La3+/La(OH)3 loaded magnetic cationic hydrogel composites for phosphate removal: Effect of lanthanum species and mechanistic study

Abstract

In this study, La3+(ion)/La(OH)3-W/La(OH)3-EW-loaded magnetic cationic hydrogel (MCH) composites were fabricated in situ and characterized to investigate the effects of lanthanum species on phosphate adsorption. The corresponding maximum P adsorption capacities of MCH-loaded La3+(ion) (MCH-La3+(ion)), La(OH)3-W (MCH-La(OH)3-W), and La(OH)3-EW (MCH-La(OH)3-EW) were 70.5 ± 2.67, 69.2 ± 3.5, and 90.2 ± 2.9 mg P/g, respectively. Furthermore, for MCH-La(OH)3-EW, the P adsorption capacity was maintained relatively stable and high at pH 4.5–11 because of the ligand exchange, electrostatic interactions, and Lewis acid–base interactions. The enhanced adsorption of P was achieved over a wide pH range, as well as in the presence of competing anions (including Cl−, NO3−, SO42−, HCO3− and SiO44−). Moreover, the exhausted MCH-La(OH)3-EW could be easily regenerated by a NaOH-NaCl desorption agent with above 72% adsorption capacity remained during five recycles. The column adsorption capacity of MCH-La(OH)3-EW reached ∼3500 bed volumes (BV) (∼67.7 mg P/g) as the concentration of P decreased from 5 mg/L to 0.1 mg/L. The ATR-IR, Raman, and XPS deconvolution results revealed that both MCH and lanthanum compounds, including La3+(ion), La(OH)3-W and La(OH)3-EW, contributed to the phosphate adsorption because of the electrostatic interactions between –N+(CH3)3 and phosphate, as well as the formation of LaPO4·xH2O.