Chloroethene oxidation

The microbial degradation of contaminants under anaerobic conditions using humic acids as electron acceptors has been demonstrated. These included the oxidations (a) chloroethene and 1,2-dichloroethene to CO2 that was confirmed using C-labeled substrates (Bradley et al. 1998) and (b) toluene to CO2 with AQDS or humic acid as electron acceptors (Cervantes et al. 2001). The transformation of l,3,5-trinitro-l,3,5-triazine was accomplished using Geobacter metallireducens and humic material with AQDS as electron shuttle (Kwon and Finneran 2006). 

Reductive biotransformation of a contaminant can occur when the contaminant serves as the terminal electron acceptor. Many contaminants that are recalcitrant to bio-oxidation will undergo reductive biotransformations. These biotransformations can lead to detoxification, mineralization, or changes in the mobility of the targeted contaminant. Hexavalent chromium and tetra-chloroethene (PCE) have been investigated as candidates for reductive biotransformation. This technology may be most applicable for in situ remediation for the following scenarios PCE contamination, low-yield aquifers, areas contaminated by both alkylbenzenes and chlorinated ethenes, and deep aquifer contamination. 

Bradley PM, Landmeyer JE, Dinicola RS (1998b) Anaerobic oxidation of [l-2-14C] di-chloroethene under Mn(IV)-reducing conditions. Appl Environ Microbiol 64 1560-1562... 

A varying and much more complex mechanistic situation exists in heterogeneous photocatalysis (Fig. 5-13). With respect to the transient oxygen species, comparable overall oxidation reactions are usually observed, but the set of primary reactive oxygen species is slightly different. It is commonly assumed, that superoxide radical anions and hydroxyl radicals are the primary species formed after photogeneration of the electron-hole pair of a semiconductor catalyst in the presence of water and air (Serpone, 1996). In the presence of ozone, ozonide radical anions or are formed by fast electron transfer reaction of superoxide radical anions with O3 molecules. The combination Ti02-03-UV/VIS is called photocatalytic ozonation (Kopf et al., 2000). For example, it was applied for the decomposition of tri-chloroethene in the gas phase (Shen and Kub, 2002). 

Emissions of phosgene most commonly arise as a result of its release during manufacture and use, its formation from the decomposition of chlorinated hydrocarbons, and its formation from the photochemical oxidation of air-borne chlorinated organic materials, particularly the C, and C chloroalkanes, and chloroethenes. The location and estimation of air emissions from sources of phosgene have been described by the US Environmental Protection Agency [2088b], Catastrophic emissions and accidental spills and leaks are discussed in Section 3.6. 

The overall reaction and product stoichiometries for the degradation of chloroalkene substrates by O2 - in DMF are summarized in Table 7-1.20 Within the limits of a reaction time of 10 min or less, chloroethene, frflws-1,2-dichloroethene, Aldrin, and Dieldrin are not oxidized by O2 - in DMF. A reasonable mechanism for these oxidations is an initial nucleophilic addition of superoxide to the chloroalkenes [e.g., tetrachloroethene (Scheme (7-7)]. Subsequent loss of chloride ion would give a vinyl peroxy radical, which can cyclize and decompose to a chloroacyl radical and phosgene. t These would undergo subsequent facile reactions with O2 - to give bicarbonate and chloride ions. 

Figure 19.1. Schematic view of the reductive and oxidative biodegradation pathway for chloroethenes.

Lohner ST, Becker D, Mangold K-M, Tiehm A (2011) Sequential reductive and oxidative biodegradation of chloroethenes stimulated in a coupled bioelectro-process. Environ Sci Technol 45 6491-6497... 

The three DCE isomers can be further degraded under reducing conditions into chloroethene (C2CIH3, also called vinyl chloride). The oxidation state of the carbon is thereby reduced to (—1) ... 

Hermans et al. (1985) and Hartmans and de Bont (1992) show that vinyl chloride can be used as a primary substrate under aerobic conditions, with vinyl chloride being directly mineralized to carbon dioxide and water. Direct vinyl chloride oxidation has also been reported by Davis and Carpenter (1990), McCarty and Scmprini (1994), and Bradley and Chapelle (1998). Aerobic oxidation is rapid relative to reductive dechlorination of dichlo-roethene and vinyl chloride. Recent research demonstrated that during direct aerobic oxidation, the interaction among different chloroethenes can be an important factor (Zhao et a/., 2010). 

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