Kinetics and thermodynamics of uranium(VI) and thorium(IV) ions adsorption from aqueous solutions onto nanokaolinite

In this work, the adsorption of uranium(VI) and thorium(IV) onto naturally occurring nanokaolinite from their aqueous solutions was studied for the first time. Nanokaolinite was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Nanokaolinite appears as nanoplatelets with randomly oriented and heterogeneous sizes of individual nano-thin plates or thick plates. The effects of pH, mass, time, temperature and metal ion concentration on the adsorption of U(VI) and Th(IV) onto nanokaolinite were studied. The kinetic studies show that the adsorption by nanokaolinite follows a pseudo-second-order kinetic model. Adsorption isotherm experiments for nanokaolinite were expressed by Langmuir, Freundlich and Dubinin–Radushkevich (D–R) models at three temperatures (25.0 °C, 35.0 °C and 45 °C). The results showed that the adsorption of U(VI) and Th(IV) onto nanokaolinite correlated better with the Langmuir model and adsorption capacities at 25.0 °C were 2.96 and 8.23 mg/g for U(VI) and Th(IV) onto nanokaolinite, respectively. Thermodynamic parameters such as ΔG°, ΔH° and ΔS° indicate that the adsorption of U(VI) and Th(IV) onto nanokaolinite was spontaneous, endothermic and involved increasing in randomness. Recovery of U(VI) and Th(IV) ions from nanokaolinite was done by using 1.0 and 0.1 M HNO₃. The best percent recovery for U(VI) is 80.4% and Th(IV) is 64.2% were found within the first recovery stage using 0.1 M HNO₃.