Sorption-induced metal isotope fractionation: Two decades of advances in methodologies, mechanisms, and kinetics

Abstract

Stable metal isotopes are widely applied as tracers to address the complex biogeochemical cycling of metallic elements, attracting attention from diverse scientific disciplines, including geology, oceanography, biology, archaeology, and environmental sciences. In the low-temperature systems of Earth's surface environment, significant fractionations of metal isotopes occur during sorption processes at solid-water interfaces, which influence variations of metal isotope compositions on Earth's surface. Over the past two decades, advanced techniques, especially X-ray absorption fine spectroscopy (XAFS) and ab initio quantum chemistry, have been applied to elucidate the mechanisms of isotope fractionation, greatly enhancing our understanding of these phenomena. This review summarized the molecular-level mechanisms underlying metal isotope fractionation induced by sorption, leveraging insights from XAFS and ab initio quantum chemical studies. It emphasizes the crucial role of relative mass difference in determining the overall isotopic fractionation across various metals. Coordination chemistry emerges as a primary determinant for this fractionation, with the valence state playing an important role, particularly for redox-sensitive metals. Temperature occasionally impacts these processes and warrants consideration. While some sorption-induced isotopic fractionation exhibits kinetic effects, the underlying mechanisms, including kinetic isotope effects due to diffusion and the evolution of sorption mechanisms, remain poorly understood. These advancements promise to improve our ability to predict isotopic variations on Earth's surface more accurately and to further the application of metal isotopes in low-temperature systems.