Adsorption behavior of U(VI)/Th(IV) by acid-leached red mud: A comparative study

Acid-leached red mud, a type of inorganic-adsorptive by-product of bauxite Bayer process via acid leaching, was used for the removal of U(VI)/Th(IV) from aqueous solutions. Variables of the adsorption such as contact time, temperature, solution pH, initial concentration and dose of acid-leached red mud were investigated. The results indicated that the adsorption is strongly affected by dosage, the solution pH, contact time and initial concentration. The isothermal data were fitted with both Langmuir and Freundlich equations, but the data fitted the former better than the latter. A pseudo-first-order kinetic model and pseudo-second-order kinetic model were used to describe the kinetic data, but the pseudo-second-order kinetic model was the better. Thermodynamic parameters such as enthalpy (Delta H-o), entropy (Delta S-o) and free energy change (Delta G(o)) were calculated, and the negative Delta G(o) values of U(VI)/Th(IV) at different temperatures confirmed the adsorption processes were spontaneous.