Rates of hexavalent chromium reduction in anoxic estuarine sediments: pH effects and the role of acid volatile sulfides.

Rates of Cr(VI) reduction were assessed in batch kinetic experiments using dilute suspensions of anoxic sediments collected from the Baltimore Harbor. Rapid and complete Cr(VI) reduction occurred on the time scale of minutes (pseudo-first-order rate constants ranged from 0.01 to 0.40 min(-1) for 1.0 g/L sediment suspensions at pH 7.0) via reaction with mineral surfaces. The reaction rate was first-order with respect to Cr(VI) concentration and proportional to the concentration of sediment acid volatile sulfides (AVS). AVS-normalized rates of Cr(VI) reduction decreased by approximately 2 orders of magnitude as the suspension pH increased from 5.0 to 8.2. The AVS-normalized rate constant was linearly correlated with the fraction of total dissolved Cr(VI) in the protonated HCrO(4)(-) form at a given pH (f(HCrO(4))((-))). The following rate law was found to describe all the experimental data: d[Cr(VI)]/dt = -k[AVS]f(HCrO(4))((-))Cr(VI), where the AVS- and pH-independent rate constant k = 3.14(+/-0.75) x 10(4) M(-1) min(-1). The results of this study suggest the importance of reactions between Cr(VI) and solid phase AVS constituents in controlling Cr(VI) fate in anoxic estuarine sediments, and indicate that Cr(VI) is unlikely to be a toxicological stressor in AVS-containing sediments.