Modeling the Kinetics, Equilibrium and Thermodynamics of Poly(acrylamide)/Sodium-Bentonite for Enhanced Phenol Removal from Aqueous Media

Abstract

A composite based on polyacrylamide and Na-bentonite, designated as PAAm/Na-bentonite was prepared by in-situ radical polymerization using ammonium persulfate as an initiator and bis-acrylamide as a crosslinker. The structural and morphological features were examined by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunner − Emmet − Teller measurements. The results showed PAAm intercalation in the Na-bentonite layers. Equilibrium, thermodynamics, and kinetic studies were conducted by considering the effects of pH, initial phenol concentration, contact time and temperature. The zeta potential of the PAAm/Na-bentonite composite was calculated to understand the mechanism of phenol adsorption onto our product. The pollutant uptake behavior was determined by UV-Vis spectrophotometry. Adsorption results showed that PAAm/Na-bentonite composite showed a maximum adsorption of 160.3 mg.g−1 of 30 mg.L−1 phenol solution at pH 6 after 15 min of adsorption using 4 g.L−1 of the adsorbent. The presence of acrylamide modified the surface characteristics of the Na-bentonite and offered more adsorption sites as confirmed by XRD and SEM. Equilibrium modeling of the phenol removal process was carried out using Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D–R) adsorption isotherms. The equilibrium adsorption data were well fitted with the Langmuir model. In addition, the kinetics of adsorption were best described by a pseudo-second-order expression rather than a first-order model. The interactions between phenol molecules and adsorbent were explained by electrostatic as well as hydrogen bonding interactions, as confirmed by the pseudo-second-order kinetic model. A model for the interactions between the composite and the phenol molecules is proposed. The negative values of Gibb’s free energy (ΔG°) confirmed that the process was spontaneous. The positive value of change in entropy (ΔS° = 0.922 JK−1mol−1) suggests that the randomness was increased at the solution/solid interface. We suggest, the results indicate that PAAm/Na-bentonite presents a significant potential as an adsorbent material for phenol removal from aqueous solutions.