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Natural organochlorines in sediments
08/2008 Recent fluvial, lacustrine and marine sediments were found to contain significant concentrations of organohalogens which cannot be explained by known anthropogenic halogen compounds. The assumption of a natural source for the major part of these organohalogens is strongly supported by the fact that in lacustrine sediments deposited some hundred years ago - where no industrial chlorinated organic compounds should be expected - concentrations between 30 and 100 mg Cl/kg of adsorbable organohalogens (AOX, expressed as equivalent chlorine) were detected. In anoxic sediments from the Black Sea, several thousand years old and having a high organic content (mainly composed of marine phytoplankton), organic bromine and iodine were detected at levels up to 313 and 467 mg/kg, respectively. High AOX concentrations in biogenic sediments rich in organic matter from various epochs within a timescale of some thousands to about 300 million years and representing different stages of coalification (peat - lignite - bituminous coal - anthracite) are clear evidence for naturally occurring organohalogens. The finding of high AOX concentrations in organic rich sediments led to the idea to investigate organic rich meteorites, i.e. the group of "carbonaceous chondrites". From a 1953 publication, it was already known that the "Cold Bokkeveld meteorite" (South Africa) contains "chlorinated resinous extracts", termed "chlorobitumens". Our measurements on four carbonaceous chondrites from four independent fall events revealed AOX concentrations between 124 and 209 mg/kg. In the Cold Bokkeveld meteorite three defined chlorinated acids (4-chloro-, 2,4- and 2,6-dichlorobenzoic acid) could be detected by gas chromatography. The occurrence of alkyl chlorides and chlorobenzene in the Cold Bokkeveld and Orgueil meteorites had already been reported in 1965. The occurrence of natural organohalogens in sediments might be explained by deposition of biogenic material (in particular plants and plant detritus) already containing organohalogens and/or by formation of chlorinated organic compounds within the sediment itself. It is now beyond question that plant material has to be considered as the most important primary source of high molecular weight organohalogens in sediments. Marine plants are known for their organically bound chlorine, bromine and iodine content, the sea being a huge reservoir for these halogens and high AOX concentrations could therefore be expected. Three common marine macrophytes from the intertidal area (Spartina, Salicornia and Fucus) were found to contain AOX levels of 114, 129 and 182 mg Cl/kg, respectively. In marine algae, maximum bromine and iodine contents of 3,400 and 2,500 mg/kg, respectively, have been measured. However, terrestrial plants also exhibited high AOX contents: in five genera (grown in a protected greenhouse to prevent atmospheric contamination) concentrations varied between 65 and 115 mg/kg Cl. In tobacco of nine different cigarette brands AOX contents between 30 and 66 mg/kg Cl were measured. It can be assumed that the vast majority of this is due to natural halogens. In addition, natural low molecular weight organohalogens produced by bacteria, fungi, and algae (such as haloalkanes, terpenes, amino acids and peptides, chlorophenols etc) will also accumulate in the sediment and become part of the primary organohalogen content. As a secondary source of organohalogens in sediments their formation in the sediment itself has to be considered. Biotic halogenation of organic substrates by haloperoxidases (occurring in algae and other marine organisms but also in terrestrial lichens and fungi) leading to volatile organohalogens has already been observed. Little knowledge exists on abiotic halogenation. In preliminary investigations in the system trichloroacetic acid - water - humic substances, chlorinated compounds could be identified. A transfer of (low molecular weight) organohalogens from the sediment into the interstitial water is obvious: if compared with the supernatant lake water, the AOX concentrations in interstitial water of Lake Constance sediments are enriched by a factor of 20 - 70. Under anaerobic conditions bacterial decomposition of organohalogens leads to the release of halide ions into the interstitial water of the sediment. The occurrence of organic halogens in the carbonaceous chondrites provides evidence for naturally produced (abiotic) halogenated organic compounds. The results raise the question about the origin of halogenated organic compounds in primitive planetary matter. The results here confirm our previous conclusion that the AOX value cannot be used as a sum parameter for anthropogenic organic halogen compounds. |
