Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO₃^2–) by Ca–Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO₃^2–-intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO₃, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl₂ shows much stronger affinity and larger distribution ratio (R_d ∼ 17800 L kg^–1) toward SeO₃^2– than AFm-SO₄ (R_d ∼ 705 L kg^–1). At stoichiometric SeO₃^2– loading for anion exchange, the newly formed AFm-SeO₃ phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ∼11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO₃^2– forms inner-sphere complexes with the Ca–Al–O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca–Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl^–, SO₄^2–, MoO₄^2–, and SeO₃^2–). Contrary to AFm-SeO₃ with inner-sphere SeO₃^2– complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO₃^2– is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.

中文翻译：

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Ca-Al LDH对亚硒酸盐的吸收：插层阴离子配位几何的描述

层状双氢氧化物（LDH）是阴离子交换剂，具有清除水生环境中阴离子污染物的强大潜力。在这里，研究了Ca-Al LDHs对硒（SeO ₃ ^2-）的吸收与硒浓度的关系。批量吸附等温线的热力学模型表明，SeO ₃ ^2-插层的AFm（水合铝酸钙单取代物）相AFm-SeO _3的形成是控制Se在中等负荷下保留的主要机制。AFM-CL ₂示出了强得多的亲和力，以及较大的分配比（[R _d〜17800公斤大号^-1）的SeO朝向₃ ^2-比AFM-SO ₄（- [R _d〜705公斤大号^-1）。在用于阴离子交换的化学计量的SeO ₃ ^2-负载下，新形成的AFm-SeO ₃相产生两个基本间距，即9.93±0.06Å和〜11.03±0.03Å。扩展的X射线吸收精细结构（EXAFS）光谱表明，嵌入的SeO ₃ ^2–与Ca–Al–O层形成内球络合物。原位X射线衍射（XRD）表明，Ca–Al LDH的基本间距与水合插层阴离子（即Cl ^–，SO ₄ ^2–，MoO ₄ ^2–和SeO ₃ ^2）的大小具有显着的线性关系。^–）。与在中间层中带有内球SeO ₃ ^{2 –}配合物的AFm-SeO ₃相反，具有氢键结合内球的SeO ₃ ^2–的相在动力学上是有利的，但在热力学上是不稳定的。这项工作为通过XRD分析确定插层阴离子配位几何提供了新的见解。