An Enhanced Study on Adsorption of Al(iii) onto Bentonite and Kaolin: Kinetics, Isotherms, and Mechanisms

In order to remove aluminum ions in bauxite processing wastewater, two natural clay minerals (bentonite and kaolin) were used as adsorbents. The adsorption behaviors including kinetics and isotherms were studied at much broader pH range, higher temperature, and higher Al(III) concentration by batch experiments. The adsorption mechanisms were investigated by optical microscope, zeta potential testing, and XRD analysis. The results show that Al(III) adsorption is strongly pH dependent and increases clearly with increasing pH. The pseudo-second-order kinetic model can best describe the adsorption process, and the intra-particle diffusion was not the sole rate-controlling step. The Langmuir-Freundlich model can fit best the adsorption equilibrium data, and the maximum capacities of Al(III) adsorbed on bentonite and kaolin reach to 8.52 mg g(-1) and 0.58 mg g(-1), respectively, at 35 degrees C. Thermodynamic studies display that the adsorption of Al(III) onto clay minerals is non-spontaneous. The adsorption of Al(III) onto bentonite is endothermic and causes an increase in the entropy, while kaolin is exothermic and causes a decrease in the entropy. The interaction mechanisms of Al(III) with bentonite and kaolin involve electrostatic attraction, cation exchange, surface complex, and precipitation. Bentonite as adsorbent has more potential to remove Al(III) than kaolin.