REDUCTION OF HEXAVALENT CHROMIUM BY FERROUS IRON

Abstract

To better understand the geochemical processes that result in the reduction of hexavalent chromium [Cr(VI)] in natural waters, the reaction of Cr(VI) with Fe(II) has been studied as a function of pH and temperature. Over the concentration range studied [0.3-100 μM Fe(II) and 0.9-600 μM Cr(VI)], the reaction follows first-order kinetics with respect to Fe(II) and Cr(VI). The relationship between the rate coefficient and solution conditions can be explained by considering the reactivity of each of the Fe(II) species that are present at significant concentrations [i.e., Fe2+, FeOH+, and Fe(OH)2]. The effect of pH on the rate of reduction of Cr(VI) can be expressed by the following relationship: -d[Cr(VI)]dt = k[Cr(VI)][Fe(II)] where k = [4.4 x 103 M-2s-1 (H+)] + [3.0 x 105 M-3s-1 (H+)2] + [3.0 x 104M-1s-1c βFeOH+(OH-)] + [1 x 108 M-1s-1c βFe(OH)2(OH-)2] and c βFeOH+ = 3.16 x 104 M-1 c βFe(OH)2 = 3.16 x 107 M-2 at low ionic strength.The value of k can be adjusted for the effects of temperature by correcting the constants using published ΔH values. These results suggest that the reduction of Cr(VI) occurs on the timescale of minutes to months in Fe(II)-containing sediments, soils, and waters.