Anion Adsorption Behavior of Rare Earth Oxide Hydrates

Abstract

The study of the anion adsorption on various hydrous metal oxides in the aqueous phase showed that the adsorption of fluoride ion on the hydrous rare earth oxides was excellent. The results were due to the relatively small ionic potential and strong basisity of rare earth ions. Therefore, they had strong tendency to dissociate OH group into hydroxy ion (Table 1, Fig.1). The adsorption on hydrous ceric oxide, which was prepared by drying at 50°C or -80°C under vacuum and consisted of microcrystallines having disordered structure (Fig.3), was studied in detail. The specific surface area of hydrous ceric oxide decreased with the elevation of drying temperature, due to the crystallization of microcrystallines (Table 2). The adsorbability decreased with the decrease of specific surface areas (Fig.5). As for monovalent ions, the adsorption decreased in the following order: F^->Cl^->NO₃^->BF₄^->Br^->Cl0₄^- (Table 3). It was suggested that the large ions suffe red from a strong steric effect because of the limited available space of the ion-exchange site. Hydrous ceric oxide was found to have the adsorbability towards F^-, HPO₄^2-, As(III) and B(OH)₄^- (Fig.7). The adsorption selectivity of coexisting anions was in the following order: F^->HPO₄^2->SO₄^2->Cl^-, Br^-, NO₃^- (Table 4). The extremly high selectivity for F^- was a ttributable to the smaller ion radious of F^- compared to that of OH^-. An infrared absorption peak (1500 cm^-1), which is attributable to the OH groups of hydrous ceric oxide, disappeared by the adsorption of anions (Fig.8). The ion exchange between the OH groups of hydrous ceric oxide and anions was confirmed to be reversible.