Equilibrium, kinetic and thermodynamic studies of Na+ adsorption from aqueous solutions on homo-ionized K- and NH4-kaolin

Egyptian kaolin clay (UnK) was homo-ionized with K⁺and NH₄⁺ to obtain K-K and N-K, respectively. The analysis of the molar leaching solutions of KCl, NH₄Cl or HCl revealed anoriginal cation exchange capacity of UnK up to 6.08meq per 100 g. The adsorption parameters of Na⁺ on K-K and N-K were examined in aqueous solutions. The equilibrium adsorption data were evaluatedusing Langmuir, Freundlich adsorption and Dubinin–Radushkevich-Kaganer isotherm models. Freundlich model was the best that described the adsorption process and the maximum capacities (q_max) are found to be enhanced to 75.6 and 17.5 mg g^-1, respectively. The experimental data fitted well the pseudo-second-order kinetics model. The thermodynamic parameters such as standard Gibbs free energy (∆G^ads), enthalpy (∆H^ads) and entropy changes (∆S^ads) for the adsorption process were calculated. The negative values of the Gibbs free energy confirm that the adsorption processes are spontaneous and thermodynamically favorable while the negative value of ∆H^ads supports the exothermic physicochemisorption of the adsorption process. Both homo-ionized kaolin samples are promising cheap adsorbents for lowering the sodicity of soil while gradually exchanging valuable nutrients for plants