Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering , Nanjing University , Nanjing 210023 , China.
State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering , Nanjing University , Nanjing 210023 , China.
Environ Sci Technol. 2018 Aug 21;52(16):9087-9096. doi: 10.1021/acs.est.8b01414. Epub 2018 Jul 31.
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 (ΔZn) during Zn sorption onto γ-AlO 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, ΔZn is 0.47 ± 0.03‰, whereas ΔZn 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. 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). In contrast, at low surface coverage (Γ < 0.8 μmol m), 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.
水合锌与矿物表面的相互作用会导致显著的锌同位素分馏，从而影响锌源示踪，这需要从原子水平上进行理解。在这项研究中，我们证明了γ-AlO 上锌吸附过程中的锌同位素分馏（ΔZn）取决于 pH 值和锌浓度，最终与表面覆盖度（Γ）相关。在 pH 值为 6.0-6.5 和/或锌浓度为 0.1-0.2 mM 的条件下，Γ < 0.8 μmol m 时，ΔZn 为 0.47 ± 0.03‰，而在 pH 值为 7.0-8.0 和锌浓度为 0.4-0.8 mM 的条件下，ΔZn 降低至 0.02 ± 0.07‰，此时 Γ 较高，范围为 1.5-3.2 μmol m。利用扩展 X 射线吸收精细结构（EXAFS）光谱，我们阐明了在高表面覆盖度（1.5 < Γ < 3.2 μmol m）下形成了锌-铝层状双氢氧化物（LDH），其中锌-氧键长为 2.06 Å。相比之下，在低表面覆盖度（Γ < 0.8 μmol m）下，吸附的锌表现为具有平均锌-氧原子间距离为 1.98 Å的四面体配位内球表面络合物。这些对比使我们从原子水平上理解了同位素分馏、局部键合结构（即配位和键距离）与溶液化学之间的紧密联系，这对于更有效地将稳定金属同位素用作环境示踪剂至关重要。
