| Hexachlorocyclohexane (HCH) isomers, as a result of the production and application of HCH containing pesticides, are globally dispersed persistent organic pollutants (POPs) and widespread in soil and groundwater. Recently, the European Environment Agency reported that approximately 342,000 contaminated field sites are in need of remediation due to the risks that pollutants pose to human and ecosystem health. And similar situation exists in the USA where there were approximately 294,000 contaminated field sites. For this reason, contamination of soil, sediment, ground and surface waters by HCH isomers is nowadays acknowledged as an important worldwide environmental issue, which mandates some management strategies and proposes urgent need of sustainable approaches to remove them from the environment. Microbial degradation is regarded as an important process of HCHs-removal from environment and can cause a sustainable and efficient decrease in pollutant mass. However, it requires accurate monitoring techniques to evaluate the biodegradation of pollutants. Compound-specific isotope analysis (CSIA) has been increasingly considered for characterizing in situ biodegradation processes qualitatively and quantitatively. The method takes advantage of the preferential transformation of lighter isotopes during a degradation reaction, thus leading to an enrichment of heavier isotopes in the residual phase during the course of biodegradation. α-HCH as many other organic components appear as a racemic mixture of enantiomers in the environment and fractionation of the enantiomers (EF) can yield information on biodegradation. The combination of CSIA and the enantiomer selective stable isotope analysis (ESIA) has potential for distinguishing transformation processes of chiral contaminants.To validate the applicability of CSIA for HCH, in this study, Firstly we investigated the isotope fractionation of the biodegradation of HCHs by Dehalococcoides strains 195 in order to elucidate the potential of CSIA for characterizing the in situ dechlorination under anoxic conditions. The anaerobic biodegradation pathway was investigated by analyzing metabolites and end products. Furthermore, the shifts in stable carbon isotope composition of HCH isomers during biotransformation were assessed and compared to a γ-HCH degrading enrichment culture. Secondly, the carbon stable isotope fractionation during aerobic degradation of the enantiomers of a-HCH by two new Sphingobium spp. strains, S. quisquilarium P25 and S. chinhatense IP26 were determined, and this was compared with the isotope fractionation of bulk α-HCH and γ-HCH. And further investigation on changes in enantiomeric composition of α-HCH during aerobic biodegradation was to verify if the Rayleigh equation can be applied for describing the fractionation of the α-HCH enantiomers, therefore propose that ESIA, in addition to CSIA, can be used as a tool for analyzing the fate of enantiomeric chemicals in the environment making use of both enantioselective degradation and isotope fractionation as indicators. Finally, CSIA was applied for assessing biodegradation of HCHs in a contaminated soil system via inoculation with S. quisquilarium P25 for the first time. The progress and sustainability of HCH biodegradation by Sphingobium spp in contaminated soil was evaluated.Reaction-specific carbon isotope enrichment factors (εc) were determined in laboratory experiments for HCH isomers during anaerobic and aerobic biodegradation. Bulk enrichment factors determined for aerobic degradation of a and γ-HCH by S. quisquilarium P25 and S. chinhatense IP26) with similar reaction mechanism were similar (εc=-1.43±0.3‰ and -0.86±0.2‰ for α-HCH, εc =-1.03±0.1‰ and -1.27±0.2‰ for γ-HCH). And within a HCHs contaminated soil, enrichment factors (εc=-2.52±0.1‰ andεc = -2.06±0.3‰, for a and γ-HCH respectively by S. quisquilarium P25 is little high than in pure culture medium, which is reported for the first time. Carbon isotope fractionation for aerobic degradation was smaller (-0.86%o to -1.43%o) as compared to anaerobic biodegradation of y-HCH experiments with D. ethenogenes strain 195 (εc=-5.5± 0.8 ‰) and enrichment culture (εc = -3.1±0.4). Furthermore, isomer and enantiomer selective stable isotope fractionation of α-HCH was analysed during aerobic biodegrading reactions. The carbon isotope enrichment factors were very different from the (+) to (-) a-HCH enantiomers for both S. quisquilarium P25 and S. chinhatense IP26, the (+) a-enantiomer (-1.72 0.8‰ and -2.27±0.6‰, respectively) reveals a higher isotope enrichment factors than (-) α-enantiomer (-0.73±0.2‰ and -1.01±0.6‰, respectively). (-) α-HCH was preferentially degraded with the enantiomeric fraction (EF) (-) values ranging from 0.447 to 0.142 in case of S.quisquilarium P25 and 0.495 to 0.236 for S. chinhatense IP26. And enantiomeric enrichment factors (εe) for aerobic a-HCH biodegradation by S.quisquilarium P25 and S. chinhatense IP26 were -44±16% and -23±6%, respectively. Enantioselective transformation and isotope fractionation of a-HCH enantiomers was observed in the aerobic biological degradation study. The enrichment factors of individual enantiomers (εe) allowed calculating an average enrichment factor in all cases which was identical with bulk enrichment factors εbulk showing the validity of the analytical approach. The extent and variability of carbon stable isotope fractionation in all laboratory investigation validate the applicability of CSIA as tool to characterize transformation of HCH in the environment. Further the ESIA method can help to distinguish biodegradation between enantiomers. The ESIA approach has probably potential for tracing the fate of other chiral contaminants in the environment. The evaluation of CSIA at a contaminated soill demonstrated its potential for the identification of source zone of HCH contaminations and estimation of the extent of degradation down gradient the source zones. In short, this study provides a concept for studying the biodegradation of HCH in the environment. The use of ESIA provides a comprehensive assessment of in situ degradation of α--HCH but also provide bases for tracing the fate of other contaminants having chiral isomer with respect to providing evidence of degradation, and distinguishing pathways as well as quantifying degradation at contaminated field sites. |
PDF Full Text Request