Effect of Protonated Media on Dye Diffusion in Chitosan–Cellulose-Based Cryogel Beads

Synthetic dyes are increasingly relevant pollutants due to their widespread use and discharge into water bodies. This study examines how the solution pH affects the morphology of chitosan-cellulose cryogel (Ch-C-EGDE) and its impact on dye transport to adsorption sites. Adsorption tests with dyes Y5, R2, and B1 over a pH range of 2-12 revealed optimal performance at pH 2.5. High hydronium ion concentrations significantly improved adsorption capacities (945-1605 mg/g), with a hierarchy B1 > R2 > Y5 at 250 mg/L initial concentration. The dependence of the dye adsorption on the acidic pH of the solution suggests that there is a mechanism of adsorption by electrostatic forces due mainly to the protonation of the amino group (NH₃⁺). During the dye adsorption studies, a decrease in the diameter of the cryogel beads was observed, as well as a possible "zipper effect" in the pores of the Ch-C-EGDE cryogel beads, which depends on the pH at which the anionic molecules of the dyes attract the positively charged chitosan-based adsorbent walls, which physically closes the pores and results in a decrease in pore size as well as a geometric and/or load-bearing impediment. The experimental data fitted well with the pseudo-second-order kinetic models and the Sips isotherm model, indicating multilayer and heterogeneous adsorption behavior. In the Sips model, a value of n > 1 was obtained, which confirms favorable adsorption conditions and suggests strong dye-adsorbent material interactions, especially at higher dye concentrations.