Dispersion-promoted synergistic cationic dye removal through the co-introduction of natural diatomite and bentonite into chitosan-based hydrogel beads

Hydrogel beads have drawn considerable attention in sustainable wastewater treatment because of their low cost, modifiability, and feasibility. Accordingly, chitosan-based hybrid hydrogel beads were fabricated by incorporating naturally available siliceous micro- and nanoparticles, namely diatomite and bentonite, with different mass ratios. Morphology, swelling behavior, mechanical stability, and ultimately, Methylene Blue (MB)-adsorption performance of the hydrogel beads were comprehensively evaluated in terms of filler dispersion and interactions between the fillers and the matrix. Results revealed that while the simultaneous incorporation of inorganic additives inside the chitosan backbone reduced the swelling degree, the mechanical stability was significantly ameliorated when the 1:1 diatomite-bentonite mass ratio was exploited inside the chitosan matrix. That can be attributed to the well-dispersed fillers and enhanced mechanical entanglements, as well as the strengthened physical interactions between diatomite-bentonite and chitosan matrix. When the hybrid filling system was applied, MB removal efficiency increased synergistically by over 246 % compared to bentonite alone and 268 % compared to diatomite alone, even at low MB concentration (10 ppm). The MB adsorption kinetics and isotherm were also studied. The adsorption kinetics of MB were well-fitted for all compositions using the pseudo-second-order model. The isotherm data for optimum beads indicate that the Freundlich isotherm model provided the best fit, suggesting the multilayer adsorption with a non-uniform distribution of adsorption heat. The cost appraisal and recyclability/reusability assessment indicate that the combination of diatomite and bentonite in chitosan-based hydrogel beads offers a sustainable, highly efficient, and cost-effective solution for cationic dye removal applications.