Comparative Study between Cryogel and Hydrogel-Based Chitosan: Relationship between Structure, Swelling Ability, and Cu(II) Adsorption Ability

Chitosan-based materials are widely known for their ability to adsorb heavy metals. However, the role of their internal porous structure in controlling the adsorption performance remains unclear. In this study, chitosan/star-shaped polycaprolactone (CS/stPCL) cryogels and hydrogels were synthesized using the same urethane/urea cross-linking chemistry, allowing the effect of the fabrication method and internal structure to be directly compared. Scanning electron microscopy revealed that the cryogel had a highly interconnected macroporous network, whereas the hydrogel had thick pore walls. These structural differences strongly affected the material performance. The cryogel exhibited a higher equilibrium swelling ratio and better Cu-(II) removal efficiency, owing to its open, accessible pore structure, which facilitated Cu-(II) transfer. In contrast, the hydrogel showed faster initial swelling but a lower final swelling ratio because its pore walls were thicker. Although Cu-(II) adsorption in both materials followed a physisorption mechanism, their kinetic and isotherm behaviors were clearly different. This finding highlights that the pore structure is as important as chemical composition in designing effective adsorbents. In addition, both materials were biodegradable in soil, which supported their potential as sustainable materials for environmental remediation.