Hybrid Microhydrogels with Sulfonic Groups for the Removal of Methylene Blue from Aqueous Solutions

In this study, sulfonic-functionalized hybrid microhydrogels (MHGs-HF) were developed from alginate via ionic crosslinking by dropwise addition, followed by functionalization at different reaction times (3–12 h). The incorporation of sulfonic groups was confirmed by FTIR and thermal analysis, revealing changes in the thermal stability and residual fraction of the material. The microhydrogels achieved 2000% swelling. Methylene blue (MB) adsorption studies performed using an initial MB concentration of 40 mg L−1 showed that MHGs-HF achieved removal efficiencies of up to 80–90% under pH 7.0 and 11.0, while increasing ionic strength significantly reduced MB removal efficiency, particularly in the presence of divalent ions, highlighting the importance of electrostatic interactions in the adsorption process. The experimental results analyzed through the DoE methodology indicated that the optimal conditions for MB removal were 129 ppm MB, an adsorbent dose of 85.5 mg, and a reaction time of 19.7. The kinetic analysis revealed a multistage process, with a rapid initial stage dominated by surface adsorption followed by intraparticle diffusion, reaching equilibrium in 14 h. The experimental data fit the pseudo-first-order model, and the Dubinin–Radushkevich model indicated that the process is governed by physisorption. The equilibrium isotherms were described using the Freundlich model, suggesting a heterogeneous surface with multiple active sites. Taken together, these results position MHGs-HF as versatile and efficient materials for the removal of cationic dyes in complex aqueous media.