Front. Environ. Sci. Eng. DOI 10.1007/s11783-015-0798-6

RESEARCH ARTICLE

Degradation of carbon tetrachloride in thermally activated persulfate system in the presence of formic acid Minhui XU, Xiaogang GU, Shuguang LU (✉), Zhouwei MIAO, Xueke ZANG, Xiaoliang WU, Zhaofu QIU, 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

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2015

Abstract The thermally activated persulfate (PS) degradation of carbon tetrachloride (CT) in the presence of formic acid (FA) was investigated. The results indicated that CT degradation followed a zero order kinetic model, and CO2– $ was responsible for the degradation of CT confirmed by radical scavenger tests. CT degradation rate increased with increasing PS or FA dosage, and the initial CT had no effect on CT degradation rate. However, the initial solution pH had effect on the degradation of CT, and the best CT degradation occurred at initial pH 6. Cl– had a negative effect on CT degradation, and high concentration of Cl– displayed much strong inhibition. Ten mmol$L–1HCO3– promoted CT degradation, while 100 mmol$L–1NO3– inhibited the degradation of CT, but SO24 – promoted CT degradation in the presence of FA. The measured Cl– concentration released into solution along with CT degradation was 75.8% of the total theoretical dechlorination yield, but no chlorinated intermediates were detected. The split of C-Cl was proposed as the possible reaction pathways in CT degradation. In conclusion, this study strongly demonstrated that the thermally activated PS system in the presence of FA is a promising technique in in situ chemical oxidation (ISCO) remediation for CT contaminated site. Keywords persulfate, carbon tetrachloride, thermal activation, formic acid, carbon dioxide radical anion

1

Introduction

Carbon tetrachloride (CT), a kind of chlorinated aliphatic hydrocarbon, has been widely utilized as cleaning Received March 5, 2015; accepted May 28, 2015 E-mail: [email protected]

solvents, degreasing agents, extractants and intermediates for commercial and industrial purposes for several decades. It has been frequently detected in the National Priority List (NPL) sites of USA [1]. The appearance of CT in soil and groundwater caused by improper use and disposal has been of great concern because of the potential toxicity and recalcitrant characteristics. CT is highly toxic to liver, lung and kidney and has been classified as a carcinogenic contaminant by the US Environmental Protection Agency (EPA) due to its high toxicity, thermal stability, and environmental persistence properties. In addition, CT is recalcitrant and does not degrade naturally and thus easily be accumulated in soil, sediment, and groundwater, causing serious hazardous waste remediation problem to the ecosystem. Therefore, the concentration of CT in drinking water has been regulated, and set by the maximum contaminant level at 0.005 mg$L–1 in the USA and 0.002 mg$L–1 in China [2], and hence it is urgent to develop an effective technique for CT destruction in contaminated soil and/or groundwater. In situ chemical oxidation (ISCO) is becoming an effective method for in situ soil and groundwater remediation, which is capable of breaking down many contaminants in solid or aqueous phases. By using ISCO, amenable hazardous organic contaminants are subject to either fast and complete destruction or partial decomposition as a means of preliminary treatment prior to further remediation. ISCO process involves the introduction of chemical oxidants such as Fenton, catalyzed hydrogen peroxide propagations (CHP, a modified Fenton’s process), permanganate, percarbonate and persulfate (PS) into the subsurface to break down organic contaminants. The application of peroxide, percarbonate or PS results in radical-based ISCO processes. Hydrogen peroxide (H2O2) and percarbonate can be activated using Fe2+ to form hydroxyl radicals (HO$, E0 = 2.8 V) [3], while PS can be activated to form sulfate radical (SO4– $, E0 = 2.6 V).

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Among the ISCO oxidants, PS is a promising oxidant. ISCO using PS has been widely documented to treat aquifers contaminated by chlorinated hydrocarbons including trichloroethane and trichloroethylene [4,5]. Generally, PS can be activated by heat, UV, transition metals, metal oxides, ultrasound, alkali, quinones or phenols to generate SO4– $ [6–8]. SO4– $ is relatively more stable and selective than HO$ for oxidation of organic contaminants, thereby allowing for greater dispersion distance and better mineralization of contaminants in groundwater. Among the transition metals, ferrous iron has been widely used to activate PS since it is relatively inexpensive, non-toxic and effective, and the activation reaction would be carried out under acidic conditions [9]. UV light cannot make effective transport in groundwater. Alkaline activation needs large amounts of alkaline substance to neutralize capacity of the groundwater. Under thermally enhanced PS oxidation condition, there is considerable evidence that the peroxide bond of PS will be broken to generate two SO4– $, as shown in Eq. (1) [10]. Then SO4– $ initiates a series of chain reactions involving other oxidative and reductive species generation. HO$ can be formed via Eq. (2), and especially generated via Eq. (3) in alkaline pH [11]. HO$ may participate in contaminant oxidation and more readily undergoes hydrogen abstraction or addition mechanism, while SO4– $ is more prone to electron transfer reactions [3,12]. H2O2 might be produced through the combination of HO$ and the hydrolysis of PS via Eqs. (4)–(5) [13]. Then perhydroxyl radical (HO2$, E0 = 1.7 V) would generate by radical involving reactions between H2O2 and HO$/SO4– $ (Eqs. (6)–(7)) [14]. HO2$ is not a reductant and is only a weak oxidant, and at the same time superoxide radical anion (O2– $, E0 = – 2.4 V) is generated from the propagation reactions as shown in Eq. (8) [15]. S2 O28 – þ heat↕ ↓2SO4– $

(1)

All  pH : SO4– $ þ H2 O↕ ↓HO$ þ Hþ þ SO24 –

(2)

Alkaline  pH : SO4– $ þ OH – ↕ ↓HO$ þ SO24 –

(3)

HO$ þ HO$↕ ↓H2 O2

(4)

S2 O28 – þ H2 O↕ ↓H2 O2 þ HSO4–

(5)

HO$ þ H2 O2 ↕ ↓HO2 $ þ H2 O

(6)

SO4– $

þ

þ H2 O2 ↕ ↓HO2 $ þ H þ

HO2 $ ↔

O2– $

þ

SO24 –

þ H pKa ¼ 4:8

oethylene [4,16], and the addition of formate to the thermally activated PS system has been successfully applied to degrade trichloroacetic acid [17]. To our best knowledge, the performance of perchlorinated hydrocarbons reduction in the thermally activated PS system in the presence of formic acid (FA) has never been studied, and it is expected that thermally activated PS system in the presence of FA may generate more reductive radicals that can degrade CT efficiently. In the presence of FA, SO4– $ and HO$ generated in the chain reaction are scavenged to yield $CO2H via Eqs. (9)–(10). Carbon dioxide radical anion (CO2– $) would form from the propagation reactions as shown in Eq. (11) [18]. CO2– $ is a strong reducing species with a redox potential of – 2.0 V vs. NHE (normal hydrogen electrode), and it is able to reduce many organic and inorganic compounds. Therefore, the objectives of this study are 1) to investigate the performance of CT degradation in the thermally activated PS system in the presence of FA; 2) to identify the main reactive oxygen species responsible for CT degradation; and 3) to evaluate the influence of the initial solution pH and solution matrix on the performance of CT degradation. SO4– $ þ HCOOH↕ ↓SO24 – þ $CO2 H þ Hþ

(9)

HO$ þ HCOOH↕ ↓H2 O þ $CO2 H

(10)

$CO2 H ↔ CO2– $ þ Hþ

(11)

2

Materials and methods

2.1

Materials

Sodium persulfate (98.0%), sodium nitrate (99.5%), sodium bicarbonate (99.5%), sodium chloride (99.5%), sodium sulfate (99.5%), formic acid (99.0%), 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., China. Methyl viologen dichloride (MV2+) was purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). Ultrapure water from a Milli-Q water process (Classic DI, ELGA, Marlow, UK) was used for preparing aqueous solutions. 2.2

Experimental procedures

(7) (8)

Thermally activated PS system has been demonstrated to effectively degrade the toxic trichloroethane and trichlor-

All reactions were conducted in a series of 24 mL borosilicate vials capped with polytetrafluoroethylene (PTFE) lined septa under 50°C. Based on our previous study, the temperature was set at 50°C due to the efficient contaminant degradation under this temperature condition

Minhui XU et al. Carbon tetrachloride degradation by persulfate with formic acid

and the characteristic of CT with the boiling point of 76.8°C [4]. CT and PS solutions were prepared separately by dissolving reagents at room temperature into Milli-Q water to make stock solutions. Each stock solution and FA were added to the volumetric flask at desired concentration, then a series of 24-mL reaction vials were fully filled immediately. All reaction vials were placed in a temperature-controlled water bath, and reaction started simultaneously. 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 solution pHs in all tests were unadjusted except in the test for investigating the influence of pH. All experiments were conducted in duplicate and the mean values reported. 2.3

3

2.77, hence inducing more CT thermolysis in much lower pH solution. In addition, less than 7.0% CT was degraded with 0.5 mol$L–1 PS dosage in the system within 180 min, as observed in Fig. 1. This was contributed to the generation of O2– $ which was responsible for the degradation of CT in the system and also had been demonstrated in our previous study [20]. Although O2– $ is a reactive radical for CT degradation in the thermally activated system, O2– $ is characterized by low reactivity in pure water due to its high degree solvation, reduced lifetime and reactivity of O2– $, therefore, CT degradation was relatively slow in the thermally activated PS system.

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 (CT in hexane) was then transferred to a 2-mL GC vial with a plastic dropper. CT was analyzed using a gas chromatograph (Agilent 7890A, Palo Alto, CA, USA) 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°C and 260°C, respectively, and the oven temperature was held constant at 100°C. The volatile organic intermediates produced during CT degradation were identified by the EPA SW-846 Method 8260B using an automatic purge and trap (Tekmar Atomx, Mason, OH, USA) coupled to a GC/MS (Agilent 7890/5975) with the same DB-VRX column. The chloride anion was analyzed by ion chromatography (Dionex ICS-I000, Sunnyvale, CA, USA). The concentration of PS was determined by a spectrophotometric method using potassium iodide [19].

3

Results and discussion

3.1

Performance of CT degradation in PS system

3.1.1 Enhanced CT degradation with the presence of FA in PS system

Figure 1 shows the CT degradation performance in the thermally activated PS system in the presence of FA as well as in different control groups. Around 2.3% and 3.4% CT lost in the CT alone group and in CT solution with the presence of FA at 50°C within 180 min reaction time, respectively, most likely due to the thermolysis and volatilization of CT in solution. When FA was mixed with CT solution, the solution pH dropped from 4.75 to

Fig. 1 CT degradation in the thermally activated PS system in the presence of FA (50°C, [CT]0 = 10 μmol$L–1, [PS] = 20 mmol$L–1, [FA] = 30 mmol$L–1)

However, the degradation of CT was significantly increased when FA was added into the thermally activated PS system, as shown in Fig. 1. Complete CT degradation was achieved for reactions carried out at PS concentration of 20 mmol$L–1 and FA concentration of 30 mmol$L–1 over 180 min. CT degradation rate followed zero order rate kinetics, as shown by the straight lines in Fig. 1. It is in agreement with the results reported by Mora et al. who studied the degradation of trichloroacetic acid by the thermal activation of PS in the presence of sodium formate, and concluded that the decay of trichloroacetic acid and initial formation of Cl– followed zero order rate kinetics [17]. The addition of FA to the solution can change the oxidative capacity of the reactive mixture into a reductive one, since CO2– $ can transfer an electron rapidly to quinones, nitro and nitroso compounds, pyridinium and viologen ions, porphyrins, oxygen, and many other organic and inorganic compounds [21]. Therefore, it is deduced that the degradation of CT in the thermally activated PS system in the presence of FA was due to the role of CO2– $.

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3.1.2 Identification of the main radicals being responsible for CT degradation

The reduction of methyl viologen by CO2– $ arising from the interaction of radiation-generated radicals with formate in aqueous solution has been demonstrated previously by Mulazzani et al. [22] Berkovic et al. also used methyl viologen as the scavenger of CO2– $ in the reduction of Hg (II) by CO2– $ through the laser flash photolysis tests, and obvious effect of methyl viologen was observed in the system [23]. Since methyl viologen is known to react with CO2– $ as shown in Eq. (12), it is employed as a competitive scavenger of CO2– $ to evaluate the role of CO2– $ for CT degradation in this study [24]. CO2– $ þ MV2þ ↕ ↓MVþ $ þ CO2

(12)

Figure 2 shows clearly that a significant decrease in the degradation rate of CT occurred when methyl viologen was added. The addition of 0.1 mmol$L–1 and 1.0 mmol$L–1 methyl viologen resulted in 61.3% and 26.1% CT removals in 180 min, respectively. CO2– $ generated in the system was scavenged by methyl viologen, which resulted in lower efficiency in CT degradation. Therefore, CO2– $ was proved to be the predominant radical responsible for CT degradation in the thermally activated PS system with the presence of FA in this study.

Fig. 2 Effect of scavenger on CT degradation (50°C, [CT]0 = 10 μmol$L–1, [PS] = 20 mmol$L–1, [FA] = 30 mmol$L–1)

3.2 Effects of PS and FA dosages and initial CT concentration on CT degradation performance

degradation was achieved in 180 min, and in contrast, CT degraded 40.0% and 9.8% with the PS dosages of 10 and 5.0 mmol$L–1, respectively. The straight lines of the degradation curves indicated that CT degradation at various PS dosages followed zero order rate kinetics. The slope values obtained from the straight lines were 0.0114, 0.0068, 0.0056, 0.0020 and 0.0008 with the PS dosages of 60, 30, 20, 10 and 5.0 mmol$L–1, respectively. The results showed that the rate of CT degradation increased as the initial PS dosage increased. With the increase of PS dosage, the generated amount of SO4– $ and HO$ were elevated and more CO2– $ produced through Eqs. (9)–(11). Gonzalez et al. studied the photochemically induced mineralization of CT with methanol addition [25]. The amount of reductive radicals increased with the increasing of oxidant H2O2 addition and thus elevated the degradation rate of CT. Liang et al. found that when PS dosage was more than 6.75 mmol$L–1 in the degradation of trichloroethylene in the ferrous activated PS system, it only resulted in less increase in trichloroethylene removal [5]. Similarly, excess amount of CO2– $ would lead to radicalradical recombination, as shown in Eq. (13), which might, in turn, inhibit CT degradation rate [26]. CO2– $ þ CO2– ↕ ↓– OOC-COO –

Various dosages of FA were applied to evaluate the effect of FA on CT degradation performance and the results are displayed in Fig. 3(b). It could be seen that CT removal efficiency increased with the increase of FA dosage from 5.0 to 100 mmol$L–1. Complete CT degradation could be achieved in 180 min when the FA dosage was over 30 mmol$L–1, while CT degraded 80.6%, 56.7% and 46.3% with the FA dosages of 20, 10 and 5.0 mmol$L–1, respectively. The increased dosage of FA induced the reactions, as shown in Eqs. (9)–(10), therefore more CO2– $ were generated in the system. The variation of CT degradation efficiency in the thermally activated system in the presence of FA under different initial CT concentrations, ranging from 0.01 mmol$L–1 to 1.0 mmol$L–1, was also investigated and the results are shown in Fig. 3(c). CT degradation rate followed zero order rate kinetics as shown in Fig. 3(c). When the initial CT concentration increased, the slopes of these lines were almost the same, suggesting the CT degradation rates increased in proportion to the initial CT concentration. 3.3

To study the effect of PS dosage on CT degradation in the thermally activated PS system in the presence of FA, various PS dosages were applied in the tests. The resulted CT removal corresponding to different PS dosages was measured and the results are shown in Fig. 3(a). When the PS dosage was more than 20 mmol$L–1, complete CT

(13)

Effect of solution matrix on CT degradation

3.3.1 Effect of initial solution pH on CT degradation performance

The initial solution pH values investigated in this study were 3, 6, 9 and 12 with PS and FA dosages of

Minhui XU et al. Carbon tetrachloride degradation by persulfate with formic acid

5

Fig. 4 Effect of initial solution pH on CT degradation. (a) Cl–; (b) HCO3– ; (c) NO3– ; (d) SO24 – . (50°C, [CT]0 = 10 μmol$L–1, [PS] = 20 mmol$L–1, [FA] = 30 mmol$L–1)

20 mmol$L–1 and 30 mmol$L–1, respectively. As shown in Fig. 4, the rate constants of CT degradation in the system were in the following order: pH 6 > pH 9 > pH 3 > unadjusted pH > pH 12. CT was degraded completely at pH 3, 6 and 9 in 150 min, while only 25.7% CT removal was acquired after 180 min at pH 12. CT removal in alkaline condition was extremely limited. Flyunt et al. studied proton-catalyzed disproportionation of CO2– $ in aqueous solution [27]. They found that the yields of the products of the bimolecular decay of CO2– $ were strongly dependent on the solution pH with an inflection point at pH 3.8. As shown in Table 1, all pHs dropped to nearly acidic condition after 0.5 h except for pH 12, and these results were consistent with Flyunt’s result that there was a peak on CT removal efficiency. As pKa of Eq. (11) is 2.8 [28], therefore CT degradations at the initial pH 6 and 9 were higher than those at pH 3 and unadjusted pH groups. Mora et al. also found that trichloroacetic acid degraded faster in pH 6–7 than pH 3–4 in the thermally activated PS system in the presence of formate [17]. 3.3.2

Fig. 3 Effect of dosages of PS and FA, and initial CT concentration on CT degradation. (a) PS (50°C, [CT]0 = 10 μmol$L–1, [FA] = 30 mmol$L–1); (b) FA (50°C, [CT]0 = 10 μmol$L–1, [PS] = 20 mmol$L–1); (c) CT (50°C, [PS] = 20 mmol$L–1, [FA] = 30 mmol$L–1)

Effect of solution matrix on CT degradation

As the reactivity of oxidant and reductant in contaminated groundwater system might be affected by the presence of background ions, the influences of Cl–, HCO3– , NO3– and SO24 – anions on CT degradation performance were tested individually with 1.0, 10 and 100 mmol$L–1 concentrations of each anion. As shown in Fig. 5(a), Cl– revealed a negative effect on CT degradation. The inhibitive effect increased with the increasing of Cl– concentration, and a significant decrease in the rate of CT degradation was observed upon addition

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Table 1 pH variation along with reaction at different initial solution pH conditions pH

initial

30 min

90 min

final (180 min)

unadjusted pH

2.59

2.52

2.45

2.33

pH = 3

3.02

2.97

2.85

2.61

pH = 6

5.98

4.86

4.44

3.50

pH = 9

8.90

5.16

4.64

3.50

pH = 12

12.00

11.97

11.92

11.86

Fig. 5

Effect of solution matrix on CT degradation (50°C, [CT]0 = 10 μmol$L–1, [PS] = 20 mmol$L–1, [FA] = 30 mmol$L–1)

of 100 mmol$L–1 Cl–. The inhibition caused by Cl– might be due to the reactions among SO4– $, HO$ and Cl– as shown in Eqs. (14)–(17), hence preventing the generation of CO2– $ [21,29]. House also supposed the inhibition of mercuric chloride reduction by halide ions through such equations as shown in Eqs. (14)–(17) [10]. SO4– $ þ Cl – ↕ ↓SO24 – þ Cl$

(14)

HO$ þ Cl – ↕ ↓ClOH – $

(15)

ClOH – $ þ Hþ ↕ ↓HClOH$

(16)

HClOH$↕ ↓Cl$ þ H2 O

(17)

The effect of HCO3– on CT degradation is displayed in Fig. 5(b). There is no obvious difference between 1.0 mmol$L–1HCO3– and the test without HCO3– . CT degradation was promoted when the HCO3– concentration was 10 mmol$L–1, while the inhibitive effect was observed with HCO3– concentration of 100 mmol$L–1. There might

Minhui XU et al. Carbon tetrachloride degradation by persulfate with formic acid

be two possible reasons accounted for the promoting effect with addition of 10 mmol$L–1HCO3– : 1) the initial solution pH changed to 3.43 when 10 mmol$L–1HCO3– was added to the solution. Under this pH condition, the CT degradation rate was higher than unadjusted pH condition; 2) HCO3– might have an activation effect on PS. When HCO3– was added to the solution, peroxymonocarbonate ion (HCO4– ) would be produced during the reactions. HCO4– is a reactive species in the reaction that may promote the degradation of pollutants during the chain reactions. Regino and Richardson studied the bicarbonatecatalyzed hydrogen peroxide oxidation of cysteine and related thiols [30]. The observed degradation rate of the target compound was increased when HCO3– was added in the system. When 100 mmol$L–1HCO3– was added to the solution, the solution pH during the reaction changed from 6.86 to 6.79, and CT degradation was less efficient than the unadjusted pH group. Under such high concentration of HCO3– , HCO3– might react with CO2– $ directly as shown in Eq. (18), therefore inhibited the degradation of CT [31]. HCO3– might also react with SO4– $ and HO$ as Eqs. (19) and (20), then reduce the generation of CO2– $ through Eqs. (4)–(8), and finally inhibited CT degradation [32]. CO2– $ þ HCO3– ↕ ↓CO2 þ CO23 – þ H$

(18)

SO4– $ þ HCO3– ↕ ↓CO3– $ þ SO24 – þ Hþ

(19)

HO$ þ HCO3– ↕ ↓CO3– $ þ H2 O

(20)

Figure 5(c) shows the effect of NO3– on CT degradation. 1.0 and 10 mmol$L–1NO3– did not display remarkable impact on CT degradation, while 100 mmol$L–1NO3– had an obvious inhibition on CT degradation. When a large amount of NO3– was present in the solution, NO3– might compete with CT for CO2– $, hence decreased CT degradation rate. The effect of SO24 – on CT degradation is shown in Fig. 5(d). The presence of SO24 – had a positive effect on CT degradation, and CT degradation rate increased with the increasing of SO24 – concentration. SO24 – might participate in the chain reactions through Eqs. (21) and (22) and increased the total radical concentration, therefore, more CO2– $ produced through Eqs. (9)–(11), and later promoted CT degradation.

3.4

SO24 – þ Hþ ↕ ↓HSO4–

(21)

HSO4– þ HO$↕ ↓SO4– $ þ H2 O

(22)

Investigation of CT degradation mechanism

The decomposition of PS and the release of Cl– along with CT degradation were analyzed. As shown in Fig. 6, complete CT degradation was achieved in 180 min with

7

the PS and FA dosages of 20 mmol$L–1 and 30 mmol$L–1 at 50°C. Around 25.0% PS was consumed in the180 min reaction time and the decomposition of PS might contain two possible reactive pathways. On one hand, the peroxide bond of PS homolysised to generate two SO4– $ (Eq. (1)) when activated by heat, then a series of propagation reactions were initiated by SO4– $, and the predominant radical species CO2– $ which was responsible for CT degradation was generated in the system. On the other hand, PS decomposition was due to the hydrolysis under acidified catalytic condition. In acidic solution, the hydrolysis of PS involves the reactions as shown in Eqs. (23) and (24) [14], i.e., PS hydrolysis to form HSO5– and the unstable intermediate will further decompose in acidic condition to form H2O2 and HSO4– . Due to the formation of acid caused by the decomposition of PS, the solution pH decreased along with the reaction. It could also be observed in Fig. 6 that PS decomposition increased after 120 min compared with the initial 120 min which might be due to the reaction between S2 O28 – and CO2– $ (Eq. (25)) when the CT concentration was relatively low [10].

Fig. 6 Performance of CT degradation versus PS decomposition and Cl– release. (50°C, [CT]0 = 0.1 mmol$L–1, [PS] = 20 mmol $L–1, [FA] = 30 mmol$L–1)

S2 O28 – þ H2 O↕ ↓HSO5– þ HSO4–

(23)

HSO5– þ H2 O↕ ↓H2 O2 þ HSO4–

(24)

S2 O28 – þ CO2– $↕ ↓SO4– $ þ SO24 – þ CO2

(25)

Assuming that complete dechlorination of 1 mol of CT would yield 4 mol of Cl–, the measured Cl– concentration in the solution was 75.8% of the total theoretical CT dechlorination yield. The organic chlorine in CT that did not release as the form of Cl– might be due to the formation of chloridated organic intermediates or inorganic oxy-

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chloride such as chlorite, chlorate and perchlorate [25]. Moreover, it is interesting to note that the concentration of Cl– began to decrease after 180 min. The Cl– released from CT dechlorination might be oxidized by SO4– $ or HO$ through Eqs. (14)–(17). Unfortunately, no chlorinated intermediate byproducts were detected by GC-MS analysis. CT degradation in the thermally activated PS system in the presence of FA is speculated to follow the pathway as below: The C-Cl bond of CT splits by CO2– $ and yields CCl3$ and Cl– [33]. CCl3$ undergoes proton abstraction to form CHCl3 or further cleaves C-Cl bond to generate dichlorocarbene [34]. Dichlorocarbene in solution may hydrolyze and decompose to CO2, Cl– and HCOO– [35].

4

Conclusions

This study confirmed that CT could be effectively degraded in the thermally activated PS system in the presence of FA at 50°C, and CT degradation followed a zero order kinetic model. Radical scavenger tests revealed that CO2– $ was responsible for the degradation of CT. CT degradation rate increased with increasing of PS or FA dosage, and no obvious effect of the initial CT concentration on CT degradation rate was observed. The effect of initial solution pH was investigated and the results showed that the fast degradation occurred at initial pH 6. Cl– had a negative effect on CT degradation, and higher concentration of Cl– resulted in higher inhibition. Ten mmol$L–1HCO3– promoted CT degradation, while 100 mmol$L–1NO3– inhibited the degradation of CT. However, SO24 – could promote CT degradation in the thermally activated PS system in the presence of FA. The measured Cl- concentration was 75.8% of the total theoretical CT dechlorination yield, but no chlorinated intermediate byproducts were detected. The split of C-Cl was proposed as the possible reaction pathways for CT degradation. In conclusion, this study demonstrated that thermally activated PS system in the presence of FA is a promising technique in ISCO remediation for degrading perchlorinated hydrocarbons, such as CT contaminated site.

3.

4.

5.

6.

7.

8.

9.

10. 11.

12.

13.

14. Acknowledgements This study was financially supported by a grant from the National Natural Science Foundation of China (Grant Nos. 41373094 and 51208199).

15.

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