A model for kinetic isotope effects during redox reactions and applications to chromium(VI) reduction
Keywords: Geological and Environmental Sciences.

Stable isotopic fractionation has been proposed as a proxy for redox reactions in both ancient and modern geochemical systems. However, reconstruction of past and present redox conditions from stable isotope signatures is complicated by variable isotopic fractionation associated with different reduction pathways. Quantitative understanding of the fundamental thermodynamic controls on redox-driven isotopic fractionation may improve the reliability of models of isotopic variations in natural systems. In this dissertation, I develop and experimentally test a mechanistic framework to model redox-driven kinetic isotopic fractionation as a function of the corresponding equilibrium fractionation and thermodynamic parameters. Marcus electron transfer theory predicts that for a set of related reactions, kinetic isotopic fractionation during electron transfer should be proportional to equilibrium isotopic fractionation and should decrease linearly in magnitude as the standard free energy of the reaction decreases and the reaction becomes more thermodynamically favorable. Furthermore, kinetic isotopic fractionation should be log-linearly correlated with the rate constant of electron transfer, such that a faster redox reaction induces less kinetic isotopic fractionation. I apply this framework to two model systems: chromium(VI) (Cr(VI)) reduction by aqueous iron(II) (Fe(II)) species, and Cr(VI) reduction by Fe(II/III)-bearing clay minerals. Chromium(VI) is a water-soluble pollutant whose mobility can be controlled by reduction to less soluble, environmentally benign Cr(III). Both aqueous Fe(II) and Fe(II/III)-bearing clays are ubiquitous in subsurface environments and potentially fast reductants of Cr(VI). First, I demonstrate that for the homogeneous reduction of Cr(VI) by aqueous Fe(II), kinetic isotopic fractionation of Cr(VI) is linearly correlated with the standard free energy of reaction, which is consistent with Marcus theory. This linear free energy relationship allows the magnitude of isotopic fractionation to be directly linked to environmental conditions such as pH and oxygen levels (Eh). I next examine the chemical and isotopic kinetics of Cr(VI) reduction by two Fe(II/III)-bearing clay minerals, an Fe-poor montmorillonite and an Fe-rich nontronite. I determine the dependence of the kinetics on ionic strength, pH, total Fe content, and the fraction of reduced structural Fe(II) (Fe(II)/Fe(total)), which controls the effective standard reduction potential of the clay. The last variable has the largest effect on Cr(VI) reduction kinetics: for both clay minerals, the rate constant of Cr(VI) reduction varies by more than three orders of magnitude with Fe(II)/Fe(total) and is described by a linear free energy relationship consistent with Marcus theory. Chromium fractionation factors similarly follow a linear free energy relationship consistent with Marcus theory. Under all conditions examined, Cr and Fe K-edge X-ray absorption near-edge structure (XANES) spectra show that the main Cr-bearing product is a Cr(III)-hydroxide and that Fe remains in the clay structure after reacting with Cr(VI). This dissertation helps to quantify our understanding of the kinetics of Cr(VI) reduction, which may improve predictions of Cr(VI) behavior in subsurface environments. More importantly, it provides a conceptual framework for redox-driven kinetic isotopic fractionation. By demonstrating that the magnitude of kinetic isotopic fractionation can be thermodynamically controlled, this study explains a large part of the variability in Cr(VI) isotopic fractionation and allows fractionation factors to be extrapolated from limited experimental data. The proposed framework may be valuable for modeling kinetic isotopic fractionation in a broad range of geochemically relevant systems.