Enhanced Degradation of Tetrachloroethylene by Green Rusts with

Apr 4, 2008 - R&D Center, Samsung Engineering Co., LTD. ... 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, South Korea ... Corresponding author pho...
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Environ. Sci. Technol. 2008, 42, 3356–3362

Enhanced Degradation of Tetrachloroethylene by Green Rusts with Platinum J E O N G Y U N C H O I † A N D W O O J I N L E E * ,‡ R&D Center, Samsung Engineering Co., LTD. 415-10, Wancheon-Dong, Youngtong-Gu, Suwan-city, 443-823, South Korea, and Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, South Korea

Received October 29, 2007. Revised manuscript received February 13, 2008. Accepted February 14, 2008.

This study presents an experiment which characterizes reductive dechlorination of tetrachloroethylene (PCE) by green rusts (GRs) in the presence of Pt using a batch reactor system. Relative to GR alone, the rate of PCE reduction in GR suspensions was greatly enhanced with the addition of Pt(IV) (95% of PCE was removed in 30 h). PCE was mostly transformed to a nonchlorinated byproduct, acetylene rather than trichloroethylene, and the carbon mass recovery was 98% at the last sampling point. The reduction of PCE was four times faster for GR-F(Pt) than for GR-CO3(Pt), mainly due to the higher Fe(II) content of GR-F. The estimated kinetic rate constants of GR-Cl(Pt) increased significantly (i.e., 0.17, 0.21, and 1.01 h-1, respectively) with an incremental addition of Pt from 0.5 to 2 mM. X-ray diffraction analysis showed the transformation of GR to magnetite as an oxidation product. X-ray photoelectron spectroscopy analysis revealed that the oxidation was coupled to the reduction of Pt (IV to 0) on the GR surfaces. The scanning electron microscope with energy dispersive spectrometer measurement showed the formation of Pt particles on the surfaces of GRs modified with the Pt(IV).

Introduction Chlorinated solvents are prevalent organic contaminants found in the soil and groundwater of the United States. They have been widely used in various industrial processes such as surface coating, dry cleaning, and metal degreasing during the past half-century. Considerable efforts have been made up to the present in order to remove these chemical contaminants from wastewater and groundwater as they are potential human carcinogens and mutagens (1, 2). In the late 1990s, a layered Fe(II)-Fe(III) hydroxide solid with a different type of anion such as Cl-, SO42-, CO32-, and F- in its interlayer was introduced to the field of environmental chemistry. Called green rusts (GRs), this solid was demonstrated to effectively reduce a wide range of soil and groundwater contaminants. GRs have been found as intermediate corrosion products of iron in natural alkaline suboxic soil and sediments (3). There are different types of GR depending on the type of the anions in the interlayer. GR 1 * Corresponding author phone: +82-42-869-3624; fax: +82-42869-3610; e-mail: [email protected]. † Samsung Engineering Co., LTD. ‡ Korea Advanced Institute of Science and Technology. 3356

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represents the GRs that have a planar anion such as Cl-, F-, and CO32-. GR 2 has an anion with a three-dimensional structure, such as SO42- and SeO42-. GR 1 and GR 2 can be identified through X-ray diffraction (XRD) analysis (4). One of the important characteristics of GR for the purpose of environmental engineering is its high reductive capacity for a variety of organic and inorganic contaminants in soil and groundwater systems. The reductions of many toxic contaminants by GRs are known to be thermodynamically favorable in suboxic condition (5). Carbon tetrachloride (CT) was reductively dechlorinated by GR-SO4 (6). Also, chlorinated ethylenes (tetrachloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), and vinyl chloride (VC)) were successively transformed by GR-SO4 (7, 8). While GRs reportedly reduce organic contaminants, the slow reaction kinetics of GRs has posed an obstacle to their application to the remediation of contaminated groundwater and soil and the treatment of wastewater. In comparison, recent studies have focused more on the enhancement of reductive dechlorination of chlorinated organics by iron-bearing reductants (zerovalent iron (ZVI) and GR) with reactive additives such as trace metals and reducing agents. The addition of transition metals to ironbearing reductants was reported to raise their reactivity for the reductive dechlorination by up to 3 orders of magnitude (5, 9–11). In particular, this increase was accelerated by the addition of noble metals (Pd, Pt, Au, and Ru) to the ironbearing reductants (9–11). The modification of GRs by the reactive additives has been explored for the enhancement of reductive dechlorination by GRs. A substantial improvement in the reductive dechlorination by GRs was observed when Ag(I), Au(III), and Cu(II) were added to GR-SO4 and GR-Cl (5, 12, 13). The transition metals were bound on GR surfaces and reduced to a zerovalent state. They were believed to act as a catalyst by facilitating the electron transfer from GRs to chlorinated organic compounds (5). However, there was no attempt to determine the effect of Pt on the reactivity of GRs to date even though Pt showed a considerable increase of reactivity for the reductive dechlorination by ZVI (10). Therefore, the reactivity of GRs in the presence of Pt for the reductive dechlorination of PCE was investigated through batch kinetic experiments in this study. Four different GRs (GR-Cl, GR-SO4, GR-CO3, and GR-F) were used to characterize the reductive dechlorination of PCE with Pt. The effect of anion type and concentration of Pt on the reaction kinetics was identified in detail. Finally, the scanning electron microscope (SEM) with energy dispersive spectrometer (EDS), XRD, and X-ray photoelectron spectroscopy (XPS) were used to explore the reaction mechanism of Pt modification on the surfaces of GRs.

Experimental Section Materials. Chemicals used in the experiment include: PCE (99.9+%, HPLC grade, Aldrich), TCE (99.6%, Sigma), cis-DCE (97%, Aldrich), trans-DCE (trans-dichloroethylene, 98%, Aldrich), 1,1-DCE (1,1-dichloroethylene, 99%, Aldrich), ferrous chloride (tetrahydrate, 99%, Aldrich), ferrous sulfate (heptahydrate, 99+%, Aldrich), sodium hydroxide (97.0% min. EM), sodium carbonate (99.5+%, Sigma), sodium fluoride (99.8%, J.T. Baker), and platinum chloride (PtCl4, 98%, Aldrich). A mixed gas (1% CO, CO2, methane, ethane, ethylene, and acetylene in nitrogen, respectively, Alltech) was used as a standard for the analysis of nonchlorinated transformation products. Four types of GRs were synthesized by the partial air oxidation method developed by Genin and co-workers (14–16). The methods of GR synthesis are described in the 10.1021/es702661d CCC: $40.75

 2008 American Chemical Society

Published on Web 04/04/2008

Supporting Information. To prevent the oxidation of GR, all experiments were conducted in an anoxic chamber (Coy Laboratory Products, Inc.) containing an atmosphere of 5% hydrogen and 95% nitrogen. Experimental Procedures. The suspension pH of washed GRs was initially adjusted to 7.5 using an appropriate amount of HCl and NaOH, and then Pt was introduced to each suspension so as to generate 0.5, 1, and 2 mM, respectively. GR suspension modified by Pt (GR(Pt)) was finally produced by mixing GR and Pt continuously for 10 min with a magnetic stirrer. Kinetic experiments for PCE degradation were conducted using borosilicate glass vials (24.3 ( 0.1 mL) with a three-layered septum system (PTFE film, lead foil, and PTFE film lined rubber septum). An aliquot amount (24.2 mL) was taken from the GR as well as from GR(Pt) suspensions (13.9 g/L) mixed with a magnetic stirrer, with each amount transferred to the glass vials. PCE stock solution in methanol (596 mM) was freshly prepared at each test. The reaction was initiated by spiking 10 µL of the PCE stock solution into the GR and GR(Pt) suspensions using a 10 µL gastight syringe (Hamilton). The vials were rapidly capped, taken out of the chamber, and placed on a tumbler that provided end-overend rotation at 7 rpm at the room temperature (25 ( 1 °C). The degradation kinetics of PCE was determined by monitoring the PCE concentration in its aqueous phase at each sampling point. GR and GR(Pt) samples were prepared in duplicate. Controls (without GR and GR(Pt)) were also prepared in duplicate to investigate the potential loss (i.e., volatilization and sorption) of target compound in the same way mentioned above. Analytical Procedures. PCE and its transformation products were analyzed by gas chromatography. PCE, TCE, and DCEs were measured with the following steps. The retrieved vials were centrifuged at 2960g for 20 min. A 50 µL volume of supernatant was transferred to 2 mL vials containing 1 mL extractant (hexane with 0.025 mM of 1,2-dibromopropane). They were extracted for 1 h on an orbital shaker and placed on an autosampler installed on a gas chromatograph (Hewlett-Packard 6890 GC) equipped with DB-VRX column and electron capture detector. Nonchlorinated byproducts (ethane, ethylene, and acetylene) were analyzed by HP 6890 GC equipped with GC-Alumina column and flame ionization detector. A 10 mL aliquot of supernatant produced by centrifugation was transferred into a 20 mL vial and capped quickly. The vials were shaken for 1 h using an orbital shaker to equilibrate the gas and liquid phases, which were then allowed to stand for 1 h at room temperature. A 100 µL sample of the gas phase was extracted with a 100 µL gastight syringe (Hamilton) and manually injected into the injection port of GC. The dimensionless Henry’s law constants used to calculate the aqueous phase concentrations of ethane, ethylene, and acetylene showed 20.4, 8.7, and 1.1, respectively (17). This study also carried out surface sensitive analyses (XPS and SEM) and XRD to identify the reaction mechanism of kinetic enhancement of GRs in the presence of Pt. The analytical procedures are described in the Supporting Information. The concentrations of iron and anions were measured using the UV–vis spectrophotometer and the ion chromatograph, respectively. More details about the methods are provided in the Supporting Information.

Results and Discussion Effect of Pt Addition on the Reductive Dechlorination of PCE by GR. Figure 1(a) shows the degradation kinetics of PCE by GR-SO4 in the absence and presence of 0.5 mM Pt at pH 7.5. The initial concentration of the target compound in the control sample decreased to 0.21 mM (89% of initial PCE concentration) at the first sampling point and remained relatively constant afterward. The initial decrease of PCE was

due to the sorption of PCE on the surface of septum lining and reactor wall and/or volatilization during the sample preparation process (7, 17). The concentration of PCE in GR-SO4 suspension did not diminish much, remaining quite similar to that in the control sample by the end of reaction (50 h). In contrast to GR-SO4, GR-SO4(Pt) showed a remarkable increase in the reductive capacity for the degradation of PCE. It degraded the target compound fully in 30 h and increased the formation of acetylene conspicuously over the reaction time. Based on the reaction pathway for the reductive dechlorination of PCE proposed by Arnold and Roberts, PCE can be initially transformed to TCE or dichloroacetylene and further degraded to acetylene, ethylene, and ethane (18). In the previous study of PCE degradation with GR and GR with a transition metal, several transformation products such as TCE, acetylene, ethylene, and ethane were observed (7, 13). However, no detectable level of transformation products except acetylene were observed in the current research, and the total carbon mass balance of target and transformation products was approximately 98% at the last sampling point. In the Figure 1(a), the gray square indicates acetylene concentration and the open circle shows the sum of PCE and acetylene concentrations. Similar observations on the rapid transformation of PCE to nonchlorinated compounds were also reported, especially when reductants were treated with a noble metal like Pd (19, 20). The kinetic data of decreasing PCE by GR-SO4(Pt) was fitted by the following pseudo first-order kinetic model. dCL,PCE ) - kapp,PCE · CL,PCE dt

(1)

where CL,PCE is the concentration of PCE in its aqueous phase; kapp,PCE is the pseudo first-order rate constant. The formation of the transformation product (acetylene) was estimated by the following equation: dCL,AC ) kAC(C0 - CL,AC) dt

(2)

where CL,AC is the concentration of acetylene formed in the aqueous phase; C0 is the initial PCE concentration; kAC is the first-order rate constant for the formation of acetylene. The kinetic rate constants (kapp,PCE and kAC) were obtained by conducting the nonlinear regression of PCE concentration in the aqueous phase using a Gauss–Newton algorithm in the MATLAB (MathWorks Inc.). The experimental error (95% confidence level) for the kinetic rate constants was calculated using the “nlparci” function in MATLAB. The results for the degradation of PCE and formation of acetylene estimated by the kinetic models for GR-SO4(Pt) are presented in Figure (1a) with solid lines. The degradation of PCE was fit well by the first-order kinetic model. The rate constant was estimated to be 0.0993 ( 0.0058 h-1, approximately 2 orders of magnitude higher than that of GR-SO4 without Pt (Supporting Information Table S2). The kinetic rate constant for the formation of acetylene was 0.0767 ( 0.021 h-1. The addition of Pt into GR in this study hence provided an enhancement in the reductive dechlorination of chlorinated aliphatic hydrocarbons similar to other transition metals (Ag, Au, and Cu) studied in the previous research (5, 12, 13). Effect of GR Type on the Reductive Dechlorination of PCE. The effect of GR type on the reductive dechlorination kinetics was not clear in the GRs suspensions without Pt, since the reaction rates of the GRs were too slow to be differentiated (Supporting Information Table S2). The concentration range of PCE removed by GRs without Pt over 10 days was 0.195 ∼ 0.211 mM when 0.245 mM of PCE was initially spiked into the GRs suspensions. VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. (a) Results of kinetic experiments on PCE degradation by GR-SO4 in the absence and presence of 0.5 mM Pt at pH 7.5, (b) Results of kinetic experiments for the PCE degradation by GR-Cl, GR-SO4, GR-CO3, and GR-F in the presence of 1 mM Pt at pH 7.5, and (c) Dependence of degradation rates of GR-Cl(Pt) and GR-SO4(Pt) on Pt concentration. The error bars for the rate constants in Figure (1c) represent the 95% confidence intervals. However, the effect of GR type on the reductive dechlorination kinetics was apparent in the GRs(Pt) suspensions at Figure (1b). GR-F(Pt) showed the fastest reaction kinetics followed by GR-Cl(Pt), GR-SO4(Pt), and GR-CO3(Pt). 85% of PCE added was removed by GR-F(Pt) in 4.8 h, whereas it took 20 h to remove it by GR-CO3(Pt). The estimated kinetic rate constant of GR-F(Pt) was greater than that of GR-CO3(Pt) by a factor of 4 as shown in Supporting Information Table S2. It has been reported that GR-Cl had faster reaction kinetics than GR-SO4 and GR-CO3 for the nitrate reduction by a factor of 20–30 (3). These results confirm that the reaction kinetics for the reductive dechlorination of PCE is dependent on the type of GRs, or more precisely on the type of structural anions in GRs. We speculate that the effect of GR type on the reductive dechlorination kinetics is due to the Fe(II) contents of the different types of GR. We noticed that GRs containing monovalent anions were more reactive than those with divalent anions for the reductive dechlorination of PCE and nitrate reduction. This can be explained by the ideal chemical formulas of GRs as shown in Supporting Information Table S1. Note that the ratio of Fe(II) to Fe(III) is three for GRs containing monovalent anions such as F- and Cl- and two for GRs with divalent anions such as SO42- and CO32- (3). GRs with monovalent anions containing a greater Fe(II) content with their higher Fe(II) 3358

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to Fe(III) ratio showed faster reaction kinetics than those with divalent anions for the reductive dechlorination of the target compound. This is experimentally confirmed by the measured Fe(II) contents of GRs(Pt) shown in Supporting Information Table S2, where the Fe(II) contents of GR-F and GR-Cl (77.1 and 78.1 mM) are higher than those of GR-SO4 and GR-CO3 (74.4 and 70.1 mM). In addition, the different reactivity of GRs might be affected significantly by the specific surface area and size of Pt on the surfaces of GRs. We observed no differences in the distribution of transformation products among the different types of GRs. Acetylene was the only transformation product observed in all experimental runs by GRs(Pt) and the carbon mass balance (PCE + acetylene) at the last sampling point varied in the range of 81-95%. This result is consistent with previous studies which have also found no significant dependence of the product distribution on the GR types. In the previous experiments on the reductive dechlorination of CT by GRs, 45.0% of CF and 4.0% of CH4 formed in GR-SO4 suspension and 65.5% of CF and 8.7% of CH4 formed in GR-Cl suspension, respectively. However, no CF and 25% of CH4 were detected when GRs was modified with Cu (5, 13). Based on our experimental results as well as from the previous studies, therefore, we can suggest that the distribution of chlorinated and nonchlorinated transformation products of

FIGURE 2. Resutls of surfac analyses (XPS and XRD) of GR-Cl(Pt). XPS analysis of GR-Cl(Pt): (a) full scan spectrum, (b) narrow region scans of Pt(4f7/2), (c) Fe(2p3/2), and (d) O(1s), and (e) XRD analysis of GR-Cl(Pt): M and G represent magnetite and GR, respectively. PCE may depend on the presence of the transition metals in GR suspensions rather than the type of GRs. Effect of Pt Cncentration on the Reductive Dechlorination of PCE by GR. Figure (1c) presents the effect of Pt concentration on the reductive dechlorination kinetics of PCE. Two types of GRs (GR-Cl and GR-SO4) were used to identify the effect of Pt concentration and its concentration was set to 0.5, 1, and 2 mM. The increase of Pt concentration raised the reductive dechlorination rate of PCE in both GR-Cl

and GR-SO4 suspensions. The estimated rate constants for the reductive dechlorination of the target compound by GR-SO4(Pt) increased linearly with a slope of 0.3 h-1 mM-1. GR-Cl(Pt) showed the substantial increase of rate constants with the increase of Pt concentration, compared to GR-SO4(Pt). The estimated rate constant of GR-Cl(Pt) rose from 0.21to 1.02 h-1 as Pt concentration increased from 1 to 2 mM, which was 2.5 times greater than the increase in GR-SO4. The reason of higher reactivity of GR-Cl than VOL. 42, NO. 9, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. SEM image and EDS spectra of GR-Cl(Pt). GR-SO4 was not clearly identified in this study. We propose that the accelerated increase may be due to its higher Fe(II) content, which implies that more Pt bound on the surfaces of GR-Cl can play a reactive role in the reductive dechlorination of PCE. The observed effects of Pt concentration may help us better understand the role of Pt in the GR suspensions during the reductive dechlorination of PCE. Since Pt was added to the experimental systems as Pt(IV), it could not serve as a reductant for the reductive dechlorination of PCE. Rather, the dependence of the reductive dechlorination rate on the Pt concentration may be explained by the role of Pt as a catalyst in assisting the electron transfer from GR surfaces to the target chlorinated compound (5, 10). The catalytic role of transition metals in the reductive dechlorination of chlorinated compounds in systems containing iron(II)-bearing soil minerals and zerovalent metals has been well recognized in the existing research. For instance, Ag(I), Au(III), and Cu(II) added into GR suspensions and Pd(II), Pt(IV), and Ni(II) into ZVI suspensions showed faster reductive dechlorination kinetics by the strong reductive capacities due to their catalytic roles (5, 10, 21). It has been conjectured that the transition metals added into solid reductant suspensions are (i) initially bound on the surfaces of the solid reductants, (ii) reduced to their zerovalent forms, and (iii) attack target chlorinated compounds adsorbed on the surfaces of the reductants, facilitating the electron transfer from the reductants to the target compounds (5, 22). Yet no direct evidence on the catalytic role of Pt during the reductive dechlorination of PCE by GRs has been reported to support this model. The results of surface analysis shown in the next section might help to explain the reaction mechanism of GR(Pt), lending strong support to the hypothesis on the catalytic role of Pt suggested in this research. 3360

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Surface Analyses to Identify the Reaction Mechanism of GRs(Pt). XPS analysis was conducted to verify the redox state of Pt on the surfaces of GR-Cl after the addition of Pt(IV) to the GR-Cl suspension. The concentration of Pt was set to 10 mM because the XPS analysis could not clearly detect low levels of Pt concentration (i.e., e 2 mM). The full scan spectra in Figure (2a) distinctly identified the four chemical elements (Fe, Cl, O, and Pt), which were the main chemical components of GR-Cl(Pt). Figure 2(b)-(d) shows the narrow scan of three elements (Pt(4f), Fe(2p3/2), and O(1s)) with their binding energies. The two peaks at 71.27 and 74.39 eV in Pt(4f) spectrum (Figure 2(b)) represent Pt(4f7/ 2) and Pt(4f5/2), respectively, which are consistent with those of metallic zerovalent Pt (Pt(0)) (23). The Pt(4f5/2) peak could not be resolved from the Al 2P peak of the Al foil used to mount the sample. The transformation of Pt(IV) to metallic Pt(0) clearly supports the hypothesis that the enhanced reductive dechlorination rates may be accounted for by the catalytic role of Pt(0). Figure 2(c) shows the narrow region spectra for Fe (2p3/2) which were composed of three identical peaks at 709.5 (area percentage ) 24.1%), 711.0 (37.8%), and 713.0 eV (38.1%), respectively. The binding energy for Fe(II)-O and Fe(III)-O has been reported to be in the range of 709-709.5 eV and 711-714 eV, respectively (24, 25). We can, therefore, conclude that iron on the GR surfaces with Pt was composed of approximately 24% of Fe(II) and 76% of Fe(III). The substantial decline in the Fe(II) to Fe(III) ratio of GR-Cl(Pt) compared to that of GR-Cl indicates that electrons were transferred via coupled surface Fe(II) oxidation to reduce Pt(IV) to Pt(0) on the GR-Cl surfaces; oxidation of GR-Cl(Pt) by atmospheric oxygen during the sample preparation may also contribute to the low Fe(II) content on the GR surfaces. The result of O 1s spectrum shown in Figure 2(d) can be resolved into two peaks at 529.68 and 531.42 eV

representing O2- and OH-, respectively (24, 25). Since GR was composed of Fe(II)-Fe(III) hydroxide layer and anions in interlayer as shown in Supporting Information Table S1, the presence of O2- species suggests that GR may be transformed to other iron bearing minerals, possibly iron oxide, during the reaction with Pt. We did not perform the XPS analysis of GR-Cl(Pt) after the reductive dechlorination of PCE in this research. XRD analysis was conducted to identify the transformation product of GR during the reaction with Pt. Figure 2(e) shows the diffractograms of GR-Cl and GR-Cl(Pt). The peak pattern of GR samples was identical to that of GR-Cl provided in the Joint Committee on Powder Diffraction Standards (JCPDS). We observed notable changes in the XRD pattern of GR-Cl when the GR-Cl was mixed with 2 mM Pt for 1 h. The d-space values of three main peaks for GR-Cl(Pt) were 2.52, 1.48, and 1.61 Å. These values are consistent with those of magnetite (FeIIFe2IIIO4) in JCPDS indicating the oxidative transformation of GR-Cl to magnetite during the reaction with Pt. The current result appears to fully support the suggestion based on the aforementioned result of XPS analysis, i.e., the formation of iron oxide as an oxidation product of GR. A few other research groups have identified magnetite as the oxidation product of GRs during the redox reaction (7, 13). The following equations can be proposed for the chemical reactions among Pt(VI), GR-Cl, and PCE in this research. Note that Pt(0) in eq 4 acts as a catalyst. 4+ 0 f 16FeII1 FeIII 12FeII3 FeIII 1 (OH)8 · Cl + 5Pt 2 O4 + 5Pt + 12Cl +

32H+ + 32H2O (3) 0 0 II III 18FeII3 FeIII 1 (OH)8·Cl + Pt + 5C2Cl4 f 24Fe1 Fe2 O4 + Pt +

5C2H2 + 38Cl- + 38H+ + 48H2O (4) SEM images of GR-Cl(Pt) and GR-SO4(Pt) were taken to demonstrate the micromorphology of zerovalent Pt on the surfaces of GRs. Figure 3(a) and (b) clearly demonstrate that hundreds of nanosized particles (see regions within the circles) were associated onto the surfaces of GR-Cl with dark gray hexagonal shapes. EDS analysis on the surfaces of the circles in Figure 3(a) and (b) shows that the main chemical components are O, Cl, Fe, and Pt (Figure 3(d)). These results of XPS, XRD, and SEM with EDS above therefore allow us to conclude that the nanosized particles on the GR surfaces are zerovalent Pt. Figure 3(c), at 10 times lower magnification than Figure 3(a), shows that the bright Pt(0) particles in the circles were aggregated on the GR surfaces. The formation of nanosized zerovalent transition metals on the surfaces of reactive solids has been reported by O’Loughlin and coworkers (26). The results from the surface analyses can help to explain the reaction mechanism of GRs(Pt). The enhanced reactivity of GRs(Pt) for the reductive dechlorination of PCE is mainly due to the formation of reactive nanosized Pt(0), which is caused by the electron transfer via coupled oxidation of Fe(II) to Fe(III) on the surfaces of GRs resulting in the oxidative transformation of GR to iron oxide. The nanosized Pt(0) may play an important role as a catalyst in enhancing the rate of reductive dechlorination. The catalytic role of the Pt(0) in the reductive dechlorination of PCE may be repeatedly carried out until the electron transfer by the surface oxidation of Fe(II) ends and/or the reductive dechlorination is fully achieved (5). The results obtained in this study demonstrate the effects of significant factors (i.e., Pt addition, GR type, Pt concentration, and surface characteristics) on the reaction kinetics and product distribution for the reductive dechlorination of PCE by GRs(Pt). The current results thus provide a key to understanding the reductive degradation of PCE by GRs (Pt). In particular, the results of XPS and SEM analyses

providing the surface characteristics of GR(Pt) are crucial for elucidating the mechanism of the catalytic effect in the enhanced reductive dechlorination.

Acknowledgments We thank Prof. Soyoung Kim of KAIST and three anonymous reviewers for the critical reviews to enhance the quality of this manuscript. This research was supported by the grants from the Korean Ministry of Science and Technology (R01-2006-000-10727-0) and Korea Research Foundation (KRF-2007-313-D00439). The contents do not necessarily reflect the views and polices of the sponsors nor does the mention of trade names or commercial products constitute endorsement or recommendation for use.

Supporting Information Available The synthetic method of GRs, Analytical method for XPS, XRD, and SEM, The changes of pH in solution and ratio of Fe(II) to Fe(III) in solid during the oxidation of Fe(OH)2 in the synthesis (Figure S1), Diffraction patterns for the synthesized GRs (Figure S2), SEM image of GRs (Figure S3), The chemical recipe for GR synthesis and ideal chemical formulas for GRs (Table S1), and Fe(II) content and obtained first order rate constants of GRs and GRs(Pt) (Table S2). This material is available free of charge via the Internet at http:// pubs.acs.org.

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