Sequential application of electron donors and humic acids for the anaerobic bioremediation of chlorinated aliphatic hydrocarbons

In situ anaerobic bioremediation of chlorinated solvents such as perchloroethene (PCE) frequently faces the problem of accumulating toxic, lower chlorinated compounds such as dichloroethene (cis-DCE) and vinyl chloride (VC). In the present study, the efficacy of the sequential application of electron donors, supporting reductive dechlorination, and of humic acids, acting as extracellular electron shuttles facilitating the anaerobic oxidation of recalcitrant intermediates, was explored in microcosm studies. Upon one initial dose of lactose, supplied in a 1000-fold superstoichiometric electron equivalent ratio, PCE was completely converted into cis-DCE within 35 days. Repeated electron donor additions did not entail exhaustive cis-DCE degradation over incubation time (120 days). Although the electron donor was quickly converted into fatty acids, about 30% of added reducing equivalents were recovered as acetate after four months of operation, indicating the inhibition of acetoclastic methanogenesis. In the next step, the substoichiometric addition of anthraquinone-2,6-disulfonate, a humic acid model compound, effected the complete removal of the accumulated cis-DCE within 15 days, probably as a result of the participation of the quinone in the biotic or abiotic anaerobic oxidation of cis-DCE. Cis-DCE degradation was not connected to the accumulation of VC, rendering the proposed two-step treatment an efficient and environmentally compliant remedy for anaerobic groundwater bodies contaminated with chlorinated solvents.