Ternary Complex Formation of Phosphate with Ca and Mg Ions Binding to Ferrihydrite: Experiments and Mechanisms,ACS Earth and Space Chemistry

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Ternary Complex Formation of Phosphate with Ca and Mg Ions Binding to Ferrihydrite: Experiments and Mechanisms
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2020-03-25 , DOI: 10.1021/acsearthspacechem.9b00320
Juan C. Mendez ₁ , Tjisse Hiemstra ₁

Affiliation

Calcium (Ca) and magnesium (Mg) are the most abundant alkaline-earth metal ions in nature, and their interaction with ferrihydrite (Fh) affects the geochemical cycling of relevant ions, including phosphate (PO₄). The interfacial interactions of Ca and Mg (M²⁺) with PO₄ have not been analyzed yet for freshly precipitated Fh. Here, we studied experimentally this interaction in binary M²⁺-PO₄ systems over a wide range of pH, M²⁺/PO₄ ratios, and ion loadings. The primary adsorption data were scaled to the surface area of Fh using a recent ion-probing methodology that accounts for the size-dependent chemical composition of this nanomaterial (FeO_1.4(OH)_0.2·nH₂O). The results have been interpreted with the charge distribution (CD) model, combined with a state-of-the-art structural surface model for Fh. The CD coefficients have been derived independently using MO/DFT/B3LYP/6-31+G** optimized geometries. M²⁺ and PO₄ mutually enhance their adsorption to Fh. This synergy results from the combined effect of ternary surface complex formation and increased electrostatic interactions. The type of ternary complex formed (anion- vs cation-bridged) depends on the relative binding affinities of the co-adsorbing ions. For our Ca-PO₄ systems, modeling suggests the formation of two anion-bridged ternary complexes, i.e., ≡(FeO)₂PO₂Ca and ≡FeOPO₃Ca. The latter is most prominently present, leading to a relative increase in the fraction of monodentate PO₄ complexes. In Mg-PO₄ systems, only the formation of the ternary ≡FeOPO₃Mg complex has been resolved. In the absence of Ca, the pH dependency of PO₄ adsorption is stronger for Fh than for goethite, but this difference is largely, although not entirely, compensated in the presence of Ca. This study enables the use of Fh as a proxy for the natural oxide fraction, which will contribute to improved understanding of the mutual interactions of PO₄ and M²⁺ in natural systems.

中文翻译：

钙和镁离子结合三水铁矿的磷酸盐三元配合物形成：实验和机理

钙（Ca）和镁（Mg）是自然界中最丰富的碱土金属离子，它们与水铁矿（Fh）的相互作用会影响相关离子的地球化学循环，包括磷酸盐（PO ₄）。Ca和Mg（M ²⁺）与PO ₄的界面相互作用尚未针对新鲜沉淀的Fh进行分析。在这里，我们通过实验研究了在广泛的pH值，M ²⁺ / PO ₄比和离子负载范围内的二元M ²⁺ -PO ₄系统中的相互作用。使用最新的离子探测方法将主要吸附数据缩放到Fh的表面积，该方法说明了这种纳米材料的尺寸依赖性化学成分（FeO _1.4（OH）_0.2 · n H ₂ O）。通过电荷分布（CD）模型和最新的Fh结构表面模型对结果进行了解释。CD系数是使用MO / DFT / B3LYP / 6-31 + G **优化的几何结构独立得出的。M ²⁺和PO ₄相互增强对Fh的吸附。这种协同作用是由于三元表面复合物形成和增加的静电相互作用共同作用的结果。形成的三元络合物的类型（阴离子桥对阳离子桥）取决于共吸附离子的相对结合亲和力。对于我们的Ca-PO ₄系统，建模提出两种阴离子桥联的三元复合物的形成，即，≡（FeO计）₂ PO ₂ Ca和≡FeOPO ₃的Ca. 后者最明显地存在，导致单齿PO ₄复合物的分数相对增加。以mg-PO _4个系统中，仅三元≡FeOPO形成₃ Mg配合物已得到解决。在没有Ca的情况下，Fh的PO ₄吸附的pH依赖性强于针铁矿，但这种差异在存在Ca的情况下得到了很大的补偿，尽管不是全部。这项研究使得可以使用Fh代替天然氧化物组分，这将有助于增进对PO相互作用的理解自然系统中为₄和M ²⁺。

更新日期：2020-04-23

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