KINETICS OF CHROMATE REDUCTION BY FERROUS IRON

Reduction of Cr(VI) to Cr(III) is environmentally favorable as the lat ter species is not toxic to most living organisms and also has a low m obility and bioavailability. Ferrous iron is one possible reductant im plicated as a major contributor to the removal of Cr(VI) from suboxic and anoxic waters and soils. Despite the importance of this redox reac tion, no mechanistic or kinetic information are available, which are n eeded 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- Row 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 t o -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) L(0.6). Based on this rate expression and th at for the oxygenation of Fe(II), Cr(VI) reduction should be unaffecte d by oxygen except at pH values in excess of 8 even at micromolar conc entrations.