Fate of the persistent organic pollutant (POP)Hexachlorocyclohexane (HCH) and remediation challenges

Persistent Organic Pollutants (POPs) are hazards to human health and exhibit harmful effects to the environment, owing to their typically lipophilic and bio-accumulative nature. Chlorinated compounds are most relevant within the classical POPs, which are both toxic and hazardous. Lindane, 'gamma' isomer of hexachlorocyclohexane (gamma-HCH), is one such organochlorine compound primarily used as a fumigant and an insecticide in agriculture. Due to its continuous use in the past decade throughout the world and severe environmental impacts in terms of toxicity and threats, there is an urgent need to develop cleaning up and remediation strategies for the lindane-contaminated sites. Being electron deficient due to its highly chlorinated nature, the degradation pathway of gamma-HCH is previously thought to be generally under anaerobic condition via reductive dechlorination. But both aerobic and anaerobic degradation pathways are suggested for HCH isomers and biodegradation pathway of lindane has been completely elucidated in Sphingomonas paucimobilis UT26. Many bioremediation technologies viz. biostimulation, bioaugmentation, bioreactor systems have been employed so far for the treatment of HCH contaminated soil. However, the potential of these strategies has been tested at the laboratory scale and should be implemented for field-scale treatment of soil and aquatic systems contaminated with lindane and other HCH isomers. This review focuses on the fundamentals about the biodegradation and also various technologies utilized for full-scale remediation of HCH contamination in sediment and soils.