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[Display omitted] • A recyclable PCNs@PCP-La was firstly fabricated for phosphate removal. • The phosphate uptake capacity was up to 121.2 mg P g−1. • PCNs@PCP-La showed rapid adsorption, high capacity, attractive selectivity and high pH adaptability. • PCNs@PCP-La presented a superior reusability for phosphate removal. • Multiple techniques were used to demonstrate the adsorption mechanisms. 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 LaPO 4 ·0.5H 2 O precipitates between adsorbent and adsorbate. [ABSTRACT FROM AUTHOR]