Effect of soil humic and fulvic acids, pH and ionic strength on Th(IV) sorption to TiO2 nanoparticles

Thorium is considered as a chemical analog of other tetravalent actinides. Herein, the sorption of Th(IV) on TiO2 in the presence or absence of soil fulvic acid (FA)/humic acid (HA) as a function of pH, ionic strength and FA/HA concentration has been studied by a batch method. The morphology was characterized by scanning electron microscopy (SEM). The results indicate that sorption of Th(IV) on TiO2 increases from 0% to ∼94% at pH 1∼4, and then maintains level with increasing pH values. Both FA and HA have a positive effect on Th(IV) sorption at low pH values and the contribution of FA on Th(IV) sorption is rather higher than that of HA at pH<4. The sorption is weakly dependent on the concentration of KNO3 in solution, but the cations K+, Na+ and Li+ influence Th(IV) sorption more obviously. The batch results indicate that the inner sphere complex formation is formed at bare surfaces or FA/HA-bound TiO2 particle surfaces. Results of SEM analysis show that the particle sizes of TiO2, Th–TiO2 and Th–HA–TiO2 colloids are quite different. Surface complexation may be considered as the main sorption mechanism.