Biotransformation of hexachlorocyclohexanes contaminated biomass for energetic utilization demonstrated in continuous anaerobic digestion system.

Lindane, the gamma-hexachlorocyclohexane (HCH) isomer, was among the most used pesticides worldwide. Although it was banned in 2009, residues of Lindane and other HCH-isomers are still found with high concentrations in contaminated fields. For clean-up, phytoremediation combined with anaerobic digestion (AD) of contaminated biomass to produce biogas and fertilizer could be a promising strategy and was tested in two 15 L laboratory-scale continuous stirred tank reactors. During operation over one year by adding HCH isomers (gamma, alpha and beta) consecutively, no negative influence on conventional reactor parameters was observed. The gamma- and alpha-HCH isomers were transformed to chlorobenzene and benzene, and transformation became faster along with time, while beta-HCH was not removed. Genus Methanosaeta and order Clostridiales, showing significant enhancement on abundance with HCH addition, may be used as bioindicators for HCH dehalogenation in AD process. The potential for HCH degradation in AD system was restricted to axial Cl atoms of HCH and it showed slight en-antioselective preference towards transformation of (+) alpha-HCH. Moreover, metabolite benzene was mineralized to CO2 and methane, deducing from tracer experiments with benzene-C-13(6). Overall, AD appears to be a feasible option for treatment of gamma and alpha-HCHs contaminated biomass.