Site-specific isotope fractionation during Zn adsorption onto birnessite: Insights from X-ray absorption spectroscopy, density functional theory and surface complexation modeling
Site-specific isotope fractionation during Zn adsorption onto birnessite: Insights from X-ray absorption spectroscopy, density functional theory and surface complexation modeling
复制标题
DOI：
10.1016/j.gca.2023.03.006
复制
发表时间：
2023-03
影响因子：
5
通讯作者：
Zhao Wang;C. Peacock;K. Kwon;X. Gu;Xionghan Feng;W. Li
中科院分区：
文献类型：
--
作者：
Zhao Wang;C. Peacock;K. Kwon;X. Gu;Xionghan Feng;W. Li
水钠锰矿矿物有助于控制锌在表面环境中的命运，并通过吸附反应轻松分馏锌同位素，但人们对各种反应位点在稳定同位素分馏中所起的作用知之甚少。在这里，我们展示了在不同反应时间、pH 值和 Zn 浓度下水钠锰矿吸附引起的 Zn 同位素分馏数据。我们观察到，在 pH 6 下反应约 120 小时后即可达到 Zn 的同位素平衡。在 pH 3–5 和 Zn 浓度 0.05–0.3 mM 时，同位素分馏（Δ66Zn 吸附水）约为 -0.46 ± 0.04‰，并在 pH 6–8 和 Zn 浓度0.2 毫米。使用表面络合模型很好地描述了 Zn 同位素组成随 pH 值和 Zn 浓度变化的变化，其中涉及两个结合位点：外部边缘位点和层间空位。根据该模型，计算了 Zn 的两种不同同位素分馏因子：空位吸附的 Δ66Znadsorbed-aqueous= -0.46 ± 0.04‰ 和边缘位点结合的 Δ66Znadsorbed-aqueous= 0.52 ± 0.04‰。扩展 X 射线吸收精细结构光谱 (EXAFS) 表明，在 pH 3 和 Zn 浓度为 0.05–0.2 mM 时，Zn 在水钠锰矿空位上形成三角共享 (TCS) 八面体复合物，其中 Zn 在一侧与 Mn 空位的三个氧原子 (∼2.03 Å) 配位，在另一侧与三个水分子 (∼2.15 Å) 配位，表明形成了扭曲的 Zn 单键 O 八面体（平均键长：∼2.09 Å）。在 pH 6 和 8 时，除了空位上的 TCS 八面体复合物之外，层边缘上还形成了双角共享 (DCS) 复合物。密度泛函理论（DFT）优化表明DCS Zn配合物以四面体配位存在。基于 EXAFS 光谱、DFT 优化和表面络合模型，Zn 的不同同位素分馏与水钠锰矿不同反应位点的 Zn 局部结构差异有关。我们的结果提供了对天然水钠锰矿环境中锌同位素分馏的分子尺度理解，以及预测其他类似金属的吸附和分馏之间的联系的新见解。
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 birnessite under different reaction times, pH values, and Zn concentrations. We observe that isotopic equilibrium 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 (Δ66Znadsorbed-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 different isotopic fractionation factors of Zn are calculated: Δ66Znadsorbed-aqueous= −0.46 ± 0.04‰ for adsorption on vacancy sites and Δ66Znadsorbed-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 Å) and to three water molecules on the other side (∼2.15 Å), suggesting the formation of distorted Znsingle bondO octahedra (average bond length: ∼2.09 Å). At pH 6 and 8, double-corner-sharing (DCS) complexes on layer edges formed in addition 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 understanding 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.