Oxidative Dissolution of Chromium(III) Hydroxide at pH 9, 3, and 2 with Product Inhibition at pH 2

Hexavalent chromium, Cr(VI), can be immobilized under neutral to alkaline conditions by reduction to Cr(III); similarly, the mobility of naturally occurring Cr in soils and sediments can be limited by its occurrence in the +III oxidation state. Conversely, the oxidation of Cr(III) to Cr(VI) increases both its toxicity and often its mobility. Dissolution of Cr(OH)_3(s) in 0.01 M NaNO₃ suspensions was examined in batch experiments in the presence and absence of the strong oxidant sodium hypochlorite (NaOCl). Dissolution of Cr(OH)_3(s) (1.0 g/L) was accelerated in the presence of excess strong oxidant (20 mM) at pH 9 by a factor of ca. 200 and to a lesser extent at pH 2 and 3. Linear kinetics of oxidative dissolution was observed at pH 9 and 3. In contrast, the rate of Cr release at pH 2 decreased rapidly with time, and within 2.5 h, the dissolution reaction was completely inhibited. Under oxidizing conditions, Cr released into solution is expected to be present as Cr(VI), which sorbs strongly to Cr(OH)_3(s) at low pH. Cr(VI) sorption followed a Langmuir isotherm and reached maximum sorption densities of 308 ± 8 and 271 ± 10 μmol/g at pH 3 and 2, respectively. However, sorption of Cr(VI) (putatively formed during oxidative dissolution) cannot explain the observed inhibition of the reaction because (1) sorption occurs at both pH 2 and 3 but inhibition only at pH 2 and (2) preequilibration of Cr(OH)_3(s) with Cr(VI) did not affect the rate of dissolution observed upon the addition of the oxidant. Thus, we hypothesize that the inhibition of (net) oxidative dissolution at pH 2 may be the result of secondary precipitation of a chromic hydroxy chromate phase.