An efficient phosphorus scavenging from aqueous solution using magnesiothermally modified bio-calcite

Bio-calcite (BC) derived from waste hen eggshell was subjected to thermal treatments (calcined bio-calcite (CBC)). The BC and CBC were further modified via magnesiothermal treatments to produce modified bio-calcite (MBC) and modified calcined bio-calcite (MCBC), respectively, and evaluated as a novel green sorbent for P removal from aqueous solutions in the batch experiments. Modified BC exhibited improved structural and chemical properties, such as porosity, surface area, thermal stability, mineralogy and functional groups, than pristine material. Langmuir and Freundlich models well described the P sorption onto both thermally and magnesiothermally sorbents, respectively, suggesting mono- and multi-layer sorption. Langmuir predicted highest P sorption capacities were in the order of: MCBC (43.33 mg g^-1) > MBC (35.63 mg g-¹) > CBC (34.38 mg g^-1) > BC (30.68 mg g^-1). The MBC and MCBC removed 100% P up to 50 mg P L^-1, which reduced to 35.43 and 39.96%, respectively, when P concentration was increased up to 1000 mg L^-1. Dynamics of P sorption was well explained by the pseudo-second-order rate equation, with the highest sorption rate of 4.32 mg g^-1 min^-1 for the MCBC. Hydroxylapatite [Ca₁₀(PO₄)₆(OH)₂] and brushite [CaH(PO₄)·2H₂O] were detected after P sorption onto the modified sorbents by X-ray diffraction analysis, suggesting chemisorption as the operating sorption mechanism.