A dual-functional cellulose-polyelectrolyte hydrogel with homogeneous ZnO loading toward sustainable and mechanistically enhanced Cr(VI) remediation

A robust composite hydrogel (i.e., ZnO@(CNC- g -PAA/PEI)) integrating cellulose nanocrystal-grafted polyacrylic acid/polyethyleneimine with uniformly dispersed ZnO nanoparticles (NPs) was synthesized through dual chemical-physical cross-linking strategies. This multifunctional hydrogel synergistically combines adsorption and photocatalytic reduction for efficient Cr(VI) removal, exhibiting exceptional stability, recyclability, and performance. Systematic optimization revealed that pH, ZnO loading, adsorbent dosage, and contact time critically influenced removal efficiency. Under optimal conditions (pH at 3, 15 wt% ZnO loading, 100 mg/L Cr(VI), 0.5 g/L adsorbent), a remarkable 99.03 % Cr(VI) removal was achieved. The enhanced efficacy arises from three synergistic features: (1) a 3D porous architecture enabling rapid Cr(VI) diffusion, (2) abundant protonated amine groups for electrostatic adsorption, and (3) homogeneously distributed ZnO NPs minimizing aggregation and maximizing photocatalytic activity. Simulated light irradiation triggered Cr(VI)-to-Cr(III) reduction, regenerating active sites and amplifying removal capacity. Kinetic analyses confirmed pseudo-second-order adsorption. The hydrogel retained 94.75 % efficiency after five cycles, demonstrating robust reusability. This work provides a scalable strategy for designing hydrogel-based adsorbent-photocatalyst hybrids, offering a sustainable solution for chromium-laden wastewater remediation.