Superior adsorption of phosphate by ferrihydrite-coated and lanthanum-decorated magnetite

Abstract

Present study reports the successful development of a novel lanthanum (La)-based magnetic adsorbent and its use for phosphate removal from water. For its synthesis, natural magnetite (Mag), Fe3O4, was subjected to partial dissolution in HCl solution and the obtained suspension was mixed with an alkaline solution for in-situ synthesis of ferrihydrite (Fh)-coated Mag (Mag@Fh). Mag@Fh was then decorated with La (hydr)oxides followed by calcination to produce Fh-coated and La-decorated Mag (Mag@Fh-La). Obtained Mag@Fh-La represented high phosphate adsorption capacity (44.8 mg P/g at 15.7% La in its structure) and La usage efficiency. Moreover, Mag@Fh-La retained its high adsorption capacity (>35.0 mg P/g) over a wide range of equilibrium solution pH (3.2–10.7). The combination of FTIR, XPS analysis and adsorption experiments revealed that ligand exchange and electrostatic attraction were the main mechanisms that jointly facilitated the adsorption of phosphate. Adsorption-desorption cycle studies confirmed the well-retained adsorption efficiency of regenerated Mag@Fh-La for repeated applications. Final experiments with real domestic wastewater (initial phosphate concentration of 1.7 mg/L) revealed that 0.2 g/L Mag@Fh-La efficiently reduced the phosphate concentration to below 0.02 mg/L. Overall, this work clearly highlights that the synthesized novel adsorbent has promising applications in phosphate removal from real wastewater.