Rapid and selective harvest of low-concentration phosphate by La(OH)3 loaded magnetic cationic hydrogel from aqueous solution: Surface migration of phosphate from –N+(CH3)3 to La(OH)3

Abstract

Phosphate is an important factor for the occurrence of surface water eutrophication, and is also a non-renewable resource which faces a potential depletion crisis. In this study, La(OH)3 loaded magnetic cationic hydrogel composite MCH-La(OH)3-EW was used to absorb low strength phosphate in simulated water and real water. The adsorption amount of MCH-La(OH)3-EW was 39.14 ± 0.31 mg P/g and the equilibrium time was 120 min at the initial phosphate concentration of 2.0 mg P/L. The adsorption process was a spontaneous endothermic reaction. MCH-La(OH)3-EW exhibited a high selectivity towards phosphate within pH of 4.0–10.0 or in the presence of co-existing ions (including Cl−, SO42−, NO3−, HCO3−, SiO32−) and humic acid. After 10 cycles of adsorption-desorption, the adsorption amount of regenerated MCH-La(OH)3-EW still remained at 63.4% of its maximum value. For the real water sample with phosphate concentration of 2.0 mg P/L, the phosphate removal efficiency could achieve 97.65–98.90% and the effluent turbidity was 2.10–4.27 NTU at the MCH-La(OH)3-EW dosage of 0.04 g/L. The adsorption mechanism analysis showed that both quaternary amine groups (–N+(CH3)3) and La(OH)3 of MCH-La(OH)3-EW were involved in the process of phosphate adsorption. The electrostatic interaction between phosphate and –N+(CH3)3 rapidly occurred at the initial stage of adsorption process, then the electrostatic absorbed phosphate migrated to La(OH)3 on the surface of MCH-La(OH)3-EW via ligand exchange to form inner-sphere complex. This phenomenon was conducive to phosphate adsorption kinetics by MCH-La(OH)3-EW.