Interaction Processes of Tetravalent Actinides in the System Humic Acid / Quartz Sand / Solution

In the present work the influence of HA on the interaction of tetravalent actinides onto quartz sand was investigated in batch and column experiments. HA can affect the sorption of actinides onto mineral surface as a complexing ligand for cations in solutions, and as an adsorbent by modifying the properties of the mineral surface.
Batch experiments were performed with Th(IV) as stable tetravalent actinide element. Column experiments were carried out with uranium which is less stable in the tetravalent state under laboratory conditions. Results of the U(IV) migration were compared to the U(VI) migration.
HA shows a pH-dependent influence on the Th(IV) sorption onto quartz sand. At pH < 4 HA may immobilize Th(IV) due to HA sorption and/or precipitation onto the surface. Between pH 4 and pH 5 the Th(IV) sorption is decreased in the presence of HA which can be attributed to the formation of dissolved Th(IV)-humate complexes. For higher pH values a comparable effect is assumed. The experimental proof was not possible by the performed batch experiments.
From the column experiments it can be concluded that HA affects the migration of both U(IV) and U(VI). In the presence of HA U(VI) is significantly mobilized. A similar effect is supposed for U(IV). This is in accordance with the results of the batch experiments.