The use of electromigration as a qualitative technique to study the migration behaviour and speciation of uranium in the Boom Clay

Under the geochemical conditions prevailing in situ in the Boom Clay Formation (pH, E_h ...), calculations predict that U(OH)₄ is the dominant uranium species present in the interstitial water and the concentration is solubility limited. However the boundary of the domain where the non solubility limited UO₂(CO₃)₃^4- species dominates is very close. It is therefore of prime interest to know the correct speciation of uranium during the migration process. Electromigration was used as technique with the advantage that it can provide information on the speciation because the movement of the species towards the electrodes depends on its charge and speciation. Electromigration experiments have been performed with preconditioned ²³³UO₂(CO₃)₃^4- sources, starting from the hypothesis that this species should migrate without retardation towards the anode. Despite relatively long electromigration times, sufficient to displace strong retarded tracers, no displacement of the migration profile towards the anode was observed. All ²³³U remained near the source position, but the electropherograms clearly showed the presence of species moving towards the cathode. This indicates the presence of neutral or positively charged uranium species. These electropherograms are interpreted as a change in uranium valence state: reduction of UO₂(CO₃)₃^4- and precipitation of U(IV) oxy-hydroxides near the source position. The solubility limited species, U(OH)₄ (aq), are carried with the pore water towards the cathode. The electromigration experiments indicate, in support of the speciation calculations, that the dominant migrating U-species is probably the solubility limited U(OH)₄.