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03B Risk Assessment and Cycling of Natural Organochlorines

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03B Risk Assessment and Cycling of Natural Organochlorines FOCS Focus on Chlorine Science Edition 2013 ISSUE 03B Risk assessment and the cycle of natural To investigate potential chlorination rates, conifer- Rapid and extensive natural chlorination & 36 ous forest soil from the organic horizon with Clin dechlorination of soil organic matter was amended, and over time the amounts of 36Cl subsequently present as chloride (Clin) and non- David Bastviken (Stockholm University, Sweden) extractable organic chlorine were monitored (Clorg; using water and 0.1M potassium nitrate as extract- Co-authors of the original paper: Frida Thomsen, Teresia ants). Three different experiments were per- Svensson, Susanne Karlsson, Per Sandén, George Shaw, formed. Miroslav Matucha and Gunilla Öberg. In the first, transformation of Cl over 133 days un- During recent decades, the extent to which soil der oxic conditions was studied. This experiment organic matter can be chlorinated by natural pro- 36 showed a rapid initial increase in Clorg and after cesses has been debated. The amounts of organi- 36 eight days, 23% of the added Clin had been trans- cally bound chlorine (Clorg) in soils are often as 36 36 36 formed into Clorg. The proportion of Cl as Clorg large as or greater than the quantities of inorganic remained at this level until day 20. After that, the chlorine (chloride; Clin). Such high levels of Clorg 36 fraction of Clorg started to decrease and reached a even in very remote areas imply that its presence plateau at 8% by day 133. The decline in 36Cl af- cannot be explained by anthropogenic emissions org 36 ter day 20 can only be explained by substantial alone. Here, the authors present evidence from Cl transformation of Cl to Cl . The long term abun- tracer experiments that natural transformation of org in dance of Clorg is controlled by several different pro- chlorine between Clin and Clorg in soils can be rapid cesses including chlorination and dechlorination. and extensive, with specific rates up to 10% of the standing stock transformed per day, and that chlo- In the third experiment, the chlorination potential rination relies primarily on microbial or enzymatic under oxic and anoxic conditions was studied. Re- processes. sults indicated much higher net rates of formation 36 of Clorg under oxic conditions than in the absence of oxygen. This indicates that the natural formation Dr David Bastviken is Assistant Professor at Stockholm University, Sweden. He received an MSc from Uppsala University, Sweden, in 1997 and a PhD from Linköping University, Sweden, in 2002. His postdoctorate in 2003-2004 was at the Institute of Ecosystems Studies, Millbrook New York. Dr Bastviken’s research covers biogeochemistry, ecology, and microbial ecology. His early research examined aquatic food web dynamics following invasion of exotic species. He has also studied aquatic carbon cycling. Recent research has focused on natural chlorine transformations in soil, and methods for high precision measurements of rapid chlorination and dechlorination in the complex soil matrix. Current research interests include chlorine biogeochemistry, natural chlorination and dechlorination of organic matter, carbon biogeochemistry, and trace gas biogeochemistry including methane dynamics. P 01 Focus on Chlorine Science ISSUE 03B Edition 2013 36 of Clorg is regulated by biotic processes requiring The biodegradability of organochlorine oxygen, or that dechlorination processes are sub- stantially greater under anoxic conditions resulting compounds in lower net chlorination rates. All experiments showed substantial transformation 36 36 of Clin to Clorg within days under oxic condi- James Field (University of Arizona, USA) tions. The first experiment also indicates rapid Key to the natural chlorine cycle is the biodegrada- dechlorination of organochlorines in soil. To esti- tion of organochlorine compounds, converting mate rates, the possibility of simultaneously occur- chlorine back to chloride. Microorganisms have ring chlorination and dechlorination has to be con- developed diverse strategies for degrading organo- sidered. The best estimates of initial specific chlo- chlorine compounds, from reductive dehalogena- rination and dechlorination rates were in similar tion to hydrolytic and oxygenolytic chloride re- ranges, but dechlorination rates were always lease. Biodegradation should be considered as a greater than chlor-ination rates. Combined with redox reaction in which an electron donor is oxi- the frequent observation that Cl exceeds Cl in org in dised at the expense of an electron acceptor. organic soils, this suggests that some of the Clorg formed is resistant to dechlorination and accumu- Lower chlorinated compounds serve as an electron lates in spite of high specific dechlorination rates. donor and carbon source by being oxidised if an Results indicate that a maximum of 10% of the adequate electron acceptor is present. Higher 36 Clorg formed during the experiment could have chlorinated compounds can serve as electron ac- been refractory and that the remainder (>90%) was ceptors by being reductively de-halogenated in the subjected to dechlorination. presence of electron-donating compounds in the halorespiration process. Chlorinated solvents can The specific rates estimated in the present study also be cometabolically biotransformed by acci- represent potential rates since our experimental dental reactions with enzymes and cofactors in- conditions were dissimilar to field conditions. volved in the de-gradation of other substrates. Ex- Hence, our estimated specific rates should be con- amples of cometabolism include oxidation of chlo- sidered with caution. However, even if specific rinated solvents by monooxygenases and reduction chlorination rates under field conditions are 10- of chlorinated solvents by commonly-occurring re- fold lower than the estimates in the present study, duced cofactors in anaerobes (vitamin B12). the whole Clin pool is turned over by transfor- mation to Clorg at least twice a year. Therefore, Some natural organochlorines have a large turno- even a conservative interpretation of our results ver in the biosphere, e.g. chloromethane, chloro- supports the conclusion that soil chlorine transfor- form and methylated derivatives of chlorophenol. mations are rapid and extensive. Chloromethane is a good carbon and energy source for aerobic bacteria (Hypho-microbium and Methylobacterium) which mineralise the compound - completely to CO2 and Cl . The doubling times of Dr James Field is a Full Professor at the Department of Chemical and Environmental Engi- neering, University of Arizona (UA), USA. He joined the UA faculty as associate professor in 2001. Prior to that, he was Assistant Professor in the Department of Environmental Technology and in the Division of Industrial Microbiology at Wageningen University, The Netherlands. Dr Field received a PhD from Wageningen University in 1989. He has published over 150 scientific articles in peer reviewed journals. Current research interests include the biodegradation of chlorinated solvents; biotransformation of perchlo- rate and arsenic compounds; and autotrophic denitrification. He has worked on the natural production of organochlorine compounds by wood and litter-degrading fungi. P 02 Edition 2 013 ISSUE 03B aerobic chloromethane-utilising bacteria range valid to assume that such compounds will persist from 5.7 to 7.7h. The anaerobe, Acetobacterium indefinitely in the environment. dehalogenans is known to ferment chloromethane - to acetate and Cl in the presence of CO2. Aerobic and anaerobic bacteria use methyltransferases as The biogeochemical cycle of chlorine the first step in chloromethane degradation. The only known biodegradation of chloroform is co- metabolic. Under aerobic conditions, chloroform is Gunilla Öberg (Linköping University, Sweden) co-oxidised by methane or ammonia oxidising bac- teria using monooxygenases. Phosgene is a transi- In the early 1990s, it was revealed that large ent intermediate of the co-oxidation reaction re- amounts of naturally produced Clorg are present in soil. Later studies showed that chlorine partici- sulting in the formation of CO2. Chloroform is also reductively dehalogenated via the intermediates pates in a complex biogeochemical cycle in which dichloromethane and chloromethane in anaerobic the terrestrial environment appears to be a key environments. component. Recent studies suggest that a large portion of the Clin that is deposited in terrestrial Numerous bacteria use chlorophenols as a carbon environments is transformed to Clorg in soil or and energy source under aerobic conditions. The vege¬tation. The majority appears to become most studied strains utilising pentachlorophenol chlorinated organic matter. The process is not un- belong to the genera Mycobacterium and Sphingo- derstood, but there is strong evidence that the monas. Extensive evidence shows that chlorophe- transformation is driven by biotic processes, alt- nols are mineralised by these bacteria. The dou- hough abiotic processes have also been shown to bling times of aerobic chlorophenol utilising bacte- exist. ria range from 2.3 to 19.8h. In temperate regions, the chlorinated organic mat- Two main strategies are used for the aerobic deg- ter is subsequently transported to deeper soil lay- radation of chlorophenols. Lower chlorinated phe- ers with the soil water. It precipitates and is even- nols are initially attacked by mono-oxygenases tually mineralised, leading to the release of Clin yielding chlorocatechols as the first intermediates. following the
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