Double network self-healing hydrogels based on carboxyethyl chitosan/oxidized sodium alginate/Ca2+: Preparation, characterization and application in dye absorption

This paper presents the formation of a self-healing hydrogel prepared by carboxyethyl modification of chitosan and crosslinking with oxidized sodium alginate. Concurrently, the incorporation of Ca2+ facilitated the formation of "calcium bridges" through intricate coordination with carboxyl moieties, bolstering the attributes of the hydrogel. Various characterization methods, including scanning electron microscopy, texture analysis, and rheological measurements, demonstrated that the introduction of carboxyethyl groups resulted in a more compact hydrogel network structure and improved the hardness and elasticity. The addition of Ca2+ helped to further enhance the mechanical performance of the hydrogel and increase its thermal stability. Then, the adsorption capacity was also investigated, showing adsorption capacities of 46.17 mg/g methylene blue and 46.44 mg/g congo red for carboxyethyl chitosan/oxidized sodium alginate hydrogel, a four-fold increase for congo red versus chitosan/oxidized sodium alginate hydrogel. In addition, the adsorption behavior of CEC/OSA/2%Ca2+ hydrogel can be well described by pseudo-second-order kinetic model and Langmuir adsorption isothermal model. Compared to traditional hydrogels, CEC/OSA/2%Ca2+ hydrogel shows superior mechanical strength, enhanced thermal stability, and improved adsorption capacity, which can effectively adsorb not only methylene blue but also congo red. These advancements demonstrate our hydrogel's innovative properties.