Influence of coexisting calcium and magnesium ions on phosphate adsorption onto hydrous iron oxide

Removal of phosphorus (P) from municipal wastewater is of vital importance to the control of eutrophication in receiving freshwater bodies. Typical cations such as Ca²⁺ and Mg²⁺ generally exist in municipal wastewater, and they may affect the sorption behavior and mechanism of iron oxide-based materials for aqueous phosphate (H_xPO₄^x - 3, x = 0, 1, 2, or 3 depending on solution pH). To better apply iron oxide-containing materials as adsorbents to eliminate H_xPO₄^x - 3 in municipal wastewater, a hydrous ferric oxide (HFEO) was prepared and characterized at first and then the impact of coexisting Ca²⁺ and Mg²⁺ on the uptake of H_xPO₄^x - 3 by HFEO was studied. The results showed that, without coexisting Ca²⁺ and Mg²⁺, the kinetic data for H_xPO₄^x - 3 sorption onto HFEO were better described by the Elovich model (R² = 0.953) than the pseudo-second-order (R² = 0.838) and pseudo-first-order (R² = 0.641) models, and the isotherm data were fitted better with the Dubinin-Radushkevich (R² = 0.966) and Freundlich (R² = 0.953) models than with the Langmuir (R² = 0.924) model. The ligand exchange of the Fe-bound hydroxyl group with H_xPO₄^x - 3 and the generation of Fe-O-P bonding played a key role in the uptake of H_xPO₄^x - 3 by HFEO in the absence of Ca²⁺ and Mg²⁺. Coexisting Ca²⁺ and Mg²⁺ greatly improved the adsorptive removal of H_xPO₄^x - 3 by HFEO, including the adsorption capacity and initial adsorption rate. According to the Langmuir isotherm equation, the predicted maximum H_xPO₄^x - 3 adsorption capacity for HFEO at pH 7 in the presence of 2 mmol/L Ca²⁺ (24.7 mg P/g) or 2 mmol/L Mg²⁺ (18.4 mg P/g) was much larger than that without coexisting Ca²⁺ and Mg²⁺ (10.7 mg P/g). The formation of aqueous CaHPO₄⁰ and MgHPO₄⁰ species firstly and then the adsorption of the formed CaHPO₄⁰ and MgHPO₄⁰ species on the HFEO surface to generate the HPO₄^2--bridged ternary complexes (i.e., Fe(OPO₃H)Ca⁺ and Fe(OPO₃H)Mg⁺) had an important role in the improvement of H_xPO₄^x - 3 adsorption onto HFEO by coexisting Ca²⁺ and Mg²⁺.