Adsorption-regeneration-recycling: A multi-site self-crosslinked hydrogel combined with carbonization for the efficient removal of cationic dyes and heavy metal ions from wastewater.

Hydrogels show great promise for the adsorption of dyes and heavy metal ions from wastewater owing to their excellent swelling capabilities. However, their adsorption performance is often limited by the density of available adsorption sites. To address this limitation, this study synthesized three capsaicin derivative monomers: 5-(acrylamidomethyl)-2,3,4-trihydroxybenzoic acid (TA), 3-(acrylamidomethyl)-5-benzoyl-4-hydroxy-2-methoxybenzenesulfonic acid (SA), and 3,5-bis(acrylamideomethyl)-2,4-dihydroxybenzoic acid (DA) rich in polar functional groups, which provide multiple adsorption sites for a self-crosslinked hydrogel (AA/TA/SA@DA) to enhance its adsorption performance effectively. The adsorption results indicate that the maximum adsorption capacities of this hydrogel for Malachite Green (MG), Crystal Violet (CV), Pb2 +, and Cd2+ reached 5650, 22501, 938, and 463 mg·g-1, at 298 K and pH 5 (MG/CV) or pH 6 (Pb2+/Cd2+). Most importantly, this study introduced an innovative carbonization strategy to overcome the regeneration challenges of dye-saturated hydrogels after acid-base elution, increasing the adsorption capacities for MG, Pb2+, and Cd2+ to 2.81, 1.55, and 1.85 times those of the original hydrogel, respectively. After four cycles, the pollutant removal rate remained above 86%, achieving the cyclic utilization of adsorbent materials. This work provides a strategy for treating dye and heavy metal pollution that integrates high adsorption efficiency, regenerability, and sustainability.