Sorption characteristics of hydrous bismuth-thorium mixed oxides

Hydrous mixed oxides having better anion sorption properties could be prepa red by substituting the host metal ion in the hydrous oxide with a cation o f higher valency. Hydrous bismuth-thorium mixed oxide (BT), which is one su ch mixed oxide, of different compositions have been prepared by coprecipita tion using NaOH and are characterised for their chemical composition, cryst allinity, surface area and the nature of the surface groups by analysing th e water sorption isotherms using the D'Arcy and Watt equation. The intrinsi c dissociation constants; pK*(ai), and the point of zero charge, PZC, have been evaluated by analysing the pH-titration curves in acid (in presence of ClO4- 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 Tn in B T, the final product becomes less crystalline and the surface area increase s; 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 characterist ics of the component hydrous oxides and the composition of BT, has been obs erved with a maximum around 20 to 30 wt% of Th. A broad minimum in pK*(ai) and PZC has been observed between 10 to 30 wt% of Th. The inability to prep are hydrous bismuth-thorium mixed oxides over the entire range of compositi on, the variation of the surface chemical and surface charge characteristic s with the composition of BT have been explained on the basis of the struct ural characteristics of the component oxides (that is, hydrous bismuth and thorium oxides) and the coordination tendencies of the central cation (that is, bismuth and thorium).