Reusable and Recyclable pH-Responsive Hydrogel Particles for Acid/Base-Regulated Adsorption/Desorption Switching of Anionic Azo Dyes

Abstract This study demonstrates the rational design of high-performance, pH-responsive hydrogel particle adsorbents synthesized via precipitation polymerization for advanced water remediation. An N,N-dimethylaminoethyl methacrylate unit was selected as the cationic functional group in the hydrogel particles for the removal of anionic azo dyes. The engineered three-dimensional polymeric network exhibited excellent adsorption efficiencies and rapidly reached adsorption equilibrium via electrostatic interactions on the adsorbent interface. Notably, mild acid/base stimuli of the hydrogel particles enabled switchable adsorption/desorption and recycling of the adsorbents with a removal efficiency of approximately 100%, which was maintained over 10 cycles. Increasing the molar composition of N,N-dimethylaminoethyl methacrylate from 20 to 60% increased the maximum adsorption capacity from 282 to 904 mg/g. The adsorption process for the anionic dyes fitted well to the pseudo-second-order kinetic model and the Langmuir isotherm model. The sorption behavior was predominantly driven by reversible, pH-dependent electrostatic associations with ion pairing between the adsorbent amino cation and adsorbate sulfonated anion. The rapid adsorption/desorption through acid/base stimuli based on pH switching provided high selectivity for anionic dyes in a mixed solution with cationic dyes. Furthermore, the adsorbents demonstrated superior removal efficiencies for various colored dyes in multicomponent contaminated solutions. These findings highlight the potential of acid/base-responsive hydrogel particles as robust, smart interfacial platforms for rapid, efficient, and selective separation processes.