Influence of the oxidizing species on the reactivity of iron-based bimetallic reductants

Anticipating which pollutants are amenable to treatment by iron-based bimetallic reductants requires an understanding of the mechanism(s) driving pollutant reduction. Here, batch studies with six bimetals (Au/Fe, Co/Fe, Cu/Fe, Ni/Fe, Pd/Fe, and Pt/Fe) and four oxidants (alkyl polyhalides, vinyl polyhalides, alkynes, and water) explored the influence of the electron acceptor on reductant reactivity. Bimetals exhibited disparate reactivity toward some oxidant classes. For example, Pt/Fe enhanced rates of cis-dichloroethylene reduction, but it inhibited the reduction of several alkyl polyhalides. Moreover, the rate increase for vinyl polyhalide reduction by Ni/Fe (similar to 100-fold) and Pd/Fe (similar to 1000-fold) was far greater than that measured for alkyl polyhalides (similar to 10-fold), and reactivity toward vinyl polyhalides exhibited a more pronounced dependence on Ni and Pd loadings than did reactivity toward alkyl polyhalides. These results suggest that the reactions of alkyl and vinyl polyhalides with iron-based bimetals involve different active reductants. Neither rates of alkyl nor vinyl polyhalide reduction correlated with rates of iron corrosion by water, contrary to expectations if galvanic corrosion was primarily responsible for organohalide reduction. Trends observed for the hydrogenation of 2-butyne did mirror the sequence we identified for 1,1,1-trichloroethane reduction, consistent with a role for atomic hydrogen as the principal electron donor in these two systems.