Halloysite nanotubes: an eco-friendly adsorbent for the adsorption of Th(IV)/U(VI) ions from aqueous solution

In this study, the halloysite nanotubes was characterized and the adsorption of Th(IV) and U(VI) on halloysite nanotubes was investigated as a function of contact time, adsorbent dosage, pH, ionic strength, initial U(VI) concentration and temperature using batch method. Meanwhile, the adsorption mechanism of uranium and thorium on halloysite nanotubes was discussed. The microscopic results displayed that halloysite nanotubes was tubular morphology and highly porous and high specific surface area of ca. 55.65 m 2 /g. The adsorption results showed that the adsorption of U(VI) and Th(IV) on halloysite nanotubes followed pseudo-second-order kinetic model. The strong pH dependent adsorption of U(VI) and Th(IV) displayed their strong surface complexation with the surface of halloysite nanotubes. The adsorption of Th(IV) and U(VI) increased with elevating temperature and was an endothermic and spontaneous process. The adsorption isotherms of Th(IV) and U(VI) can be better described by Freundlich and D–R model. The effect of ionic strength on the adsorption of thorium on halloysite nanotubes was much greater than that on uranium(almost unaffected), which suggested the adsorption of Th(IV) on halloysite nanotubes was most probably based on the formation of inner-sphere complexes, while that of U(VI) was based on the formation of outer-sphere complex on the edge surfaces. Th(IV) and U(VI) adsorption–desorption isotherm on halloysite nanotubes indicated adsorption process was irreversible. The selectivity order of adsorption by the halloysite nanotubes was Th(IV) > U(VI). The higher adsorption efficiency of the halloysite nanotubes for Th(IV) could be utilized for selective separation of Th(IV) from U(VI) aqueous with pH 4.1–4.3. The novel and environmentally friendly adsorption material is feasible to extract thorium from waste aqueous solution.