Kinetics of Chromate Reduction by Ferrous Iron

Reduction of Cr(VI) to Cr(III) is environmentally favorable as the latter species is not toxic to most living organisms and also has a low mobility and bioavailability. Ferrous iron is one possible reductant implicated as a major contributor to the removal of Cr(VI) from suboxic and anoxic waters and soils. Despite the importance of this redox reaction, no mechanistic or kinetic information are available, which are needed to determine the rate of Cr(VI) reduction and to assess the role of oxygen in limiting this reaction. In this study we used a stopped-flow kinetic technique monitored by UV−VIS spectroscopy and an initial rate method to ascertain the rate constant and the rate dependence of each reactant. We observed that the rate of Cr(VI) removal conformed to −d[Cr(VI)]/dt = k cr[Fe(II)]0.6[Cr(VI)]1 where k cr = 56.3 (±3.7) mmol-0.6 min-1 L0.6. Based on this rate expression and that for the oxygenation of Fe(II), Cr(VI) reduction should be unaffected by oxygen except at pH values in excess of 8 even at micromolar concentrations.