Selenium isotope fractionation during adsorption by Fe, Mn and Al oxides

Adsorption plays an important role in the biogeochemical cycling of selenium (Se) in natural environments and Se isotope fractionation during adsorption is a significant but poorly studied part of Se isotope system. This paper examined Se(IV) and Se(VI) adsorption onto four naturally occurring metal oxides (hematite, manganese dioxide (β-MnO₂), and α and γ-alumina oxides) and revealed Se isotope fractionation as a function of the absorbents used, Se species adsorbed, exposure time, and pH. The results show that Se(IV)/(VI) adsorption was initially rapid and was accompanied with kinetic isotope fractionations as large as 3‰, but slowed down as adsorption equilibrium was approached, eventually approaching isotopic equilibrium. The Se(IV) adsorption onto the studied metal oxides took at least 12 h to reach adsorption equilibrium, longer than Se(VI) adsorption (<60 min). At adsorption equilibrium, Se(IV) adsorption onto Fe and Mn oxides induces significant isotope fractionation, with lighter Se isotopes preferentially adsorbed, whereas Se(IV) adsorption onto Al oxides causes only small fractionation: Δ^82/76Se_{dissolved-adsorbed} = δ^82/76Se_dissolved − δ^82/76Se_adsorbed is 0.87 ± 0.12 ‰ for hematite, 1.24 ± 0.05 ‰ for β-MnO₂, 0.08 ± 0.10 ‰ for α-alumina, and 0.05 ± 0.09 ‰ for γ-alumina at pH 5. In contrast to Se(IV) adsorption, Se(VI) adsorption does not induce detectable Se isotope fractionation. The contrasting Se isotope fractionation between Se(IV) and Se(VI) adsorption is likely related to the mechanism of adsorption onto metal oxides, which causes a structural difference between dissolved and adsorbed Se(IV)/Se(VI). In addition, pH had a strong influence on Se isotope fractionation during Se(IV) adsorption onto β-MnO₂: Δ^82/76Se_{dissolved-adsorbed} varied from 1.24‰ to −0.08‰ as pH increased from 5 to 8. However, there was little pH effect on Se isotope fractionation during adsorption onto Fe (consistently at 0.7–0.9‰) and Al oxide (consistently at ∼0‰). Our findings show that, beside abiotic and biotic reduction, Se(IV) adsorption onto Fe-Mn oxides is potentially another important process that can induce Se isotope fractionation in Earth's surface environments. This moves an important step toward correctly reconstructing the Se isotopic composition of seawater using Fe-Mn nodules or crusts.