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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 Cr-53/Cr-52 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 epsilon (approximate to delta Cr-53(product flux) - delta Cr-53(reactant)) is -4.20 +/- 0.11 parts per thousand 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. epsilon = -3.11 +/- 0.11 parts per thousand 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 gamma-Al2O3 at pH = 4.0). (C) 2012 Elsevier Ltd. All rights reserved.