Diversity of contaminant reduction reactions by zerovalent iron: role of the reductate.

The reactions of eight model contaminants with nine types of granular Fe(0) 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(0). 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(0) 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(0) suggests that removal of CrO2(2-) and NO3(-) (the inorganic anions) involves adsorption to oxides on the Fe(0), 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 eta).

Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal.

The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe0) were studied in batch reactors for a range of Fe0 surface area concentrations and solution pH values (5.5-9.0). At pH 7.0, there was only a modest difference (2-4x) in first-order rate coefficients (k(obs)) for each contaminant among the three Fe0 types investigated (Fisher, Peerless, and Connelly). The k(obs) values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 x 10(-3) and 2.2 x 10(-1) L m(-2) h(-1) were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe0, which reduced nitrate at slower rates at higher Fe0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe0 decreased with increasing pH with apparent reaction orders of 0.49 +/- 0.04 for nitrate, 0.61 +/- 0.02 for nitrite, and 0.72 +/- 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr(VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h.