Enhanced Selective Removal of Cd (II) from Aqueous Solution by Dual Ionic Thermosensitive Hydrogel: Synthesis, Characterization and Isotherm Analysis

Abstract In this study, hydrogels based on N-isopropylacrylamide (NIPAM) copolymerized with acrylic acid (AA) or with chitosan (Chi) were synthesized at various temperatures (30°C and 50°C) and evaluated for their structural, swelling, and adsorption properties. FTIR analysis confirmed successful copolymerization and indicated thermal effects on functional group expression. Swelling tests revealed that hydrogels synthesized at 30°C exhibited significantly higher equilibrium swelling ratios, attributed to the expanded polymer network below NIPAM's LCST. SEM and BET analyses demonstrated that lower synthesis temperatures promoted a larger surface area and enhanced porosity. Adsorption experiments using CdCl₂ solutions showed that blended gels achieved equilibrium within 4 h. In addition, the blending of NIPAM/AA and NIPAM/Chi at a synthesis temperature of 30°C resulted in the highest swelling ratio of 5.8 and the highest removal of 91.90%. Isothermal modelling revealed that the Freundlich model best described the adsorption behaviour, suggesting multilayer and heterogeneous surface adsorption. The kinetics study for cadmium (Cd2+) and chloride (Cl-) ion adsorptions was best fit with the Avrami kinetic model, describing the adsorption of Cd2 + and Cl- ions occurred through both physical and chemical mechanisms sustainably. These findings support the development of temperature-sensitive and eco-friendly hydrogel-based adsorbents for heavy metal remediation in aqueous environments.