Poly(vinyl pyrrolidone-co-acrylamide)-Based Hydrogels for Methyl Orange Adsorption: Equilibrium Analysis and Isotherm Modeling

The removal of synthetic dyes from industrial wastewater remains an important challenge in water treatment. In this work, poly(vinyl pyrrolidone-co-acrylamide) hydrogels were synthesized by free-radical polymerization using 2–6 wt% N,N′-methylenebisacrylamide (MBA) as a crosslinking agent and evaluated for methyl orange (MO) adsorption. The hydrogels were characterized by attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Their swelling behavior was studied at different temperatures, and adsorption experiments were carried out at 25 °C using different hydrogel dosages. Equilibrium data were fitted to the Freundlich, Sips, and Flory–Huggins (FH) models by nonlinear regression. The FH model was fitted using weighted least squares, while model comparison was performed using the corrected Akaike information criterion (AICc). Swelling was highest at 25 °C and decreased as the MBA content increased. The thermal analyzes showed similar behavior among the hydrogels, with glass transition temperatures close to 182 °C. MO removal increased rapidly at low adsorbent dosages and then tended toward a plateau. H18, containing 5 wt% MBA, showed the highest Freundlich affinity constant KF=1.4221 (mg/g) (L/mg)0.4928. This result indicates that an intermediate crosslinker content provides suitable network accessibility while maintaining the hydrogel structure. Freundlich gave the best statistical fit for most hydrogels, whereas Sips performed better for H16. Overall, the results indicate that the amount of crosslinker affects both swelling and MO adsorption.