Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd²⁺ adsorption on edge sites (Δ^114/110Cd_{edge-solution} = −1.54 ± 0.11‰) is smaller than that on vacancies (Δ^114/110Cd_{vacancy-solution} = −0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd–O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.

中文翻译：

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叶锰酸盐边缘位点和空位吸附过程中的镉同位素分馏


镉 (Cd) 的地球化学行为受到其在天然页锰酸盐上的吸附的强烈影响，天然页锰酸盐同时包含层边缘位点和空位；然而，这些位点的镉同位素分馏机制尚未得到解决。在目前的工作中，结合批量吸附实验、扩展 X 射线吸收精细结构 (EXAFS) 光谱、表面络合建模和密度泛函理论（DFT）计算。轻Cd同位素优先富集在固体表面，Cd ²⁺ 在边缘位点吸附引起的同位素分馏（Δ ^114/110 Cd _{edge-solution} = -1.54 ± 0.11 ¡ Cd K 边缘 EXAFS 和 DFT 结果均表明在层边缘位点上形成双角共享复合物，在空位上主要形成三角共享复合物。两种配合物的变形都会导致固体上的负同位素分馏，并且稍长的第一 Cd-O 距离和边缘位置 Cd 周围最近的 Mn 原子数量较少可能是与空位相比更大的分馏幅度的原因。这些结果为镉与页锰酸盐相互作用过程中的同位素分馏机制提供了深入的见解。