Accumulation Of Vinyl Chloride Research Articles

Effect of Fe(III) on 1,1,2,2-Tetrachloroethane Degradation and Vinyl Chloride Accumulation in Wetland Sediments of the Aberdeen Proving Ground

1,1,2,2-Tetrachloroethane (TeCA) contaminated groundwater at the Aberdeen Proving Ground discharges through an anaerobic wetland in West Branch Canal Creek (MD), where dechlorination occurs. Two microbially mediated pathways, dichloroelimination and hydrogenolysis, account for most of the TeCA degradation at this site. The dichloroelimination pathways lead to the formation of vinyl chloride (VC), a recalcitrant carcinogen of great concern. The goal of this investigation was to determine whether microbially-available Fe(III) in the wetland surface sediment influenced the fate of TeCA and its daughter products. Differences were identified in the TeCA degradation pathway between microcosms treated with amorphous ferric oxyhydroxide (AFO-treated) and untreated (no AFO) microcosms. TeCA degradation was accompanied by a lower accumulation of VC in AFO-treated microcosms than untreated microcosms. The microcosm incubations and subsequent experiments with the microcosm materials showed that AFO treatment resulted in lower production of VC by (1) shifting TeCA degradation from dichloroelimination pathways to production of a greater proportion of chlorinated ethane products, and (2) decreasing the microbial capability to produce VC from 1,2-dichloroethene (DCE). VC degradation was not stimulated in the presence of Fe(III). Rather, VC degradation occurred readily under methanogenic conditions and was inhibited under Fe(III)-reducing conditions.

Carbon isotopes as a tool to evaluate the origin and fate of vinyl chloride: laboratory experiments and modeling of isotope evolution.

Accumulation of vinyl chloride (VC) is often a main concern at sites contaminated with chlorinated ethenes and ethanes due to its high toxicity. Since there can be several possible sources of VC and ethene at such sites, assessing the origin and fate of VC can be complicated. Aim of this study was to evaluate carbon isotope fractionation associated with various anaerobic processes that lead to the production of VC and ethene in view of using isotopes to evaluate the origin and fate of these compounds in groundwater. Microcosms were constructed using sediments and groundwater from a contaminated site and amended with potential precursors for VC and ethene production. In the microcosms with dichloroethene isomers, sequential reductive dechlorination was observed, and isotopic enrichmentfactors of -19.9 +/- 1.5 per thousand for cis-1,2-dichloroethene, -30.3 +/- 1.9 per thousand for trans-1,2-dichloroethene, and -7.3 +/- 0.4 per thousand for 1,1-dichloroethene were obtained. In microcosms with chlorinated ethanes, 1,2-dichloroethane (1,2-DCA) and 1,1,2-trichloroethane (1,1,2-TCA) were predominantly transformed by dichloroelimination to ethene and VC, respectively, and enrichmentfactors of -32.1 +/- 1.1 per thousand for 1,2OCA and -2.0 +/- 0.2 per thousand for 1,1,2-TCA were observed. Except for 1,1,2-TCA, a strong 13C enrichment in each of the potential precursor of VC was observed, which opens the possibility to trace the origin of VC based on the isotope ratio of potential precursors. Furthermore, it was possible to model the isotope evolution of VC present as substrate or intermediate product as a function of time. The study demonstrates that carbon isotope ratios can potentially be used for qualitative and possibly quantitative evaluation of the origin and fate of VC at sites with complex contaminant mixtures.

Enhanced bioremediation of trichloroethene contaminated by a biobarrier system

The industrial solvent trichloroethylene (TCE) is among the most ubiquitous chlorinated compounds found in groundwater contamination. The objective of this study was to develop a barrier system, which includes a peat (used as the primary substrates) layer to enhance the aerobic cometabolism of TCE in situ. A laboratory-scale column experiment was conducted to evaluate the feasibility of using this peat biobarrier to remediate aquifers contaminated by TCE. This system was performed using a series of continuous-flow glass columns including a soil column, a peat column, followed by two consecutive soil columns. Activated sludges were inoculated in all three soil columns to provide microbial consortia for TCE cometabolism. Simulated TCE contaminated groundwater with a flow rate of 0.25 L/day was pumped into the system. Effluent samples from each column were analyzed for TCE and its degradation byproducts [cis-dichloroethylene (cis-DCE) and vinyl chloride (VC)]. Average removal efficiency was 96% for TCE over a 60-day operating period. Accumulation of VC was observed due to the depletion of oxygen in the system. Results from this laboratory study reveal that the developed biobarrier treatment scheme would be expected to provide a more cost-effective alternative to remediate chlorinated-solvent contaminated aquifers.

Cobalamin-Mediated Reduction of cis- and trans-Dichloroethene, 1,1-Dichloroethene, and Vinyl Chloride in Homogeneous Aqueous Solution: Reaction Kinetics and Mechanistic Considerations

Since cobalamin is involved in the enzymatic reduction of halogenated ethenes by a variety of anaerobic bacteria and since cobalamin has been suggested as electron transfer mediator for the treatment of halogenated solvents, its reactions with such compounds are presently of great interest. In this paper, it is shown that, in homogeneous aqueous solution containing titanium(III) citrate as the bulk electron donor, superreduced cobalamin reductively dechlorinated cis- and trans-dichloroethene (cis-DCE and trans-DCE), 1,1-dichloroethene (1,1-DCE), and vinyl chloride (VC) in pH-dependent reactions to ethene and ethane. Evidence is given that the initial step was the addition of cob(I)alamin to the chlorinated ethenes (CEs) with simultaneous protonation. Only for 1,1-DCE at high pH, a dissociative electron transfer mechanism as suggested for tetrachloroethene (PCE) and trichloroethene (TCE) in earlier work was important. 1,1-DCE reacted about 30 times faster than VC, 600 times faster than trans-DCE, and 3000 times faster than cis-DCE. Acetylene and ethene were found to react at similar rates as 1,1-DCE and VC, respectively. However, at more positive redox potentials, the reductive cleavage of the addition products, particularly of the adducts of acetylene, ethene, and VC with cob(I)alamin, may become very slow, thus preventing the regeneration of cob(I)alamin. The results of this study demonstrate that, at more negative potentials and at low pH, cobalamin is a potent electron transfer mediator for the complete dehalogenation of PCE and TCE without significant accumulation of VC.

Use of ethylene and ethane as primary substrates for aerobic cometabolism of vinyl chloride

A significant problem encountered with anaerobic reductive dechlorination of polychlorinated ethylenes during groundwater remediation is accumulation of vinyl chloride (VC). Even when reduction of VC proceeds to ethylene and/or ethane, low levels of VC may persist. The purpose of this study was to examine use of ethylene and ethane as primary substrates for aerobic cometabolism of VC. Both ethylene‐ and ethane‐grown enrichment cultures (developed with activated sludge inoculum) readily consumed VC. The ethylene culture exhibited an initial preference for VC over ethylene but then switched after several weeks. This culture was unable to use ethane or VC as sole substrates. Although VC inhibited ethylene use, growth on ethylene still occurred in the presence of VC. The ethane‐grown culture was able to use both VC and ethylene as sole substrates. When all three compounds were present, ethylene was consumed first, followed by VC and ethane. Thus, the presence of ethylene and/or ethane with VC may eliminate the need to add a primary substrate (for example, methane, toluene, or phenol) to sustain cometabolism of VC below the regulatory limit.

Anaerobic Mineralization of Vinyl Chloride in Fe(III)-Reducing, Aquifer Sediments

Within anaerobic aquifer systems, reductive dehalogenation of polychlorinated ethenes commonly results in the accumulation of vinyl chloride, which is highly toxic and carcinogenic to humans. Anaerobic reduction of vinyl chloride is considered to be slow and incomplete. Here, we provide the first evidence for anaerobic oxidation of vinyl chloride under Fe(III)-reducing conditions. Addition of chelated Fe(III) (as Fe−EDTA) to anaerobic aquifer microcosms resulted in mineralization of up to 34% of [1,2-14C]vinyl chloride within 84 h. The results indicate that vinyl chloride can be mineralized under anaerobic, Fe(III)-reducing conditions and that the bioavailability of Fe(III) is an important factor affecting the rates of mineralization.