Uranium Release from Boom Clay in Bicarbonate Media

The release of natural uranium from Boom Clay was studied to better understand the in situ retention mechanisms. Batch leaching experiments suggested that natural uranium is present in colloidal form at the bicarbonate concentration prevailing in Boom Clay. At higher bicarbonate concentration level, uranium was present predominantly as dissolved species indicating a destruction of colloids and a formation of uranium carbonate complex. The in situ uranium concentration of the pore water was found being 2 to 3 orders of magnitudes lower than the one measured by the batch techniques. These results illustrated that the batch techniques may cause a detachment of uranium colloids from clay surfaces when Boom Clay is suspended, agitated, and mechanically perturbed. These laboratory attainable colloids are however not mobile in situ because of the high compaction of Boom Clay. Due to the presence of colloids, a laboratory derived partitioning coefficient is not equivalent to and cannot simply be converted to the distribution coefficient Kd defined in a solute transport equation and therefore not applicable in predictive calculations.