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Hexachlorocyclohexanes in the Environment: Mechanisms of Dechlorination

The hexachlorocyclohexanes (HCHs) are a very well known and widely studied class of organochlorine pesticides. Because of their widespread global usage over the past 5 decades, HCHs are ubiquitous in the environment. With the molecular formula of C6H6Cl6, there are 8 known HCH isomers, which differ in the axial versus equatorial orientation of the chlorine substituents. Among these isomers, alpha (α), gamma (γ), beta (β), and delta (δ), HCHs are of major environmental significance. The authors’ primary objective is to review the properties, reactivity, and chemical reaction pathways of HCHs in the context of natural attenuation and accelerated remediation of HCHs. The structural differences contributing to the varying physical and chemical properties of the isomers, classical reaction pathways of substitution, nonreductive elimination, and reductive elimination, are summarized and discussed. In addition, recent research results on the application of nanoscale zero valent iron for treatments of HCH-contaminated groundwater are also presented.