Sorption characteristics of hydrous bismuth-thorium mixed oxides

Abstract

Hydrous mixed oxides having better anion sorption properties could be prepared by substituting the host metal ion in the hydrous oxide with a cation of higher valency. Hydrous bismuth-thorium mixed oxide (BT), which is one such mixed oxide, of different compositions have been prepared by coprecipitation using NaOH and are characterised for their chemical composition , crystallinity, surface area and the nature of thc surface groups by analysing the water sorption isotherms using the D'Arcy and Watt equation . The intrinsic dissociation constants; <i style="mso-bidi-font-style:normal">p</i><i>K*_ai </i>,and the point of zero charge, PZC, have been evaluated by analysing the <i style="mso-bidi-font-style:normal">p</i>H-titration curves in acid (in presence of CIO^-_4, NO₃^-, and Cl^- and alkaline (Na⁺) media. BT samples having more than 40 wt% of Th could not be prepared. With increase in the content of Th in BT, the final product becomes less crystalline and the surface area increases; the amount of strong primary water sorption sites and the interaction of water with these primary sites also increase. An enhancement in the anion sorption compared to the expected value, based on the sorption characteristics of the component hydrous oxides and the composition of BT, has been observed with a maximum around 20 to 30 wt% of Th. A broad minimum in <i>pK</i><i>*_ai </i>and PZC has been observed between 10 to 30 wt% of Th. The inability to prepare hydrous bismuth-thorium mixed oxides over the entire range of composition, the variation of the surface chemical and surface charge characteristics with the composition of BT have been explained on the bas is of the structural characteristics of the component ox ides (that is, hydrous bismuth and thorium oxides) and the coordinati on tendencies of the central cation (that is, bismuth and thorium).