ASSESSMENT OF METAL INHIBITION OF REDUCTIVE DECHLORINATION OF HEXACHLOROBENZENE AT A SUPERFUND SITE

Laboratory studies were conducted to identify easily measured paramete rs that would predict the potential for metal inhibition in sediments co-contaminated by heavy metals and hexachlorobenzene (HCB) at the Pet ro Processors Superfund site. Metal inhibition was investigated using HCB-contaminated sediment and a range of added metal concentrations (1 -1,000 mg/kg). Cadmium, Cu, Pb, and Zn all caused inhibition of HCB re ductive dechlorination and methane production with inhibiting concentr ations of Cd and Pb equivalent to those found in portions of the site. In the second phase of studies, laboratory experiments investigated t he relationship between Cd and Pb availability on the kinetics of HCB degradation. Cadmium and Pb affected both dechlorination rates and lag times. In addition, Cd affected the appearance of lower chlorinated b enzene metabolites. Inhibition of HCB dechlorination was well correlat ed to ratios of acid-volatile sulfide (AVS) to simultaneously extracta ble metals (SEM) and water-soluble Cd. Only treatments with a measurab le concentration (>6 mu g/L) of free, uncomplexed Cd exhibited inhibit ion. Lead inhibited HCB dechlorination at all concentrations tested (1 0-1,000 mg/kg) and inhibition did not correlate to either water-solubl e Pb concentrations or AVS/SEM ratios. For both Cd and Pb, methane pro duction was less sensitive to metals than HCB dechlorination, making p roduction of methane a poor indicator of inhibition. Measurement of fr ee, uncomplexed Cd was the best predictor of Cd inhibition while no su itable indicators were found for Pb. These results demonstrate the imp ortance of metal bioavailability on the inhibition of reductive dechlo rination.