Diversity of Contaminant Reduction Reactions by Zerovalent Iron: Role of the Reductate

The reactions of eight model contaminants with nine types of granular Fe(O) were studied in batch experiments using consistent experimental conditions. The model contaminants (herein referred to as "reductates" because they were reduced by the iron metal) included cations (Cu²⁺), anions (CrO₄^2-, NO₃^-, and 5,5′,7,7′-indigotetrasulfonate), and neutral species (2-chloroacetophenone, 2,4,6-trinitrotoluene, carbon tetrachloride, and trichloroethene). The diversity of this range of reductates offers a uniquely broad perspective on the reactivity of Fe(O). Rate constants for disappearance of the reductates vary over as much as four orders of magnitude for particular reductates (due to differences in the nine types of iron) but differences among the reductates were even larger, ranging over almost seven orders of magnitude. Various ways of summarizing the data all suggest that relative reactivities with Fe(O) vary in the order Cu²⁺, 5,5′,7,7′ -indigotetrasulfonate > 2-chloroacetophenone, 2,4,6-trinitrotoluene > carbon tetrachloride, CrO₄^2- > trichloroethene > NO₃^-. Although the reductate has the largest effect on disappearance kinetics, more subtle differences in reactivity due to the type of Fe(O) suggests that removal of CrO₂^2- and NO ₃^- (the inorganic anions) involves adsorption to oxides on the Fe(O), whereas the disappearance kinetics of all other types of reductants is favored by reduction on comparatively oxide-free metal. Correlation analysis of the disappearance rate constants using descriptors of the reductates calculated by molecular modeling (energies of the lowest unoccupied molecular orbitals, LUMO, highest occupied molecular orbitals, HOMO, and HOMO-LUMO gaps) showed that reactivities generally decrease with increasing E_LUMO and increasing E_GAP (and, therefore, increasing chemical hardness η).