Simultaneous and synergistic removal of Cu²⁺ and methylene blue from aqueous solution using ZIF-8-decorated diatomite/alginate/polyvinyl alcohol hydrogel beads

Efficient wastewater treatment requires simultaneous removal of coexisting contaminants such as dyes and heavy metals. Natural polymer-based hybrid hydrogels offer stability, low toxicity, and biodegradability, yet most studies still focus on single-pollutant systems or specific nanoparticles. To address this, hybridized ZIF-8/diatomite incorporated into alginate/polyvinyl alcohol (PVA) hydrogel beads were developed for simultaneous adsorption of Cu²⁺ and methylene blue, offering a novel approach for multi-pollutant wastewater treatment. The beads were characterized using FE-SEM, XRD, FTIR, and BET surface area analysis. Hybridization with ZIF-8 significantly enhanced the surface area of diatomite from 6.2 to 36.3 m²/g. The resulting hydrogel beads were systematically characterized to assess their swelling behavior, mechanical stability, and adsorption performance. The ZIF@diatomite incorporated alginate and PVA beads (PAZDH) exhibited a swelling ratio of approximately 200 % and maintained their mechanical integrity when subjected to mechanical waves for 8 min, up to 70 %. In the single-pollutant system, samples with higher diatomite content were more effective for MB removal (up to 92 %), while Cu²⁺ adsorption was enhanced in samples containing both ZIF-8 and diatomite functional groups (95 %). The PAZDH beads achieved 94 % MB and 93.5 % Cu²⁺ removal in the single-pollutant system. In the simultaneous adsorption of both pollutants, PAZDH demonstrated a balanced performance, achieving 91 % removal of Cu²⁺ and 63 % removal of MB. Adsorption kinetics of PAZDH sample followed pseudo-first-order and pseudo-second-order models for MB and Cu²⁺ removal, respectively. These findings suggest that hybrid hydrogel beads offer an efficient, eco-friendly solution for wastewater treatment, effectively removing both organic dyes and heavy metal ions.