Formation of volatile organohalogens by microbially catalyzed redox reactions of iron minerals and humic substances

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Organohalogens, organic compounds containing halogens such as chlorine, are of environmental concern. In particular because they influence atmospheric chemistry and thus the earth climate, the identification of the sources of organohalogens and an understanding of the mechanisms that lead to formation of organohalogens are necessary. From recent studies it became evident that not only biotic processes but maybe to an even larger extent also abiotic redox reactions involving humic substances and iron minerals can lead to the formation of organohalogens. However, the mechanisms of these reactions and in particular the identity of the reactive organic and iron species leading to the formation of organohalogens are unclear. Whether microbial redox transformation of humic substances and iron minerals can produce such reactive organic and iron species and thereby stimulate the formation of organohalogens is not known at all. The main goal of this research project therefore is to determine whether microbial redox reactions of Fe(ll)- and Fe(lll)-minerals and humic substances can produce reactive iron species and humic compounds that lead to the formation of polar and volatile organohalogens (POX and VOX). For this purpose microcosms will be setup with either pure cultures of ironmetabolizing bacteria or with microbially active soil samples from an organohalogen-releasing field site. These biotic systems will be compared to abiotic ones. One particular goal of this research project is to determine the reactive iron species and to identify the reactive functional groups in the humic substances. Because of the ubiquity of humic substances and Fe(lll)-minerals and the variety of chemical and microbiological processes that are able to reduce and oxidize humic substances and iron minerals, these reactions have the potential to play a key role in the formation of volatile organohalogens and for their release into the atmosphere.