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  • Title: Influence of coexisting calcium and magnesium ions on phosphate adsorption onto hydrous iron oxide.
    Author: Lin J, Zhao Y, Zhan Y, Wang Y.
    Journal: Environ Sci Pollut Res Int; 2020 Apr; 27(10):11303-11319. PubMed ID: 31965506.
    Abstract:
    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+.
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