Remarkable phosphate recovery from wastewater by a novel Ca/Fe composite: Synergistic effects of crystal structure and abundant oxygen-vacancies

Calcium-based materials are considered to be promising adsorbents for phosphate removal in the water environment due to their environmental friendliness and low price. However, improving the efficiency and rate of P adsorption of calcium-based materials still needs further exploration. In this study, a high-efficiency and eco-friendly Ca/Fe composite was rationally designed and fabricated by a co-precipitated method. Batch adsorption experiments showed that Ca/Fe composites with a Ca: Fe molar ratio of 3: 1 exhibited a remarkable phosphate sorption capacity of 161.4 mg P/g. Furthermore, the phosphate adsorption capacity of Ca/Fe-3/1 composite was maintained relatively high at pH 3–11 due to the ligand exchange, electrostatic and chemical precipitation. In addition, the experiment performed to determine the effect of coexisting ions shows that only carbonate ions slightly inhibit the phosphate adsorption effect of the Ca/Fe-3/1 composite. The newly prepared Ca/Fe composites have a fast phosphate removal efficiency. The XPS and EPR analysis showed that a large number of oxygen vacancies were formed on Ca/Fe composites due to the introduction of magnetic Fe. This is the first time to introduction oxygen vacancies into Ca/Fe composites by co-precipitation. The existence of oxygen vacancies can promote electron transfer rate and reduce the bonding energy barrier for phosphate adsorption, thereby increasing the phosphate absorption rate of the Ca/Fe composites. The enhanced phosphate removal by Ca/Fe composites with abundant oxygen vacancies provides a new strategy for the preparation of commercial phosphate -controlling materials.