Hexachlorocyclohexanes in the Environment: Mechanisms of Dechlorination

Abstract

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 C 6 H 6 Cl 6 , 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.

The effect of distribution processes on the isomeric composition of hexachlorocyclohexane in a contaminated riverine system

“…Consequently, higher reactivity of a-HCH and c-HCH over b-HCH can be assumed. Biotic degradation of HCHs was observed under aerobic (aqueous-phase) and anaerobic (sediment-phase) conditions (Phillips et al 2005), and numerous pathways were proposed (Li et al 2011b). General isomeric tendency to degradation is similar to abiotic reactions.…”

“…Both abiotic and biotic degradation pathways of HCHs were observed (Bhatt et al 2009). Briefly, isomers of lower chlorinated cyclohexane derivatives such as pentachlorocyclohexene are formed by hydrolysis of HCHs (Li et al 2011b), the preferred abiotic degradation pathway. This hydrolysis is triggered by the potential of anti-elimination within the cyclic molecular system (Rodriguez-Garrido et al 2004).…”

“…In-depth examinations of structural properties of HCH are described by Li et al (2011b). Different chemical and physical properties such as the environmentally relevant partition coefficient K OW and water solubility S WL are caused by the stereochemical orientation of the chlorine atoms around the cyclohexane ring (see Table 1).…”

“…Various studies have been performed on metabolization (Macrae et al 1967;Willett et al 1998;Rodriguez-Garrido et al 2004;Li et al 2011b) and isomerization (Huhnerfuss et al 1992;Wu et al 1997;Phillips et al 2005) of HCH isomers under different environmental conditions (redox environment, biotic/abiotic). Slightly different biotic transformations under aerobic and anaerobic conditions were proposed involving stepwise dechlorination to less chlorinated derivatives such as pentachlorocylohexene (e.g., Bachmann et al 1988;Rodriguez-Garrido et al 2004;Phillips et al 2005;Ricking and Schwarzbauer 2008;Bhatt et al 2009;Li et al 2011b). Hydrolysis was reported as the dominant abiotic degradation pathway for HCHs (Bhatt et al 2009).…”

Degradation of γ-Hexachlorocyclohexane Using Carboxymethylcellulose-Stabilized Fe/Ni Nanoparticles

“…The amount of CB increased continuously, suggesting that CB is a final product. Iron-mediated reductive transformations mainly follow two major reaction mechanisms: dihaloelimination and hydrogenolysis (Vogel et al 1987;Li et al 2011). Specifically, alkyl halides with axially oriented vicinal halogens undergo much more rapid dihaloelimination than would be the case if one or both were equatorial.…”