Uranium(VI) and Humic Acid Sorption onto Phyllite and Ferrihydrite

We studied the sorption of uranium and humic acid onto phyllite and ferrihydrite both by steady-state experiments in the pH range 3.5 to 9.5 and by kinetic experiments at pH 6.5. Furthermore, the sorption of uranium onto ferrihydrite in the presence of humic acid was studied by extended X-ray absorption fine structure (EXAFS).
Phyllite was used as site-specific rock material because it is closely associated with uranium deposits of the former uranium mining areas in East Germany. Ferrihydrite is known to be formed as secondary mineral phase due to weathering of phyllite. Its sorption behavior was therefore studied and compared to that of phyllite. As humic acid, we applied both a natural humic acid (Kranichsee HA) and a 14C-labeled synthetic humic acid (14C-M1).
The steady-state experiments have shown that humic acid is strongly taken up by both solids over the entire pH range. The uranium sorption is affected both by the pH and by the presence of organic material.
The kinetic experiments have shown that sorption of uranium and humic acid onto phyllite and ferrihydrite is rapid. The initial uranium sorption rates depend on the sequence of addition of uranium and humic acid. For ferrihydrite the initial uranium sorption rate decreases with increasing amounts of uranyl humate complexes in solution. For phyllite the dependence of the uranium sorption rate on the sequence of addition of uranium and humic acid does not exhibit a clear pattern which is attributed to the complex nature of the phyllite rock. The results have shown that the amount of uranium sorbed on solids in equilibrium is determined by the total number of surface sites and by their affinity and accessibility to solutes.
The results of the steady-state and the kinetic experiments show that the sorption behavior of phyllite for uranium and humic acid is dominated by small amounts of ferrihydrite. We conclude that the uranium mobility in an environment with phyllite as a major constituent is controlled by its sorption onto amorphous iron minerals.