Heterogeneities in Adsorption from Aqueous Solution — An Example of the Effect of Surface Coverage

Studies of the interaction of uranyl species with the surface of quartz are reported. Two approaches were applied. These were (i) the classical static batch adsorption approach in which only the forward adsorption uptake reaction is usually studied and (ii) a dynamic column approach in which the retention of a solute during its migration through a porous medium is investigated and where adsorption and desorption are both relevant for the final breakthrough curve. In this paper, we present the results of a column study at pH 4. Under these conditions, the interference of the carbonate ion (via the formation of dissolved or adsorbed uranyl-carbonato species), the effects of quartz surface charge (quartz/silica is virtually uncharged under these conditions) and the potential effects of quartz dissolution on uranyl speciation (via the formation of dissolved or adsorbed uranyl-silicato species) are minimised. The reported batch adsorption results are reasonably predicted by a model which under similar conditions successfully described batch and column data on a quartz sample with grains smaller than the quartz particles used in the present study by a factor of ca. 25. However, this model was incapable of describing the breakthrough behaviour of uranyl species. This failure was attributed to the heterogeneity of the quartz particles used, which may be more important for column breakthrough than for batch adsorption data.