An apparent stall in the biodegradation of 1,2-cis-dichloroethene (cDCE) is often observed at many natural attenuation sites where cDCE accumulates and is not further degraded. The lack of further breakdown of cDCE is often attributed to a lack of available hydrogen donor and/or absence of a suitable microbial community to further degrade the contaminant. Bradley et al. (1998) reported that addition of Mn(IV) could enhance microbial oxidation of cDCE under anaerobic conditions. However, the extent to which this process occurs in groundwater and whether it can be enhanced by manganese dioxide (MnO2) addition in aquifers with persistent cDCE is unknown.
The objective of this project was to examine the effect of MnO2 and other amendments in promoting biological oxidation of cDCE under anaerobic, aerobic, or cometabolic conditions. Efforts were also made to find and enrich for naturally occurring microbial populations that could biodegrade cDCE using MnO2 as an electron acceptor.
Historical groundwater data from 16 sites with known chlorinated volatile organic compound (CVOC) groundwater contamination was examined. Groundwater and/or saturated soil from the water bearing subsurface zone in plumes contaminated with chlorinated ethenes were collected from Hill Air Force Base (AFB) in Utah, Myrtle Beach AFB in South Carolina, Navy Base Kitsap in Keyport, Washington (Keyport), two locations near Launch Complex 34 at Cape Canaveral Air Force Station in Florida (CCAS LC-34 Plume and CCAS LC-34 ESB), and the Alamac American Knits LLC (Alamac) textile manufacturing facility in North Carolina. At each of these locations, there was evidence of cDCE stall. Laboratory microcosm and/or enrichment cultures were constructed at the Shaw Biotechnology Facility in Lawrenceville, New Jersey, using the matrices collected from these locations to evaluate the rate and extent of contaminant biodegradation under ambient conditions and with added manganese and organic substrates. Two indicators were used to measure the effectiveness of the treatments: (1) changes in cDCE and VC concentrations and (2) changes in the concentration of soluble Mn(II).
Changes in the CVOC concentrations (specifically cDCE and VC) were recorded over the prescribed incubation period. Concentrations of dissolved Mn(II) were analyzed in solution. An increase in the rate and/or extent of cDCE loss in incubations amended with MnO2 coupled with an increase in dissolved Mn was considered evidence that MnO2 addition stimulated cDCE degradation. Conversely, the disappearance of cDCE with concurrent production of VC or ethene was considered evidence that anaerobic reductive dechlorination was the operational biodegradation pathway.
The different microcosm and enrichment studies were incubated and monitored for 2 to 9 months. There was some indication that the background conditions at several sites led to VC formation by reductive dechlorination. This was more apparent at sites with residual total organic carbon (e.g., Keyport, Myrtle Beach, and Alamac) and the addition of additional carbon sources (acetate or humic acids) may have further enhanced this reaction. The presence of an aerobic headspace appeared to promote the best biodegradation of cDCE, apparently through aerobic oxidation. MnO2 addition appeared to inhibit cDCE biodegradation in matrices from Keyport and Alamac. There was little evidence that increases in the concentration of soluble Mn(II) from the biological oxidation of cDCE occurred.
Multiple treatments were prepared to promote the anaerobic biological oxidation of cDCE. None of these treatments were effective in enhancing the anaerobic oxidation of cDCE using MnO2 as an electron acceptor. Based on these results, there is no evidence that addition of MnO2 to aquifers will enhance cDCE biodegradation. Further pilot testing of MnO2 addition as a technology to enhance cDCE biodegradation is not recommended at this time.