Microbial Degradation of Toxaphene
Sammanfattning: This works deals with the degradation of a group of recalcitrant compounds known as toxaphene. Toxaphene is a complex mixture of chlorinated compounds including hexa- and octachlorocamphenes, hepta-, nona- and decachlorobornanes. Some components of toxaphene are very persistent while others are degraded rapidly by both abiotic and biotic processes. Not all components of toxaphene have been identified. The main purpose of this research was to develop inexpensive and robust technology for degradation of toxaphene using microorganisms. To achieve the degradation two approaches have been used: anaerobic dechlorination with a subsequence metabolism of the carbon skeleton, and use of strong oxidizing reagents from fungal peroxidase/laccase to at least initiate degradation. A sequential anaerobic-aerobic bioreactor configuration was used for the purpose of toxaphene degradation. Aerobic processes permit the attainment of higher organic matter removal rates and are often less sensitive to pollutant toxicity. In this study, COD removal efficiencies of up to 90% were attained in the anaerobic step, confirming the potential of this process for the initial attack of toxaphene. No significant enhancement in the removal of COD was recorded in the aerobic step. The anaerobic-aerobic system implemented to degrade toxaphene in water was effective and after 42 days, the total toxaphene concentration was reduced by 87%. Final degradation, achieved after 269 days, was very similar to the results obtained already from the anaerobic reactor (98%).The anaerobic step was responsible for high removal efficiency and small improvements were obtained in the aerobic step. Toxaphene isomers having more chlorine substitutions were degraded relatively fast, resulting in an initial rise in concentration of isomers with less chlorine substitutions. White-rot fungi often need the presence of easily biodegradable co-substrate to trigger the production of extracellular enzymes. Wheat husk, wood chips and molasses were compared for their ability to support toxaphene degradation by a Bjerkandera sp. Toxaphene biodegradation was intrinsically linked with the production of lignin peroxidase (LiP). Approximately 85% of toxaphene was removed when wheat husk was used, while lower removal efficiencies were recorded when using wood chips and molasses, respectively. There are several evidences that ligninolytic enzymes produced by Bjerkandera sp. are the oxidizing agents of recalcitrant compounds; however, the capability of toxaphene removal has not been studied before. Future work should focus on the development of continuous fungal-based processes and further comparison with bacterial based processes. Toxaphene biodegradation in bio slurry reactors is often limited by pollutant bioavailability. Removal efficiencies of toxaphene isomers of up to 96% were achieved in 79 days of operation. In our study, no improvement in the biodegradation of less chlorinated isomers was observed when adding the surfactant Triton X-114 or lactic acid. The removal of heavily chlorinated compounds was enhanced by the addition of the compounds above mentioned. From toxaphene-contaminated soil it was possible to identify one species that was capable of degrading toxaphene on its own. Enterobacter sp. Strain D1 is a facultative anaerobic, Gram negative heterotrophic bacterium. Based on 16S rDNA analysis, the strain D1 was clustered closely with the species Enterobacter cloacae subsp. dissolvens and E. cloacae. The ability of this microorganism to utilize and transform the different chlorinated camphenes present in toxaphene was investigated under anaerobic conditions. Levels of hexachlorocamphenes and heptachlorobornanes were initially reduced, but the levels increased again during the last 20 days of cultivation.
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