Molecular-scale insight into selenium isotope fractionation caused by adsorption on Fe (oxyhydr)oxides

Selenium (Se) isotopes have recently emerged as a potential proxy for tracing biogeochemical processes and reconstructing the evolution of global Se cycle in the oceans. However, little is known about Se isotope fractionation mechanism during the sequestration of dissolved Se oxyanions by marine ferromanganese oxides that are mainly composed of Fe and Mn (oxyhydr)oxides. Here we elucidate the molecular mechanism governing equilibrium Se isotope fractionation during adsorption on 2-line ferrihydrite, by combining isotope ratio measurements, extended X-ray absorption fine structure (EXAFS) analyses, and high level quantum chemical calculations. Results show that Se isotopes can be fractionated 0.89 parts per thousand (Delta Se-82/76(aqueous-adsorbed)) with enrichment of lighter isotopes in the solid phase during the adsorption of Se(IV) on 2-line ferrihydrite, which is primarily driven by the formation of bidentate-binuclear inner-sphere complexes. By contrast, little or no Se isotope fractionation (<0.2 parts per thousand) was observed during Se(VI) adsorption due to the outer-sphere complexation. In combination with previous results, our findings would provide molecular-scale insights into Se isotope compositions in marine ferromanganese oxides and lead to an improved understanding of Se biogeochemical cycle in the ocean. Our study also has an implication for the systematical understanding of mechanisms governing isotope fractionations of other metal oxyanions like Mo, which highlights the controls of their proton dissociation constants, electronic configurations, and radius ratios. This would improve our understanding for isotope signatures of metals in the dissolved species and the adsorbed sinks in modern and ancient environments.