Site-specific isotope fractionation during Zn adsorption onto birnessite: Insights from X-ray absorption spectroscopy, density functional theory and surface complexation modeling

초록

Birnessite minerals help control the fate of Zn in surface environments and readily fractionate Zn isotopes through adsorption reactions, yet little is known about the role played by various reactive sites in stable isotopic fractionation. Here we present the Zn isotope fractionation data cause by adsorption on birnes-site under different reaction times, pH values, and Zn concentrations. We observe that isotopic equilib-rium of Zn is attained after-120 h of reaction time at pH 6. At pH 3-5 and Zn concentrations of 0.05- 0.3 mM, the isotopic fractionation (D66Znadsorbed-aqueous) is around-0.46 +/- 0.04%., and gradually increases to-0.09 +/- 0.05%. at pH 6-8 and Zn concentrations of 0.2 mM. The change in Zn isotopic compositions as a function of pH and Zn concentration is well described using the surface complexation model, where two binding sites are involved: external edge sites and interlayer vacancies. According to this model, two dif-ferent isotopic fractionation factors of Zn are calculated: D66Znadsorbed-aqueous =-0.46 +/- 0.04%. for adsorp-tion on vacancy sites and D66Znadsorbed-aqueous = 0.52 +/- 0.04%. for binding to edge sites. Extended X-ray absorption fine structure spectroscopy (EXAFS) demonstrates that Zn forms triple-corner-sharing (TCS) octahedral complex on birnessite vacancies at pH 3 and Zn concentrations of 0.05-0.2 mM, where Zn is coordinated on one side to three oxygen atoms of the Mn vacancy (-2.03 angstrom) and to three water mole-cules on the other side (-2.15 angstrom), suggesting the formation of distorted ZnAO octahedra (average bond length:-2.09 angstrom). At pH 6 and 8, double-corner-sharing (DCS) complexes on layer edges formed in addi-tion to the TCS octahedral complex on vacancies. Density functional theory (DFT) optimisations suggest that DCS Zn complex exist in tetrahedral coordination. Based on EXAFS spectroscopy, DFT optimisations and surface complexation modeling, the distinct isotopic fractionation of Zn is related to the differences in Zn local structure at different reactive sites of birnessite. Our results provide a molecular-scale under-standing of Zn isotopic fractionation in natural birnessite-containing settings, as well as new insights into predicting the links between adsorption and fractionation of other similar metals.(c) 2023 Elsevier Ltd. All rights reserved.

키워드