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 (Δ⁶⁶Zn_{sorbed-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, Δ⁶⁶Zn_{sorbed-solution} is 0.47 ± 0.03‰, whereas Δ⁶⁶Zn_{sorbed-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 Å 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 surface complex with an average Zn-O interatomic distance of 1.98 Å. Such contrasts lead to an atomic level understanding of the strong links between isotope fractionation, local bonding structures (i.e., coordination and bond distances), and solution chemistry, which is crucial for more effective applications of stable metal isotopes as environmental tracers.