Equilibrium isotopic fractionation and isotopic exchange kinetics between Cr(III) and Cr(VI)

Abstract We determined the equilibrium isotope fractionation between Cr(III) and Cr(VI), defined as Δ 53 Cr VI-III  = δ 53 Cr(VI) − δ 53 Cr(III), and the rates of isotopic exchange between the two redox species under different conditions. In high Cr concentration, low-pH experiments we determined the Δ 53 Cr V-III between CrO 4 2− and Cr(H 2 O) 6 3+ to be 5.2 ± 0.3‰ and 5.5 ± 0.3‰ at 60 °C and 40 °C, respectively. At 25 °C, the system only progressed 25% toward isotopic equilibrium after 684 days. By extrapolating from the 60 °C and 40 °C experiments we estimated the Δ 53 Cr VI-III between CrO 4 2− and Cr(H 2 O) 6 3+ to be 5.8 ± 0.5‰ at 25 °C. Isotope exchange rates between dissolved Cr(III) and dissolved Cr(VI) at 25 °C, 40 °C, and 60 °C were determined to be 3.13 × 10 −5  M day −1 , 6.83 × 10 −4  M day −1 , and 8.37 × 10 −3  M day −1 , respectively. In low concentration, neutral-pH experiments we determined the isotopic exchange rates between dissolved Cr(VI) and solid Cr(III) oxyhydroxide at 25 °C. In these experiments, significant isotopic exchange was found on time scales of months, though the magnitude of isotopic shifts was limited by the small mass of Cr(III) available for exchange on the surfaces of Cr(III) oxyhydroxide particles. Exchange rates were relatively fast, compared to rates obtained from high concentration, low-pH experiments. This faster isotopic exchange is attributed to adsorption of Cr(VI) to Cr(III) particle surfaces, which keeps Cr(III) and Cr(VI), and potentially intermediate species Cr(V), in close proximity long enough to allow multiple electron transfers. The isotopic exchange rate at neutral-pH was found to conform to the rate law R = k ·[Cr(VI)] adsorbed , in which R is the isotopic exchange rate (M day −1 ); k is the rate constant, determined to be 0.00047 day −1 ; [CrO 4 2− ] adsorbed is the concentration of Cr(VI) adsorbed to Cr(III) oxyhydroxide (M). The impact of isotopic exchange on the 53 Cr/ 52 Cr ratio of the dissolved Cr(VI) depends on the relative masses and 53 Cr/ 52 Cr ratios of the starting Cr(III) and Cr(VI), as well as the fraction of Cr(III) atoms exposed to solution. In many natural settings, isotopic exchange will have little impact, because only very small amounts of Cr(VI) are adsorbed onto Cr(III) oxyhydroxide surfaces. However in certain settings with low Cr(VI) concentrations and abundant, exposed Cr(III)-bearing solids, significant isotopic exchange could happen within about 20 years.