Biotite has the most important role for the sorption of radionuclides in granitic rocks. Experiments on the sorption of uranium (VI) on biotite were conducted to understand the fundamental controls on uranium sorption on biotite mineral, including the effects of pH and uranium concentration in solution. Biotite powder (mesh 32 – 60) were washed with 1N HC1 for a week and were rinsed twice with deionized water for a week. This HC1 treatment was necessary to avoid the effects by other minerals. The agreement between surface adsorption coefficient, Ka, of both biotites with and without HC1 treatment was within one order of magnitude. The peak Ka value was in the range of 0.1 to 0.01 cm3/cm2 around pH 6. A comparison of aqueous uranium speciations and sorption results indicates that neutral uranyl hydroxide could be an important species sorbed on the biotite.
Sequential desorption experiments with KC1 and HC1 solutions were also carried out after sorption experiments to investigate sorption forms of uranium. Approximately 20 % of uranium in solution were sorbed on the biotite as an exchangeable ion. The fraction of exchangeable uranium had a little dependence on pH. The other uranium could not be extracted even by 6N HC1 solution. It is possible that most of the uranium could be precipitated as U(IV) via Fe(II) reduction on the biotite surface.