Sorption Behavior of Uranium(VI) on a Biotite Mineral
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SORPTION BEHAVIOR OF URANIUM (VI) ON A BIOTITE MINERAL
K. IDEMITSU, K.OBATA, H.FURUYA, AND Y.INAGAKI Department of Nuclear Engineering, Kyushu University 6-10-1 Hakozaki, Fukuoka 812, JAPAN.
ABSTRACT
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 IN HCl 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 HCI treatment was within one order of magnitude. The peak Ka value was in the range of 0.1 to 0.01 cm 3/cm 2 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 KCI and HCI 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 HCI solution. It is possible that most of the uranium could be precipitated as U(IV) via Fe(II) reduction on the biotite surface. INTRODUCTION
In the safety assessment of high-level radioactive waste disposal, a significant retardation of nuclides can be expected. In granitic rock, biotite has an important role for sorption of radioactive elements 1,2,3.4. Biotite has the highest sorption coefficient for many kinds of
elements in most of pH range 1,2. And biotite grains were connected each other by micro fissures in granite 4. Thus most of radionuclides that diffuse by pore diffusion mechanism can be trapped by biotite. The pH dependence of sorption coefficients could be controlled not by kind of minerals but by chemical form of elements in the solution 1. The sorption of alkali and alkaline earth elements could be explained by ion exchange mechanism. Recently surface complexation model was applied to sorption of uranium on goethite 5,6. On the model special species of uranium will make some complexes with the surface of the mineral. And sorption experiments of uranium on zeolite suggested that neutral uranyl hydroxide could be a important species 7. Anyway it is necessary to obtain sorption data in various pH. 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. The biotite used in these experiments was treated with HCI to avoid the sorption onto other minerals such as iron oxides and to minimize pH buffer capacity. And sequential desorption ex
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