCH2C(O)Cl] dt (66) d[ClCH(OH)CHCl2] ) -ruv,TCE,4 dt k6_b[ClCH(OH)CHCl2] (67) d[HOOCCOOH] ) ruv,oxalic + k35[OHCCOOH] (68) dt d[HCHO] ) k29[•OCH2COOH] + k41[CH3COOH] dt k34[HCHO] (69) d[Cl2CHCHO] ) k6_b[ClCH(OH)CHCl2] dt k20[Cl2CHCHO] - k21-24[Cl2CHCHO] (70) d[•OCH2COOH] ) k26-28[•CH2COOH] dt k29[•OCH2COOH] - k30[•OCH2COOH] (71) pH Calculation. The pH calculation in the UV system is based on the charge balance of ions in the system. The acidbase equilibrium of the acids is calculated at each simulation step based on their pKa. The acids considered in the pH simulation over the irradiation time are mono- and dichloroacetic, formic, glyoxylic, and oxalic acids and carbonate species. The calculation of carbonate species is based on the mass balance of carbon and pH of the system. Since it is an oxygenrich system, CO2(aq) in the form of H2CO3* is assumed to be final product and is not monitored in the experiments. Carbon monoxide (CO) was ignored, although it is a possible photolysis product of formyl chloride, formic acid, and oxalic acid. As a result, the carbon dioxide formation can be calculated at each exposure time by subtracting the carbon VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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in the intermediates and byproducts from the carbon in the degraded TCE:
TIC (H2CO3*/HCO3-/CO32-) ) 2(∆TCE) -
∑µ C
i i
(72)
where TIC is the total inorganic carbon including H2CO3*, carbonate, and bicarbonate species; ∆TCE is the degradation of TCE, which is calculated by subtracting TCE concentration at each step from the initial TCE concentration; subscript i denotes the carbon-containing species, which include both radicals and ionic species; µi is the number of moles of carbon in 1 mol of carbon-containing species; and Ci is the concentration of the species under consideration. The concentration of carbonate anions can be then represented according to TIC, pKa, and pH. Table 1 gives the pKa values used in these calculations. The nonlinear equation below describes the pH change in each step:
10-pH - total negative charge - 10(pH-14) ) 0 (73) where the total negative charge is the summary of the carbonate anions and all negative charged radicals and organic anions. The solid line in Figure 2 shows the predicted pH change in the system and can be compared with the experimental data. The good fit proves indirectly that the model includes most of the species that can dissociate in the water. The pH of the system drops sharply in the first 10 min of irradiation and remains relatively constant at longer exposure times. The formation of DCA and HCl contribute most to the pH decrease; DCA is a strong acid (pKa ) 1.26), and it reached a maximum concentration before 10 min irradiation. The fit of pH confirms that most organic acids generated in the system were included in the proposed and modeled reaction mechanism. After 50 min irradiation, the predicted pH of 2.46 is somewhat higher than that measured experimentally (pH 2.37). A possible explanation could be that the model has not considered the release of CO2 from the reactor to the atmosphere. TCE and Degradation Byproducts. Figure 1a-d compares the model fits to the data for TCE and degradation byproducts. In general, the agreement is very good and validates the proposed mechanism as an appropriate description of the reaction kinetics in this system. The kinetic model indicates that the decay of TCE over up to ∼80% of its initial concentration follows a zero-order kinetics with a rate constant of 1.67 × 10-6 M s-1. The quantum yields for the photolysis pathways 1, 6a, 7, and 8 in the reaction mechanism and Table 1 were determined as 0.13, 0.10, 0.032, and 0.092, respectively. The other major pathway for the decay of TCE is the attack by chlorine radical, which has a second-order rate constant of 4.88 × 1010 M-1 s-1 according gas-phase studies (32). The model describes the TCE degradation rate after 10 min as being faster than the experimental data indicates. This could be due to some competition for the light from the aqueous chlorine as well as from peroxyl radicals, which are predicted by the reaction mechanism but not measured experimentally or accounted for in the kinetic modeling. Also, at such low concentrations of TCE, some experimental analytical errors may occur. As shown in Figure 1a, the formation and fate of monoand dichloroacetylenes are described well by the model. The overestimation of the TCE decay rate after approximately 12 min of irradiation produces model simulated concentrations of acetylenes that are lower than the experimental measurements at longer reaction times. The simulated dichloroacetic acid (DCA) data (Figure 1c) suggest that DCA is formed mainly through pathways in which 6692
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no other stable species are involved. Comparing the two proposed pathways for the formation of DCA, one observes that the photoassisted hydrolysis of TCE generates dichloroacetaldehyde first, which continues to persist after TCE is degraded and is slowly oxidized to DCA, whereas the chlorine radical attack (reaction 8) appears to be the major route for the DCA generation through the fast hydrolysis of dichloroacetyl chloride. The kinetic model simulates DCA photolysis well, and the quantum yield was determined to be 0.60. This value agrees well with the reported value of 0.52 (35). To fit the experimental monochloroacetic acid data, a wavelength-averaged quantum yield of 0.9 is needed. This is larger than the generally accepted value of 0.34 at 254 nm (24). The assumption that the quantum yields of species are wavelength-independent might be a reason for the use of this large quantum yield. The literature values of the quantum yield for the photolysis of monochloroacetic acid are 0.34 ( 0.05 and 1.1 ( 0.1 at 253.7 and 184.9 nm, respectively (24, 35). It was also reported that the quantum yield was sensitive to the irradiation wavelength and intensity, concentration, temperature, and presence of other species. Another possible reason for the discrepancy between these values and the one required by the model might be the simplified reaction scheme used with the model, which ignores other possible degradation pathways of monochloroacetic acid, such as the reaction with chlorine radical (24). On the other hand, the kinetic model fits monochloroacetic acid data very well using a quantum yield of 0.9. The set of reactions below (reaction 74)
summarizes the photolysis paths for formic acid. Calvert and Pitts (36) determined the overall quantum yield for formic acid photolysis as 1.0. In the aqueous phase, only the third pathway is important. When the experimental data was fit by the kinetic model, a quantum yield of 0.7 was found, which is somewhat lower than that reported by Calvert and Pitts (36). Dichloroacetaldehyde is formed from chlorohydrin ClHC(OH)CHCl2, which is the product of the H2O addition to the excited state of TCE. The model predicts correctly the experimental pattern of dichloroacetaldehyde, supporting the mechanistic pathways proposed in this work. The model fits the experimental data for glyoxylic acid reasonably well but does not predict well the patterns for formaldehyde and oxalic acid. Possible explanations could be: (i) the model does not account for any hydrolysis processes of some of the short-lived radicals, such as that generated through reaction 31; (ii) the presence of some lightabsorbing species/compounds that the model does not account for, such as aqueous chlorine or radicals (e.g., HO2•, O2•-), which could impact the fraction of light absorbed by the byproducts; (iii) the byproduct photolysis may follow several pathways, which also could be wavelength-dependent (37); (iv) the model does not consider the dark reactions between byproducts and chlorine radical; (v) the concentration levels of these intermediates are very low; therefore, the analytical errors in their measurement should not be excluded. The rate constants predicted for the decay of oxyl radicals involved in reactions 11, 12-17, 29, 30 are in the order of 102-103 s-1, comparable with those characteristic to the 1,2-H shift and β-scission processes undergone by such species. The radical reaction sequences starting with O2 addition and
formation of peroxyl radicals and ending with either stable products or oxyl radicals, such as those comprising reactions 2 and 3, 2 and 4, and 10 and 26-28 (identified in Table 1 as 2-3, 2-4, and 10 and 26-28, respectively) were assumed to proceed with first-order rate constants of 103-104 s-1. Considering that the O2 concentration in the system was in the order of 10-4-10-5 mol/L, these rate constants would be equivalent to second-order rate constants of 107-109 M-1 s-1, consistent with the fact that the peroxyl radical formation approaches diffusion-controlled processes (18). The small value estimated for the rate constant of the reaction leading to glyoxylic acid (see sequence 32-33) may indicate that the rate-limiting step is the elimination of chlorine radical Cl•, a relatively slow process, as indicated in the literature for the decay of Cl(O)C-CCl2O• (20). The rate constants for hydrolysis reactions (such as 18a, 18b, 19a, and 19b) were predicted by the model in the order of 10-3-10-4 s-1 which are comparable with those reported for hydrolysis kinetics of chlorinated species (38). The overall oxidation rate constants of molecules, such as those for the reaction sequences 20, 21-24, and 35 were calculated in the order of 10-3-10-4 s-1, which is the order of magnitude of the rate constants for molecular reactions. The reaction between carbonic acid H2CO3* and chlorine radical was found comparable to that of HCO3-. The rate constants and the quantum yields listed in Table 1 are mostly obtained by fitting the experimental data. The lack of such information in the published literature makes impossible any comparison or assessment on their validity. However, our mechanistic study on the •OH radical-induced degradation of TCE, which comprises some of the reactions described in this work, supports the rate constants and the quantum yields reported here.
Acknowledgments We would like to thank Dr. J. R. Bolton, University of Alberta, Edmonton, AB, Canada (formerly affiliated with the University of Western Ontario, London, ON, Canada), for his helpful comments during the execution of this work.The modeling work was supported by the Center for Clean Industrial and Treatment Technologies (CenCITT) sponsored by the U.S. Environmental Protection Agency and the Department of Civil and Environmental Engineering of Michigan Technological University. Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the supporting organizations.
Supporting Information Available Additional information on the analytical methods and TCE and its byproducts characteristics, including figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review January 2, 2004. Revised manuscript received August 31, 2004. Accepted September 14, 2004. ES040304B
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