Sorption of Uranyl Ions on Hydrous Silicon Dioxide

Sorption of uranyl ions on SiO[sub 2].[chi]H[sub 2]O (silica gel) is investigated in absence and in presence of carbonate as function of pH. The curves obtained are very similar to those observed for sorption of uranyl ion on TiO[sub 2].[chi]H[sub 2]O, indicating the dominating influence of the uranium species in solution. Between pH 2 and 5 the sorption ratio R[sub s] increases with hydrolysis of uranyl ions (formation of UO[sub 2]OH[sup +]), around pH 7 it is nearly independent of pH, and at higher pH it decreases again. The equilibrium constants are calculated for these ranges. In presence of carbonate R[sub s] decreases drastically above pH 6, due to the formation of carbonato complexes in solution. Sorption of uranyl ions on SiO[sub 2].[chi]H[sub 2]O, on TiO[sub 2].[chi]H[sub 2]O, and on cryst. SiO[sub 2] and Al[sub 2]O[sub 3] is compared. The problems of 'surface complexation' modelling are discussed. (orig.).