Zinc Isotope Fractionation during Sorption onto Al Oxides: Atomic Level Understanding from EXAFS

Interactions between aqueous Zn and mineral surfaces can lead to notable Zn isotope fractionation that affects Zn source fingerprinting, which needs an atomic-level understanding. In this study, we demonstrate that Zn isotope fractionation (Δ66Znsorbed-aqueous) during Zn sorption onto γ-Al2O3 depends on both pH and Zn concentration and ultimately correlates to surface coverage (Γ). At pH values of 6.0–6.5 and/or Zn concentrations of 0.1–0.2 mM, where Γ < 0.8 μmol m–2, Δ66Znsorbed-solution is 0.47 ± 0.03‰, whereas Δ66Znsorbed-aqueous decreases to 0.02 ± 0.07‰ at pH values of 7.0–8.0 and Zn concentrations of 0.4–0.8 mM, with a high Γ ranging from 1.5 to 3.2 μmol m–2. Using extended X-ray absorption fine structure (EXAFS) spectroscopy, we elucidated that a Zn–Al layered double hydroxide (LDH) with a Zn–O bond length of 2.06 A forms at high surface coverage (1.5 < Γ < 3.2 μmol m–2). In contrast, at low surface coverage (Γ < 0.8 μmol m–2), the sorbed Zn occurs as a tetrahedrally coordinated inner-sphere surfac...