Lanthanum ion modification of aminated cyclomatrix polyphosphazene-coated porous carbon nanosheets for rapid, efficient and selective removal of phosphate

Phosphate removal has been the top priority in eutrophic wastewater treatment. Herein we reported a fast, efficient and selective adsorbent, PCNs@PCP-La, for phosphate removal from an aqueous medium. The adsorbent was fabricated via two steps: (1) synthesis of PCNs@PCP composite through in situ polymerization of hexachlorocyclotriphosphazene and polyethyleneimine on the porous carbon nanosheets; and (2) lanthanum ion (La³⁺) modification of PCNs@PCP via an impregnation process to create PCNs@PCP-La hybrid. Multiple characterizations showed that the atomically dispersed La³⁺ ions could be facilely chelated with nitrogen-rich cyclomatrix polyphosphazene framework, significantly improving the application efficiency of La³⁺ ions as active adsorption sites. Comparative tests confirmed that PCNs@PCP-La had a high phosphate uptake capacity of up to 121.2 mg P g⁻¹ (pH = 4.0) at 298 K and performed well even in a wide pH range (3.0–9.0). Also, PCNs@PCP-La showed a particularly short saturated adsorption time (5 min), indicating a fast adsorption kinetic process. Meanwhile, PCNs@PCP-La presented a superior adsorption selectivity for phosphate under interference with coexisting anions/cation and humic acid, and owned an attractive reusability (after 5th cycles, 85% capacity retention). The plausible adsorption mechanism can be ascribed to electrostatic attraction, inner-sphere complexation, and the formation of LaPO₄·0.5H₂O precipitates between adsorbent and adsorbate.