Reductive dechlorination of hexachlorocyclohexane (HCH) isomers in soil under anaerobic conditions

The biological anaerobic reductive dechlorination of -hexachlorocyclohexane under methanogenic conditions was tested in a number of contaminated soil samples from two locations in the Netherlands. Soils from a heavily polluted location showed rapid dechlorination of -hexachlorocyclohexane to benzene and chlorobenzene with lactate as electron donor. Soils from an adjacent slightly polluted location did not show substantial dechlorination of -hexachlorocyclohexane within 4months. A heavily polluted sample was selected to optimise the dechlorination. All tested hexachlorocyclohexane isomers (-, -, -, and -), either added separately or simultaneously, were dechlorinated in this soil sample. The most rapid dechlorination was observed at a temperature of 30°C. Dechlorination of -hexachlorocyclohexane was observed with acetate, propionate, lactate, methanol, H2, yeast extract and landfill leachate as electron donors. In a soil percolation column, packed with a selected heavily polluted soil sample, the presence of 10mM sulphate in the influent led to simultaneous dechlorination of -hexachlorocyclohexane and sulphate reduction. When the column was fed with 10mM nitrate instead of sulphate, dechlorination ceased immediately. After omitting nitrate from the influent, dechlorination activity recovered in about 1month. Also in a separate column, the addition of nitrate from the start of the experiment did not result in dechlorination of -HCH. The significance of these experiments for in situ bioremediation of polluted soils is discussed