One-step room temperature synthesis of cyclotriphosphazene crosslinked tannic acid spherical adsorbent for efficient removal of cationic dyes in a wide pH range

A new tannic acid-based adsorbent (cyclotriphosphazene crosslinked tannic acid, TAH) for effective and selective removal of cationic dyes from aqueous solution was facilely synthesized employing rigid molecule hexacyclotriphosphazene (HCCP) as cross-linker at room temperature. The resulting typical sample TAH-3 was spherical with mean size of about 480 nm. Using cationic methylene blue (MB) as target dye, the adsorption property of TAH-3 was assessed systematically by investigating the effects of various key factors on the MB removal, including solution pH, adsorption time, con-existing ions, initial dye concentration and temperature. The experimental results showed that at 298.15 K and an initial MB concentration of 150 mg/L, the adsorption capacities of TAH-3 for MB ranged from 510 to 562 mg g⁻¹ across a wide pH range of 3 to 10. The adsorption rate was very swift, and 72 % of equilibrium adsorption amount could be achieved within the initial 30 min. The effect of different valence co-existing ions on MB adsorption was quite different. The addition of monovalent Na⁺ could promote the MB adsorption, while the introduction of high-valence Ca²⁺ and Al³⁺ could inhibit the MB adsorption. After five adsorption–desorption cycles, TAH-3 presented a small loss in adsorption capacity for MB and maintained good structural stability. The remarkable adsorption of MB on TAH-3 might arise from the synergistic contribution of ion exchange, electrostatic attraction, H-bonding and π-π interaction. In addition, the TAH-3 spherical adsorbent could separate MB dye from binary mixed solution polluted by MB and anionic dye with a relative separation factor ranging from 281 to 11556. This work offers a facile and cost-effective synthetic route for negatively charged TA-based spherical adsorbent with high adsorption capacity and swift adsorption rate for cationic dye over a wide solution pH range.