| Scientists have not halted the investigations on the hexachlorocyclohexanes (HCHs) in their physical and chemical properties, environmental behaviors, toxicology and influences on the health of human, since the HCHs were employed as pesticides discharging into the environments. Recent studies suggest that compound-specific isotopic analysis of organic contaminants is a promising tool for environmental research. It is importance of expansion the database of isotopic fractionation during environmental process for organic contaminants of interests. In this work, three most ubiquitous isomers of HCHs (α, β, γ) in natural environment were selected as target compounds. A series experiments were designed in the lab to study the kinetics of, and carbon isotopic fractionation during, transformations of HCHs in aqueous systems with FeS or without FeS present. The main objective of this study is to provide useful information on evaluation of environmental residue half-life and the fate of HCHs and to probe into the feasibility that the stable isotope analysis is as a novel technique to investigate the HCHs' environmental behaviors.In the first part of the study, the initial carbon isotope compositions of HCHs were determined by GC-C-IRMS, EA-IRMS and conventional Dual-Inlet IRMS, respectively. Comparison of the δ13C values obtained from these three isotopic analytic systems indicated that the GC-C-RIMS method used in this study is sufficiently accuracy and precise. To evaluate if isotopic fractionation occurs during the course of preparation and sampling, the isotopic compositions of HCHs were monitored during these processes and the results suggest no isotope fractionation occurring.In the second part of the study, the products of transformation of HCHs were identified; the concentrations and carbon isotope compositions of reactants and products were monitored during the experiment. The results showed that the concentrations and isotopic compositions of the β-HCH in both reaction systems with and without FeS present remained unchanged during the cause of experiment (more than 200 days). In the heterogeneous FeS-water systems, transformation of α- and γ-HCH involves two possible reaction pathways. The first pathway is hydrolysis reaction in aqueous phase and the process can be described as HCHs→PCCHs→TCBs. The dehydrochlorination is the main reaction type in this pathway.The second pathway is believed to be reductive dechlorination occurring in the surface of FeS. The process can be described as HCHs→TeCCHs→DCBs and HCHs→TeCCHs→ (DCCD) →CB. There are two main possible reaction types involving in this process: vicinal dichloroelimination and dehydrochlorination. The solution pHs control the reaction rates and pathways: at lower pH (<8.3), the reaction rate is slow with half-life more than 100 days and the second pathway is major; while at higher pH (>9.3), the reaction rate is quick with half-life less than 20 days and the first pathway predominate. Significant carbon isotopic fractionations were observed during transformation of α- and γ-HCH both in homogeneous and heterogeneous reaction systems and followed Rayleigh behavior well. The total isotope enrichment factors ranged from -6.9 to 13.6‰. The experimental results showed that temperature influences the dimension of isotopic fractionation during hydrolysis of α- and γ-HCH in homogeneous aqueous system. The relationship between isotopicfractionation factors and temperature can be molded by In a = In A* +(12Ea-13Eα) / RT. pHalso affect the isotopic fractionation, generally, the isotopic kinetic effect decreasing with the pH increasing in our experimental systems.In the third part of the study, the preliminary work on transformation of lindane associated with clay minerals and humic acid was performed. The results showed that the clay minerals and organic compounds existing in natural environment would lower or promote the transformation of HCHs.The results of the study made a contribution to understand the environmental chemistry behaviors of HCHs in natural environment. The significant isotopic fractionation during abiotic transformation of HCHs indicates that use of compound-specific isotope analysis may be a potential tool to study the fate of HCHs in environment.
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