Preparation Of MAPTAC-based Lanthanum-loaded Magnetic Cationic Hydrogel And Its Adsorption Properties And Mechanism Towards PO₄^3--P In Water

At present,the global phosphate rock resources are facing exhaustion,whereas the fate of phosphorus in traditional wastewater treatment plants,which forms phosphorus-containing compounds in the sludge,is not only difficult to be resourced,but also leads to excessive phosphate in receiving water bodies and eutrophication problems.Therefore,the phosphate recovery from wastewater and the reduction/fixation of phosphorus in receiving water are the main ways to solve the above problems,which require efficient phosphate removal technology as support.During phosphate removal,adsorption technologies have the advantages of simple process,high removal efficiency,low cost and easy separation and recovery,while highly positive charged cationic hydrogels loaded with rare earth lanthanum for phosphate adsorption have been a hot topic in recent years.In this study,3-（methacryloylamino）propyl-trimethylammonium chloride（MAPTAC）was used as monomer to synthesize a highly positive charged La（OH）₃-loaded MAPTAC-based magnetic cationic hydrogel（La（OH）₃@MMCH）,and the adsorption characteristics and mechanisms for PO₄^3--P in water were studied.The results were shown as follows:In the p H range of 3.0-11.0,the surface charge densities of MAPTAC-based magnetic cationic hydrogel（MMCH）were 34.38-59.38 meq/kg and the zeta potential values were 27.75-46.7 m V,while the swelling ratio reached 3014.57%within 24 h.After loading with La（OH）₃,La（OH）₃@MMCH showed high zeta potential values of 36.0-46.75 m V and the lanthanum content of 315 mg/g.Furthermore,the release amount of lanthanum and iron could be neglected under neutral and alkaline conditions.For the simulated wastewater with initial concentration of 50.0 mg P/L,0.2 g/L La（OH）₃@MMCH was added and 95.28%of the saturated adsorption capacity（105.72±5.99 mg P/g）could reach within 2 h.The corresponding adsorption isotherm and adsorption kinetics fitted well with the Sips model and the pseudo-second-order kinetic model,respectively,which meant the multilayer adsorption process was mainly chemically controlled.La（OH）₃@MMCH had stable adsorption capacity of 94.33-99.37 mg P/g in the p H range of 3.0-9.0 and showed strong selective adsorption towards PO₄^3--P in the presence of different anions.The adsorption capacity of La（OH）₃@MMCH maintained 73.3%of the initial value after reuse for 5 cycles.In addition,when the dosage of La（OH）₃@MMCH was 0.06 g/L and the initial concentration was 2.0 mg P/L,the PO₄^3--P removal rate was 95.71%in the simulated water derived from the secondary sedimentation tank and the effluent reached the A standard of"Discharge standard of pollutants for municipal wastewater treatment plants"（GB18918-2002）,while the PO₄^3--P removal rate was 95.21%in the simulated surface water and the effluent reached Class II standard of"Environmental quality standards for surface water"（GB3838-2002）.The Donnan membrane effect produced by MMCH strengthened the mass transfer process of PO₄^3--P at the solid-liquid interface and promoted the enrichment of PO₄^3--P into the hydrogel phase.FTIR and XPS analysis showed that-N⁺（CH₃）₃ groups and La（OH）₃ were both involved in the adsorption process,among which-N⁺（CH₃）₃ groups produced the electrostatic attraction with PO₄^3--P,and La PO₄·x H₂O formed by ligand exchange between La（OH）₃ and PO₄^3--P.The ligand exchange was the main adsorption mechanism.