Enhanced adsorption behaviors of Co²⁺ on robust chitosan hydrogel microspheres derived from an alkali solution system: kinetics and isotherm analysis

Chitosan hydrogel microspheres derived from the LiOH/KOH/urea aqueous system demonstrate great characteristics of high mechanical strength, relative chemical inertness, renewability and 3-D fibrous network, making them promising functional supports. This work aims to investigate the tunable Co²⁺ adsorption behaviors on these robust chitosan microspheres in detail, providing the theoretical basis for optimizing the preparation procedure of chitosan microspheres supported Co₃O₄ catalysts in the future. The experimental results revealed that the fabricated original chitosan microspheres with more extended chain conformation could display enhanced adsorption capacity for Co²⁺ at determined concentration both in water and alcohol solutions, which is about 2-7 times higher than that of the conventional chitosan hydrogel microspheres prepared from the acetic acid solution. The kinetic experiments indicated that the adsorption process in water solution agreed with the pseudo-second-order kinetic equation mostly, while the chemical and physical adsorptions commonly contribute to the higher Co²⁺ adsorption on chitosan microspheres in alcohol solution. Moreover, in both cases, the film diffusion or chemical reaction is the rate limiting process in the initial adsorption stage, and the adsorption of Co²⁺ on chitosan microspheres can well fit to the Langmuir isotherm. Thermodynamic analysis demonstrated that such adsorption behaviors were dominated by an endothermic (ΔH° > 0) and spontaneous (ΔG° < 0) process.