A site had a plume that contained both PCE (500 ug/l) and Cr[VI] (up to 90 mg/l). The Cr[VI] plume was to be chemically reduced with a mixed reductant solution containing 1% calcium polysulfide (CaSx) with corn syrup to biologically regenerate sulfide in situ. The consultant recommending the technology claimed PCE would simultaneously undergo reductive dechlorination with this application. I disagreed; I believed the CaSx concentration (10,000 mg/l) would be inhibitory, Cr(VI) might also inhibit reductive dechlorination, bioaugmentation was needed, and biological reduction of Cr[VI] would be more compatible with PCE degradation. A lab study was designed using site soil/ground water to test the hypotheses.
The results showed 1) PCE would not biodegrade in microcosms treated with CaSx, 2) PCE degradation didn't occur until the Cr[VI] concentration had decreased, 3) corn syrup and other donors could stimulate biological reduction of Cr[VI], but 4) only after bioaugmentation. (Freedman et. al., 2005a).
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