Synthesis of a novel magnetic calcium-rich biochar nanocomposite for efficient removal of phosphate from aqueous solution

Phosphorus (P) scarcity and eutrophication have triggered the development of new materials for P recovery. In this work, a novel magnetic calcium-rich biochar nanocomposite (MCRB) was prepared through co-precipitation of crab shell derived biochar, Fe²⁺ and Fe³⁺. Characteristics of the material demonstrated that the MCRB was rich in calcite and that the Fe₃O₄ NPs with a diameter range of 18–22 nanometers were uniformly adhered on the biochar surface by strong ether linking (C–O–Fe). Batch tests demonstrated that the removal of P was pH dependent with an optimal pH of 3–7. The MCRB exhibited a superior P removal performance, with a maximum removal capacity of 105.6 mg g⁻¹, which was even higher than the majority lanthanum containing compounds. Study of the removal mechanisms revealed that the P removal by MCRB involved the formation of hydroxyapatite (HAP-Ca₅(PO₄)₃OH), electrostatic attraction and ligand exchange. The recyclability test demonstrated that a certain level (approximately 60%) was still maintained even after the six adsorption–desorption process, suggesting that MCRB is a promising material for P removal from wastewater.