Role of Reactive Oxygen Species for 1,1,1-Trichloroethane

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Role of Reactive Oxygen Species for 1,1,1-Trichloroethane Degradation in a Thermally Activated Persulfate System Minhui Xu, Xiaogang Gu, Shuguang Lu,* Zhaofu Qiu, and Qian Sui State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: A thermally activated persulfate (PS) system was applied to degrade 1,1,1-trichloroethane (TCA) in aqueous solution. The generation of reactive oxygen species (ROS) in the system and their roles in TCA degradation were investigated. The experimental results showed that TCA (0.15 mM) could be completely oxidized in 1 h at 50 °C with a PS concentration of 30 mM. TCA degradation and PS decomposition well fitted a pseudo-first-order kinetic model. In addition, the chemical probe method was developed to identify the ROS. The results showed that SO4•−, HO•, and O2•− were all generated in the system and the generation intensities could be strengthened with the increase of PS concentration. The tests for PS persistence in solution indicated that oxidative species were intensified during the initial 2 h, suggesting more SO4•− and HO• were generated, whereas after 12 h SO4•− and HO• intensities were slightly reduced. In contrast, O2•− generated in the system was maintained at a stable level after reaction but at a slightly lower intensity simply due to quenching by PS or other species. Radical scavenger tests showed that HO• was the predominant radical species responsible for TCA degradation, and this was also confirmed by electron paramagnetic resonance (EPR) spectrum analysis in the system. Finally, two different pathways, dechlorination and C−C bond breakage, were proposed as the main TCA degradation mechanism. In conclusion, a thermally activated PS process is a highly promising technique for TCA degradation, and the potential to degrade highly oxidized organic contaminants greatly increases its application in in situ chemical oxidation (ISCO) remediation in contaminated sites.

1. INTRODUCTION In the late 1950s 1,1,1-trichloroethane (TCA) became the replacement for carbon tetrachloride (CT) and trichloroethene in metal degreasing applications and was used popularly in the 1990s.1,2 Even though its classification as a hazardous air pollutant and an ozone depleting chemical in the 1990 Clean Air Act Amendments led to its discontinuation as an industrial solvent and a complete phase-out by 2002 under the Montreal Protocol, TCA was widely detected in contaminated soil and groundwater sites. For instance, it has been reported that TCA was present in 393 of 1723 National Priorities List Sites in the United States due to its widespread usage as organic solvents and improper disposal in the past.3 TCA is denser than water (specific gravity of 1.34), and when it escapes to the subsurface as a pure phase or mixed with other chlorinated solvents it typically forms dense nonaqueous phase liquids (DNAPLs) that can migrate through an aquifer vertically by force of gravity. In addition, TCA is more recalcitrant than common chlorinated ethenes due to the presence of a single C−C bond,4 therefore, being one of the more persistent sources as groundwater contaminants. So far, there are many effective ex situ or in situ techniques for TCA remediation, in which in situ chemical oxidation (ISCO) is a widely employed alternative that involves the injection of strong chemical oxidants into the contaminated zone to destroy the target contaminants. Fenton, catalyzed hydrogen peroxide propagations (CHP, a modified Fenton’s process), permanganate, ozone, peroxymonosulfate, and persulfate (PS) have all been common oxidants applied in ISCO processes. Each of these ISCO reagents has various reactivities with the contaminants of concern. For example, H2O2 is a strong oxidant with high nonselective reactivity to © 2014 American Chemical Society

most organic contaminants; therefore a larger quantity of H2O2 would be rapidly consumed during transportation through the soil and aquifer before reaching the contaminants.5 Permanganate is relatively more stable; however, it is selectively reactive to unsaturated chlorinated compounds and, therefore, limited in its application.6 Gates-Anderson et al.7 observed very little degradation of TCA with either peroxide or permanganate. In contrast, Huang et al.8 and Liang et al.9 found that thermally activated PS could completely degrade chlorinated solvent compounds including TCA. PS has advantages as an ISCO reagent because of its high redox potential (E0 = 2.01 V) and its long persistence in the subsurface. It can diffuse into lower permeability strata and contact with contaminants in soil and groundwater. Even though PS can act as a direct oxidant, its reaction rates are limited for more recalcitrant contaminants. Fortunately, the kinetics of PS oxidation can be enhanced significantly through the generation of sulfate radicals (SO4•−, E0 = 2.6 V) by PS activation processes including heat,10 UV light,11,12 transition metals,13 base,14 soil minerals,15 ultrasound,16 and radiolysis,17 etc. So far, numerous investigations have been focused on the activation of PS. For example, thermal activation of PS has been studied for treating diuron and poly(vinyl alcohol).18,19 In addition, Liang et al.20 found that iron(II), but not iron(III), activated PS and proposed using thiosulfate to regenerate iron(II) after it was oxidized by PS activation. Recently, Received: Revised: Accepted: Published: 1056

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and the oven temperature of 100 °C. Extracts containing AN and NB were analyzed on a GC fitted with a HP-5 column (30 m length, 320 μm i.d., and 0.25 μm thickness) and a flame ionization detector. The injector and detector port temperatures were 200 and 250 °C, respectively. The oven temperatures for AN and NB were 155 and 170 °C, respectively. Electron paramagnetic resonance (EPR) detection was conducted to confirm the main free radicals present in the thermally activated PS system in which DMPO was used to trap the free radicals in solution. Typically, samples (0.2 mL) were taken at the desired time and thoroughly mixed with DMPO solution (1.8 mL, 80 mM). After mixing liquid was transferred to a capillary tube with a microinjector for further detection with the EPR instrument. The concentration of PS was determined by a spectrophotometric method using potassium iodide.23

zerovalent iron activated PS was investigated for the oxidation of poly(vinyl alcohol).19 Also much attention has been paid to base activated PS by maintaining the system at pH 12 or adding an equal or excess molar ratio of base to PS, and the advantage of the latter is that the pH of the groundwater returns to nearneutral when the reactions complete. Our previous research has already studied the degradation of TCA in a thermally activated PS system, and reductive degradation of TCA by zerovalent iron in a soil slurry system.22 The results strongly suggested that TCA could be degraded through both oxidative and reductive ways. However, some reactive oxygen species (ROS) are generated during PS propagation reactions in thermally activated PS systems, and their roles in TCA degradation have not been fully investigated. Therefore, the purposes of this research were (1) to investigate the generation of the ROS in thermally activated PS systems by using chemical probes to detect ROS generation and assess ROS intensity by probe degradation rates and (2) to evaluate ROS roles in TCA degradation performance by comparing the changes of TCA degradation rates with or without ROS scavengers.

3. RESULTS AND DISCUSSION 3.1. Performance of TCA Degradation by PS Oxidation. 3.1.1. Performance of TCA Degradation. The normalized remaining TCA concentrations (Ci/C0) versus reaction time are illustrated in Figure 1. The experiments were

2. MATERIALS AND METHODS 2.1. Materials. 1,1,1-Trichloroethane (99.0%), sodium persulfate (98.0%), tert-butyl alcohol (TBA, 99.0%), nitrobenzene (NB, 99%), isopropanol (IPA, 99.7%), sodium bicarbonate (99.5%), and potassium iodide (99.0%) were purchased from Shanghai Jingchun Reagent Co. Ltd. (Shanghai, China). Carbon tetrachloride (CT, 99.5%) and n-hexane (97%) were purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd. Anisole (AN, 99%) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were purchased from Alfa Aesar, A Johnson Matthey Co. All of the reagents were used without purification. Ultrapure water from a Milli-Q water process (Classic DI, ELGA, Marlow, U.K.) was used for preparing aqueous solutions. 2.2. Experimental Procedures. All reactions were conducted in 24 mL borosilicate vials capped with poly(tetrafluoroethylene) (PTFE) lined septa under controlled temperature (50 °C). TCA, PS, NB, AN, and CT solutions were prepared by dissolving reagents at room temperature into deionized water to make stock solutions. Each stock solution was added to the volumetric flask at a desired concentration; then a series of reaction vials was fully filled. Control tests were carried out in parallel without PS addition. At each designated time the samples were removed from the reaction vials, chilled to 4 °C in an ice bath for 5 min to quench the reaction, and then analyzed. The initial pH in all tests was unadjusted. 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. The organic phase (TCA in hexane) was then transferred to a 2 mL gas chromatography (GC) vial with a plastic dropper. TCA was analyzed using a gas chromatograph (Agilent 7890A, Palo Alto, CA) equipped with an electron capture detector, an autosampler (Agilent 7693), and an DB-VRX column (60 m length, 250 μm i.d., and 1.4 μm thickness). The temperatures of the injector and detector were 240 and 260 °C, respectively, and the oven temperature was held constant at 75 °C. Hexane extracts were analyzed for CT using GC with electron capture detector by injections into a 60 m length, 250 μm i.d., and 1.4 μm thickness capillary column. Chromatographic parameters included injector and detector temperatures of 240 and 260 °C,

Figure 1. TCA degradation in thermally activated PS system (50 °C, [TCA]0 = 0.15 mM).

carried out by fixing the TCA concentration to 0.15 mM and varying the concentration of PS from 3 to 300 mM (the molar ratio of PS/TCA, from 20/1 to 2000/1). The control test without PS addition revealed less than 2% TCA loss due to thermolysis and/or volatilization at 50 °C. The results showed clearly that TCA degradation was highly PS dependent. For example, complete TCA degradation was observed for reactions carried out at PS concentrations of 300 and 30 mM over 20 and 60 min, respectively, whereas, partial (60%) but apparent TCA degradation happened after 180 min at 3 mM PS concentration. It is deduced that more SO4•−, HO•, or O2•− were generated in solution as PS concentration increased, therefore leading to a considerably higher TCA degradation rate. These results were in agreement with previous findings in ibuprofen removal by heated PS system in which ibuprofen degraded faster when the initial PS concentration increased.24 The TCA degradation pattern in a thermally activated PS system exhibited pseudo-first-order kinetics at 50 °C. Under pseudo-first-order kinetics the rate of TCA degradation is 1057

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directly proportional to the amount of undegraded TCA, which can be expressed as follows:

−dC i /dt = kobsC i

(1)

Equation 1 can be modified as follows: ln(C i /C0) = −kobst

(2)

Half-life (t1/2) can be obtained as:

t1/2 = ln 2/kobs

(3)

where C0 is the initial TCA concentration in aqueous phase (mM); Ci is the TCA concentration at time t (mM); kobs is the pseudo-first-order reaction rate constant that represents an overall rate of TCA degradation by a variety of oxidizing or reducing species (e.g., SO4•−, HO•, and O2•−) generated in the system (min−1); and t is the reaction time (min). The observed reaction rate constants, the correlation coefficient (R2), and the calculated t1/2 are summarized in Table 1. All of the R2 values were greater than 0.93, showing

Figure 2. PS decomposition in a thermally activated PS system (50 °C; [TCA]0 = 0.15 mM).

Table 1. TCA Degradation Performance [PS]0 (mM)

kobs (min−1)

R2

t1/2 (min)

kobs,PS × 105 (min−1)

3 30 300

0.0050 0.0305 0.0964

0.99 0.96 0.93

138 23 7.2

39.8 26.7 8.4

other reaction responsible for PS consumption might be an acid-catalyzed reaction involving the unsymmetrical rupture of the peroxide bond of the persulfate anions to form sulfur tetroxide and bisulfate,28 as shown in eq 5. The critical complex for the acid-catalyzed reaction is believed to be HS2O8−.28 With the influence of the associated hydrogen ion, it decomposes unsymmetrically, giving both electrons of the O−O bond to one fragment to form sulfur tetroxide and a bisulfate ion.

that the data well fitted the pseudo-first-order kinetic model. For an initial TCA level of 0.15 mM, the pseudo-first-order reaction rate constant was increased from 0.0050 to 0.0305 and 0.0964 min−1 with the increase of PS concentration from 3 to 30 and 300 mM, respectively, which indicated that the rate of TCA degradation was directly related to the initial PS concentration. The t1/2 was calculated upon eq 3, and t1/2 reached less than 8 min for the reaction undertaken at 300 mM PS concentration and were about 23 and 138 min for 30 and 3 mM, respectively. The above results strongly implied that higher PS concentration might generate more SO4•−, HO•, or O2•− in solution, therefore promoting TCA degradation significantly. 3.1.2. PS consumption. As shown in Figure 2, the overall consumption of PS was less than 8% of the PS dosage during the whole 3 h reaction period. The consumption of PS also followed the pseudo-first-order kinetics, similar to the previous report by other researchers.9 The values of kobs,PS obtained from this study ranged between 8.4 × 105 and 39.8 × 105 min−1, which was consistent with the data reported by Johnson et al. at similar experimental conditions.25 One can notice from the data assembled in Table. 1 that kobs,PS values for all three PS concentrations varied depending on the initial concentration of PS. Even the PS consumption rate decreased with increasing of the initial PS concentration; the net lost quantities of PS were 1.19, 0.351, and 0.0544 mmol at 300, 30, and 3 mM of initial PS concentrations after 3 h reaction, showing the most PS consumption in the initial PS concentration of 300 mM. This observation was consistent with Sra et al.’s finding.26 Two main independent reactions may have occurred simultaneously in relation to PS consumption. Homolysis of the peroxide bond in an aqueous phase during thermal PS activation generates two SO4•− as shown in eq 4. Couttenye et al. proved SO4•− formation under heat-catalyzed PS decomposition by EPR.27 Once SO4•− was formed, it might initiate a series of chain reactions in a thermally activated PS system. The

S2 O82 − + heat → 2SO4•−

(4)

S2 O82 − + H+ → HS2 O8− → SO4 + HSO4 −

(5)

In general, a higher PS concentration generates a larger H+ concentration (eq 6). More H+ generated in the system will accelerate the proceedure of eq 5 and later lead to more PS consumption. This was consistent to our finding that more PS was consumed when the PS dosage was high. S2 O82 − + H 2O → 2HSO4 − + 1/2 O2

(6)

It is believed that during PS consumption, some ROS such as SO4•−, HO•, and O2•− are generated, and it is assumed that these generated oxidative or reductive species are responsible for TCA degradation. Therefore, the investigation of the presence of those ROS and their contribution to TCA degradation was emphasized in the following sections. 3.2. Generation of Reactive Oxygen Species. 3.2.1. Detection of ROS in a Thermally Activated PS System. Chemical probe method is applied to investigate the presence of ROS in a thermally activated PS system. As the probe compound AN is highly reactive with both SO4•− and HO•,29,30 and no compounds are available that react solely with SO4•−, therefore, AN was selected as a probe for both SO4•− and HO•. The combined generation of SO4•− and HO• in the PS (0.3 M) system measured by AN loss is shown in Figure 3a. More than 99% of AN was oxidized in 180 min, indicating that significant fluxes of SO4•−, HO•, or both were generated in the system. The generation of HO• in a thermally activated PS (0.3 M) system, measured by NB loss, is shown in Figure 3b. Complete NB loss was reached in 300 min, proving that HO• was generated in the system. It is reported28 that HO• can be generated in acid and neutral pH or alkaline conditions through 1058

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SO4•− + OH− → SO4 2 − + HO•

(8)

However, NB oxidation was likely to be slower because NB reacts only with HO•, whereas AN reacts with both HO• and SO4•−. In addition, HO• reacts slightly more rapidly with AN (kHO• = 5.4 × 109 M−1 s−1) than it does with NB (kHO• = 3.9 × 109 M−1 s−1). Furthermore, our previous study showed that 1 M tert-butyl alcohol could sweep almost all of the HO• in the system, but AN was still partially oxidized in the test with the presence of HO• scavenger tert-butyl alcohol (data not shown), therefore demonstrating the presence of both SO4•− and HO• in the system. It is also noted that an activated PS system is sometimes more efficient than other ISCO systems, as the stoichiometry of radical generation in the PS system is significantly more productive than in the CHP system, which is attributable to the large number of propagation reactions involved in the system.31 Besides oxidative species generation, a thermally activated PS system may also have potential generating reducing species such as O2•−. In order to detect the presence of O2•−, CT, a highly oxidized compound, was used and the presence of O2•− was assessed by CT loss, as shown in Figure 3c. In the control test, 12% CT was lost likely due to volatilization and/or hydrolysis; in contrast, CT was completely degraded in 4 days in a thermally activated PS (30 mM) system, indicating the presence of O2•− in the system. Based on the experimental data and literature survey, possible comprehensive pathways for the generation of O2•− in a thermally activated PS system could be derived as eqs 9−11.32,33 2HO• → H 2O2

(9)

H 2O2 + HO• → H 2O + HO2•

(10)

HO2• ↔ H+ + O2•−

(11)

O2•−

It is also reported that is a weak nucleophile and reductant that can be generated in a CHP system34 and in a base activated PS system.35 In a CHP system O2•− is responsible for the degradation of highly oxidized organic contaminants (e.g., CT),36 for the enhanced desorption of hydrophobic organic contaminants,37 and the enhanced treatment of DNAPLs.38 O2•− generation has also been documented in base activated PS systems35 and is likely to be an important pathway that contributes to the widespread reactivity of PS formulations. Due to these characteristics, the potential to degrade highly oxidized organic contaminants greatly increases PS application in ISCO techniques. 3.2.2. Effect of PS Concentration on the Intensity of the Generated ROS. Four PS concentration levels (30, 100, 300, and 500 mM) were employed in the experiments to investigate the influence of oxidant concentration on the intensity of generated ROS, which could be evaluated by the calculated rate constant of chemical probe degradation using the pseudo-firstorder kinetics model. Apparently the degradation of three chemical probes showed a positive dependence on PS concentration (Figure 3 and Table 2). As shown in Figure 3a, increasing PS concentration significantly enhanced the removal of AN, and the rate constants increased from 0.00155 to 0.00432, 0.01355, and 0.02798 min−1 with the PS concentrations increasing from 30 to 100, 300, and 500 mM, respectively (Table 2). It is easily understood that an increased level of PS after activation by heat would lead to an increased generation level of SO4•− and HO•,

Figure 3. Effect of PS concentration on the intensity of the generated ROS: (a) AN; (b) NB; (c) CT. (Conditions: 50 °C; [AN]0 = 1 mM; [NB]0 = 1 mM; [CT]0 = 50 μM; [PS] = (■) control and (●) 30, (▲) 100, (▼) 300, and (◆) 500 mM.)

the reactions between SO4•− and H2O or SO4•− and OH−, as shown in eqs 7 and 8. SO4•− + H 2O → SO4 2 − + HO• + H+

(7) 1059

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Table 2. kobs of Three Chemical Probe Compounds in Various PS Concentrations PS (mM)

30

100

300

500

AN kobs (min−1) R2 NB kobs (min−1) R2 CT kobs (day−1) R2

0.00155 0.9984 0.00053 0.9714 0.00739 0.9689

0.00432 0.9871 0.00185 0.9912 0.10377 0.9719

0.01355 0.9737 0.00677 0.9925 0.68637 0.9977

0.02798 0.9529 0.01268 0.9649 1.4346 0.9560

which in turn, promote an increased degree of organic contaminants destruction. The results shown in Figure 3b and Table 2 demonstrate that higher concentration of PS could promote the rapid loss of NB in a thermally activated PS system. This conclusion was also demonstrated from the results shown in Figure 1; i.e., a higher concentration of PS resulted in higher levels of SO4•− or HO• formation and therefore higher TCA degradation rate. The above results were also confirmed by other researchers when treating aniline and acid orange 7 contaminants at various PS concentrations.39,40 Xie et al. studied four different PS concentrations and found that the rates of aniline degradation were directly proportional to the initial PS concentration. Yang et al. found that an increased level of PS would lead to generation of an increased level of SO4•−, which in turn, may promote increased acid orange 7 destruction. Figure 3c illustrated that, upon increasing the PS concentration, a faster and more efficient transformation of CT occurred. With the increase of PS concentration, CT conversion improved from 10 to 99% within 2 days. It is found that there was less CT degradation when keeping the PS concentration at 30 mM in this study. Huang et al.8 studied the degradation of CT by 5 g/L PS at 40 °C for 120 h and observed no CT concentration change in a deionized water matrix, but CT was degraded in a volatile organic compounds (VOCs) mixture, which implied that CT degradation in VOCs mixture was not through the oxidative mechanisms associated with PS. It was suspected that the discrepancies in results between the early work and this study were due to the different conditions applied in the tests. The generation of O2•− may be highly related with the temperature and PS concentration in thermally activated PS systems; i.e., low temperature and less PS concentration would not generate O2•− radicals effectively. It should be noted that 99% TCA was degraded within 60 min when the PS concentration was kept at 30 mM (Figure 1). However, in the initial 60 min there was hardly any CT degradation, and the same observation was also proved in the PS concentration of 300 mM. Hou et al.41 studied the removal of tetracycline by magnetite activated PS in the presence of ultrasound irradiation and also found that the formation of O2•− during the reaction was significantly delayed. Therefore, it could be confirmed that TCA degradation in a thermally activated PS system was through the oxidation mechanism rather than the reduction mechanism. 3.2.3. Effect of PS Persistence on the Intensity of the Generated ROS in the System. In order to investigate the effect of PS persistence on the generation of ROS in the subsurface, chemical probe compounds were added to the prethermally activated PS solution (300 mM) at different times and the probe compounds’ degradation rate constants were compared with the control tests (zero time). Figure 4 shows the probe compounds degradation results at 0, 1, 2, and 12 h

Figure 4. Effect of PS persistence on the intensity of the generated ROS in the system: 9a) AN; (b) NB; (c) CT. (Conditions: 50 °C; [AN]0 = 1 mM; [NB]0 = 1 mM; [CT]0 = 50 μM; [PS]0 = 300 mM; (■) 0, (●) 1, (▲) 2, and (▼) 12 h.)

after PS activation by heat, respectively. From the results presented in Figure 4a,b, the calculated rate constants for AN degradation were 0.0136, 0.0235, 0.0240, and 0.0126 min−1 and those for NB degradation were 0.0068, 0.0093, 0.0134, and 0.0065 min−1 at 0, 1, 2, and 12 h, respectively. Apparently, 1060

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within the initial 2 h the intensities of oxidative species increased slightly, indicating more SO4•− and HO• were generated in the system, but after 12 h SO4•− and HO• intensities were slightly reduced. The possible reasons maybe as follows: (a) the free radicals quenched with the reaction proceeding according to eqs 9, 12, 13, and 14; and (b) slightly less residual PS was left after 12 h (the PS concentrations were 298, 296, and 289 mM at 1, 2, and 12 h, respectively, compared to 300 mM at the beginning), reducing the rate of SO4•− and HO• generation. S2 O82 − + SO4•− → SO4 2 − + S2 O8•−

(12)

SO4•− + HO• → HSO5−

(13)

HO2• + H 2O2 → OH• + O2 + H 2O

(14)

In contrast, as shown in Figure 4c, the calculated CT degradation rate constants were 0.0286, 0.0093, 0.0134, and 0.0065 h−1 for 0, 1, 2, and 12 h, respectively, indicating a slight decrease compared with the control test. It is possible that O2•− generation in thermally activated PS systems maintained a roughly stable level after reaction, and this O2•− intensity reduction may be simply due to the quenching by PS or other species (eqs 15−17). HO• + O2•− → OH− + O2

(15)

HO2• + O2•− → HO2− + O2

(16)

S2 O82 − + O2•− → product

(17)

Figure 5. Effects of scavengers on the degradation of TCA (50 °C; [TCA]0 = 0.15 mM; [PS]0 = 30 mM; [TBA] = 30 mM; [IPA] = 30 mM).

EPR technique using DMPO as a spin trapping agent was also applied to detect and identify the radical species in a thermally activated PS system in Figure 6. Compared with the

3.2.4. Identification of Predominant ROS and Reaction Mechanism for TCA Degradation in a Thermally Activated PS System. According to our previous study, O2•− has less effect on TCA degradation due to its late generation, or quenching by PS or other species, so SO4•− and HO• are considered to be the main ROS responsible for TCA degradation when PS is activated thermally. In order to evaluate the contribution of the reactive species to TCA removal, two kinds of scavengers were employed: TBA and IPA, which have been successfully applied to distinguish SO4•− vs HO• activity in an iron activated PS system.42 Literature investigation indicated that alcohols containing an α-hydrogen, such as IPA, react at high and comparable rates with SO4•− and HO• (kHO• = 1.9 × 109 M−1 s−1 and kSO4•− = 8.2 × 107 M−1 s−1).30,43,44 However, TBA, without an α-hydrogen has much different reaction rate constants, and the rate constant for HO• (kOH• = 5.2 × 108 M−1 s−1) is 600-fold higher than the rate constant for SO4•− (kSO4•− = 8.4 × 105 M−1 s−1). Based on these properties, TBA could be used to scavenge HO•, while IPA, for both SO4•− and HO•. As shown in Figure 5, scavenging with IPA resulted in only 1% TCA loss and scavenging with TBA resulted in 3% TCA loss. The results clearly showed that the dominant reactive oxygen species responsible for TCA degradation in thermally activated PS systems is HO• rather than SO4•−. Liang and Su found that HO• was a dominant ROS in a thermally activated PS system at basic pH and SO4•− was the predominant radical at pH < 7.10 Although SO4•− and HO• have traditionally been considered the primary ROS in an activated PS system, which also has been confirmed in the present study, the results of this research clearly showed that HO• was responsible for TCA degradation in the thermally activated PS system.

Figure 6. EPR spectrum of radicals generated in the thermally activated PS system after 10 min reaction.

standard spectra, a peak for HO• was evident in the EPR spectrum; unfortunately, the presence of SO4•− or O2•− was difficult to determine, likely due to low fluxes of the radicals in the system. Zhao et al.45 also used EPR spectra to illustrate the characteristic spectra for SO4•−, HO•, and O2•− produced in PS treatment system. They found that HO• was the main species generated in thermal activation of PS, while SO4•− and O2•− were also generated but in weak intensities. In our previous study, TCA dechlorination performance and various intermediates were analyzed in a thermally activated PS system in TCA degradation.21 Compared with the theoretical TCA mineralization efficiency, only 61.3% of the total theoretical chloride ions yielded and some chlorinated intermediates such as trichloromethane, dichloromethane, and formic acid were identified by GC/MS. The proposed two different pathways, dechlorination and C−C bond breakage being responsible for TCA degradation, were due to the effect 1061

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of HO• which was the predominant radical species in the thermally activated PS system. Neta et al.46 reported that SO4•− is a relatively specific oxidant; it does not react rapidly with oxidized organic compounds such as those with a high degree of chlorine and nitrate substitution. Generally, SO4•− is more prone to electron transfer reactions than HO•. In contrast to SO4•−, HO• more readily undergoes hydrogen abstraction or addition. This suggested, from another aspect that HO• is responsible for TCA degradation in the thermally activated PS system.

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4. CONCLUSION This study made an effort to explore TCA degradation in a thermally activated PS system in aqueous solution at 50 °C. Experimental results showed that TCA degradation and PS decomposition could be well fitted by a pseudo-first-order kinetics model, and increasing PS concentration would result in an increase in the TCA degradation rate. The chemical probe method was applied to assess the ROS generation of SO4•−, HO•, and O2•− in the system and found more PS concentration will generate more SO4•−, HO•, and O2•−. The tests for PS persistence in solution showed oxidative species were strengthened during the initial 2 h, indicating more SO4•− and HO• were generated in the system, whereas after 12 h SO4•− and HO• intensities were slightly reduced. O2•− generated in a thermally activated PS system maintained at a roughly stable level after reaction, but at a slightly lower intensity simply due to the quenching by PS or other species. Finally, HO• was identified as the predominant radical species responsible for TCA degradation, and two different pathways, dechlorination and C−C bond breakage, were proposed as the TCA degradation mechanism in a thermally activated PS system. The above encouraging results strongly demonstrated that thermally activated PS can rapidly and effectively degrade the target contaminant, and this technique provides a viable alternative to TCA contaminated groundwater remediation.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 64250709. Fax: +86 21 64252737. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by a grant from the National Environmental Protection Public Welfare Science and Technology Research Program of China (Grant No. 201109013), the National Natural Science Foundation of China (Grant Nos. 41373094 and 51208199), the Shanghai Natural Science Funds (Grant No. 12ZR1408000), China Postdoctoral Science Foundation (Grant No. 2013T60429), and the Fundamental Research Funds for the Central Universities.

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