Phosphate reclaim from simulated and real eutrophic water by magnetic biochar derived from water hyacinth

In this study, the efficiency and mechanism of aqueous phosphate removal by magnetic biochar derived from water hyacinth (MW) were investigated. The MW pyrolyzed at 450 °C (MW450) exhibited the most prominent phosphate sorption capacity, which was estimated to be 5.07 mg g^-1 based on Langmuir-Freundlich model. At an initial phosphorus (P) concentration of 1 mg l^-1, >90% P removal was achieved over pH 3-9, but the efficiency decreased sharply at pH > 10. The presence of arsenate and carbonate could remarkably decrease P sorption, while the inhibition effects of antimonate, nitrate and sulfate were less significant. In further application of MW450 to reclaim P from eutrophic lake waters (0.71-0.94 mg l^-1 total P), ∼96% P removals were attained in the batch studies and the effluent P concentrations in the column tests were reduced to <0.05 mg l^-1 within 509-1019 empty bed volumes. As indicated by XRD, MW450 surface was dominated by Fe₃O₄ and Fe₂O₃, resulting in a good ferromagnetic property of this composite (saturation magnetization 45.8 emu g^-1). Based on XPS, P sorption onto MW450 occurred mainly by surface complexation with the hydroxyl via ligand exchange. These results highlighted that MW derived from highly damaging water hyacinth could provide a promising alternative for P removal from most eutrophic waters.