Microcosm Evaluation of Vinyl Chloride Bio-Oxidation Under Anaerobic Conditions

The vast majority of hazardous waste sites in the US have groundwater contaminated with halogenated organic solvents, primarily tetrachloroethene (PCE) and trichloroethene (TCE). Because the cost to remediate these sites is projected to exceed hundreds of billions of dollars, there is considerable interest in utilizing bioremediation as a low-cost remedy. Under anaerobic conditions, the best characterized biodegradation pathway for chlorinated ethenes is reductive dechlorination, whereby an electron donor such as hydrogen provides the reducing equivalents needed to replace chlorine atoms with hydrogen atoms. The ultimate daughter products from chlorinated ethenes are non-hazardous ethene and ethane, a process easily documented with mass balances. Unfortunately, there are numerous sites where a mass balance is inconclusive. There appears to be a significant loss of chlorinated ethenes without a corresponding increase in ethene and ethane, a situation in which most regulators will not allow bioremediation as a remedy due to possible inadequate contaminant plume detection. A more recent explanation for the lack of a mass balance has emerged - anaerobic oxidation of vinyl chloride (VC) and in some cases cis-1,2-dichloroethene (cDCE) and ethene. The occurrence of anaerobic oxidation of these lesser chlorinated compounds and ethene has been demonstrated in laboratory studies using 14C-labeled substrates. Oxidation has been shown under a variety of terminal electron accepting conditions, including Fe(III)-reducing, humic acid reducing, sulfate-reducing, and methanogenic conditions. At the present time, documenting anaerobic oxidation in situ continues to be limited to time-consuming and costly laboratory studies using 14C-labeled substrates. In situ, the products of oxidation (CO2 or Cl-) are not released in adequate amounts to permit their detection above background levels and there are no diagnostic tools available to document the occurrence of VC, cDCE, and ethene anaerobic oxidation. Without such a tool, bioremediation will not be permitted as a remedy when a mass balance based on ethene and ethane comes up short. The main objective of this research was to identify at least one microcosm prepared with groundwater and/or soil from a contaminated site that showed anaerobic oxidation of VC. To find the activity, 470 microcosms were prepared, covering six sites, nine different sources of groundwater, seven different types of soil and/or sediment, and addition of five types of terminal electron acceptors. A large percentage of the microcosms were prepared with groundwater from the same industrial site where oxidative activity had previously been observed in a similar microcosm study conducted at Clemson University. In spite of this wide range of effort, none of the microcosms prepared exhibited anaerobic oxidation of VC. The initial approach used to prepare microcosms was to assemble them in the anaerobic chamber, the atmosphere of which contained a low concentration of hydrogen. The microcosms were then removed and approximately 5 mg/L of VC was added (and in some cases [14C]VC). After incubating these microcosms for more than one year and not observing oxidative behavior, a different approach to microcosm preparation was tested. After assembling bottles in the anaerobic chamber, they were removed and the headspaces were aggressively purged with oxygen-free N2 to remove any hydrogen from the anaerobic chamber. A lower concentration of VC was then added (0.44 mg/L). Regardless, this alternative method of microcosm preparation did not yield any evidence of anaerobic VC oxidation either. Three subsets of the microcosms exhibited robust reductive dechlorination of VC to ethene. For two of these subsets, the presence of Fe(III) or EDTA-Fe(III) slightly enhanced the rate of ethene formation, even though only moderate levels of iron reduction were detected. For one of the subsets, addition of sulfate enhanced the rate of reduction somewhat, while for the other two subsets addition of sulfate resulted in a slower rate of VC conversion to ethene. Addition of Mn(IV) had a slightly negative effect on the rate of VC reduction, while adding AQDS slowed the rate of reduction even further. In general, the results are consistent with the expectation that alternate terminal electron acceptors tend to compete with dechlororespiration for available electron donor and in so doing may slow the rate of reductive dechlorination. Methanogenesis was a dominant process in two subsets that were prepared with the same groundwater. The level of methane output was several times higher than expected based on the initial COD of the groundwater. The source of the additional COD remains unclear at this time. One potential source was a volatile contaminant in the atmosphere of the anaerobic chamber used to prepare the microcosms. A suspect in this regard was glycerol, since an open container of glycerol had been used to adsorb sulfides; the container was later removed after these microcosms were prepared. If indeed the source of the exogenous COD was the anaerobic chamber, it raises uncertainty about how that may have affected the behavior of the other microcosms. Further investigation of this issue is warranted