Chromium isotope fractionation factors for reduction of Cr(VI) by aqueous Fe(II) and organic molecules

Chromium stable isotope ratios are useful as indicators of Cr redox reactions and Cr sources in both modern and ancient geochemical systems. Correct interpretation of Cr isotope data requires a quantitative understanding of isotopic fractionation by various processes, the most important of which is reduction of Cr(VI) to Cr(III). We determined the magnitude of isotopic fractionation, for the 53Cr/52Cr ratio, induced by abiotic, dark reduction of Cr(VI) by aqueous Fe(II) and a few organic substances. The isotopic fractionation for reduction by dissolved Fe(II), expressed as ε (≈δ53Crproduct flux−δ53Crreactant) is −4.20±0.11‰ from pH=4.0 to 5.3. Lesser fractionation was observed in preliminary experiments with very rapid reaction; we attribute this to transient heterogeneity and diffusive limitation of the reaction as reactants were mixed. This phenomenon is a general problem with batch isotopic fractionation experiments, if significant reaction occurs before mixing of reactants is complete. ε=−3.11±0.11‰ for reduction by three organic reductants (a humic acid at pH=4.5 and 5.0, a fulvic acid at pH=5.0, and mandelic acid catalyzed by goethite or γ-Al2O3 at pH=4.0).