Environ. Sci. Technol. 2003, 37, 2575-2581
Reductive Dechlorination of Carbon Tetrachloride and Tetrachloroethylene by Zerovalent Silicon-Iron Reductants R U E Y - A N D O O N G , * ,† KUN-TIEN CHEN,‡ AND HSIAO-CHUNG TSAI† Department of Atomic Science and Department of Chemistry, National Tsing Hua University, Hsinchu, 30013 Taiwan
Reductive dechlorination of carbon tetrachloride (CT) and tetrachloroethylene (PCE) by zerovalent silicon (ZVS, Si0) and the combination of Si0 with metal iron (Fe0) was investigated as potential reductants for chlorinated hydrocarbons. The X-ray photoelectron spectroscopy (XPS) was used to identify the surface characteristics of Si0. CT and PCE can be completely degraded via sequential reductive dechlorination to form lesser chlorinated homologues by Si0. Productions of chloroform (CF) and trichloroethylene (TCE) accounted for 80% of CT and 65% of PCE dechlorination, respectively. The degradation of CT and PCE by Si0 at pH 8.3 followed pseudo-first-order kinetics, and the normalized surface rate constants (ksa) were 0.288 and 0.003 L m-2 h-1, respectively, which react more efficiently than zerovalent iron in CT and PCE dechlorination. A linear relationship was also established between pH and the ksa value. The XPS results showed that the hydrogenated silicon surface and silicon oxides on the silicon surface were removed during the dechlorination processes, thus providing a relatively clean silicon surface for dechlorination reactions. The combination of zerovalent silicon with iron influences both the dechlorination rate and the distribution of products. Sequential reductive dechlorination was still the main reaction for CT dechlorination by Si0/Fe0, while reductive dechlorination and β-elimination were the dominant reaction pathways for PCE dechlorination with ethane and ethene as the major end products. Also, the combination of silicon and iron constitutes a buffer system to maintain the pH at a stable value. A 0.3 unit of pH changed upon increasing the amount of Fe by a factor of 35 was observed, depicting that Si0 serves as a pH buffer in Si0/Fe0 system during dechlorination processes.
Introduction The remediation of contaminated aquifers has recently received much attention. A number of studies (1-5) including laboratory-scale and field studies have demonstrated that the abiotic dechlorination using zerovalent metals as the electron donor is a potential technology for the remediation * Corresponding author phone: +886-3-5726785; fax: +886-35718649; e-mail:
[email protected]. † Department of Atomic Science. ‡ Department of Chemistry. 10.1021/es020978r CCC: $25.00 Published on Web 04/22/2003
2003 American Chemical Society
of contaminated groundwater. Numerous halogenated compounds including chlorinated methanes, ethenes, and aromatic compounds such as chlorinated phenols and polychlorined biphenyls (PCBs) (6, 7) are susceptible to reduction by metal iron (Fe0). For chlorinated methanes, sequential hydrogenolysis is believed to be the major pathway for the degradation of chlorinated aliphatic compounds (1). Several studies (8-10) reported that β-elimination appeared to dominate the chlorinated ethylenes, leading to the formation of C2 hydrocarbons such as acetylene, ethene, and ethane. Also, reductive R-elimination may occur, resulting in a carbene intermediate when a carbon atom is multichlorinated (11). Although zerovalent iron can decompose chlorinated hydrocarbons effectively, several limitations have also been reported in the dechlorination process of the chlorinated compounds (12). One of the limitations is the increase of pH to 10-12 during the dechlorination reactions (1, 5, 13). The increase in pH causes the oxidation of ferrous ion and the precipitation of iron oxides. A number of iron species including hematite, goethite, magnetite, and green rust are formed as a result of anaerobic corrosion of iron in water (14, 15). When Fe0 surface was covered with iron precipitates, the transport and adsorption of chlorinated compounds could be severely impeded, subsequently decreasing the reaction rates of chlorinated hydrocarbons. Several attempts have been made to enhance the dechlorination rate using zerovalent iron by controlling the pH (9) or using bimetallic systems such as Pd/Fe and Ni/Fe (16-19). Chlorinated hydrocarbons are shown to be rapidly dechlorinated in water with Pd-based catalysts using dissolved hydrogen gas as the reductant. However, surface analyses of Ni/Fe and Pd/Fe systems suggest that the reactivity of catalysts decreased over time because of the formation of a thick oxide layer (20, 21). In addition to Fe0, several zerovalent metals such as tin (Sn) and zinc (Zn) can also dechlorinate the chlorinated hydrocarbons effectively (4, 9, 22). However, dissolved metal ion species are released into the groundwater during the dechlorination processes. Silicon is present in the earth’s crust at 27.7% of the total and, after oxygen, is the second most abundant element. Silicon compounds are most often found in soil and groundwater environments, either combined with oxygen as silicon dioxide (silica, SiO2) or combined with other elements such as aluminum (Al), magnesium (Mg), calcium (Ca), or iron. Although zerovalent silicon (Si0) is not a natural product in the environment, it has been widely used in semiconductor industry and is the subject of numerous investigations such as self-assembled monolayer (23, 24). Like zerovalent iron, Si0 is also a strong reducing agent and can undergo oxidative dissolution reaction to form native silicon dioxide (SiO2) on the surface of silicon by longtime exposure to humid air at room temperature (25). In a Si0-H2O system, the redox couple formed by Si0 and SiO2 has a standard reduction potential (EH°) of -0.857 V (SHE) (26). This makes Si0 a possible candidate as an electron donor to react with chlorinated hydrocarbons:
Si0 + 3H2O f H2SiO3 + 4H+ + 4e-
(1)
RX + H+ + 2e- f RH + X-
(2)
Si0 + 2RX + 3H2O f H2SiO3+ 2RH + 2H+ + 2X-
(3)
The reaction of eq 3 is thermodynamically favorable and will produce 2 mol of protons when 1 mol of Si0 reacts with 2 mol of chlorinated hydrocarbons. Zhou et al. (27) depicted that VOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. X-ray photoelectron spectra of Fe(2p) of iron surface. hydroxyl ions (OH-) could enhance the reaction of silicon with water and that the reaction of zerovalent silicon will be faster at higher pH, partly due to the solubilization of silicon dioxide by the hydroxide ions. This gives the impetus to combine zerovalent silicon and zerovalent iron for the dechlorination of chlorinated hydrocarbons since the increase in pH by Fe0 may be beneficial to the reaction with silicon. The objective of this study was to investigate the feasibility of using zerovalent silicon (Si0) as a novel reductant to degrade chlorinated compounds. Carbon tetrachloride (CT) and tetrachloroethylene (PCE) were selected as the model compounds. X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical species on the silicon surface. Moreover, the combination of silicon with metal iron on the dechlorination of chlorinated hydrocarbons was examined to understand the role of Si0 in permeable iron barrier. Also, the kinetics and degradation pathways of CT and PCE by Si0 and Si0/Fe0 are discussed in this study.
Materials and Methods Chemicals. All chemicals were used as received without further treatment. Tetrachloroethylene (PCE) (>99.8%, GC grade), trichloroethylene (TCE) (>99.8%, GC grade), and 2-(Ncyclohexylamino) ethanesulfonic acid (CHES buffer) (reagent grade) were purchased from Aldrich Co. (Milwaukee, WI). Methylene chloride (DCM) (>99.8%, GC grade) and ethanol (HPLC grade) were obtained from J. T. Baker Co. (Phillipsburg, NJ). Carbon tetrachloride (CT) (>99.8%, GC grade), chloroform (CF) (>99.8%, GC grade), tris(hydroxymethyl) aminomethane (Tris buffer), and zerovalent silicon (>99.5% purity, 99.5% purity,
