Phosphate removal by a La(OH) loaded magnetic MAPTAC-based cationic hydrogel: Enhanced surface charge density and Donnan membrane effect.

Cationic hydrogels have received great attention to control eutrophication and recycle phosphate. In this study, a type of La(OH) loaded magnetic MAPTAC-based cationic hydrogel (La(OH)@MMCH) was developed as a potential adsorbent for enhanced phosphate removal from aqueous environment. La(OH)@MMCH exhibited high adsorption capacity of 105.72±5.99 mg P/g, and reached equilibrium within 2 hr. La(OH)@MMCH could perform effectively in a wide pH range from 3.0 to 9.0 and in the presence of coexisting ions (including SO, Cl, NO, HCO, SiO and HA). The adsorption-desorption experiment indicated that La(OH)@MMCH could be easily regenerated by using NaOH-NaCl as the desorption agent, and 73.3% adsorption capacity remained after five cycles. Moreover, La(OH)@MMCH was employed to treat surface water with phosphate concentration of 1.90  mg/L and showed great removal efficiency of 95.21%. Actually, MMCH showed high surface charge density of 34.38-59.38 meq/kg in the pH range from 3.0 to 11.0 and great swelling ratio of 3014.57% within 24 h, indicating that MMCH could produce the enhanced Donnan membrane effect to pre-permeate phosphate. Furthermore, the bifunctional structure of La(OH)@MMCH enabled it to capture phosphate through electrostatic attraction and ligand exchange. All the results prove that La(OH)@MMCH is a promising adsorbent for eutrophication control and phosphate recovery.