Synergistic N/O coordination-driven selective separation and dynamic enrichment of Sn(IV) from strongly acidic multi-ion media using a covalent phosphazene-based silica composite

Precise separation and dynamic enrichment of Sn(IV) from complex strongly acidic waste liquids are important for strategic metal recovery, but remain challenging because high acidity and competing ions can severely weaken adsorption selectivity. Herein, a covalent phosphazene-based silica composite adsorbent, CPP-O/SiO 2 , was prepared through the in situ solvothermal polymerization of cyanuric acid and hexachlorocyclotriphosphazene on porous SiO 2 . The SiO 2 support preserved a regular spherical morphology and favorable column adaptability, while the CPP-O network supplied abundant N/O coordination sites. CPP-O/SiO 2 retained high Sn(IV) removal efficiencies above 95% in 0.5–1 M HNO 3 and achieved approximately 98% Sn(IV) removal in a simulated acidic multi-ion matrix mimicking waste printed circuit board (WPCB) leachate/high-level liquid waste (HLLW). Adsorption equilibrium was reached within 120 min, and the saturated adsorption capacity reached 188.41 mg/g at 318 K. Fixed-bed column experiments further demonstrated selective dynamic retention and enrichment of Sn(IV) from simulated HLLW, giving an enrichment factor of approximately 116 after elution with 0.1 M NaOH and maintaining stable performance over eight adsorption–desorption cycles. FT-IR, XPS, and DFT analyses revealed that Sn(IV) capture was mainly governed by synergistic coordination involving O and N sites, especially Sn O interactions. This work provides a robust covalent composite adsorbent for selective Sn(IV) recovery from strongly acidic multi-ion media.