pH-Sensitive Hydrogel Membrane-Based Sodium Alginate/Poly(vinyl alcohol) Cross-Linked by Freeze-Thawing Cycles for Dye Water Purification

Hydrogel membranes are currently attracting significant attention due to their versatile 3D network structures capable of adsorbing water and various dissolved compounds. In this study, a pH-sensitive hydrogel membrane was developed by combining sodium alginate (SA) molecules with poly(vinyl alcohol) (PVA) for water purification applications. We investigated the physicochemical characteristics of the SA/PVA membranes, including equilibrium swelling, gel fraction, and water absorption. We also examined the influence of freeze-thaw cycles on pore size and morphology. SEM imaging revealed that the pore size increases with the number of cycles, while FTIR results confirmed the presence of intermolecular hydrogen bonding between the PVA and SA chains. The SA/PVA hydrogel membrane demonstrated remarkable pH sensitivity from pH 1 to 11. The SA/PVA hydrogel membrane showed significant potential for removing toluidine blue (TB) dye from aqueous solutions. The adsorption kinetics were studied using pseudo-first-order and pseudo-second-order models, with the latter offering a superior fit to the experimental data (R² = 0.97). Furthermore, adsorption isotherm modeling indicated monolayer adsorption on homogeneous sites and multilayer adsorption on heterogeneous sites. The Langmuir isotherm model provided a maximum adsorption capacity (q_m) of 74.1 mg/g, and the Freundlich isotherm model exhibited a correlation coefficient (R²) of 0.89.