Neoformation of DLH During Impregnation of α-Alumin

Abstract
The adsorption of Co(II) or Ni(II) ammine complexes from aqueous solutions onto α-alumina at neutral pH and ambient temperature was investigated. The formation of coprecipitates including Al (III) ions extracted from the support was demonstrated by EXAFS for contact times and Ni or Co loadings higher than 0.5 h and about 2.0 wt %, respectively. The EXAFS technique makes it possible to distinguish the Ni or Co hydroxides and basic nitrates from coprecipitates with a double layer hydroxide (DLH) structure. Not only is EXAFS shown to be sensitive to the presence of aluminum in the coprecipitates, but in most cases, the M(lI)/Al(III) ratio (M= Ni or Co) in the supported coprecipitates can be estimated. Thus, alumina should not be considered systematically as inert even at pH values close to its isoelectric point. It is suggested that a dissolution-precipitation mechanism is involved and that the rate of alumina dissolution is promoted by adsorbed Ni(II) or Co(II) ions. Site-binding models have a considerable value for the early stages of impregnation, whereas a geochemical approach involving surface rehydration and coprecipitation have probably a greater validity for the later stages.