Factors Influencing Rates and Products in the Transformation of Trichloroethylene by Iron Sulfide and Iron Metal

Batch experiments were performed to assess (i) the influence of pH, solution amendments, and mineral aging on the rates and products of trichloroethylene (TCE) transformation by iron sulfide (FeS) and (ii) the influence of pretreatment of iron metal with NaHS on TCE transformation rates. The relative rates of FeS-mediated transformation of TCE to different products were quantified by branching ratios. Both pseudo-first-order rate constants and branching ratios for TCE transformation by FeS were significantly influenced by pH, possibly due to a decrease in the reduction potential of reactive surface species with increasing pH. Neither Mn²⁺, expected to adsorb to FeS surface S atoms, nor 2,2‘-bipyridine, expected to adsorb to surface Fe atoms, significantly influenced rate constants or branching ratios. FeS that had been aged at 76 °C for 3 days was completely unreactive with respect to TCE over 6.5 months, yet this aged FeS transformed hexachloroethane to tetrachloroethylene with a rate constant only slightly lower than that for nonaged FeS. This finding suggests that the oxidation state of iron sulfide minerals in the environment will strongly influence the potential for intrinsic remediation of pollutants such as TCE. Treatment of iron metal with bisulfide significantly increased the pseudo-first-order rate constant for TCE transformation at pH 8.3. This effect was attributed to formation of a reactive FeS coating or precipitate on the iron surface.