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b. Anaerobic and aerobic degradation pathways for carbon tetrachloride (modified from Majcher and others, 2007).
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... The higher electron acceptor demand from the presence of the mobile co-contaminants nitrate and perchlorate could exacerbate the difficulty of achieving and maintaining suitable reducing conditions for degradation of RDX in fractured rock and affect the survival of RDX-degrading microorganisms. Previous tests with WBC-2 have shown an unusual tolerance to oxygen exposure, demonstrating a continued ability to degrade chlorinated solvents through reductive dechlorination even after saturation with oxygen (Majcher et al., 2009). Perchlorate concentrations began to rise near the end of the field test, most likely because fresh groundwater entering the injection radius increased contaminant concentrations and diluted the injected donor below effective concentrations. ...
The potential neurotoxic and carcinogenic effects of the explosives compound RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) on human health requires groundwater remediation strategies to meet low cleanup goals. Bioremediation of RDX is feasible through biostimulation of native microbes with an organic carbon donor but may be less efficient, or not occur at all, in the presence of the common co-contaminants perchlorate and nitrate. Laboratory tests compared biostimulation with bioaugmentation to achieve anaerobic degradation of RDX, perchlorate, and nitrate; a field pilot test was then conducted in a fractured rock aquifer with the selected bioaugmentation approach. Insignificant reduction of RDX, perchlorate, or nitrate was observed by the native microbes in microcosms, with or without biostimulation by addition of lactate. Tests of the RDX-degrading ability of the microbial consortium WBC-2, originally developed for dehalogenation of chlorinated volatile organic compounds, showed first-order biodegradation rate constants ranging from 0.57 to 0.90 per day (half-lives 1.2 to 0.80 days). WBC-2 sustained degradation without daughter product accumulation when repeatedly amended with RDX and lactate for a year. In microcosms with groundwater containing perchlorate and nitrate, RDX degradation began without delay when bioaugmented with 10% WBC-2. Slower RDX degradation occurred with 3% or 5% WBC-2 amendment, indicating a direct relation with cell density. Transient RDX daughter compounds included methylene dinitramine, MNX, and DNX. With WBC-2 amendment, nitrate concentrations immediately decreased to near or below detection, and perchlorate degradation occurred with half-lives of 25–34 days. Single-well injection tests with WBC-2 and lactate showed that the onset of RDX degradation coincided with the onset of sulfide production, which was affected by the initial perchlorate concentration. Bioegradation rates in the pilot injection tests agreed well with those measured in the microcosms. These results support bioaugmentation with an anaerobic culture as a remedial strategy for sites contaminated with RDX, nitrate, and perchlorate.
... In the column studies of Lorah et al. (2008), it is possible that CF was transformed near the column inlet, thereby allowing PCE and DCE transformation to proceed downstream. Differences in microbial communities, the potential for spatial segregation of key populations and biotransformations in columns, or a combination of both factors, could explain why the toxicity/inhibition effects observed in the microcosm study reported here are more pronounced than in other studies with the WBC-2 culture (Lorah et al., 2008;Majcher et al., 2009). Nevertheless, batch microcosm assays like the ones conducted with contaminant mixtures in this study may reveal the potential for inhibition effects that could be important during remediation of field sites. ...
Bioremediation strategies, including bioaugmentation with chlorinated ethene-degrading enrichment cultures, have been successfully applied in the cleanup of subsurface environments contaminated with tetrachloroethene (PCE) and/or trichloroethene (TCE). However, these compounds are frequently found in the environment as components of mixtures that may also contain chlorinated ethanes and methanes. Under these conditions, the implementation of bioremediation may be complicated by inhibition effects, particularly when multiple dehalorespirers are present. We investigated the ability of the 1,1,2,2-tetrachloroethane (TeCA)-dechlorinating culture WBC-2 to biotransform TeCA alone, or a mixture of TeCA plus PCE and carbon tetrachloride (CT), in microcosms. The microcosms contained electron donors provided to biostimulate the added culture and sediment collected from a wetland where numerous "hotspots" of contamination with chlorinated solvent mixtures exist. The dominant TeCA biodegradation mechanism mediated by the WBC-2 culture in the microcosms was different in the presence of these wetland sediments than in the sediment-free enrichment culture or in previous WBC-2 bioaugmented microcosms and column tests conducted with wetland sediment collected at nearby sites. The co-contaminants and their daughter products also inhibited TeCA biodegradation by WBC-2. These results highlight the need to conduct biodegradability assays at new sites, particularly when multiple contaminants and dehalorespiring populations are present.
... A mixed, anaerobic culture was enriched from the site (Lorah et al., 2008) and incorporated into an organic-based matrix that was placed at the sediment-water interface. This bioreactive mat successfully treated the chlorinated contaminants prior to discharge (Majcher et al., 2009). Although the bioreactive mat was constructed on the banks of a tidal wetland (i.e., not completely subaqueous) and the design is not immediately suitable for submergence (e.g., buoyancy restrictions, delivery of bioaugmentation culture), the success of the approach supports the concept of bioreactive capping as an in situ remedial technique. ...
... However, at sites where diffusive conditions exist, or where groundwater seepage rates are significantly slower than those employed here, complete dechlorination could be achieved. For instance, at the USGS biomat pilot test described by Majcher et al. (2009), a bioreactive layer successfully dechlorinated a range of chlorinated aliphatics at a site where average hydraulic residence times in the reactive mat were assumed to be 8 to 14 days. Thissystem also included an organic layer composed of a mixture of peat, compost, and chitin to provide long-term electron donor. ...
The development of bioreactive sediment caps, in which microorganisms capable of contaminant transformation are placed within an in situ cap, provides a potential remedial design that can sustainably treat sediment and groundwater contaminants. The goal of this study was to evaluate the ability and limitations of a mixed, anaerobic dechlorinating consortium to treat chlorinated ethenes within a sand-based cap. Results of batch experiments demonstrate that a tetrachloroethene (PCE)-to-ethene mixed consortium was able to completely dechlorinate dissolved-phase PCE to ethene when supplied only with sediment porewater obtained from a sediment column. To simulate a bioreactive cap, laboratory-scale sand columns inoculated with the mixed culture were placed in series with an upflow sediment column and directly supplied sediment effluent and dissolved-phase chlorinated ethenes. The mixed consortium was not able to sustain dechlorination activity at a retention time of 0.5 days without delivery of amendments to the sediment effluent, evidenced by the loss of cis-1,2-dichloroethene (cis-DCE) dechlorination to vinyl chloride. When soluble electron donor was supplied to the sediment effluent, complete dechlorination of cis-DCE to ethene was observed at retention times of 0.5 days, suggesting that sediment effluent lacked sufficient electron donor to maintain active dechlorination within the sediment cap. Introduction of elevated contaminant concentrations also limited biotransformation performance of the dechlorinating consortium within the cap. These findings indicate that in situ bioreactive capping can be a feasible remedial approach, provided that residence times are adequate and that appropriate levels of electron donor and contaminant exist within the cap.
Challenges to sediment remediation include not only the sheer scope of contamination but also technical limitations and escalating costs associated with cleanup. The development of in situ sediment remediation technologies, mirroring the development of successful in situ groundwater remediation approaches, has recently been identified as a priority research need (SERDP and ESTCP, 2004) and could result in treatments that are more effective compared to traditional methods. Implementation of technically feasible and cost-efficient in situ remediation approaches, such as in situ biotransformation, provides numerous potential advantages which could contribute to successful contaminated sediment management. Most notably, in situ biotransformation can directly reduce contaminant concentrations and/or toxicity. When occurring naturally, in situ biotransformations could serve as a key component for management strategies based on monitored natural recovery (MNR) and could potentially be incorporated into capping and combined remedy designs. In other scenarios, engineering may be required to stimulate particular microbial populations and/or manipulate environmental conditions to optimize biotransformation (and biodegradation) activity.
Anaerobic, fixed film, bioreactors bioaugmented with a dechlorinating microbial consortium were evaluated as a potential technology for cost effective, sustainable, and reliable treatment of mixed chlorinated ethanes and ethenes in groundwater from a large groundwater recovery system. Bench- and pilot-scale testing at about 3 and 13,500 L, respectively, demonstrated that total chlorinated solvent removal to less than the permitted discharge limit of 100 μg/L. Various planned and unexpected upsets, interruptions, and changes demonstrated the robustness and reliability of the bioreactor system, which handled the operational variations with no observable change in performance. Key operating parameters included an adequately long hydraulic retention time for the surface area, a constant supply of electron donor, pH control with a buffer to minimize pH variance, an oxidation reduction potential of approximately -200 millivolts or lower, and a well-adapted biomass capable of degrading the full suite of chlorinated solvents in the groundwater. Results indicated that the current discharge criteria can be met using a bioreactor technology that is less complex and has less downtime than the sorption based technology currently being used to treat the groundwater.
