A model for kinetic isotope fractionation during redox reactions

Abstract

Stable isotopes are used to track redox reactions in both ancient and modern geochemical systems. However, because the magnitude and even the direction of fractionation depend on the reaction pathway, extracting information about redox conditions from isotopic signatures is difficult. Quantitative understanding of the fundamental thermodynamic controls on redox-driven fractionation may improve the reliability of fractionation models in natural systems. Here we develop a mechanistic framework based on Marcus electron transfer theory to model redox-driven kinetic isotope fractionation as a function of the corresponding equilibrium fractionation and thermodynamic parameters. The framework predicts that for related reactions, (1) kinetic fractionation during electron transfer should be proportional to and in the same direction as equilibrium fractionation and (2) the magnitude of kinetic fractionation should decrease linearly as the standard free energy of the reaction decreases and the reaction becomes more thermodynamically favorable. Furthermore, kinetic fractionation should be log-linearly correlated with the rate constant of electron transfer, such that a faster redox reaction induces less kinetic fractionation. To illustrate the application of the model to stable isotope systems, we discuss examples of kinetic isotope fractionation during reduction of chromium (VI) and nitroaromatics by aqueous and solid-phase reductants respectively. For both these systems, altering the standard reduction potential of the reductant to vary the standard free energy of the redox reaction produces large changes in the magnitude of kinetic fractionation, which can be quantitatively predicted with our model. The proposed framework thus may be valuable for modeling redox-driven kinetic isotope fractionation in a broad range of geochemically relevant systems for both traditional light stable isotopes (e.g., nitrogen) and nontraditional metal isotopes (e.g., chromium).