ARTICLE pubs.acs.org/IECR
Oxidation of 1,1,1-Trichloroethane Stimulated by Thermally Activated Persulfate Xiaogang Gu,† Shuguang Lu,*,† Lin Li,† Zhaofu Qiu,† Qian Sui,† Kuangfei Lin,† and Qishi Luo‡ †
State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, Shanghai Key Laboratory of Functional Materials Chemistry, East China University of Science and Technology, Shanghai 200237, China ‡ Shanghai Academy of Environmental Science, Shanghai 200233, China ABSTRACT: In this study, thermally activated persulfate (PS) to stimulate the oxidation of 1,1,1-trichloroethane (TCA) in groundwater remediation was investigated. The effects of various factors including temperature; initial TCA concentration; PS/ TCA molar ratio; solution pH; and common constituents in groundwater such as Cl, HCO3, SO42, and NO3 anions and humic acid (HA) were evaluated. The experimental results showed that TCA can be completely oxidized in 2 h at 50 °C with a PS/ TCA molar ratio of 100/1, indicating the effectiveness of thermally activated PS oxidation for TCA removal. TCA oxidation was fitted with a pseudo-first-order kinetic model, and the rate constant was found to increase with increasing temperature and PS/TCA molar ratio, but to decrease with increasing initial TCA concentration. In addition, acidic conditions were favorable to TCA removal and elevating, the initial solution pH value (from pH 3 to 11) decreased the TCA degradation rate. Anions Cl and HCO3 had negative effects on TCA removal, whereas the effects of both SO42 and NO3 were negligible. With 510 mg L1 concentrations of HA in solution, an inhibitive effect was observed, indicating that dissolved organic matter consumed some of the oxidant. However, the anticipated effective thermally activated PS oxidation of TCA in groundwater from a real contaminated site was not achieved because of the complex solution matrix. On the other hand, the TCA degradation mechanism derived from GC/MS analytical results confirmed formic acid, dichloromethane, and trichloromethane as the primary intermediates, and therefore, two TCA decomposition pathways were proposed. In conclusion, thermally activated PS oxidation is a highly promising technique for TCA-contaminated groundwater remediation, but more complex constituents in in situ groundwater should be carefully considered for its practical application.
1. INTRODUCTION The chlorinated solvent 1,1,1-trichloroethane (TCA) has historically been used widely as a major chemical solvent in metal degreasing, adhesives, aerosols, textile processing, extraction solvents, and industrial solvent blends.1,2 As a result of its extensive production and usage in the past two decades, TCA is currently one of the most commonly identified organic pollutants in contaminated groundwater and at hazardous waste sites,3,4 so that it presents an extremely high contamination potential even though it was phased out for most uses as an ozone-depleting compound under the Montreal Agreement.5 TCA in groundwater undergoes both abiotic and biotic transformations, as impacted groundwater is often under anoxic conditions. Biotransformation of TCA can lead to the occurrence of toxic intermediates such as 1,l-dichloroethane (1,1-DCA), chloroethane (CA), 1,1-dichloroethylene (1,1-DCE), and vinyl chloride (VC) through the function of microorganisms.68 Because of the potential adverse effects of TCA and its transformation derivatives, the maximum contaminant level (MCL) of TCA in drinking water has been set at 0.2 mg L1.9 In situ chemical oxidation (ISCO) is a promising technology for the remediation of groundwater contaminated by chlorinated solvents that is relatively faster and more cost-effective than conventional treatment technologies such as pump and treat.10,11 Several chemical oxidative reagents, for instance, ozone, permanganate, and Fenton’s reagent, have been successfully applied for in situ treatment of groundwater contaminated by chlorinated solvents including trichloroethylene (TCE), tetrachloroethylene r 2011 American Chemical Society
(PCE), DCE, and VC; however, some of the above-mentioned reagents showed very poor performance when treating TCAcontaminated groundwater.12,13 Persulfate anion (S2O82) is one of the strongest oxidizing agents, with a redox potential (E0) of 2.01 V.14 The sulfate free radical (SO4•) is particularly more important in ISCO applications that can be achieved through heat,1,1520 ultraviolet (UV) irradiation,19,21,22 and transitionmetal (e.g., cobalt and iron) activation of persulfate (PS).17,2325 When sulfate radical serves as an oxidant, it has a higher redox potential (E0 ≈ 2.6 V).26 Thermally activated PS has been found to provide a prominent alternative for the decomposition of many organic contaminants including chlorinated ethenes, trichloroethanes, and BTEX compounds (benzene, toluene, ethylbenzene, and xylenes).1,16,25 However, the efficiency of thermally activated PS processes would be strongly affected by solution pH in the reaction system. For example, sulfate radicals and hydroxyl radicals (•OH) are generated as a result of heat decomposition of PS in aqueous phases. Using radical probe compounds, Liang et al.27 reported that, in acidic solutions (pH < 7), SO4• was the predominant radical, whereas both SO4• and •OH were present at pH 9, and at basic pH (i.e., pH 12), •OH was the predominant radical. In addition, the reactivity of SO4• in groundwater systems might be affected by the presence of background anions. Received: May 17, 2011 Accepted: August 25, 2011 Revised: August 24, 2011 Published: August 25, 2011 11029
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Industrial & Engineering Chemistry Research Yang et al.19 observed that HCO3, HPO4, Cl, and CO32 have the potential to activate peroxymonosulfate in azo dye Acid Orange 7 degradation. Moreover, the efficiency of PS oxidation in groundwater is also significantly influenced by natural organic matter (NOM), such as humic acid (HA), which can compete with target contaminants for oxidation. Therefore, the influence of the aqueous matrix on in situ groundwater remediation performance should also be considered when applying the ISCO technique in practice. 1,1,1-TCA is naturally recalcitrant as a saturated hydrocarbon with one single bond. Thermally activated PS oxidation has been demonstrated to be able to remediate TCA contamination in pure water solution.28 Huang et al.16 found that thermally activated PS oxidation is effective in degrading many volatile organic compounds (VOCs) including TCA. Liang et al.1 observed a significant degradation of TCA in 6 h in an aqueous system with a PS/TCA molar ratio of 10/1 and an activation temperature of 50 °C and found the activation energy for the degradation of TCA to be 163.86 ( 1.38 kJ mol1 at pH 6. However, all of their studies were conducted in solutions prepared from pure water, and the influence of the complex solution matrix encountered in practice was not considered. Therefore, this study was undertaken to thoroughly investigate the effects of solution conditions such as reaction temperature, concentrations of TCA and PS, solution pH, anions (Cl, HCO3, SO42, and NO3), and HA on the TCA removal performance. Furthermore, TCA removal in groundwater collected from a real contaminated site was also tested to assess its potential application in real remediation processes. Finally, the intermediates of TCA oxidation were determined using gas chromatography/ mass spectrometry (GC/MS), and accordingly, a possible TCA oxidation pathway is proposed.
2. MATERIALS AND METHODS 2.1. Materials. 1,1,1-Trichloroethane (TCA, 99.0%), sodium persulfate (Na2S2O8, 98.0%), sodium chloride (NaCl, 99.5%), sodium sulfate (Na2SO4, 99.0%), sodium bicarbonate (NaHCO3, 99.5%), sodium carbonate (Na2CO3, 99.5%), sodium phosphate dibasic dodecahydrate (Na2HPO4 3 12H2O, 99.0%), sodium dihydrogen phosphate dihydrate (NaH2PO4 3 2H2O, 99.0%), sodium nitrate (NaNO3, 99.0%), potassium iodide (KI, 99.0%), sodium thiosulfate (Na2S2O3, 99.0%), hexane (C6H14, 97%), and HA (fulvic acid > 90%, as fulvic acid is one of the main fractions of dissolved organic matter with high solubility) were purchased from Shanghai Jingchun Reagent Co. Ltd. (Shanghai, China). Ultrapure water from a Milli-Q water process (Classic DI, ELGA, Marlow, U.K.) was used for preparing aqueous solutions. The in situ TCA-contaminated samples (see Table 1) from a contaminated site in Pudong, Shanghai, China, were collected from a layer approximately 3 m below the ground surface and filtered using fiber filters (0.45 μm, Waters Corporation, Shanghai, China) before the experiments. 2.2. Experimental Procedures. Stock solution of TCA was prepared by allowing the pure nonaqueous-phase liquid TCA to equilibrate with Milli-Q water overnight with gentle stirring in the dark and later diluted to the desired concentration. A predetermined amount of PS was added to the TCA-containing solution, mixed immediately for a few minutes, and later distributed by fully filling a series of 24-mL screw-cap borosilicate glass vials (reaction vials) with TFE/silicone liners. All sample vials were placed in a temperature-controlled water bath shaker
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Table 1. Characteristics of Groundwater at Contaminated Site
a
parameter
value
pH
6.7
TCA concentration (mg L1) total organic carbon (TOC, mg L1)
180.0 54.0
Fe concentration (mg L1)
3.1
Mn concentration (mg L1)
1.5
Cl concentration (mg L1)
1340
HCO3 concentration (mg L1)
745.7
NO3 concentration (mg L1)
NDa
SO42 concentration (mg L1)
78.6
Not determined.
(SHZ-B, Shanghai, China) under stirring at 150 rpm. At desired time intervals, two sample vials were sacrificed for immediate duplicate analyses. Control tests were carried out in parallel without PS addition. The initial pH in all experiments was unadjusted except in the tests for investigating the influence of pH. 2.3. Analytical Methods. Aqueous samples (1 mL) were analyzed following extraction with hexane (1 mL) for 3 min using a vortex stirrer and standing for 5 min for separation. TCA was analyzed using a gas chromatograph (Agilent 7890A, Palo Alto, CA) equipped with an electron capture detector (ECD), an autosampler (Agilent 7693), and an HP-5 column (30-m length, 320-μm i.d., 0.25-μm thickness). The temperatures of the injector and detector were 240 and 260 °C, respectively, and the oven temperature was constant at 60 °C. The amount of sample injected was 1 μL with a split ratio of 10:1. Intermediates formed in the TCA oxidation process were identified by gas chromatography/ mass spectrometry (GC/MS, Shimadzu GC/MS-QP 2010, Kyoto, Japan) equipped with a DB-5MS column (30-m length, 250-μm i.d., 0.25-μm thickness) using a headspace sampling procedure. The temperature of the injection port was 200 °C, and the oven temperature was constant at 40 °C. The ion source and interface temperatures were 250 and 200 °C, respectively, and the mass range was from 40 to 350 amu. The chloride anion was analyzed by ion chromatography (Dionex ICS-I000, Sunnyvale, CA). The concentration of persulfate was determined by both titration and spectrophotometric methods.29,30 The total organic carbon (TOC) was determined with a TOC analyzer (Elementar Analysensysteme GmbH, Hanau, Germany). The pH was measured with a pH meter (Mettler-Toledo DELTA 320, Greifensee, Switzerland).
3. RESULTS AND DISCUSSION 3.1. Performance of TCA Oxidation by PS. Figure 1 presents the results of TCA oxidation by PS at various temperatures. The results from the control tests in the absence of PS showed less than 5% loss of TCA during the whole experimental period under all test conditions (data not shown). In contrast, at 20 °C, the reaction of TCA with PS was very slow, and only 7.6% of TCA was oxidized in 10 h. However, significant TCA removal was observed as the temperature was increased from 30 to 50 °C. For example, 31.6% TCA removal was achieved after 10 h at 30 °C, and complete oxidation of TCA by PS was observed in 6 and 2 h at 40 and 50 °C, respectively, suggesting that increasing the temperature could significantly increase the TCA removal rate. 11030
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Table 2. Results of TCA Oxidation Performance under Various Conditions operating conditions pseudo-firstpH
order
correlation
rate constant,
coefficient,
(initialfinal) k 102 (min1)
test no.
R2
C0 = 20 mg L1, PS/TCA = 100/1
From previous studies,1,16 TCA oxidation by PS has been postulated to follow pseudo-first-order reaction kinetics and can be determined by
Ct ¼ kt C0
0.02
0.7774
T = 30 °C
0.07
0.9748
3
T = 40 °C
0.57
0.9229
4
T = 50 °C
4.82.8
1.77
0.9658
5
C0 = 10 mg L1
2.13
0.9547
6
C0 = 20 mg L1
4.82.8
1.77
0.9658
7 8
C0 = 50 mg L1 C0 = 100 mg L1
1.64 1.27
0.9667 0.9826
T = 50 °C, C0 = 20 mg L1
ð1Þ
Equation 1 can be rewritten as ln
T = 20 °C
2
T = 50 °C, PS/TCA = 100/1
Figure 1. Oxidation of TCA at various temperatures (initial TCA concentration = 20 mg L1, PS/TCA molar ratio = 100/1).
d½TCA ¼ k½TCA dt
1
9
PS/TCA = 20/1
0.45
0.9976
10
PS/TCA = 50/1
1.09
0.9656
11
PS/TCA = 100/1
4.82.8
1.77
0.9658
12
PS/TCA = 200/1
4.76
0.9220
ð2Þ
1
T = 50 °C, C0 = 20 mg L , PS/TCA = 100/1
where Ct and C0 are the concentrations of TCA (mg L1) at time t and time zero, respectively, and k is the pseudo-first-order decay rate constant (min1). In this study, TCA oxidation performance could be well-fitted to the pseudo-first-order kinetic model, as indicated by high correlation coefficients (presented as R2), and the rate constants at 20, 30, 40, and 50 °C were calculated to be 0.0002, 0.0007, 0.0061, and 0.0177 min1, respectively (Table 2). Based on the above calculations, the activation energy over the range of 2050 °C was determined to be 122.44 kJ mol1 (R2 = 0.99), which is comparable to 163.86 kJ mol1 (from 4060 °C) from Liang et al.’s research.1 On the other hand, it is worth noting that, at 50 °C, the initial pH value of TCA solution with PS was 4.8, and the final pH value decreased to 2.8 after reaction, which was possibly due to the release of protons when sulfate radical reacted with water (see eqs 3 and 4) leading to the formation of acidic products.31,32
pH 3.0a
3.02.8
2.38
0.9627
14 15
pH 6.0 pH 6.5
6.05.8 6.56.4
1.15 0.90
0.9659 0.9814
16
pH 7.0
7.06.9
0.70
0.9892
17
pH 7.5
7.57.4
0.25
0.9639
18
pH 8.0
8.07.7
0.15
0.9255
19
pH 11.0b
11.010.8
0.03
0.4226
T = 50 °C, C0 = 20 mg L1, PS/TCA = 100/1
k1
SO4 • þ H2 O sf • OH þ HSO4 , k1 ¼ ð6:5 ( 1:0Þ 107 M1 s1
13
ð3Þ
20
[Cl ] = 1 mM
4.82.8
1.63
0.9712
21
[Cl] = 10 mM
4.82.8
0.96
0.9924
22 23
[Cl] = 100 mM [HCO3] = 1 mM
4.83.1 7.73.1
0.07 0.08
0.8490 0.9620
24
[HCO3] = 10 mM
8.18.0
0.02
0.5703
25
[HCO3] = 100 mM
8.18.1
0.008
0.1973
26
[SO4] = 1 mM
4.82.8
1.78
0.9414
27
[SO4] = 10 mM
4.93.0
1.43
0.9437
28
[SO42] = 100 mM
5.13.3
1.31
0.9550
29
[NO3] = 1 mM
4.72.8
2.08
0.9281
30 31
[NO3] = 10 mM [NO3] = 100 mM
4.72.8 4.72.8
1.83 1.71
0.9400 0.9452
ð4Þ
a Initial pH was unbuffered. b Initial pH was adjusted with 0.1 M sulfuric acid and 0.1 M sodium hydroxide.
Because the pollutant concentration could be an important parameter controlling the oxidation process, the effect of the initial TCA concentration at 50 °C was investigated by varying the initial TCA concentration in the range of 10100 mg L1 while keeping the initial PS/TCA molar ratio constant at 100/1 (Figure 2). It can be seen that, with an increase of the initial TCA concentration, the oxidation efficiency slightly declined. Moreover, the influence of initial PS/TCA molar ratios of 20/1, 50/1,
100/1, and 200/1 was also investigated at a temperature of 50 °C, and the results are shown in Figure 3. Complete oxidation of TCA was observed in 4, 2, and 1 h at PS/TCA molar ratios of 50/1, 100/1, and 200/1, respectively, whereas only 66.2% TCA was removed in 4 h at PS/TCA = 20/1. The results strongly suggested that the oxidation of TCA proceeded more rapidly with increasing PS/TCA molar ratio and that this factor played a much more important role in TCA oxidation rate than the initial
HSO4 f Hþ þ SO4 2
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Figure 2. Effect of initial TCA concentration on oxidation performance (temperature = 50 °C, PS/TCA molar ratio = 100/1). Figure 4. Effect of initial pH on TCA oxidation performance (temperature = 50 °C, initial TCA concentration = 20 mg L1, PS/ TCA molar ratio = 100/1).
values and various temperatures (10, 20, and 30 °C) and found that neutral conditions were conductive to TCE degradation. Huang et al.15 investigated the effect of pH (phosphate buffered) on PS oxidation of MTBE at 40 °C and found that the reaction rate decreased with increasing pH. Therefore, it was assumed that different characteristics of the contaminants and mechanisms of degradation might be responsible for the different results. For example, House reported that the observed rate constant of decomposition of peroxydisulfate increased with decreasing pH under various temperatures.14 In addition, PS can be further catalyzed under acidic conditions as in the equations Figure 3. Effect of initial PS/TCA molar ratio on oxidation performance (temperature = 50 °C, initial TCA concentration = 20 mg L1).
TCA concentration. These conclusions were also demonstrated by other researchers when treating TCE and BTEX contaminants at various PS and pollutant molar ratios.20,25 In addition, the results of persulfate determination showed that the overall consumption of persulfate at 50 °C was less than 5%, which was similar to the results for BTEX and TCE oxidation by thermally activated PS processes.25,33 3.2. Effect of Solution pH on TCA Removal Performance. Because the pH of groundwater is nearly neutral, initial solution pH values of 6.0, 6.5, 7.0, 7.5, and 8.0 [all solutions phosphatebuffered (0.1 M)] were investigated, and at the same time, a solution without pH adjustment (pH 4.8) and two solutions with extreme pH values of 3 and 11 (adjusted with 0.1 M sulfuric acid and 0.1 M sodium hydroxide, respectively) were also tested at 50 °C to assess the influence of pH (Figure 4). The pH variation was within 0.2 unit during the course of experiments in buffered solutions. The oxidation rate was highest at pH 3 and decreased with increasing initial solution pH (see Figure 4), with TCA removals of only 33.0% and 11.3% being obtained after 4 h at pH 8 and 11, respectively. In contrast, TCA was oxidized completely at pH 3 and 6 after 2 and 4 h, respectively, which means that acidic conditions are propitious for the TCA oxidation. However, Liang et al.33 investigated the oxidation of TCE at different pH
S2 O8 2 þ Hþ f HS2 O8
ð5Þ
HS2 O8 f SO4 • þ SO4 2 þ Hþ
ð6Þ
However, PS is also highly reactive at high pH, and base activation has been widely used for in situ PS oxidation application.34 Block et al.35 reported that base activation of PS was dependent not only on high pH conditions, but also on the molar ratio of the pH modifier to PS; therefore, 82.1% of TCA removal was achieved after 14 days with a KOH/PS molar ratio of 0.8 in their study. They also suggested that excess base was required to consume any acid in soil. In our present study, the quantity of base was less compared with PS dosage, and the alkaline conditions (pH 11) were not much stronger than typical base-activated pH conditions (pH g 13); hence, we speculated that the base that was used simply consumed PS as an inhibitor instead of functioning as an activator. In addition, when the pH was increased from 7 to 7.5, the TCA removal efficiency decreased significantly (from 80.3% to 47.8%), also indicating that caustic conditions might inhibit the oxidation of TCA. Moreover, under basic conditions, the SO4• formed can react with OH in accordance with the equation k2
SO4 • þ OH sf • OH þ SO4 2 , k2 ¼ ð6:5 ( 1:0Þ 107 M1 s1 11032
ð7Þ
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chloride ion was demonstrated to be relative to the generation rate of SO4•.41 In this study, the generation rate of SO4• was much higher because of the higher temperature applied, thus resulting in a significant scavenging degree of the sulfate free radicals at 100 mM chloride ion concentration. k3
SO4 • þ Cl T Cl• þ SO4 2 , k4
k3 ¼ 4:7 108 M1 s1 , k4 ¼ 2:5 108 M1 s1
ð9Þ
k5
Cl• þ Cl T Cl2 • , k6
k5 ¼ 8 109 M1 s1 , k6 ¼ 4:2 104 M1 s1
ð10Þ
k7
Figure 5. Effect of anions on TCA oxidation performance (temperature = 50 °C, initial TCA concentration = 20 mg L1, PS/TCA molar ratio = 100/1).
Cl2 • þ Cl2 • sf 2Cl þ Cl2 , k7 ¼ 1:3 109 M1 s1
ð11Þ
•
to generate OH, and the presence of various anions in solution (e.g., SO42) inhibits the reactivity of •OH.33,36 In contrast, when SO4• converts OH into •OH, two radicals can recombine as in eq 8,31 thereby decreasing the efficiency of TCA removal. SO4 • þ • OH f HSO4 þ
1 O2 2
ð8Þ
It should be noted that the rate constants of SO4• with HPO42 and H2PO4 in buffered systems are 1.2 106 and 7 106 M1 s1, respectively.37 Maruthamuthu and Neta37 found that SO4• can react with phosphate anions to propagate a chain reaction and form various phosphate radicals, the oxidation potentials of which are all less than that of SO4•. Therefore, it can be deduced that SO4• reacts slowly with phosphate anions and the impact of buffered phosphate on TCA oxidation performance in this study is expected to be minimal. The results are consistent with the conclusions of some other researchers.38,39 3.3. Effects of Solution Matrix on TCA Oxidation. As the reactivity of PS in contaminated groundwater system might be affected by the presence of background ions, the influences of Cl, HCO3, SO42, and NO3 anions on TCA oxidation performance were investigated individually at 50 °C with 1.0, 10, and 100 mM concentrations of each anion. The results demonstrated that both Cl and HCO3 have significant scavenging effects on SO4• during TCA oxidation (Figure 5a,b), whereas the influences of SO42 and NO3 were found to be negligible at the tested ion strength ranges (Figure 5c,d). The corresponding values of apparent reaction rate constants are also listed in Table 2. As shown in Figure 5a, the effect of chloride ions on the oxidation of TCA was not obvious at low concentration (1.0 mM). However, marked inhibition occurred at the concentrations of 10 and 100 mM, with the apparent reaction rate constant decreasing to 0.0096 and 0.0007 min1, respectively. Possible chemical scavenging mechanisms are shown as eqs 911.40 Chloride ion at an elevated concentration can react with SO4• and, therefore, result in competition with SO4• for reaction with TCA. This inhibitive phenomenon was also observed in a test of TCE degradation when the concentration of chloride exceeded 0.2 M at 20 °C, and the interference of
However, Adewuyi and Owusu42 found that positive effects on the sonochemical removal of NO occurred at low NaCl concentration (10 mM), and Adewuyi and Khan39 reported that NO absorption was significantly improved by addition of NaCl at 70 °C because of the formation of a large number of chlorinated reactive radicals (Cl•, Cl2•, and ClOH•). However, in our study, apparent inhibition was observed at high Cl concentrations (10 and 100 mM), whereas this effect at low concentrations [0.1 mM (data not shown) and 1.0 mM] was negligible. We speculate that the scavenging effect of the radicals formed from Cl is more significant than the positive effect due to both the low reactivity of the chlorinated radicals and the recalcitrance of TCA. Compared with chloride ion, a much more significant inhibitive effect was observed for bicarbonate ion concentrations ranging from 1 to 100 mM (Figure 5b). This adverse effect can be explained by the following two reasons: (a) When bicarbonate was added, the initial pH value of the PS solution changed to around 8.0 instead of pH 4.8 without any anion addition, and the oxidation rate was suppressed under basic conditions as discussed in section 3.2. (b) Bicarbonate performs as a radical scavenger in the aqueous phase as in the equation43 k8
SO4 • þ HCO3 sf CO3 • þ SO4 2 þ Hþ , k8 ¼ ð2:8 9:1Þ 106 M1 s1
ð12Þ
On the other hand, NOM always exists in groundwater, so its effect on TCA oxidation by PS was investigated at 50 °C (Figure 6). In these experiments, various HA dosages of 1.0, 5.0, and 10.0 mg L1 were added along with a 20 mg L1 initial TCA concentration. The results showed that 1 mg L1 HA had a less adverse effect on TCA removal, whereas an apparent inhibitive effect was observed at HA additions of 5.010.0 mg L1. However, complete TCA removal was still achieved after 4 h at all of the above HA dosages. It should be noted that an obvious lag time (40 and 60 min) was observed in the initial TCA removal curve at high HA concentrations (5.0 and 10.0 mg L1, respectively), 11033
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Figure 6. Effect of HA on TCA oxidation performance (temperature = 50 °C, initial TCA concentration = 20 mg L1, PS/TCA molar ratio = 100/1).
indicating that sulfate radical was initially consumed by HA rather than by TCA. Therefore, the performance of TCA oxidation was not well-fitted to the pseudo-first-order model in the presence of high HA concentrations, and accordingly, these kinetic data were not included in Table 2. Costanza et al.44 evaluated in situ heatactivated PS oxidation of PCE in soils and reported that a retardation was observed in reactors containing Great Lakes and Appling soils, further suggesting soil NOM as a potential contributor to oxidant consumption. Therefore, for practical in situ remediation applications using the PS oxidant technique, the sulfate radical consumed by NOM both dissolved in groundwater and attached on soil should be considered in determinations of the PS dosage. 3.4. Tests in Real TCA-Contaminated Groundwater. Figure 7a shows the TCA oxidation performance from a sample of a real contaminated site at 50 °C with PS/TCA molar ratios of 50/1, 100/1, and 200/1. The TCA oxidation efficiency clearly increased slightly with increasing PS dosage, but the TCA removal efficiencies were only 13.0%, 25.6%, and 27.2%, respectively, after 6 h of treatment at the above PS/TCA molar ratios. The presence of various constituents in the sample is speculated to have consumed the oxidant through the existence of organic carbon in contaminated groundwater (TOC = 54 mg L1) or inhibited the reaction between SO4• and TCA because of the high anions contents of Cl and HCO3 (see Table 1). To ascertain the effects caused by the groundwater matrix the, samples from the real contaminated sites were pretreated by air sparging (sample S1) and by dilution with tap water (sample S2) to reach a TCA concentration around 20 mg L1, and both were further oxidized under 100/1 and 500/1 of PS/TCA molar ratio conditions at 50 °C. It is apparent that, for sample S1, the TCA removal efficiencies after 8 h reached only 7.7% and 20.1% at PS/TCA molar ratios of 100/1 and 500/1, respectively. It is reasonable that the pretreatment by air sparging merely decreased the TCA concentration to 20 mg L1 but that solution contents such as anions and NOM were not changed, so that the oxidation of TCA was inhibited to the same extent as in the original sample. In contrast, for sample S2, TCA removal efficiencies of 30.0% and 96.7% were achieved after 8 h at 100/1 and 500/1 PS/TCA molar ratios, respectively. These results were well anticipated because the significantly reduced adverse effects were due to the simultaneous dilution of organic materials and
Figure 7. In situ TCA-contaminated groundwater treatment performance using thermally activated persulfate oxidation. (Samples S1 and S2 were pretreated by air sparging and tap water dilution, respectively.)
inhibitive components along with TCA dilution by tap water. From the above experimental results, it is clear that the thermally activated PS technique is applicable for contaminated groundwater remediation but that the matrix in in situ groundwater plays an important role in the TCA removal performance. In addition, one should take site characteristics into consideration when thermally activated persulfate technology is applied for in situ TCA remediation. An important cost component for activated PS applied in situ is the cost of the oxidant, which depends on the mass of PS required for remediation and the unit cost of PS. The mass of PS is determined by the amount of contaminant to be destroyed, as well as the naturally occurring organics, reduced metal species, and other components that all consume a portion of PS. Therefore, analysis of PS demand is an important factor in determining the efficacy and cost of thermally activated PS process used in a specific contaminant site. 3.5. Mechanism of the TCA Oxidation Pathway. Several mechanisms have been proposed in elucidating the TCA decomposition pathways under reduction and atmospheric oxidation conditions.5,4547 However, so far, there are no reports related to TCA oxidation pathway by PS and its intermediates. Therefore, some samples were collected at desired time intervals for the analyses of TCA, chloride ion, and intermediates. The results revealed that the concentration of chloride ion increased with decreasing TCA concentration and remained approximately steady after 2 h, whereas TCA was completely oxidized at the same time (Figure 8). Assuming that complete dechlorination of 1 mol of TCA would yield 3 mol of Cl, the measured Cl 11034
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SO42, and NO3; and NOM such as HA. TCA decomposition was found to fit a pseudo-first-order kinetic model. The experimental results showed that TCA removal is significantly influenced by temperature, PS/TCA molar ratio, and solution pH. In contrast, the presence of Cl and HCO3 has inhibitive effects on TCA decomposition, with Cl having less of an effect than HCO3, and the influence of NO3 and SO42 is negligible. However, NOM has a negative effect on TCA removal at relatively high concentrations. In conclusion, thermally activated PS oxidation for TCA-contaminated groundwater remediation is a highly promising technique, but more complex constituents in in situ groundwater significantly lower the TCA removal efficiency. Approaches to overcome these obstacles and improve the PS oxidation performance will be emphasized in our future work. Figure 8. TCA decomposition and released Cl versus treatment time (temperature = 50 °C, initial TCA concentration = 20 mg L1, PS/TCA molar ratio = 100/1).
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86 21 64250709. Fax: +86 21 64252737. E-mail: lvshuguang@ ecust.edu.cn.
’ ACKNOWLEDGMENT This study was financially supported by a grant (201109013) from the National Environmental Protection Public Welfare Science and Technology Research Program of China, the National Natural Science Foundation of China (Grant 40871223), and a Shanghai Postdoctoral Grant (11R21412500). ’ REFERENCES
Figure 9. Proposed TCA decomposition pathway.
concentration in our study was 61.3% of the total theoretical yield. Liang et al.1 also reported that the release of Cl was 60% when TCA was completely oxidized, suggesting that some other chlorinated intermediates were generated. Our further investigation through GC/MS analysis confirmed the existence of three primary intermediates, namely, trichloromethane (1), dichloromethane (2), and formic acid (3), in agreement with the TCA and Cl analytical results. Based on the above results, TCA decomposition is speculated to follow two different pathways, one through dechlorination and the other through CC bond breakage (see Figure 9). In dechlorination, 1,1-dichloroethylene is formed initially then the CdC bond breaks, forming dichloromethane and formic acid. In CC bond breakage, trichloromethane is formed in the first step and then dechlorinated later, forming dichloromethane. In both TCA oxidation pathways, the generated formic acid can be further decomposed to carbon dioxide and water.
4. CONCLUSIONS This study thoroughly investigated TCA removal performances using thermally activated PS oxidation by considering various influence factors, namely, initial TCA concentration; PS/ TCA molar ratio; solution pH; anions such as Cl, HCO3,
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