Lacl3 Performed Best in Phosphate Adsorption Among the La-Based Materials Due to its Abundant Leaving Groups of Cl− for Ligand Exchange

Promoting the ligand exchange between lanthanum and phosphate is an effective strategy to construct efficient La-containing adsorbents. Here, four magnetic composites of La3+/La2(CO3)3/La(OH)3/La2O3 hybridized with Fe3O4 were fabricated by in situ embedded approach and used for adsorption and recovery of phosphate at low concentrations in water. La3+@Fe3O4 (MLI) with high leaving-groups-ability of Cl− presented enhanced adsorption performance. The adsorption capacity (46.28 mg/g) and initial adsorption rate (132.53 mg/g⋅min) of MLI were 1.5-2 times and 1.46-2.22 times higher than those of La2(CO3)3@Fe3O4, La(OH)3@Fe3O4 and La2O3@Fe3O4. Detailed adsorption result showed that MLI-41 with La-to-Fe molar ratio of 4:1 exhibited a competitive phosphate adsorption capacity of 77.85 mg P/g, good phosphate adsorption activity over a broad pH range from 5 to 10, excellent selectivity for phosphate adsorption even in the presence of 5-50 times higher concentrations of co-existing ions. In four actual P-containing environmental water bodies (TP<5 mg P/L), 0.1 g/L of MLI-41 could efficiently reduce the phosphate concentration to below the specified P standard limits. The ligand exchange between KHPO4-/ H2PO4- and Cl– on MLI to form mononuclear monodentate complexation was the main adsorption mechanism. Moreover, the exhausted MLI-41 could be easily regenerated by 3 h treatment of the 2.5 M NaOH and 1.875 M NaCl mixture. The total phosphorus in the continuously used desorbent was gradually enriched, corresponding to the loss of the total phosphorus in the water.