Sorption and desorption of uranyl ions by silica gel: pH, particle size and porosity effects

Uranium sorption by silica gel was shown to be very sensitive to pH; the optimum pH range is ca. 5-5.5, which coincides with the appearance of hydrolysed forms of uranyl. A two-phase surface mechanism is proposed: first the adsorption of hydrolysed forms, which precipitate later on the surface of the oxide, then the removal of residue at the new sorbing surface by adsorption or precipitation. Particle size and pore characteristics have a limited effect on equilibrium concentration, but greatly influence sorption kinetics. A two-phase kinetic mechanism is proposed which gives external and intraparticle mass transfer coefficients of the order of 10(-7)-10(-5) and 10(-8)-10(-7) m min(-1), respectively. Both a mesoporous and a microporous silica gel were examined in order to determine the influence of pore size on sorption kinetics: the ratio between solute size and pore diameter appears to be the major factor in governing the uptake rate. The desorption of silica gel was also studied, particularly concerning the nature and concentration of the elution agent. Acid solutions are most effective at removing uranium. Using a batch system the number of moles of acid needs to be eight times greater than those of uranium in order to obtain a desorption efficiency higher than 90%. In dynamic desorption, on the other hand, 0.5 M hydrochloric acid gives both total desorption and optimal metal recovery. Eluate concentrations as high as 100-200 gl(-1) can be obtained. Furthermore, when seven sorption-desorption cycles were carried out using the column system, removal performances were maintained and the sorbent could be re-used.