Isotope fractionation and environmental applications of metal isotopes in terrestrial systems.

This study reviews isotope fractionation and its environmental applications in water and soil systems for seven metal isotope systems-chromium (Cr), cadmium (Cd), copper (Cu), mercury (Hg), molybdenum (Mo), nickel (Ni), and zinc (Zn)-that have been increasingly investigated and applied in environmental studies. This review focuses on common and contrasting isotope fractionation behaviors among these metal isotope systems, as well as their applications to contamination source tracing. For most geochemical processes and studied solid phases, Cd, Cr, Hg, Mo, and Ni isotopes generally exhibit higher isotopic ratios in aqueous phases than in solid phases, whereas aqueous Cu and Zn isotopic ratios may decrease as a result of adsorption. Adsorption strongly influences isotope fractionation of Mo and Zn. In redox processes, Cr, Cu, and Hg isotopes are particularly sensitive, with greater isotope fractionation often occurring during reduction than during oxidation. These characteristics suggest that isotopes of these metals can be used to constrain redox and adsorption processes in water and soil environments, with isotope fractionation generally being more pronounced in aqueous systems than in solid phases. Additionally, limitations in the current scope of isotope fractionation experiments are identified, highlighting priorities for future research.