IMPREGNATION OF GAMMA-ALUMINA WITH NI(II) OR CO(II) IONS AT NEUTRAL PH - HYDROTALCITE-TYPE COPRECIPITATE FORMATION AND CHARACTERIZATION

The adsorption of Co(II) or Ni(II) ammine complexes from aqueous solut ions onto gamma-alumina at neutral or near neutral pH (7 < pH < 8.2) a nd ambient temperature is investigated as a function of the ion concen tration in the impregnating solutions, the contact and aging times, an d the drying conditions. The formation of coprecipitates including Al( III) ions extracted from the support is demonstrated by EXAFS and IR s pectroscopy for contact times and Ni or Co loadings higher than 0.5 h and about 2.0 wt %, respectively. The EXAFS technique makes it possibl e to distinguish the Ni or Co hydroxides and basic nitrates from copre cipitates with the hydrotalcite-type structure. In other words, EXAFS is shown to be sensitive to the presence of aluminum in the coprecipit ates. Furthermore, in most cases, the M(II)/Al(III) ratio (M = Ni or C o) in the supported coprecipitates can be estimated. Infrared spectros copy can also be used as a fingerprint of coprecipitate formation thro ugh lattice octahedral modes characteristic of the NiAl and CoAl hydro talcite-type compounds in the 400-450 cm(-1) region. Thus, alumina sho uld not be considered systematically as inert even at pH values close to its isoelectric point. Possible mechanisms responsible for the form ation of coprecipitates at pH values nonaggressive for alumina are dis cussed. Dialysis experiments leading to the observation of hydrotalcit e crystallites at a distance from alumina suggest that a dissolution-p recipitation mechanism is possibly involved and that the rate of alumi na dissolution is promoted by adsorbed Ni(II) or Co(II) ions. Three ph enomer a are suggested to occur at the same time at the oxide/water in terface; the adsorption of ions, as described by the site binding mode ls; the alumina dissolution, promoted by the adsorbed ions and kinetic ally limited; the coprecipitation of M(II) with Al(III) ions extracted from the support. Site binding models have a considerable value for t he early stages of impregnation, whereas approaches which use geochemi stry as a basis and involve surface rehydration and coprecipitate form ation have probably a greater validity for the later stages.