Kinetics of Nitrate, Nitrite, and Cr(VI) Reduction by Iron Metal

Abstract

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 (2−4×) 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 (k_SA) of 9.0 × 10^-3 and 2.2 × 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.