Abstract 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 fractionation 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‰ 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.

中文翻译：

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含铁 (II/III) 粘土矿物还原铬 (VI) 期间的铬同位素分馏

摘要 铬稳定同位素比率用于追踪古代和现代系统中 Cr(VI) 至 Cr(III) 的还原。然而，Cr 同位素特征的定量解释受到了不同还原剂还原 Cr(VI) 的同位素分馏因子的巨大可变性的阻碍。在这里，我们确定了在地下环境中丰富的含 Fe(II/III) 粘土矿物在 Cr(VI) 还原过程中的 Cr 同位素分馏因子。测试了几个变量：pH、粘土的总铁含量和粘土中还原铁的分数（Fe（II）/Fe（总））。后者控制粘土的标准还原电位。我们的结果表明，粘土的 pH 值和总 Fe 含量都对同位素分馏没有主要影响。相比之下，随着粘土的有效标准还原电位和 Cr(VI) 还原的标准自由能变得更负，Cr 同位素分馏因子根据线性自由能关系从 -4.9 下降到 -1.3‰。这种线性自由能关系可以从 Marcus 电子转移理论中预测，并允许根据标准还原电位或粘土的 Fe(II)/Fe(total) 对 Cr 同位素分馏因子进行一阶预测，从而可能提高我们的能力模拟地球化学系统中的 Cr 同位素特征。铬是第一个在各种还原剂（包括水相和固相还原剂）上显示出这种线性自由能关系的同位素系统，