Dechlorination of Tetrachloroethylene in Aqueous Solutions Using

Chlorinated hydrocarbons such as tetrachloroethylene (PCE) and trichloroethylene (TCE) are the most often found toxic chemicals in contaminated aquife...
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Environ. Sci. Technol. 2008, 42, 4752–4757

Dechlorination of Tetrachloroethylene in Aqueous Solutions Using Metal-Modified Zerovalent Silicon CHUN-CHI LEE AND RUEY-AN DOONG* Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101, Section 2, Kuang Fu Road, Hsinchu, 30013, Taiwan

Received June 25, 2007. Revised manuscript received February 29, 2008. Accepted April 9, 2008.

The combination of zerovalent metal with a catalytic second metal ion (bimetallic materials) to enhance the dechlorination efficiency and rate of chlorinated compounds has received much attention.Bimetallicmaterialsnotonlyenhancethedechlorination process but also alter the reduction pathway and product distribution. In this study, the efficiency and rate of tetrachloroethylene (PCE) dechlorination by metal-modified zerovalent silicon was investigated as a potential reductant for chlorinated hydrocarbons under anoxic conditions. The X-ray photoelectron spectroscopic (XPS) results showed that metal ions including Ni(II), Cu(II), and Fe(II) could be reduced to their zerovalent forms on the Si surface. The dechlorination of PCE obeyed the pseudofirst-order kinetics, and the pseudo-first-order rate constants (kobs) for PCE dechlorination followed the order Ni/Si > Fe/Si > Cu/Si. Addition of Cu(II) lowered the dechlorination efficiency and rate of PCE by Si, while the kobs values for PCE dechlorination in the presence of 0.1 mM Fe(II) and Ni(II) were 1.5-3.8 times higher than that by Si alone. In addition, the efficiency and rate of PCE dechlorination increased upon increasing the mass loading of Ni(II) ranging between 0.05 and 0.5 mM and then decreased when the Ni(II) loading was further increased to 1 mM. The scanning electron microscopic (SEM) images and electron probe microanalytical (EPMA) maps showed that the Ni nanoparticles deposited on the Si surface and aggregated to a large particle at 1 mM Ni(II), which clearly depicts that the Ni(II) loading of 0.5 mM is the optimal value to enhance the efficiency and rate of PCE dechlorination by Si. Also, the reaction pathways for PCE dechlorination changed from hydrogenolysis in the absence of Ni(II) to hydrodechlorination when Ni(II) concentrations were higher than 0.05 mM. Results obtained in this study reveal that the metal-deposited zerovalent silicon can serve as an environmentally friendly reductant for the enhanced degradation of chlorinated hydrocarbons for long-term performance.

Introduction Chlorinated hydrocarbons such as tetrachloroethylene (PCE) and trichloroethylene (TCE) are the most often found toxic chemicals in contaminated aquifers (1). Research including laboratory-scale and field studies has demonstrated that the use of zerovalent metals such as Fe, Cu, and Si as reductants * Corresponding author phone: +886-3-5726785; fax: +886-35718649; e-mail: [email protected]. 4752

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is a promising strategy for the removal of chlorinated hydrocarbons in contaminated subsurface environments (2–5). In addition, the feasibility of combining zerovalent iron to dechlorinate PCE and TCE in typical enhanced solubilization matrices such as alcohols and surfactants has been examined (6–8). However, drawbacks including the accumulation of chlorinated byproduct and the decrease in surface activity over time have been reported (1, 9). Several attempts have been made to enhance the dechlorination rate of chlorinated hydrocarbons and to prevent the accumulation of toxic byproduct using hydrogen gas or bimetallic systems (10–12). Nutt et al. (11) showed that a bimetallic treatment approach involving palladium supported on gold increased the reaction rate in the dechlorination of TCE. A rapid and complete dechlorination of chlorinated solvents with the production of nonchlorinated hydrocarbons was also reported by using nanoscale bimetallic Pd/Fe particles (12). In addition, bimetallic Ni/Fe nanoparticles have been found to rapidly dechlorinate chlorinated ethylenes with the formation of ethane as the main products (11, 12), showing that the bimetallic system is an effective technology for accelerating the dechlorination processes and converting the chlorinated hydrocarbons to the nontoxic end products. Several bimetallic materials such as Pd/Fe, Ni/Fe, Cu/Al, and Cu/Fe have been synthesized and applied to the reduction of a wide variety of priority pollutants including chlorinated hydrocarbons (13–18), nitrobenzenes (19), pentachlorophenol (20), and anions (21). The deposition of a catalytic second metal such as Pd, Ni, and Cu onto the surface of a reductive metal could enhance the dechlorination efficiency and rate of chlorinated hydrocarbons and prevent the formation of toxic products by dechlorinating the chlorinated hydrocarbons via hydrogen reduction rather than through electron transfer (13, 22). In addition, the type of products obtained during the dechlorination reaction is dependent on the identity and mass loading of the second metal employed (13). When the bimetallic material was employed to decompose the chlorinated hydrocarbons, the dechlorination efficiency could be enhanced by increasing the loading of the second metal (11, 13–15), and an optimal loading of second metal ion usually exists for the dechlorination of chlorinated hydrocarbons. Several plausible explanations including the formation of galvanic cell (13), the surface coverage of catalytic metal on the reductive metal (14), and the absorbed atomic hydrogen (16, 23) have been proposed to explain this phenomenon. Silicon is the second most abundant element following oxygen. Silicon compounds are most often found in soil and groundwater environments. In addition, zerovalent silicon (Si) has a lower redox potential (EH° ) -0.807 V) compared with iron (EH° ) -0.44 V). This indicates that Si can be used as a strong reductant for effectively reducing contaminants. Our previous study (3) showed that Si reacts more efficiently than zerovalent iron (Fe) in the dechlorination of carbon tetrachloride (CT) and PCE under anaerobic conditions. More recently, the coupled reduction of chlorinated hydrocarbons and metal ions has attracted much attention (24–26). The low redox potential of zerovalent silicon can theoretically reduce metal ions such as Fe(II), Ni(II), and Cu(II) to form zerovalent metals, and then the reduced metal species accelerate the dechlorination efficiency and rate of chlorinated hydrocarbons. This gives the impetus to understand the synergistic effect of Si and metal ions on the dechlorination of chlorinated hydrocarbons under anoxic conditions. 10.1021/es071545x CCC: $40.75

 2008 American Chemical Society

Published on Web 05/30/2008

However, the dechlorination of chlorinated hydrocarbons by Si in the presence of metal ions has received less attention. In this study, the dechlorination of PCE by microscale Si in the presence of metal ions was investigated to understand the catalytic effect of bimetallic materials on PCE dechlorination under anaerobic conditions. Three metal ions including Ni(II), Cu(II), and Fe(II) were selected as model second dopant metals. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) were used to characterize the change in surface microstructures of the Si. In addition, the combination of various concentrations of Ni(II) with Si was examined to clarify the role of second metal in determining thereactionrateandproductdistributionofPCEdechlorination.

Materials and Methods Dechlorination Experiments. A complete list of all chemicals used as well as a detailed description of the experimental systems and analytical methods can be found in the Supporting Information. Briefly, N2-purged 120 mL serum bottles were filled with 50 mL of deoxygenated Tris buffer and 0.1 g of Si at pH 8.3 ( 0.1. The deoxygenated stock solutions of metal ions were delivered into serum bottles by gastight syringes to obtain the desired concentrations (27, 28). After 24 h of equilibrium, the stock solution of PCE dissolved in deoxygenated methanol was introduced into the bottles to get the final concentrations of 8-32 µM. Serum bottles were incubated in an orbital shaker at 25 ( 1 °C in the dark. Control experiments were also carried out without the addition of zerovalent silicon or metal ions. Analytical Methods. Detailed procedures of the analytical method are described in the Supporting Information and ref 27. The headspace analytical technique was used for the determination of chlorinated hydrocarbons and nonchlorinated hydrocarbons. The concentrations of PCE and byproducts in the headspace of the test bottles were monitored by injecting 40 µL of gas in the headspace into a gas chromatograph equipped with an electron capture detector (ECD) and a flame ionization detector (FID). The column temperature was isothermally maintained at 120 °C using ultrahigh-purity nitrogen as the carrier gas. The temperatures of the ECD and FID were maintained at 325 and 220 °C, respectively. The XPS measurements were performed by an ESCA PHI 1600 photoelectron spectrometer using an Al KR X-ray source (1486.6 eV photon energy). During data acquisition, the pressure in the sample chamber was maintained below 2.5 × 10-8 torr. The binding energies of the photoelectrons were determined under the assumption that carbon has a binding energy of 284.8 eV. After analysis of the chemical composition on the Si surface, argon etching operating at 3 kV voltage and 30 mA current was used to remove the surface layer of samples. This corresponds to the sputter rate of ca. 5.0-5.5 nm/min measured by SiO2 standards. The distribution of Ni species onto the surface of Si was determined by an electron probe X-ray microanalyzer. In addition, concentrations of dissolved metal ions including Fe(II), Cu(II), and Ni(II) in aqueous solutions were determined by the inductively coupled plasma-optical emission spectrometer.

Results and Discussion Effect of Metal Ions on Dechlorination of PCE by Si. The reaction of Si with water produces large amounts of hydrogen gas (Supporting Information, Table S1), which may significantly influence the headspace analytical procedure of chlorinated hydrocarbons. To minimize the interference of hydrogen gas on the quantification of chlorinated hydrocarbons, the produced hydrogen gas in the serum bottle was released after 12 h of incubation prior to the addition of metal ions. After a 24 h equilibrium of metal ions with Si, the

FIGURE 1. Effect of metal ions on the dechlorination of tetrachloroethylene (PCE) by Si(0) in the buffered solution at pH 8.3. The metal ions used were 0.1 mM Ni(II), Fe(II), and Cu(II).

TABLE 1. Dechlorination Products of PCE by 0.1 g of Si at pH 8.3 in the Presence of 0.1 mM Metal Ions under Anaerobic Conditions after 420 h of Incubation systems Si Si Si Si

only + Cu(II) + Fe(II) + Ni(II)

product distribution

carbon mass balance (%)

PCE (20%), TCE (64%) PCE (52%), TCE (34%), ethane (6%) PCE (11%), TCE (72%), ethane (15%) ethane (93%)

84 92 98 93

hydrogen gas was released again before the injection of the stock solution of PCE. Figure 1 shows the effect of 0.1 mM metal ions on the dechlorination of PCE by Si in the buffered solutions at pH 8.3. Of the original PCE 80% was dechlorinated by Si alone after 420 h of incubation. Addition of 0.1 mM Cu(II) lowered the dechlorination efficiency of PCE, and only 48% of the original PCE was removed after 420 h. A slight increase in the dechlorination efficiency (89%) of PCE was observed in the presence of Fe(II). On the contrary, addition of Ni(II) significantly enhanced the efficiency and rate of PCE dechlorination. A nearly complete dechlorination of PCE was achieved within 320 h when 0.1 mM Ni(II) was added. Trichloroethylene and ethane comprise the majority of the compounds observed from the dechlorination of PCE. As depicted in Table 1, only TCE, which corresponds to 64% of PCE dechlorination, was detected as the dechlorination product in the presence of Si alone. Both TCE and ethane were found to be the major dechlorination products when solutions contained 0.1 mM metal ions. However, only 6-15% of ethane were determined as the dechlorination product for PCE dechlorination in Cu/Si and Fe/Si systems. In the presence of Ni(II), TCE concentration increased in the first 70 h and then decreased with time, followed by the production of ethane (93%) as the major product. The carbon mass balances of PCE dechlorination by Si in the presence of metal ions were in the range of 84-98%. In addition, TCE was rapidly dechlorinated to ethane by Si in the presence of Ni(II) (Supporting Information, Figure S1). These results clearly show that PCE would be dechlorinated to TCE by Si first, and then the produced TCE is rapidly converted to ethane in the presence of Ni(II). The dechlorination of chlorinated hydrocarbons by zerovalent metals can be explained by a pseudo-first-order reaction kinetics (11, 13). The pseudo-first-order rate constant for (kobs) for PCE dechlorination by Si in the presence of Cu(II) was 0.0021 h-1, which is lower than that by Si alone (0.0038 ( 0.0005 h-1). A slight enhancement in the dechlorination efficiency of PCE was observed when 0.1 mM Fe(II) VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Pseudo-first-order rate constants (kobs) for PCE dechlorination as a function of Ni(II) concentration under anoxic conditions. concentration increased from 1 to 4 mM, clearly depicting that the existence of nickel ion accelerates the oxidation of zerovalent silicon (eq 1). Si0 + 2Ni2+ + 2H2O f SiO2 + 2Ni0 + 4H+ E° ) +0.557 V (1)

FIGURE 2. XPS spectra of (a) Ni(2p) and (b) Fe(2p) species on the surface of Si at pH 8.3 under anoxic conditions. The inset in a is the XPS spectra of Si species as a function of Ni(II) concentration. was added. The pseudo-first-order rate constant for PCE dechlorination in the presence of Fe(II) was 0.0053 h-1, which is 1.5 times higher than that by Si alone. Different from Fe(II) and Cu(II) ions, Ni(II) was observed to accelerate the dechlorination rate of PCE. The kobs for PCE dechlorination by Si in the presence of Ni(II) was 0.0145 h-1, which is 3.8 times higher than that by Si alone. In addition, the dechlorination pattern of PCE by Si in the presence of Ni(II) is similar to that by the Ni/Si system (Supporting Information,Figure S2), depicting that Ni(II) could be reduced to its metallic form by Si. A detailed description of the information on kinetics and thermodynamics of metal-modified Si can be found in the Supporting Information,. Characterization of Metal Species. To further understand the catalytic effect of metal ions, XPS was utilized to characterize the change in metal species onto the Si surface. Figure 2 shows the XPS spectra of Ni and Fe species onto the Si surface under anoxic conditions. The Ni(2p) spectrum after Ar sputtering for 3 min showed two peaks at binding energies of 853 and 870.4 eV, which can be assigned as Ni0(2p3/2) and Ni0(2p1/2), respectively. In addition, the ratio of peak areas of Ni0(2p1/2) to Ni0(2p3/2) is calculated to be 1:2.3, depicting that zerovalent nickel was formed on the surface of Si. The Si(2p) peak at 99.5 eV originates mainly from a single chemical form of Si(2p3/2) which is the binding energy of unreacted bulk silicon. The broad peak at 103-105 eV is attributed to silicon atoms in more positive oxidation states than those in the bulk. Silicon oxidized by oxygen and/or water gives rise to peaks at 103-105 eV due to SiO2 and SiO groups. It is noted that the peak intensity at 103-105 eV increased as the Ni(II) concentration increased, and the ratio of Si/SiO2 decreased from 5.0 to 0.5% when the Ni(II) 4754

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The XPS spectra for Fe/Si samples were slightly different from those for Ni/Si samples. The Fe(2p) spectra showed two peaks at the binding energies of 710.9 and 724.5 eV, demonstrating that the iron species were iron oxides. After sputtering by Ar for 1.5 min to remove the surface layer, however, peaks at 707 and 720 eV appeared remarkably, which reveals the existence of zerovalent iron (Fe). Previous studies (29, 30) have demonstrated the core-shell structure of iron nanoparticles in water. The shell in contact with water is in the form of iron oxyhydroxide, while the core remains metallic iron, which is consistent with the results of the bimetallic Fe/Si system obtained in this study. Effect of Concentration on PCE Dechlorination. The addition of metal ions has different effects on the enhancement of the efficiency and rate of PCE dechlorination by Si. Nickel ion was found to be the most effective second dopant metal for enhancing PCE dechlorination. Therefore, Ni(II) was selected for further experiments. Figure S3 shows the concentration effect of Ni(II) on the dechlorination of PCE by Si. The Ni(II) concentrations used in this study were in the range of 0.05-1.0 mM, which corresponds to the mass loading of 0.15-2.9 wt %. The dechlorination efficiency of PCE by Si increased from 40% in the absence of Ni(II) to 99.9% at 0.5 mM Ni(II) after 150 h of incubation and then deceased to 73% when the Ni(II) concentration further increased to 1 mM. Figure 3 shows the kobs for PCE dechlorination as a function of Ni(II) concentration. The kobs values increased linearly from 0.0038 ( 0.0005 h-1 in the absence of Ni(II) to 0.0516 ( 0.0041 h-1 at 0.5 mM Ni(II) and then decreased to 0.0085 ( 0.0004 h-1 when the Ni(II) loading was further increased to 1.0 mM. This result reflects the fact that addition of 0.5 mM Ni(II) has the maximum enhancement effect on PCE dechlorination by Si. Several studies have addressed the effect of additive loadings toward the dechlorination efficiency of chlorinated hydrocarbons by Fe (11, 13–15, 31). An optimal mass loading exists for a wide variety of bimetallic catalysts. Tee et al. (14) investigated the role of bimetallic Ni/Fe nanoparticles on the dechlorination of TCE. A higher degradation rate of TCE was observed upon increasing the Ni loading from 2 to 25 wt %, and then the kobs decreased when greater Ni loading was added. Moreover, Lin et al. (15) reported that the

FIGURE 4. SEM images and EPMA analysis of Ni-modified Si in the presence of various concentrations of Ni(II) at pH 8.3, Panels a and b show the SEM images of zerovalent silicon at 0.5 and 1 mM Ni(II), respectively. Panels c and d show the EPMA elemental maps of Ni on the Si surface with the addition of 0.5 and 1 mM Ni(II), respectively. dechlorination rate of TCE by bimetallic Ru/Fe increased as the Ru loading increased from 0.25 to 1.5 wt %. A decrease in kobs was also observed when Ru loading increased to 2.0 wt %. In this study, an optimal mass loading of Ni at 0.5 mM (1.5 wt %) was also obtained for PCE dechlorination by Si. The possible explanation is that the addition of metal ions may increase the numbers of Ni nanoparticles on the surface of Si, thus enhancing the catalytic activity for hydrodechlorination. The high loading of second dopant metal, however, leads to the aggregation of fine catalytic nanoparticles into large ones, and subsequently decreases the reaction rate of chlorinated hydrocarbons. To further understand the distribution patterns of Ni species on the Si surface, SEM and EPMA were employed to characterize the microstructures of the metal-modified Si. Figure 4 shows the SEM image and EPMA elemental maps for Ni species onto the Si surface. The SEM images clearly showed that Ni can be deposited onto the surface of Si. The particles sizes were in the range of 20-30 nm when 0.5 mM Ni(II) was added and increased to 40-80 nm at 1 mM Ni(II). In addition, EPMA elemental maps showed that the Ni nanoparticles aggregated at 1 mM Ni(II). These results clearly support the hypothesis that addition of low concentration of second metal ion increases the catalytic activity of Si, while the nanoparticles aggregate into large particles at high loading of Ni(II). Our previous studies (3, 24) indicated that the dechlorination of chlorinated hydrocarbons by Si involves the hydrogenolysis process. Reductive β-elimination and hydrogenation are also plausible reactions for reduction of PCE by zerovalent metal (6, 7, 32). Arnold and Roberts (32) examined the dechlorination of PCE by Fe alone and found that reductive β-elimination and hydrogenolysis accounted for 87 and 13%, respectively, of PCE dechlorination, resulting in the formation of ethylene, ethane, and TCE as the major products. In addition, chlorinated hydrocarbons can undergo

FIGURE 5. Product distribution of PCE dechlorination by Ni/Si after 150 h of incubation. The concentrations of Ni(II) were in the range of 0-1 mM. hydrodechlorination reaction to yield ethane in the presence of noble metal catalyst such as Pd, Pt, and Ni and hydrogen gas (10, 13). In this study, most hydrogen gas was released prior to the injection of metal ion and PCE, and addition of Ni(II) was found to obviously alter the reaction pathway and product distribution of PCE by Si. Figure 5 shows the product distribution of PCE dechlorination by Si in the presence of various concentrations of Ni(II) after 150 h of incubation. In the absence of Ni(II), only TCE was found to be the major product obtained after PCE dechlorination. The increase in Ni(II) concentration not only increased the extent and rate of PCE dechlorination but also changed the end product from TCE to ethane. The carbon mass balances were in the range of 85-95%, which is also in good agreement with the previous results of chlorinated ethylenes obtained using bimetallic Ni/Fe (11, 14). Since Ni(II) can be reduced to its VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. (a) Effect of initial concentration on the dechlorination of PCE by Ni-deposited Si and (b) the initial dechlorination rate as a function of PCE concentration at pH 8.3 under anoxic conditions. The Ni(II) concentration used was 0.5 mM. metallic form onto the Si surface, and PCE is transformed quantitatively to ethane without formation of detectable chlorinated intermediate compounds when Ni(II) concentration is higher than 0.1 mM, hydrodechlorination may be the major reaction pathway for PCE dechlorination by Ni/Si. The PCE concentrations in the contaminated aquifers have been reported to be in the range of 0.28-8.0 mg/L (33, 34). To understand the feasibility of using metal-deposited Si to dechlorinate various concentrations of PCE, the effect of initial PCE concentrations on the efficiency and rate of PCE dechlorination by Si in the presence of 0.5 mM Ni(II) was also examined. Figure 6 shows the effect of initial concentration on the dechlorination of PCE and the initial dechlorination rate as a function of PCE concentration. The removal efficiencies of PCE were in the range of 91-99.9% after 100 h of reaction when the initial concentrations of PCE were in the range of 8-32 µM. The kobs for PCE dechlorination decreased from 0.0533 h-1 at 8 µM to 0.0241 h-1 at 32 µM. Although the kobs decreased upon increasing the initial concentration of PCE, the initial dechlorination rates increased positively from 0.321 µM h-1 at 8 µM to 0.658 µM h-1 at 24 µM and then leveled off to a plateau. Arnold and Roberts (32) investigated the pathways and kinetics of chlorinated ethylene reactions with Fe particles. They found that the kobs for PCE dechlorination decreased with increasing initial concentration, while the initial rate increased with the increase in initial concentration, which is in agreement with the results obtained in this study. Environmental Implication. In this study, we have demonstrated the first report of using metal-deposited microscale Si for the effective dechlorination of PCE under anaerobic conditions. The presence of nickel ion serves as a catalyst to enhance the efficiency and rate of PCE chlorination as well as alter the reaction mechanism from hydrogenolysis to hydrodechlorination by the quantification of ethane formation. Unlike the iron system, which believes that pH is a significant factor influencing the reactivity of zerovalent iron, the pHs in Si systems after the dechlorination reaction were in the range of 8.0-8.2, which means that the change in solution pH in the presence of Si can be negligible. In addition, the production of environmentally friendly product (SiO2) is one of the advantages when silicon was employed as a reductant. These results suggest that metaldeposited Si(0) could be a useful bimetallic material for dechlorination of chlorinated hydrocarbons in contaminated sites. The combined removal of organic pollutants and heavy metals has recently received great attention (24–26, 35, 36). 4756

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Our results also showed that Si could effectively reduce Ni(II) and Fe(II) ions in the concentration range of 0.05-1 mM after 24 h of equilibrium (Supporting Information, Table S2), suggesting the potential use of Si to treat mixtures of chlorinated hydrocarbons and heavy metals at various concentrations. In the contaminated aquifers, the Fe(II) can be as high as 1.4-3 mM (37), while Ni(II) is an inorganic contaminant commonly found at contaminated sites (38). It is also noted that the kobs for PCE dechlorination by Si in the presence of 0.1 mM Fe(II) and Ni(II) were 1.5-3.8 times higher than that by Si alone. In addition, low input of Ni(II) concentration at mass loading of 0.05 mM has been shown to be effective on enhancing the efficiency and rate of PCE dechlorination and changing the product distribution in the presence of Si. This gives great impetus to coupled reduction of heavy metals and chlorinated hydrocarbons by Si under anaerobic conditions. Zerovalent silicon would adsorb metal ions such as nickel and ferrous ion first and then convert to the zerovalent forms via the electron transfer from Si. This process would significantly accelerate the dechlorination rate of chlorinated hydrocarbons under anoxic conditions and would be helpful in facilitating the development of processes that could be useful for the enhanced degradation of cocontaminants for long-term performance.

Acknowledgments The authors thank the National Science Council, Taiwan, for financial support under Contract No. NSC 95-2221-E-007077-MY3.

Supporting Information Available Description of analytical and experimental procedures, gas production of metal-modified zerovalent silicon, description of information on kinetics of PCE by metal-deposited Si(0), dechlorination of PCE and TCE by Si(0) in the presence of Ni species, removal of metal ions by Si(0), and the initial rate of PCE dechlorination as a function of initial PCE concentration (pdf). This information is available free of charge via the Internet at http://pubs.acs.org.

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