Double macrocyclic cationic crosslinked network enabling ultrafast and efficient aqueous Cr(Ⅵ) remediation

The efficient Cr(VI) remediation from industrial wastewater is of critical importance in water purification. Herein, we rationally constructed a dual-macrocyclic cationic cross-linked network (P1) through a quaternization-driven nucleophilic substitution strategy. The incorporation of calixarene units suppresses excessive aggregation of porphyrin macrocycles, thereby improving the accessibility of adsorption sites, while the cross-linked network provides mesoporous features and interparticle voids that facilitate the diffusion of chromium species. In addition, the porphyrin and calixarene building blocks offer abundant functional sites, including hydroxyl groups, NH groups, nitrogen atoms, and cationic N⁺ centers, which interact with Cr(VI) through electrostatic attraction, ion exchange, and hydrogen-bonding interactions. As a result, P1 enables ultrafast and efficient Cr(VI) capture over a wide pH range (3-11) with good resistance toward coexisting anions. P1 exhibits a Cr(VI) adsorption capacity of 265.96 mg g^-1 and a high pseudo-second-order kinetic constant (k₂ = 2.016 g mg^-1 min^-1), reaching adsorption equilibrium within 4 min. Additionally, P1 treatment substantially reduced the toxicity of chromium-containing wastewater, enabling its potential reuse for agricultural irrigation, including wheat and mung bean cultivation. Mechanistic investigations reveal that Cr(VI) removal by P1 proceeds through synergistic electrostatic attraction, ion exchange, hydrogen bonding, and partial reduction to Cr(III), accompanied by the retention of a fraction of the generated Cr(III) species through surface complexation and coordination interactions.