Enhanced method for the removal of U (VI) and Th (IV) from aqueous solutions using chemically modified kaolinite

Jordanian kaolinite was treated with two phosphate modifiers: sodium polyphosphate (MK 1) and sodium hexametaphosphate (MK 2). Both raw and modified kaolinites were characterised by XRD and FTIR techniques. The adsorption behaviour of Th(IV) and U(VI) ions onto the two modified kaolinites were studied as a function of dosage, contact time, pH and temperature. The adsorption isotherm of both metal ions onto modified kaolinite was investigated using Langmuir, Freundlich and Dubinin���Radushkevich (D-R) models, which fits better to the Langmuir model. MK 1 kaolinite showed higher metal ion uptake than MK 2 kaolinite for both ions. The metal ion uptake of Th(IV) ions on both modified kaolinites was significantly higher than that of U(VI) ions. The % adsorption efficiency (73.8 and 69.0%) of Th(IV) ions on MK 1 and MK 2 was significantly higher than that of U(VI) ions (35.0 and 13.3%). The adsorption capacities for Th(IV) (188.7 and 9.6 (mg/g)) and U(VI) (53.5 and 15.3(mg/g)) were improved when MK 1 and MK 2 kaolinites were used at 25��C. The thermodynamic studies indicated that the removal of U(VI) and Th(IV) ions using both modified kaolinites is spontaneous and favourable at higher temperatures. The positive entropy values indicated that the adsorption process increased the disorder of the system. The kinetic study had indicated that the pseudo-second-order kinetic model fit the adsorption data, suggesting a chemisorption nature of the adsorption process. Binary system experiments under optimised conditions showed higher removal of Th(IV) ions compared to U(VI) ions. The desorption study showed that the elution of both metal ions increased as the concentration of HNO3 increased. Both the modified kaolinites were regenerated three times, a slight reduction of their adsorption efficiency was observed, and the presence of interferences has the same effect.