Chitosan/oxidized sodium alginate/Ca2+ hydrogels: Synthesis, characterization and adsorption properties

Hydrogels are widely used in various fields due to their exceptional water retention and biocompatibility. However, enhancing their mechanical strength and thermal stability remains a challenge. In this study, a hydrogel by oxidatively modifying sodium alginate and cross-linking it with chitosan, while simultaneously introducing Ca2+ to form "calcium bridges" through coordination with carboxyl groups, thereby enhancing the hydrogel's performance. Structural characterizations using Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the cross-linking between amino and aldehyde groups to form Schiff base bonds and the coordination between carboxyl groups and calcium ions. Further analyses using Scanning Electron Microscopy, texture analysis, and rheology indicated that the addition of Ca2+ resulted in a denser network structure of the gel, thereby improving its mechanical properties and thermal stability. Specifically, an increase in Ca2+ concentration to 2% doubled the hydrogel's hardness. Additionally, the study examined the adsorption capacity of the hydrogel for methylene blue. With a 2% concentration of Ca2+, the adsorption capacity of the hydrogel increased to 2.6 times its original value. The adsorption kinetics and isotherm results indicate that the CS/OSA/2%Ca2+ hydrogel's adsorption behavior aligns with the pseudo-second-order kinetic model and Freundlich isotherm model.