Reductive Dechlorination of Tetrachloroethene Following Abiotic Versus Biotic Reduction of Hexavalent Chromium.

A feasibility evaluation identified chemical reduction and biostimulation as a potential remedy for a plume containing hexavalent chromium (Cr(VI)) and tetrachloroethene (PCE) at an industrial site in southern California. The objectives of this laboratory study were to determine the stoichiometry of calcium polysulfide (CaS x ) reaction with Cr(VI) in the presence of sediment, the effect of CaS x on the potential for in situ biological reductive dechlorination of PCE, and the potential to reduce Cr(VI) and PCE by addition of only an electron donor. Approximately 1 L of CaS x solution (containing 50 g S 2- /L) was required per 1000 L of groundwater containing 45 mg/L of Cr(VI) (i.e., 1.8 mol S 2- per mol Cr(VI)). The sediment also exerted a sulfide demand (≥0.38 g S 2 - per kg sediment), but at a slower rate than the Cr(VI). In microcosms prepared with lactate, corn syrup, soybean oil, or methanol, but no CaS x , the Cr(VI) was biologically reduced in the treatments with lactate and corn syrup, but much more slowly than with CaS x . Even after 20 months of incubation, no significant reductive dechlorination of PCE occurred in any of the microcosms, including those in which the Cr(VI) was removed with CaS x . Bioaugmentation was tested with the microcosms that received lactate and corn syrup (following 20 months of incubation), using an enrichment culture that actively dechlorinates trichloroethene. PCE dechlorination began within 1 month in the lactate-only treatment; in the corn syrup–amended treatment, PCE dechlorination occurred in only one of the three bottles. However, no PCE dechlorination occurred following bioaugmentation of the lactate and corn syrup microcosms that were initially treated with CaS x , indicating that CaS x (and/or its reaction products) exerted a negative impact on the chlororespiring microbes. This outcome highlights the need to evaluate sites on a case-by-case basis when in situ chemical treatment is applied prior to microbial reductive dechlorination. [ABSTRACT FROM AUTHOR]