Temperature-Tunable Adsorption of Methylene Blue by Poly(AAc-Co-AAm) Hydrogels: Swelling Behavior, Kinetics, and Isotherm Studies

Abstract

In this study, poly(acrylic acid-co-acrylamide) [poly(AAc-co-AAm)] hydrogels were synthesized via free radical polymerization using acrylic acid (AAc), acrylamide (AAm), and N, N′-methylenebisacrylamide (MBA) as a crosslinker. The synthesized hydrogels were characterized by FTIR, 1H-NMR, TGA, and SEM to confirm structural integrity, crosslinking, and thermal stability. Swelling behavior was evaluated at varying temperatures (30°C, 35°C, and 45°C) and pH (3.0–8.0). Maximum equilibrium swelling was observed for poly(AAc-co-AAm)3 due to higher hydrophilic group content, reaching saturation within 3 h. Swelling increased with both temperature and pH due to hydrogen bond disruption and ionic repulsion. TGA demonstrated a three-step decomposition, indicating stability up to ~180°C. Adsorption studies were performed by using methylene blue (100 mg/L) with 12 mg of hydrogel at pH 8.0. Optimal dye uptake occurred within 8 h. Adsorption increased with increase in dye concentration (20–100 mg/L), and the hydrogel showed enhanced adsorption at higher pH due to deprotonation of carboxylic groups. Kinetic analysis confirmed the pseudo-second-order model (R2 > 0.995) best described the process, indicating chemisorption. Freundlich isotherm (R2 = 0.998) best fit equilibrium data, suggesting multilayer adsorption on a heterogeneous surface. These findings validate the hydrogels as efficient, pH-sensitive, and thermo-responsive adsorbents for dye removal applications. © 2025 Wiley Periodicals LLC.