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 (HxPO4x - 3, x = 0, 1, 2, or 3 depending on solution pH). To better apply iron oxide-containing materials as adsorbents to eliminate HxPO4x - 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 HxPO4x - 3 by HFEO was studied. The results showed that, without coexisting Ca2+ and Mg2+, the kinetic data for HxPO4x - 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 HxPO4x - 3 and the generation of Fe-O-P bonding played a key role in the uptake of HxPO4x - 3 by HFEO in the absence of Ca2+ and Mg2+. Coexisting Ca2+ and Mg2+ greatly improved the adsorptive removal of HxPO4x - 3 by HFEO, including the adsorption capacity and initial adsorption rate. According to the Langmuir isotherm equation, the predicted maximum HxPO4x - 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 HPO42--bridged ternary complexes (i.e., Fe(OPO3H)Ca+ and Fe(OPO3H)Mg+) had an important role in the improvement of HxPO4x - 3 adsorption onto HFEO by coexisting Ca2+ and Mg2+.