Bacterial oxidation of low-chlorinated compounds under anoxic conditions

Chlorinated hydrocarbons belong to the most frequently encountered contaminants in soil and groundwater. Many of them were found to be toxic and recalcitrant, which causes a potential threat to the environment. Therefore, it is of great importance that sites contaminated with chlorinated hydrocarbons are remediated.In this thesis we focus on anaerobic microbial oxidation of low-chlorinated hydrocarbons. These low-chlorinated hydrocarbons can enter soil and groundwater by direct release or by partial dechlorination of higher chlorinated hydrocarbons. Their fate strongly depends on microbial processes in soil and groundwater systems. Since many contaminated aquifers contain large volumes that are dominated by anaerobic processes it is apparent that anaerobic microorganisms play an important role in the degradation of contaminants. The tendency of low-chlorinated hydrocarbons to undergo oxidative dehalogenation increases when the number of chlorine substituents decreases. Whether low-chlorinated hydrocarbons are more likely to be degraded by oxidation than by reduction processes under anoxic conditions depends primarily on environmental conditions (e.g. redox conditions, availability of electron acceptors/donors, present microorganisms).The potential for anaerobic oxidation of different pollutants was studied by performing both up-flow soil columns and batch experiments, inoculated with contaminated soil and supplied with a mixture of different (chlorinated) hydrocarbons. They were operated in the absence of oxygen under nitrate-,iron(III)- or manganese(IV)-reducing conditions or in the absence of an externally supplied electron acceptor.It was found that 1,2-dichloroethane (DCA), 2-chloroethanol (CE), vinyl chloride (VC), monochlorobenzene (MCB) and octane could be removed in the absence of oxygen.Fromaenrichment culture that showed DCA degradation with nitrate as sole electron acceptor, Pseudomonas stutzeri stain JJ was isolated. This bacterium is the first isolate known that can grow anaerobically with CE as sole carbon and energy source and nitrate as electron acceptor. Strain JJ was however not able to grow on DCA. The degradation pathway of CE under denitrifying conditions by strain JJ seems to be the same as found in aerobic bacteria that degrade CE . According to this pathway, CE is first oxidized to chloroacetaldehyde, then chloroacetaldehyde is oxidized to chloroacetate and subsequently chloroacetate is dehalogenated to glycolate, which enters the central metabolic pathways.MCB removal was studied in a nitrate-reducing up-flow soil column. Different observations and calculationssuggeststhat MCB was oxidized in the absence of oxygen with nitrate as electron acceptor.These results underline the potential importance of anaerobic oxidation processes for the clean-up of sites polluted with low-chlorinated hydrocarbons as pointed out by others. Anaerobic oxidation pathways can be of particular importance when plumes contaminated with chlorinated compounds spread from reduced (e.g. methanogenic, sulfate-reducing) to more oxidized (e.g. iron-reducing and denitrifying) redox zones. These redox situations frequently occur at polluted sites in Western Europe.