Tetrachloroethene (PCE) and trichloroethene (TCE) are among the most frequently detected groundwater contaminants at industrial sites, including many Department of Defense (DoD) facilities. PCE and TCE are susceptible to reductive dechlorination by microorganisms as well as reduced minerals such as iron sulfide (FeS). Unlike biological reductive dechlorination, which often results in accumulation of harmful intermediates such as cis 1,2-dichloroethene (cis-DCE) and vinyl chloride (VC), abiotic mineral-mediated dechlorination of PCE and TCE tends to result in complete transformation to nontoxic products such as acetylene. To more accurately apply natural attenuation and other remediation technologies, a greater understanding of the geochemical factors affecting the rates of purely abiotic reductive dechlorination of PCE and TCE is needed. Additional tools also are needed to determine whether abiotic reductive dechlorination is occurring at a particular site and its relative importance compared to microbial reductive dechlorination under a variety of geochemical conditions.
The overall objective of this project was to develop and apply methods for quantifying the rates of abiotic natural attenuation at sites contaminated with PCE and TCE in order to allow a quantitative estimate of the potential for abiotic transformation of these compounds based on analysis of subsurface geochemistry. Specific project objectives included: (1) assess whether stable (i.e., non-radioactive) carbon isotope fractionation can be used to distinguish between abiotic and biotic reductive dechlorination of TCE and PCE; (2) identify the geochemical conditions most strongly correlated with high rates of abiotic PCE and TCE reductive dechlorination in well-defined microcosm studies; and (3) validate and apply the findings at a series of DoD field sites contaminated with PCE or TCE.
In Task 1, researchers conducted PCE and TCE reductive dechlorination experiments using pure minerals and well-characterized pure and mixed cultures of bacteria. In Tasks 2 and 3, they studied PCE and TCE reductive dechlorination in well-defined microcosms prepared with aquifer materials from three locations. Electron donors and terminal electron acceptors were added to both stimulate microbial activity and generate reactive minerals via microbial iron and sulfate reduction. Researchers assessed the relative importance of abiotic and biotic PCE and TCE reductive dechlorination by analysis of reaction products, reaction kinetics, and stable carbon isotope fractionation.
In Task 1, significant carbon isotope fractionation was observed during FeS-mediated reductive dechlorination of PCE and TCE as well as during transformation of TCE by chloride green rust (GR-Cl) and pyrite. Bulk enrichment factors (εbulk) for PCE transformation by FeS were -30.2 ± 4.3 ‰ (pH 7), -29.54 ± 0.83 ‰ (pH 8), and -24.6 ± 1.1 ‰ (pH 9). For TCE, εbulk values were -33.4 ± 1.5 ‰ (pH 8) and -27.9 ± 1.3 ‰ (pH 9). Bulk enrichment factors (εbulk) for TCE transformation by GR-Cl and pyrite at pH 8 were -23.0 ± 1.8 ‰ and -21.7 ± 1.0 ‰, respectively.
A smaller magnitude of carbon isotope fractionation resulted from microbial reductive dechlorination by two isolated pure cultures (Desulfuromonas michiganensis strain BB1 [BB1] and Sulfurospirillum multivorans [Sm]) and a bacterial consortium (BioDechlor INOCULUM [BDI]). The εbulk values for biological PCE microbial dechlorination were -1.39 ± 0.21 ‰ (BB1), -1.33 ± 0.13 ‰ (Sm), and -7.12 ± 0.72 ‰ (BDI), while those for TCE were -4.07 ± 0.48 ‰ (BB1), -12.8 ± 1.6 ‰ (Sm), and -15.27 ± 0.79 ‰ (BDI). Researchers interpreted the results by calculating the apparent kinetic isotope effect for carbon (AKIEC), and the results suggest that differences in isotope fractionation for abiotic and microbial dechlorination resulted from differences in rate limiting steps during the dechlorination reaction.
Task 1 results suggest that measurement of very negative εbulk values at contaminated sites undergoing passive or active remediation may indicate the occurrence of abiotic reductive dechlorination of PCE and TCE. Isotope fractionation is one tool that can be used in conjunction with other tools such as microbial, geochemical, and reaction product analysis to provide evidence about the predominant PCE or TCE transformation pathway at a contaminated site (i.e., abiotic or biotic). There is too much variability in εbulk values for different minerals and different microbial cultures, however, for isotope fractionation to be a stand-alone tool.
In Tasks 2 and 3, the predominant PCE and TCE transformation pathway in most microcosms was microbial reductive dechlorination. Abiotic transformation was similar in magnitude to microbial reductive dechlorination only under conditions where the activity of dechlorinating bacteria was low. Comparison of geochemical conditions with abiotic product recoveries showed that the greatest extents of abiotic reductive dechlorination occurred under iron- and sulfate-reducing conditions where high concentrations of Fe(II) and S(-II) mineral species were present, confirming the involvement of these minerals in abiotic reductive dechlorination.
Results indicate that microbial PCE and TCE transformation is typically faster than abiotic transformation under the geochemical conditions of the microcosms (typically no more than 1 g/L of solid phase Fe(II) or S(-II)), which were set up to model natural environments. Active remediation technologies should therefore aim to either generate significantly higher mass loadings of reactive minerals in situ or else focus on microbial degradation of PCE and TCE by biostimulation or bioaugmentation. Similarly, for monitored natural attenuation, microbial PCE and TCE transformation are likely to outpace abiotic transformation, except in locations where the activity of dechlorinating bacteria is low. Testing for the presence of specific dechlorinating bacteria could provide insight into the most important PCE or TCE transformation process at a given site (i.e., biotic or abiotic). For locations without active microbial reductive dechlorination of PCE or TCE, higher concentrations of reactive Fe(II) and S(-II) minerals generated via microbial iron and sulfate reduction will be correlated with higher rates of abiotic transformation of PCE and TCE. (Project Completed – 2009)