Preparation and Characterization of Anionic Composite Hydrogel for Dyes Adsorption and Filtration: Non-linear Isotherm and Kinetics Modeling

An anionic, low-cost, and environmentally friendly composite PVA/Agar/Bentonite (PAB) hydrogel was prepared via self-assembly method, then evaluated as an adsorbent for both cationic methylene blue (MB) and anionic Congo red (CR) dyes using static (batch adsorption) and dynamic (column filtration) procedures. The structural/textural and morphological features of the hydrogel were investigated by X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, zeta potential measurements and scanning electronic microscopy. The nonlinear forms of first-order and second-order kinetic model, and nonlinear forms of Langmuir–Freundlich, Redlich–Peterson, Langmuir and Freundlich isotherm model were used to study the equilibrium adsorption. The adsorption kinetics of MB and CR dyes onto composite hydrogel followed the nonlinear form of pseudo-second-order model with the adsorption capacity of 107.45 and 42.05 mg/g, respectively. The equilibrium data were fitted well to the nonlinear Langmuir–Freundlich model for MB dye and nonlinear form of Langmuir model for CR dye, which were confirmed by R2, MSE and EABS values. Using the column purification method, the composite hydrogel PAB promotes the removal of the MB and CR pollutants by maintaining the concentrations of the filtrate solutions below 0.1 mg/L, even when the volume of the dye reaches 20 mL.