Reductive dechlorination of trichloroethene mediated by humic-metal complexes

Experiments were conducted to determine if transition metal-humic acid complexes can act as e-transfer mediators in the reductive dechlorination of trichloroethene (TCE) using Ti(lll) citrate as the bulk reductant. In the presence of Ni-Aldrich humic acid (AHA) complexes, TCE reduction was rapid, with complete removal of TCE in less than 23 h. Cu-AHA complexes were less effective as e(-) mediators than NI-AHA complexes; only 60% of TCE was reduced after 150 h. Partially dechlorinated intermediates were observed during TCE reduction; however, they were transitory, and at no time accounted for more than 2% of the initial TCE mass on a mole C basis. Ethane and ethene were the primary end products of TCE reduction; however, a suite of other non-chlorinated hydrocarbons consisting of methane and C-3 to C-6 alkanes and alkenes were also observed. The results suggest that humic-metal complexes may represent a previously unrecognized class of electron mediators in natural environments.