P. Michard et al., SORPTION AND DESORPTION OF URANYL IONS BY SILICA-GEL - PH, PARTICLE-SIZE AND POROSITY EFFECTS, Microporous materials, 5(5), 1996, pp. 309-324
Uranium sorption by silica gel was shown to be very sensitive to pH; t
he optimum pH range is ca. 5-5.5, which coincides with the appearance
of hydrolysed forms of uranyl. A two-phase surface mechanism is propos
ed: first the adsorption of hydrolysed forms, which precipitate later
on the surface of the oxide, then the removal of residue at the new so
rbing surface by adsorption or precipitation. Particle size and pore c
haracteristics 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 coeffic
ients of the order of 10(-7)-10(-5) and 10(-8)-10(-7) m min(-1), respe
ctively. 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 maj
or factor in governing the uptake rate. The desorption of silica gel w
as also studied, particularly concerning the nature and concentration
of the elution agent. Acid solutions are most effective at removing ur
anium. Using a batch system the number of moles of acid needs to be ei
ght times greater than those of uranium in order to obtain a desorptio
n efficiency higher than 90%. In dynamic desorption, on the other hand
, 0.5 M hydrochloric acid gives both total desorption and optimal meta
l recovery. Eluate concentrations as high as 100-200 gl(-1) can be obt
ained. Furthermore, when seven sorption-desorption cycles were carried
out using the column system, removal performances were maintained and
the sorbent could be re-used.