Chromium isotope fractionation during reduction of Chromium(VI) by Iron(II/III)-bearing clay minerals

Chromium stable isotope ratios are used to trace the reduction of Cr(VI) to Cr(III) in both ancient and modern systems. However, quantitative interpretation of Cr isotopic signatures has been stymied by the large variability in isotopic fraction-ation factors for Cr(VI) reduction by different reductants. Here we determine Cr isotope fractionation factors during Cr(VI) reduction by Fe(II/III)-bearing clay minerals, which are abundant in subsurface environments. Several variables were tested: pH, total Fe content of the clay, and the fraction of reduced Fe within the clay (Fe(II)/Fe(total)). The latter controls the standard reduction potential of the clay. Our results demonstrate that neither pH nor total Fe content of the clay have major effects on isotopic fractionation. In contrast, as the effective standard reduction potential of the clay and thus the standard free energy of Cr(VI) reduction become more negative, Cr isotope fractionation factors decrease in magnitude from -4.9 to-1.3 parts per thousand according to a linear free energy relationship. This linear free energy relationship can be predicted from Marcus electron transfer theory and allows first-order predictions of Cr isotope fractionation factors to be made from the standard reduction potential or Fe(II)/Fe(total) of a clay, potentially improving our ability to model Cr isotope signatures in geochemical systems. Chromium is the first isotope system to show such a linear free energy relationship over a diverse range of reductants, including both aqueous and solid-phase reductants, and may provide a model for determining other redox-driven kinetic isotope effects in environmentally important isotope systems. (c) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Chromium stable isotope ratios can be used to trace the reduction of Cr(VI) to Cr(III) in both ancient and modern systems. The study found that the fractionation factors during Cr(VI) reduction by Fe(II/III)-bearing clay minerals can be predicted from the standard reduction potential or Fe(II)/Fe(total) of the clay, potentially improving the ability to model Cr isotope signatures in geochemical systems. This linear free energy relationship may provide a model for determining other redox-driven kinetic isotope effects in environmentally important isotope systems.