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^-, SO₄^2-, NO₃^-, HCO₃^-, SiO₃^2-) 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.

Keywords: Adsorption performance; Low strength phosphate; MCH-La(OH)(3)-EW; Mechanism; Real water treatment; Regeneration.