环境地球化学国家重点实验室知识库

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(≈δ⁵³Cr_{product flux} − δ⁵³Cr_reactant) 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).