Enhanced Dechlorination of Tetrachloroethylene by Zerovalent Silicon

Feb 22, 2011 - 'INTRODUCTION. Chlorinated hydrocarbons such as tetrachloroethylene (PCE) and trichloroethylene (TCE) are commonly used for cleaning...
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Enhanced Dechlorination of Tetrachloroethylene by Zerovalent Silicon in the Presence of Polyethylene Glycol under Anoxic Conditions Chun-chi Lee and Ruey-an Doong* Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu, 30013, Taiwan

bS Supporting Information ABSTRACT: The combination of zerovalent silicon (Si(0)) with polyethylene glycol (PEG) is a novel technique to enhance the dechlorination efficiency and rate of chlorinated hydrocarbons. In this study, the dechlorination of tetrachloroethylene (PCE) by Si(0) in the presence of various concentrations of PEG was investigated under anoxic conditions. Several surfactants including cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Tween 80 were also selected for comparison. Addition of SDS and Tween 80 had little effect on the enhancement of PCE dechlorination, while CTAB and PEG significantly enhanced the dechlorination efficiency and rate of PCE by Si(0) under anoxic conditions. The Langmuir-Hinshelwood model was used to describe the dechlorination kinetics of PCE and could be simplified to pseudo-first-order kinetics at low PCE concentration. The rate constants (kobs) for PCE dechlorination were 0.21 and 0.36 h-1 in the presence of CTAB and PEG, respectively. However, the reaction mechanisms for CTAB and PEG are different. CTAB could enhance the apparent water solubility of PCE in solution containing Si(0), leading to the enhancement of dechlorination efficiency and rate of PCE, while PEG prevented the formation of silicon dioxide, and significantly enhanced the dechlorination efficiency and rate of PCE at pH 8.3 ( 0.2. In addition, the dechlorination rate increased upon increasing PEG concentration and then leveled off to a plateau when the PEG concentration was higher than 0.2 μM. The kobs for PCE dechlorination by Si(0) in the presence of PEG was 106 times higher than that by Si(0) alone. Results obtained in this study would be helpful in facilitating the development of processes that could be useful for the enhanced degradation of cocontaminants by zerovalent silicon.

’ INTRODUCTION Chlorinated hydrocarbons such as tetrachloroethylene (PCE) and trichloroethylene (TCE) are commonly used for cleaning purposes. These chemicals can be released into the environment by improper disposal or accidental leakage.1,2 Permeable reactive barriers (PRBs) have been proven to be one of the useful in situ techniques to dechlorinate the chlorinated hydrocarbons in contaminated subsurface environments.3 Several reductive metals including iron (Fe), aluminum (Al), and silicon (Si) have been utilized as the reductants to decompose chlorinated hydrocarbons because of their suitable redox potential and environmental friendliness.4,5 Zerovalent iron (Fe(0), ZVI) is one of the most often used materials for remediation of a wide variety of organic and inorganic compounds.6-9 In addition, previous studies have shown that zerovalent silicon (Si(0)) reacts more efficiently than ZVI in the dechlorination of carbon tetrachloride and PCE under anoxic conditions.5,10 The dechlorination rate of PCE by Si(0) was found to be 1.5-3.8 times higher than that by ZVI.5 However, the formation of metal oxides such as silicon dioxide after the reaction would decrease the dechlorination efficiency and rate of chlorinated hydrocarbons. r 2011 American Chemical Society

Several attempts have been made to enhance the dechlorination rate of chlorinated hydrocarbons. The combination of second catalytic metals such as Ni and Pd significantly enhanced the dechlorination efficiency and rate of chlorinated hydrocarbons by zerovalent metals.11,12 The addition of electron shuttling compounds such as quinones and humic acid also could accelerate the electron transfer rate to decompose contaminants.13-15 The utilization of surfactants is another approach to enhance the dechlorination efficiency and rate of chlorinated hydrocarbons by ZVI.16,17 Surfactants including cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Tween 80 are commonly used in iron systems.18-20 Alessi et al.16 reported that the rate constant for PCE dechlorination increased by a factor of 4-19 after the addition of octyltrimethylammonium, a cationic surfactant, to the zerovalent iron system. The dechlorination of trichlorobenzene by Pd/Fe was also enhanced by 1.5-2.5 times Received: September 3, 2010 Accepted: January 26, 2011 Revised: January 10, 2011 Published: February 22, 2011 2301

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in the presence of CTAB.20 However, the toxicity and the formation of foam limited the application of surfactants in the environment.21 Polyethylene glycol (PEG) is a bifunctional amphiphilic polymer with reactive OH groups on both ends and has been widely used in biological and food chemistry because of the advantages of high water solubility, nontoxicity, low cost, facile biodegradation, and environmental friendliness. In addition, PEG has been employed as a cross-linker to immobilize the nanoscale ZVI onto a support.22,23 However, the role of PEG in the enhancement of dechlorination efficiency and rate of PCE by Si(0) under anoxic conditions is rarely reported. In this study, the enhancement effect of PEG on the dechlorination efficiency and rate of PCE by Si(0) under anoxic conditions was investigated. The enhancement effect of three different surfactants including CTAB, SDS, and Tween 80 on the dechlorination of PCE was also conducted for comparison. The adsorption behavior of PEG and the change in silicon species in the presence and absence of surfactant were examined. In addition, the concentration effects of PCE and PEG on the dechlorination efficiency and rate of PCE by Si(0) were evaluated.

’ MATERIALS AND METHODS Dechlorination Experiments. A complete list of all the chemicals used as well as a detailed description of the experimental systems and analytical methods can be found in the Supporting Information. Briefly, 60-mL serum bottles were filled with 0.05 g of HF-washed Si(0). After being capped with aluminum crimp caps containing Teflon-lined rubber, serum bottles were transported outside the glovebox and anoxically filled with 50 mM deoxygenated Tris buffer to maintain the pH at 8.3 ( 0.2. The stock solutions of PCE and surfactants were introduced into the silicon system to attain PCE concentrations ranging from 16 to 128 μM. Concentrations of CTAB, SDS, and Tween 80 were set at 1 critical micellar concentration (CMC), while the final concentrations of PEG were in the range of 0.001-400 μM. The CMC values of surfactants used in this study are listed in Table S1 (Supporting Information). The total volume of the liquid phase was 25 mL, resulting in a 35-mL headspace for analysis. The bottles were incubated with an orbital shaker at 130 rpm and at 25 ( 1 °C in the dark to effectively suspend the microscale Si(0) particles and to minimize the mass transfer limitation in the batch experiments. The adsorption experiments of PEG onto the Si(0) surface were performed by addition of various concentrations of PEG ranging from 0.01 to 0.3 μM into aqueous solutions containing 2 g L-1 Si(0). Parallel experiments were also carried out without the addition of Si(0). It is known that the dechlorination of chlorinated hydrocarbons by zerovalent metals is a surface-mediated reaction, and the Langmuir-Hinshelwood kinetic model can be used to elucidate the reaction kinetics on the Si(0) surface:20

r ¼

dC KL St C ¼ kapp dt 1 þ KF C

ð1Þ

where C is the aqueous concentration of PCE, kapp is the limitingstep rate constant of reaction at maximum coverage under the given conditions, St is the abundance of reactive sites, and KL is the Langmuir adsorption coefficient of PCE on reactive sites. Zhu et al.20 used Langmuir-Hinshelwood kinetic model to elucidate the reaction kinetics of trichlorobenzene on the

Figure 1. Dechlorination of PCE by Si(0) in the presence of different surfactants. The surfactants used in this study were CTAB, SDS, Tween 80, and PEG. The blank controls shown in Figure S1 (Supporting Information) indicated that no PCE loss was observed after 16 h of incubation in the presence of various surfactants without the addition of Si(0).

amphiphile-modified Pd/Fe surface and found a reasonable consistency between the changes of the pseudo-first-order rate constants (kobs) and kappSt with changing amphiphile concentrations. When the concentration C is low and KLC ,1, eq 1 can be simplified to pseudo-first-order kinetics. Analytical Methods. The headspace analytical technique was used for the determination of chlorinated hydrocarbons. The concentrations of PCE and the byproducts in the headspace of the test bottles were monitored by withdrawing 40 μL of gas from the headspace using a 50-μL gastight syringe. The headspace sample was immediately injected into a gas chromatograph (GC) equipped with an electron capture detector and a flame ionization detector. A 60-m VOCOL fused-silica megabore capillary column was used to separate the organic compounds. The column temperature was isothermally maintained at 120 °C using ultrahigh purity nitrogen (>99.9995%) as the carrier gas. Control samples were also used to check for possible leakage of target compounds during the incubation process. The XPS measurements were performed by an ESCA PHI 1600 photoelectron spectrometer using Al KR X-ray source (1486 eV photon energy). During the data acquisition, the pressure in the sample chamber did not exceed 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.

’ RESULTS AND DISCUSSION Dechlorination of PCE by Si(0) in the Presence of Surfactants. The enhancement effect of surfactants on the dechlorina-

tion efficiency and rate of PCE by Si(0) at pH 8.3 ( 0.2 were first examined by the addition of different surfactants including cationic (CTAB), anionic (SDS), and nonionic surfactants (Tween 80 and PEG-35000). As depicted in Figure 1, addition of 1 CMC SDS and Tween 80 had little effect on the dechlorination efficiency of PCE by Si(0), while 99% of the original PCE were dechlorinated within 18 h when 1 CMC CTAB was amended. Addition of PEG has a significant effect on the dechlorination efficiency of PCE by Si(0), and a nearly complete

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Environmental Science & Technology dechlorination was observed within 8 h when 400 μM PEG was added. In addition, no PCE was dechlorinated after 16 h of incubation in the presence of various surfactants without the addition of Si(0), clearly showing that the decrease in PCE concentration is mainly attributed to the reducing ability of Si(0). The kobs for PCE dechlorination was 0.21 and 0.36 h-1 in the presence of CTAB and PEG, respectively. It is noteworthy that the kobs for PCE dechlorination by Si(0) in the presence of PEG is 106 times higher than that in the absence of PEG obtained in our previous work,10 clearly showing that addition of PEG significantly enhanced the dechlorination efficiency and rate of PCE by Si(0) at pH 8.3 under anoxic conditions. Surfactant is an amphiphilic compound that can lower the surface tension of water as well as change the interfacial properties. Several studies have depicted that the introduction of surfactant can enhance the dechlorination rate of chlorinated hydrocarbons by ZVI because of the increase in apparent water solubility and the adsorbed surfactant on the iron surface.16,20,24 Addition of cationic surfactant has been reported to be more effective than nonionic and anionic surfactants on the dechlorination of chlorinated hydrocarbons by ZVI.20,25 In this study, the dechlorination efficiency of PCE by Si(0) in the presence of surfactants followed the order PEG > CTAB > Tween 80 > SDS, which is in agreement with those reported using ZVI as the reductant.24 The enhancement effect of CTAB on the dechlorination efficiency may be attributed to the enhanced solubilization of PCE by CTAB in the Si(0)-H2O system. The reaction of Si(0) with water would generate silicon dioxide (SiO2), and the isoelectric point of SiO2 is 2-3,26 which means that the Si(0) surface is negatively charged at pH 8.3 ( 0.2. When CTAB was at CMC value, micelles were formed on the solid-water interface and resulted in the increase in PCE partitioning near the interfacial region of Si(0). The adsorption experiments showed that 25% of the CTAB was adsorbed onto the surface of Si(0), clearly showing that the cationic CTAB can be easily adsorbed onto the Si(0) surface and leads to the increase in the adsorbed amounts of PCE onto the modified silicon surface. Table S2 (Supporting Information) shows the enhanced ratio of water solubility of PCE under various CTAB and PEG concentrations. The enhanced ratio of apparent water solubility of PCE increased with the increase in CTAB concentration ranging between 0.1 and 5 CMCs, demonstrating that the addition of CTAB can enhance the apparent water solubility of PCE in solution. To further understand the role of CTAB in dechlorination of PCE, various concentrations of CTAB ranging from 0.1 to 10 CMCs were precoated on the surface of Si(0) and then were used for PCE dechlorination at pH 8.3. Figure S2 (Supporting Information) shows the dechlorination of PCE by CTAB-coated Si(0). Less than 20% of the original PCE was dechlorinated within 12 h when Si(0) was precoated with CTAB. These results clearly indicate that when CTAB and PCE were added into the solution simultaneously, CTAB would solubilize PCE first in the solution and then adsorb onto the negatively charged Si(0), resulting in the enhancement of dechlorination efficiency and rate of PCE. On the contrary, the precoating of CTAB onto the Si(0) surface competes for reactive sites with PCE, resulting in little enhancement in PCE dechlorination. Effect of PEG Concentration on Dechlorination of PCE. Different from the behaviors of CTAB, addition of PEG has little effect on PCE solubilization (Table S2, Supporting Information). However, the precoating of PEG onto the Si(0) surface still enhanced the dechlorination efficiency and rate of PCE (Figure

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Figure 2. The kobs for PCE dechlorination as a function of PEG concentration. The concentrations of PCE and Si(0) were 62 μM and 2 g L-1, respectively.

S3, Supporting Information). Figure 2 shows the kobs for PCE dechlorination by Si(0) as a function of PEG concentration under anoxic conditions. The dechlorination efficiency and rate of PCE increased upon increasing PEG concentrations and a nearly complete dechlorination of PCE was observed within 15 h when the PEG concentration was higher than 0.2 μM (Figure S4, Supporting Information). In addition, the kobs for PCE dechlorination increased from 0.049 h-1 (R2 = 0.938) at 0.001 μM PEG to 0.36 h-1 (R2 = 0.994) at 0.2 μM PEG, and then leveled off to a plateau (0.35-0.39 h-1) (R2 = 0.985-0.998) when higher PEG concentration was introduced. The saturation of reactivity of Si(0) occurred at PEG concentrations higher than 0.2 μM. Several studies have depicted that the increase in rate constant of chlorinated compounds by ZVI in the presence of surfactants may be due to enhanced interaction between the reductant and accumulated contaminants at the interface.16,20 In this study, however, solubilization is not the main mechanism for enhanced dechlorination of PCE by Si(0) in the presence of PEG. The surface coverage of the adsorbed PEG onto the Si(0) surface may be one of the possibilities for saturation of reactivity. The extent of PCE dechlorination is dependent on the abundance of available reactive sites on the Si(0) surface. The added PEG may adsorb onto the surface of Si(0) to maintain the reactivity of sites on Si(0). At high concentrations of PEG, the adsorbed polymer can reach a maximum coverage and form a protection layer to retain the activity of most reactive sites on Si(0) surface, leading to saturation of reactivity. The reaction intermediates and pathways for PCE dechlorination by PEG-modified Si(0) were also determined. TCE was found to be the dominant product, which corresponded to 65% of the carbon mass balance when PCE was dechlorinated by Si(0) in the presence of 0.2 μM PEG (Figure S5, Supporting Information). In addition, cis-DCE (11%) and VC (12%) were observed in Si(0)-PEG system, and the total carbon mass balance was 98% after the dechlorination. Previous studies have depicted that the dechlorination of chlorinated hydrocarbons by pure Si(0) involved the hydrogenolysis process, and the carbon mass balances were in the range of 75-84%.5,10 This result clearly shows that the product yield in PEG-modified system is higher than that of the reported values. In addition, hydrodechlorination was reported to be the major reaction pathway for PCE dechlorination by Si(0) in the presence of second catalytic 2303

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Figure 3. The kobs for PCE dechlorination as a function of adsorbed PEG on the surface of zerovalent silicon.

Figure 4. (a) Effect of initial PCE concentration on the dechlorination of PCE by Si(0) in the presence of 0.2 μM PEG under anoxic conditions, and (b) the initial rate of PCE dechlorination (r) as the function of initial PCE concentration (C). The solid line was the simulated results of L-H model and (0) represented as original data. Inset in Figure (b) is the linear relationship between 1/C and 1/r.

metal ions.10 In this study, TCE and cis-DCE were found to be the major products and no nonchlorinated hydrocarbon was detected, clearly indicating that hydrogenolysis is the major reaction pathway for PCE dechlorination in the PEG-modified Si(0) system.

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To understand the adsorption behavior of PEG onto the surface of Si(0), various concentrations of PEG ranging from 0.01 to 0.3 μM were added to the solution containing 2 g L-1 of Si(0). Figure S6 (Supporting Information) shows the adsorption isotherm of PEG onto the silicon surface after the incubation of 3 h. The adsorption density increased positively from 0.16 mg g-1 at 0.01 μM PEG to 4.1 mg g-1 at 0.3 μM PEG. The adsorption behavior of PEG can be well-fitted by Freundlich model, and the adsorption constant (KF) and intensity (1/n) of 1.73 L g-1 and 0.46, respectively, with a correlation coefficient (R2) of 0.99 were obtained. Figure 3 shows the kobs for PCE dechlorination as a function of adsorbed PEG on the surface of Si(0). Although the adsorption of PEG onto the Si(0) surface may occupy the reactive sites to negatively influence the dechlorination efficiency of PCE, a positive correlation between the observed kobs values and adsorbed PEG amounts was also observed. This result clearly indicates that the adsorbed PEG may form a protection layer to accelerate the electron transfer from reactive sites to PCE under anoxic conditions. Effect of Initial PCE Concentration. The effect of initial PCE concentrations on the dechlorination efficiency and rate of PCE by Si(0) in the presence of 0.2 μM PEG was further examined. Figure 4 and Table S3 (Supporting Information) shows the effect of initial PCE concentrations on the dechlorination efficiency of PCE under anoxic conditions. The removal efficiencies of PCE by Si(0) in the presence of 0.2 μM PEG were all higher than 99% after 12 h of reaction when the initial concentrations of PCE were in the range 16-128 μM. The kobs for PCE dechlorination decreased from 0.41 h-1 at 16 μM to 0.27 h-1 at 128 μM. After the incubation of 12 h, the same PCE concentrations were respiked into the solution, and 83-99% of the respiked PCE were dechlorinated within 8 h. The kobs for PCE dechlorination was in the range 0.22-0.34 h-1, which is 6-28% lower than the initial stage. The decrease in rate constant after respiking of the PCE concentration may probably be attributed to the formation of silicon dioxide on the surface of Si(0). Silicon dioxide is an inert layer which can lower the electron transfer from the Si(0) to PCE,27 leading to a decrease in dechlorination efficiency and rate of PCE. However, the kobs for respiked PCE is still 58.6 times higher than that of our previous reports which used pure Si(0) for dechlorination of PCE in the absence of PEG,5,10 clearly indicating that Si(0) retains good reducing ability for PCE dechlorination in the presence PEG under anoxic conditions. Figure 4b shows the initial rate constants for PCE dechlorination as a function of initial PCE concentration. Although the kobs decreased upon increasing the initial PCE concentration, the initial dechlorination rates increased positively from 7.02 μM h-1 at 16 μM to 33.75 μM h-1 at 128 μM. A good fit between the initial PCE concentration and the initial rate with kapp and KL of 83 μM h-1 and 5.94  10-3 μM-1, respectively, was obtained (R2 = 0.99, n = 5). Note that the KL value indicates the extent of PCE equilibrium adsorption onto the surface of Si(0). At initial PCE concentration lower than 64 μM, the KLC values are much lower than 1, which suggests that a linear relationship between initial rate and PCE concentration would also work sufficiently well. Role of PEG in PCE Dechlorination by Si(0). The dechlorination of chlorinated hydrocarbons by Si(0) is a surfacemediated reaction that is largely controlled by the available surface sites. The deposition of a catalytic second metal such as Pd, Ni, and Cu onto the surface of a zerovalent metal is the frequently used method to enhance the dechlorination efficiency 2304

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Figure 5. (a) The ratio of SiO2 to Si total amount and (b) kobs for PCE dechlorination as the function under anoxic conditions.

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reflecting that fact that PEG may adsorb onto the Si(0) surface to reduce the direct contact between Si(0) and water. To further examine the effect of PEG on the formation of silicon dioxide, various loadings of PEG ranging between 0.01 and 0.6 μM were added into Si(0)-H2O system. After incubation for 3 h, the released gases in all the tested bottles containing PEG were all around 4-5 mL, which were lower than that in the absence of PEG (10-15 mL). However, the ratio of the produced SiO2 (SiO2/(Si0þSiO2), SiO2/Sitotal), determined by the ratio of peak areas of SiO2 and total Si species, decreased upon increasing PEG concentration. As depicted in Figure 5a, the ratio of SiO2/Sitotal decreased from 0.77 in the absence of PEG to 0.11 when the PEG loading was 0.6 μM. A good relationship between the SiO2/Sitotal ratio and the kobs for PCE dechlorination was also established (Figure 5b). The interaction between PEG and metal particles may involve several interaction mechanisms including ion exchange, chelation, electrostatic interaction, and hydrogen bonding. Addition of PEG can adsorb onto the Si(0) surface quickly to form a protection layer and subsequently decreases the reaction rate of Si(0) with water to inhibit the production of silicon dioxide onto the surface of Si(0). Therefore, electrons produced from the surface of Si(0) can be easily transferred to PCE, resulting in the enhancement of dechlorination efficiency and rate of PCE (eq 2). Figure S8 (Supporting Information) shows the dechlorination of PCE by Si(0) in the presence of different molecular weights of PEG. It is clear that PEG with different molecular weights has a similar effect on the enhancement of dechlorination efficiency and rate of PCE by Si(0). However, PEG with a low molecular weight (PEG-350) has a slower reaction rate than that of PEG-35000, which supports the hypothesis that PEG can adsorb onto the Si(0) and form a protection layer to inhibit the formation of SiO2 onto the Si(0) surface. Si0 þ 2C2 Cl4 þ 3H2 O f H2 SiO3

28,29

and rate of chlorinated hydrocarbons. Several plausible explanations including the formation of galvanic cell, the surface coverage of catalytic metal on the reductive metal,30 and the absorbed atomic hydrogen31 have been proposed to explain this phenomenon. Note that PEG cannot react with atomic hydrogen, and hydrogenolysis is the major reaction mechanism for PCE dechlorination, which means that the plausible explanations mentioned above are not the possible reasons for the enhancement of PCE dechlorination by PEG. The enhanced solubilization of chlorinated hydrocarbons and the change in surface hydrophobicity of zerovalent metal in the presence of surfactant has also been reported to enhance the dechlorination of chlorinated hydrocarbons by ZVI.16,20,24 In this study, however, the aqueous solubility of PCE was unchanged after addition of various concentrations of PEG ranging from 0.005 to 400 μM (Table S1), supporting the hypothesis that the enhanced solubilization is not the key mechanism in acceleration of dechlorination rate of PCE. Zerovalent silicon has a low redox potential (EH0 = -0.807 V) and can easily react with water to produce hydrogen gas and silicon dioxide. It is known that the silicon dioxide can serve as the insulator layer to reduce the electron transfer rate and reactivity of Si(0).27,32 Interestingly, addition of PEG can prevent the formation of SiO2 onto the surface of Si(0). Figure S7 (Supporting Information) shows the XPS spectra of Si2p of Si(0) before and after the addition of 100 μM PEG under anoxic conditions. The peak intensity of SiO2 at 103-105 eV decreased significantly when PEG was added into the solution containing Si(0),

þ 2C2 HCl3 þ 2Hþ þ 2Cl-

ð2Þ

Environmental Implication. In this study, we have first demonstrated that the reactivity of Si(0) can be significantly enhanced by the addition of PEG at pH 8.3 under anoxic conditions. Both CTAB and PEG were found to enhance the dechlorination efficiency and rate of PCE by Si(0). However, the role of these two compounds in the enhancement of PCE dechlorination is different. CTAB can enhance the apparent water solubility of PCE in a solution containing Si(0), leading to the acceleration of dechlorination efficiency and rate of PCE. Different from the role of CTAB in enhancement of PCE dechlorination, PEG can prevent the formation of silicon dioxide and subsequently significantly enhance the dechlorination efficiency and rate of PCE at pH 8.3 under anoxic conditions. The maximum reaction rate was obtained when PEG concentration was higher than 0.2 μM, and the kobs for PCE dechlorination was 106 times higher than that by silicon alone. The PCE concentrations in the contaminated aquifers have been reported to be in the range 0.28-8.0 mg/L33,34 and our results showed that addition of trace amounts of PEG (0.2 μM) can significantly enhance the dechlorination efficiency and rate of various concentrations PCE under anaerobic conditions, suggesting the potential use of PEG to remediate chlorinated hydrocarbons in the contaminated aquifers. It is interesting to note that PEG is an environmentally friendly chemical which is biodegradable under both aerobic and anaerobic conditions.35 Biodegradation of 2305

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Environmental Science & Technology PEGs with molecular weights ranging from 200 to 40 000 has been extensively investigated, and anaerobic biodegradation of PEG with the formation of acetate, acetaldehyde, and ethanol was also reported.36 Usually the half-life of PEG biodegradation ranges between weeks and months, which is longer than that of PCE dechlorination by PEG-modified Si(0) obtained in this study. This gives great impetus to the application of PEG for contaminated aquifers. The dechlorination of PCE by Si(0) can be significantly enhanced upon addition of PEG. After the complete dechlorination of PCE, PEG would be biodegraded to form low-molecular-weight organic compounds, which could serve as electron donors to increase the microbial activity of indigenous anaerobes in the degradation of other organic contaminants in aquifers. However, only low-molecular-weight homologues were produced during the dechlorination of PCE by Si(0), which may pose a threat to human health. Our previous study showed that the reaction pathways for PCE dechlorination by Si(0) changed from hydrogenolysis to hydrodechlorination when solutions contained low concentrations of Cu(II) or Ni(II).10,37 PEG is often used as the cross-linker to bind metal ions, and some metal ions may exist in the contaminated aquifers. This means that PEG may also adsorb metal ions, and then the adsorbed metal ions could be reduced to zerovalent form by Si(0) to form a bimetallic system, which results in a change in reaction kinetics and pathways of chlorinated hydrocarbons. Further study of the dechlorination of chlorinated hydrocarbons by PEG-modified Si(0) in the presence of metal ions is now in process.

’ ASSOCIATED CONTENT

bS

Supporting Information. Materials and methods, dechlorination of PCE by Si(0) in the presence of surfactants, enhanced ratio of PCE water solubility, effect of PEG concentration on dechlorination of PCE, product distribution of PCE dechlorination, adsorption isotherm of PEG onto Si(0), XPS spectra of Si2p before and after addition of PEG, and dechlorination of PCE in the presence of PEG with different molecular weights. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ886-3-5726785; fax: þ886-3-5718649; e-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank the National Science Council, Taiwan, for financial support under Contract No. NSC 95-2221-E-007-077MY3. ’ REFERENCES (1) Burris, D. R.; Campbell, T. J.; Manoranjan, V. S. Sorption of Trichloroethylene and Tetrachloroethylene in a Batch Reactive Metallic Iron-Water System. Environ. Sci. Technol. 1995, 29 (11), 2850–2855. (2) Orth, W. S.; Gillham, R. W. Dechlorination of trichloroethene in aqueous solution using Fe0. Environ. Sci. Technol. 1996, 30 (1), 66–71. (3) Henderson, A. D.; Demond, A. H. Long-term performance of zero-valent iron permeable reactive barriers: A critical review. Environ. Eng. Sci. 2007, 24 (4), 401–423.

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