Environ. Sci. Technol. 2002, 36, 3270-3274
Carbon Isotope Fractionation during Permanganate Oxidation of Chlorinated Ethylenes (cDCE, TCE, PCE) SIMON R. POULSON* Department of Geological Sciences, MS-172, University of NevadasReno, Reno, Nevada 89557-0138 HIROSHI NARAOKA Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan
Permanganate oxidation of chlorinated ethylenes is an attractive technique to effect remediation of these important groundwater contaminants. Stable carbon isotope fractionation associated with permanganate oxidation of trichloroethylene (TCE), tetrachloroethylene (PCE), and cis1,2-dichloroethylene (cDCE) has been measured, to study the possibility of applying stable carbon isotope analysis as a technique to assess the efficacy of remediation implemented by permanganate oxidation. Average carbon isotope fractionation factors of RTCE ) 0.9786, RPCE ) 0.9830, and RcDCE ) 0.9789 were obtained, although the fractionation factor for PCE may be interpreted to change from a value of 0.9779-0.9871 during the course of the reaction. The fractionation factors for all three compounds are quite similar, in contrast to the variation of fractionation factors vs degree of chlorination observed for other degradative processes, such as microbial dechlorination. This may be due to a common rate-determining step for permanganate oxidation of all three compounds studied. The large fractionation factors and the relative lack of dependence of the fractionation factors upon other environmental factors (e.g. oxidation rate, presence of multiple contaminants, incomplete oxidation, presence of chloride in solution) indicate that monitoring δ13C values of chlorinated ethylenes during oxidation with permanganate may be a sensitive, and potentially quantitative, technique to investigate the extent of degradation.
the following: oxidation with permanganate is rapid and results in essentially complete mineralization to produce environmentally benign compounds (CO2, chloride); permanganate is not scavenged by carbonate or bicarbonate as may be the case with other oxidants; permanganate has a high aqueous solubility in contrast to some other oxidants; permanganate has a long history of use for treatment of drinking water; and remediation using permanganate may be implemented at relatively low cost (2-10). In the case of ex situ implementation of remediation strategies, quantification of the extent of remediation is typically quite straightforward. However, in the case of in situ remediation strategies, quantification of the extent of remediation effected may be difficult, due to the complex geochemical behavior of chlorinated ethylenes in the subsurface. Stable isotope analysis has been increasingly applied as a technique to assist in monitoring the extent of remediation, as changes in the isotopic composition of a contaminant may be associated with particular degradation processes. Implementation of stable isotope analysis as a technique to monitor the extent of remediation, in particular, and to study contaminant geochemical behavior in the subsurface, in general, requires experimental determination of the stable isotope fractionation factor associated with the reaction under consideration. A burgeoning database of fractionation factors is currently available for a number of contaminants of interest (e.g. chlorinated ethylenes, chlorinated methanes, monoaromatic hydrocarbons, polycyclic aromatic hydrocarbons), for a variety of relevant processes under both field and laboratory conditions (e.g. vaporization, dissolution, adsorption, microbial degradation, reductive dechlorination by iron, zinc, and hydrogen (11-20)). A general consensus appears to be that isotope fractionation associated with processes such as vaporization, dissolution, and adsorption is small or zero, whereas isotope fractionation associated with (bio)chemical reactions (e.g. reductive dechlorination) that occur during microbial degradation or engineered remediation processes is significant. Hence, it may be possible to correlate changes in contaminant isotopic composition to the extent of degradation. To date, the isotopic fractionation factors associated with the oxidation of chlorinated ethylenes by permanganate have not been determined. Hence, the objective of this study is to measure the carbon isotope fractionation factor associated with the oxidation of TCE, PCE, and cis-1,2-dichloroethylene (cDCE) by potassium permanganate, to determine the potential utility of carbon isotope analysis as a technique to monitor the extent of remediation at contaminated field sites.
Experimental Methods Introduction Chlorinated ethylenes, particularly trichloroethylene (TCE) and tetrachloroethylene (PCE), are groundwater contaminants of great concern due to their suspected carcinogenic nature. Chlorinated ethylenes are some of the most frequently detected groundwater contaminants in the U.S. (1) and are often recalcitrant with respect to degradation. Hence, significant attention has been devoted to the investigation of the efficacy of various biological and abiological remediation processes which may accelerate the degradation of chlorinated ethylenes. In particular, remediation of chlorinated ethylenes by oxidation using permanganate has recently been the subject of numerous studies because of * Corresponding author phone: (775)784-1104; fax: (775)784-1833; e-mail:
[email protected]. 3270
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 15, 2002
Oxidation experiments were performed in 50 mL glass serum bottles. Forty milliliters of distilled, deionized water was added to each serum bottle, which was then capped with a Mininert valve (Vici Precision Sampling Co. Inc., Baton Rouge, LA). One experiment was performed using 40 mL of 5 molar NaCl solution instead of distilled water, to identify if the fractionation factor showed any dependence upon ionic strength (hence, a very high ionic strength was used, to magnify any effect upon ionic strength that may exist). Neat chlorinated compounds (TCE, 99+% anhydrous; PCE, 99+% anhydrous; cDCE, 97%; Sigma-Aldrich, Milwaukee, WI) were added by means of a pipetting syringe (Hamilton Co., Reno, NV) via the Mininert valve to provide nominal dissolved concentrations (i.e. assuming all of the chlorinated ethylene is present in the dissolved phase) of 30-60 mg/L, and the solution and headspace were allowed to reach equilibrium. Previous 10.1021/es0205380 CCC: $22.00
2002 American Chemical Society Published on Web 06/17/2002
experiments using TCE indicate that equilibrium between headspace and the aqueous phase is attained within an hour (21). Preliminary time-series measurements of concentrations and δ13C values for PCE, TCE, and cDCE in this study confirmed that attainment of equilibrium for all three compounds is very rapid (less than 1 h). Stock solutions of KMnO4 (99+% ACS reagent, Sigma-Aldrich, Milwaukee, WI) were prepared at concentrations of 25 g/L and 60 g/L and were added to the serum bottles by means of a glass syringe (Hamilton Co., Reno, NV) to provide dissolved concentrations of 50-1460 mg/L. All experiments were conducted at a room temperature of 23 ( 2 °C. The experiments were not pHbuffered. Chlorinated ethylene concentrations and δ13C compositions were determined using a headspace technique (21, 22). Headspace samples of 0.1-1.0 mL were taken from the serum bottle using a Pressure-Lok gastight syringe (Vici Precision Sampling Co. Inc., Baton Rouge, LA) and injected into a Hewlett-Packard 5890 gas chromatograph (GC) equipped with a 0.32 mm ID × 0.25 µm film thickness × 60 m long HP-5TA column (Hewlett-Packard, Palo Alto, CA). The oven temperature program was 40 °C for 6 min, increased to 160 °C at 30 °C/min, and held at 160 °C for 1 min. Compoundspecific isotope analysis was conducted using a Finnigan delta S mass spectrometer combined with the HP5890 GC through a microvolume combustion furnace (0.5 mm ID × 1.5 mm OD × 34 cm) operated at 840 °C and loaded with CuO and Pt wires as oxidant and catalyst, respectively. The ion currents of the resulting CO2 for m/z 44, 45, and 46 were recorded every 0.125 s. Chlorinated ethylene concentrations were quantified by integrating the m/z 44 peak and normalizing to m/z 44 peak areas determined before addition of KMnO4. Concentration measurements had a typical reproducibility of ( 5%. Isotope analyses were compared against external CO2 isotope standards using ISODAT software, and δ13C values are reported in the usual δ notation relative to the VPDB standard. The reproducibility of isotope analyses is dependent upon concentration, with a reproducibility of 0.3‰ for initial concentrations, and a typical reproducibility of 1‰ at low concentrations (
