Mineralization of hexachlorocyclohexane in soil during solid-phase bioremediation

Soil containing hexachlorocyclohexane (HCH) was spiked with (14)C-gamma-HCH and then subjected to bioremediation in bench-scale microcosms to determine the rate and extent of mineralization of the (14)C-labeled HCH to (14)CO(2). The soil was treated using two different DARAMEND amendments, D6386 and D6390. The amendments were previously found to enhance natural HCH bioremediation as determined by measuring the disappearance of parent compounds under either strictly oxic conditions (D6386), or cycled anoxic/oxic conditions (D6390). Within 80 days of the initiation of treatment, mineralization was observed in all of the strictly oxic microcosms. However, mineralization was negligible in the cycled anoxic/oxic microcosms throughout the 275-day study, even after cycling was ceased at 84 days and although significant removal (up to 51%) of indigenous gamma-HCH (146 mg/kg) was detected by GC with electron capture detector. Of the amended, strictly oxic treatments, only one, in which 47% of the spiked (14)C-HCH was recovered as (14)CO(2), enhanced mineralization compared with an unamended treatment (in which 34% recovery was measured). Other oxic treatments involving higher amendment application rates or auxiliary carbon sources were inhibitory to mineralization. Thus, although HCH degradation occurs during the application of either oxic or cycled anoxic/oxic DARAMEND treatments, mineralization of gamma-HCH may be inhibited depending on the amendment and treatment protocol.