Probing acid/base chemistry and adsorption mechanisms of hydrolysable Al(III) species with a clay system in aqueous solution

The acid/base chemistry of hydrolysable Al(III) species with a clay (bentonite and kaolin) system was investigated at 35 °C in the expanded solution pH range from 1 to 9. The adsorption capacity (qe) and mechanism of Al(III) species on clays were examined by means of UV-Vis spectra, optical microscope, zeta potential testing, SEM, XPS and XRD analysis. The results demonstrate that the qe of Al(III) increases with increasing solution pH, and the maximum adsorptions of bentonite and kaolin are 13.64 mg g−1 and 1.06 mg g−1, respectively. Each Al(III) species offers a different contribution to the total adsorption capacity. The cation Al3+ is the dominant species below pH 4 and the anion Al(OH)4− is the main species above pH 6. The solution pH values change in the single clay or hybrid clay/Al(III) solution because of the acid–base dissolution and competitive adsorption of H+/OH− with Al(III) species. The particle sizes (d80) of the clays after adsorption at different pH were first associated with thermodynamics. The zeta potential variation of clays was first connected with the total charge numbers of Al(III) species in solution. Zeta potential, XPS and XRD studies indicate that charge neutralization and ion exchange dominate the adsorption process at lower pH and surface complexation and precipitation at higher pH.