Carboxymethyl Cellulose-Based Hydrogels Containing 2-Acrylamido-2-methylpropanesulfonic Acid, Polyvinylpyrrolidone, and Variable Acrylic Acid Content Display pH-Responsive and Superlative Adsorption of Methylene Blue Dye

Carboxymethyl cellulose (CMC)-based hydrogels are attractive adsorbents for dye removal but are often limited by poor stability, weak pH responsiveness, and modest adsorption capacity. In this work, we developed CMC-based hydrogels termed CAPA by incorporating 2-acrylamido-2-methylpropanesulfonic acid (AMPS), polyvinylpyrrolidone (PVP), and varying proportions of acrylic acid (Ac). We sought to modulate the structural, surface, and adsorptive properties of the hydrogel by varying the proportion of Ac. BET analysis of the optimized CAPA-2 hydrogel revealed a surface area of 0.428 m²/g, a total pore volume of 2.68 × 10^–3cm³/g, and an average pore diameter of ∼25 nm. Zeta potential analysis showed strongly negative surface charges (−63.5 mV for CAPA-2), arising from deprotonated carboxyl and sulfonic groups. The mesoporous character of CAPA hydrogels and the presence of strong negative charges resulted in distinctive adsorption of methylene blue (MB) dye. The maximum adsorption capacity of 475 mg/g for MB by CAPA-2 is among the highest reported for CMC-based hydrogels. Adsorption of MB by CAPA hydrogels followed the Langmuir isotherm, indicating monolayer adsorption, and pseudo-first-order kinetics consistent with a physisorption mechanism involving hydrogen bonding, electrostatic, and hydrophobic interactions. CAPA hydrogels achieved high removal efficiencies for crystal violet (99%) and rhodamine B (97%), retaining strong performance in mixed-dye systems and showing good reusability over multiple cycles. This study demonstrates that varying the Ac content provides a simple and effective strategy to tune porosity, surface charge, and adsorption in CMC-based hydrogels. The CAPA hydrogels, with their high capacity, mesoporous structure, and reusability, are promising candidates for the efficient removal of dye from effluents and wastewater.