Uranium(VI) Sorption onto Phyllite and Selected Minerals in the Presence of Humic Acid

We studied the influence of humic acid (HA) on the uranium(VI) sorption onto the rock material phyllite and onto its main mineralogical constituents quartz, muscovite, chlorite, and albite at an ionic strength of 0.1 M in the pH range of 3.5 to 9.5 under aerobic conditions. The uranium(VI) concentration was 1·10-6 M and the HA concentration was 5 and 60 mg/L, respectively. The mass loading was 12.5 g/L. Furthermore, we studied the uranium and HA sorption on ferrihydrite (3·10-4 M Fe) and compared the results to the sorption behavior of phyllite. The study showed that the uranium sorption onto phyllite and onto its mineralogical constituents is influenced by the pH-dependent sorption behavior of the HA. Due to high HA sorption onto the solids in the acidic pH range the uranium uptake is enhanced compared to the uranium uptake in the absence of HA. A high concentration of dissolved HA in the near-neutral pH range may reduce the uranium sorption due to formation of aqueous uranyl humate complexes. Furthermore, we could show that the high uranium and HA sorption on phyllite is primarily caused by minor amounts of the secondary mineral ferrihydrite that is formed due to weathering of phyllite. Thus, the ferrihydrite predominates the contributions of the main minerals quartz, muscovite, chlorite, and albite, that are naturally present in the rock material phyllite.