Anaerobic oxidation of chlorinated compounds

Aerobic, Anaerobic, and Combined Systems. The vast majority of in situ bioremediations ate conducted under aerobic conditions because most organics can be degraded aerobically and more rapidly than under anaerobic conditions. Some synthetic chemicals are highly resistant to aerobic biodegradation, such as highly oxidized, chlorinated hydrocarbons and polynuclear aromatic hydrocarbons (PAHs). Examples of such compounds are tetrachloroethylene, TCE, benzo(a)pyrene [50-32-8] PCBs, and pesticides. 

Reductions such as these usually do not completely mineralize a pollutant. Their greatest significance lies in the removal of chlorine or other halogen atoms, rendering the transformed chemical more subject to oxidation if it is ultimately transported back into an aerobic environment. Figure 2-25 shows some types of anaerobically degraded compounds. 

Compounds with a high oxidation state such as those with a lot of chlorine are recalcitrant to further oxidation. These compounds must therefore be degraded anaerobically. Chlorine is substituted by hydrogen or HC1 is removed and a double bond is introduced, for example, DDT that may be dechlorinated to 4,4 -dichlorodiphenyl-dichloroethane (DDD) by anaerobic processes or slowly converted to 4,4 -dichlorodiphenyldichloroethylene (DDE) (Stenersen, 1965). Compounds such as mirex and hexachlorobenzene are extremely recalcitrant to degradation, and microorganisms do not attack the highly fluorinated or chlorinated polymers, such as Teflon and PVC. 

Biooxidation is decomposition of organic matter with oxidizing of its carbon. Organic matter in these reactions is donor of electrons, and the acceptors are elements or compounds outside it O, NO3. NO T Fe, iron hydroxide Fe(OH)3>, CO, some chlorinated solvents, etc. There may be aerobic and anaerobic oxidizing. In the former case acceptor of electrons is directly molecular oxygen O, in the latter oxidized forms of nitrogen (NOj", NO3 ), manganese (Mn ), iron (Fe +), sulphur (SO ), etc. 

At many chlorinated ethene sites, concentrations of cis-1,2-DCE are often higher than any of the parent chlorinated ethene compounds. The reason for the accumulation of 1,2-DCE may be due to either slower rates of DCE halorespiration, or the prevalence of organisms that reduce PCE as far as cis-1,2-DCE over ones that can reduce PCE all the way to ethene. Although many researchers have commented that reductive dechlorination will result in the accumulation of VC (e.g., see 84, 89), at many field sites VC accumulation is much lower than cis-1,2-DCE. This may occur because the vinyl chloride in many chlorinated solvent plumes can migrate to zones that can support direct oxidation of VC oxidation, either aerobically and/or anaerobically. 

More oxidized compounds, such as chlorinated benzenes, are susceptible to biologically mediated reduction in environments under anaerobic conditions, such as in lake and river sediments. It is known that highly polychlorinated biphenyl (PCB) congeners, for example, are susceptible to reductive dehalo-genation, the result of the interaction of syn-trophic microbial communities that are active under methanogenic and sulfate-reducing... 

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