Eco-Friendly and Easily Synthesized Amorphous Fe–Ca (Oxy)hydroxide for Selective Phosphate Removal from Synthetic and Real Effluents: Synthesis, Optimization, and Application

The contamination of surface and groundwater by nutrients due to the discharge of untreated effluents is a serious environmental issue, as it can promote eutrophication and compromise water quality. This study aimed to optimize the synthesis of Fe-Ca (oxy)-hydroxide (CaFe) and evaluate its adsorptive capacity for phosphate ions. The coprecipitation route was chosen for the material synthesis. Structural characterizations revealed that the material exhibits high roughness, a large number of active sites, thermal stability, and a predominance of the amorphous phase, all of which favor its efficiency in phosphate adsorption. Optimization using response surface methodology (RSM) yielded a high adjusted coefficient of determination (<i>R</i> _adj ² = 0.97), indicating excellent fit to the experimental data. The adsorption process followed the pseudo-second-order kinetic model (<i>R</i> _adj ² > 0.99) and was best described by the Sips isotherm (<i>R</i> _adj ² > 0.99), suggesting multilayer adsorption in a heterogeneous system. The maximum adsorption capacity (<i>q</i> _m = 41 mg/g) was achieved when the process temperature was adjusted to 30 °C. Thermodynamic parameters confirmed that the adsorption is spontaneous and endothermic (Δ<i>H</i>° = 17.38 kJ/mol). CaFe exhibits a high capacity for phosphate adsorption, even in the presence of other anions, demonstrating excellent selectivity and robustness in complex aqueous matrices. Moreover, CaFe was capable of removing up to 84.86% of phosphate from a real effluent sample by simply applying a dosage of 1.5 g/L and adjusting the effluent pH (pH = 4). In conclusion, the results demonstrated that CaFe is a promising material for phosphate removal, showing high potential for application in the treatment of water bodies susceptible to eutrophication. The findings indicated that CaFe exhibits excellent performance as an adsorbent, with high adsorption capacity, strong selectivity in the presence of competing anions, and satisfactory regeneration efficiency. Additionally, the development of sustainable materials such as CaFe is essential for achieving the Sustainable Development Goals (SDGs), particularly SDG 6, SDG 9, SDG 12, and SDG 14.