Ternary Complex Formation of Phosphate with Ca and Mg Ions Binding to Ferrihydrite : Experiments and Mechanisms

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.