Influence of pH, soil humic/fulvic acid, ionic strength and foreign ions on sorption of thorium(IV) onto γ-Al2O3

The sorption of Th(IV) onto γ-Al2O3 in the absence and presence of soil humic acid (HA)/fulvic acid (FA) was studied by batch technique. The influence of pH from 2 to 12, ionic strength from 0.01 M to 0.1 M KNO3, soil HA/FA concentrations from 2.5 mg/L to 17.5 mg/L, and foreign cations (Li+, Na+, K+) and anions (image, Cl−) on the sorption of Th(IV) onto γ-Al2O3 was also tested. The sorption isotherms of Th(IV) were determined at pH 3.50 (±0.02) and analyzed with the linear, Freundlich, and Langmuir sorption models, respectively. The results demonstrated that the sorption of Th(IV) onto γ-Al2O3 increases steeply with increasing pH from 2 to 4. HA/FA was shown to enhance Th(IV) sorption at low pH, but to reduce Th(IV) sorption at high pH. It was assumed that the significantly positive influence of HA/FA on Th(IV) sorption onto γ-Al2O3 at pH 2–4 is attributable to the strong surface binding of HA/FA on γ-Al2O3 and subsequently to the formation of ternary surface complexes such as triple bond; length of mdashSO–O–HA–Th or triple bond; length of mdashSO–O–FA–Th. The results also demonstrated that the sorption is strongly dependent on the concentration of HA/FA, and slightly dependent on ionic strength. The sorption of Th(IV) onto γ-Al2O3 was also dependent on foreign ions in solution under the experimental conditions.