Efficient Sequential Removal of Cationic and Anionic Metals or Dyes Using a pH-Responsive Magnetic Hydroxy-azobenzene Porous Organic Polymer Adsorbent

Aquatic contamination caused by heavy metals and dyes poses critical ecological and health risks. Herein, we report a magnetic hydroxy-azobenzene porous organic polymer (TAPT-PAR-MHPOPs) adsorbent synthesized via an environmentally benign aqueous-phase diazo-coupling method for the efficient removal of heavy metals and dyes. The adsorbent features a highly porous network with abundant aromatic frameworks and N/O active sites, facilitating the capture of various contaminants through multiple interactions. The adsorption experiments demonstrate pH-regulated adsorption selectivity: at pH 2, protonated functional groups (═NH+–, −OH2+) facilitate selective removal of anionic contaminants including Cr(VI) oxygen-containing anions and methyl orange (MO) dyes; at pH 5–8, deprotonated N/O moieties facilitate efficient removal of cationic methylene blue (MB) and Pb2+; at pH 12, synergistic electrostatic attraction and π–π stacking interactions enhance MB adsorption. The adsorbent achieves maximum adsorption capacities of 219, 292, 398, and 1220 mg g–1 for Cr(VI), MO, Pb2+, and MB, respectively. The pH-triggered charge reversal mechanism enables selective separation of cationic and anionic pollutants, demonstrated by the sequential removal of MB and Pb2+ at pH 6, and MO and Cr(VI) at pH 2. Key operational advantages include rapid equilibrium kinetics, robust tolerance to salt interference, high chemical stability, and superior cyclic stability. Besides, the adsorbent exhibits a broad-spectrum contaminant affinity, effectively scavenging other ionic pollutants (Hg2+ and malachite green cationic dyes) and nonionic persistent organic contaminants (bisphenol A, dimethyl phthalate, doxycycline hydrochloride, and 2-naphthol) under near-neutral conditions. This work underscores the immense potential of pH-responsive adsorbents as a versatile and highly efficient platform for multiplexed pollutant management in complex aquatic matrices.