Zinc isotopes equilibrium fractionation induced by α-Al2O3 surface adsorption and its implications: a density functional theory study

Abstract

Although Zn coordination structures at mineral surfaces are well documented, the theoretical framework for Zn isotope equilibrium fractionation induced by Al-OH sites on weathering-derived particles is poorly constrained, which hinders application of Zn isotopes to weathering studies. Here, we investigate Zn coordination structures and isotopic properties of aqueous fluid and representative α-Al ₂O ₃(001) and (110) surfaces using first-principles molecular dynamics (FPMD) simulation and density functional theory (DFT) calculations. Our results indicate that there is ∼80 % Zn(H ₂O) ₆ ²⁺and ∼20 % Zn(H ₂O) ₅ ²⁺in the aqueous fluid, in accord with previous FPMD simulations. With different surface-terminating structures, the α-Al ₂O ₃(001) and (110) surfaces form four-coordinated ( ^IVZn) and six-coordinated ( ^VIZn) adsorption structures of Zn, respectively. As with Zn-bearing minerals, a good linear correlation exists between the 1000lnβ values of Zn isotopes and the average Zn–O bond lengths. The linear relationships between average Zn–O bond lengths and the structure distortion parameters show that the larger coordination number (CN) increases the Zn–O bond length and induces greater structural distortion, which contributes to enrichment of light Zn isotopes. The Zn isotopic fractionation between the mineral surface and aqueous fluid induced by adsorption on the α-Al ₂O ₃(001) and (110) surfaces are −0.01 ± 0.24 ‰ and 0.98 ± 0.23 ‰ at 27 °C (300 K), respectively. The calculated Zn isotope equilibrium fractionation factors were used to estimate the CNs of Zn on Al-O, Fe-O and Si-O surfaces in different Zn isotope adsorption experiments, and are consistent with extended X-ray absorption fine structure (EXAFS) spectra measurements. Hence, these results may be used to predict the CN of Zn on natural mineral surfaces, and have been used to develop a quantitative model for Zn isotope fractionation in aqueous solution-soil–bedrock systems that can be compared with watershed-scale Zn isotope data.

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