Poly(Acrylic Acid-Co-Acrylamide) Hydrogel-Enabled Removal of Crystal Violet from Water: Adsorption Isotherms, Interaction Mechanisms, and Process Optimization

Abstract

The present study investigated the adsorption behavior of crystal violet (CV) from aqueous solution using poly(acrylic acid-co-acrylamide) [P(AAc-co-AAm)] hydrogel as an adsorbent. Batch adsorption experiments were conducted at 30°C using different initial CV concentrations to evaluate adsorption kinetics and equilibrium behavior. The hydrogel exhibited rapid CV uptake during the initial contact period, followed by a gradual increase in adsorption until equilibrium was approached, indicating progressive occupation of available adsorption sites. Kinetic analysis was performed using first- and second-order models, while equilibrium data were evaluated using Langmuir, Freundlich, and Temkin isotherms. Among the equilibrium models, the Freundlich isotherm provided the best fit to the experimental data (R² = 0.9631), followed by the Temkin (R² = 0.9313) and Langmuir (R² = 0.8988) models. The Freundlich constant (KF = 26.26 L g⁻¹) and adsorption intensity parameter (n = 1.844) indicated favorable adsorption of CV onto the hydrogel. The Langmuir model estimated a maximum adsorption capacity of 172.41 mg g⁻¹. Overall, the results demonstrated that P(AAc-co-AAm) hydrogel possesses appreciable adsorption capacity for CV and that its adsorption behavior is predominantly associated with heterogeneous surface sites and variable adsorbate–adsorbent interactions. The findings highlight the potential of P(AAc-co-AAm) hydrogel as an effective polymeric adsorbent for the treatment of dye-contaminated aqueous systems.