Characteristics of isothermal adsorption and desorption of aluminum ion to/from humic acids

The adsorption and desorption characteristics of Al3+ to/from humic acids at different pH, ionic strength, and temperature were studied by the C-25 glucosan-gel chromatography method. The results showed that the maximum adsorption amount (Qmax) and adsorption constant (k) increased, whereas, the absolute value of standard thermodynamic molar free energy change (ΔG0m) decreased with the increase of pH at constant ionic strength and temperature. With ionic strength increasing from 0 to 0.15 mol/L, Qmax and k increased and the absolute value of ΔG0m decreased at constant pH and temperature. High temperature was unfavorable for the adsorption reaction, as indicated by the dramatic decrease of Qmax and the absolute value of ΔG0m with an increase in temperature. The standard thermodynamic molar free energy change (ΔG0m) and the standard thermodynamic enthalpy change (ΔH0m) of the adsorption reaction were both negative, suggesting that adsorption reaction was spontaneous and exothermic. The desorption rate of HA-Al3+ complex accelerated with the decrease of pH, and a significant linear relationship could be obtained between pH and the desorption rates of Al3+ from humic acids. These results demonstrated that the Al3+ adsorption reaction was a “biphase” reaction, and adsorption occurred at both the interior and exterior adsorption sites of humic acids.