Targeted synthesis of a high-stability anionic cyclophosphazene-based porous organic polymer for highly efficient remediation of methylene blue

Developing ionic porous polymers with stable structure is of paramount significance for the efficient removal of charged organic contaminants. Herein, an ionic cross-linked polymer (POP-SO₃Na) with rich mesopores/macropores was facilely synthesized by copolymerization of 4,4′-diamino-2,2′-stilbenedisulfonic acid and hexachlorocyclotriphosphazene, followed by an easy-to-implement alkali soaking strategy. POP-SO₃Na showed an ultra-fast adsorption speed and a high adsorption capacity for cationic model contaminant methylene blue (MLB). At the initial concentration of 250 mg L⁻¹ and temperature of 25 °C, the adsorption capacity of POP-SO₃Na for MLB were up to 785.8 mg g⁻¹, and the adsorption equilibrium time was only 20 min. Also, POP-SO₃Na maintained a high level of adsorption capacity (608.0–833.2 mg g⁻¹) for MLB over a wide pH range (3.0–8.0), presenting excellent pH adaptability. Furthermore, in the binary system of MLB mixed with three different anionic dyes, POP-SO₃Na showed superior adsorption selectivity for MLB with high values of relative separation factor (1635.9, 1194.2 and 125.6) at 10 min. The adsorption behavior of POP-SO₃Na towards MLB aligned well with Langmuir isotherm model and pseudo-second-order kinetics, and the plausible adsorption mechanism consisted of electrostatic interaction, hydrogen bonding, π-π stacking and pore filling.