Experimental Study and Modeling of Uranium (VI) Sorption onto a Spanish Smectite
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1124-Q07-05
Experimental Study and Modeling of Uranium (VI) Sorption onto a Spanish Smectite T. Missana, U. Alonso, M. Garcia-Gutierrez, N. Albarran, T. Lopez CIEMAT, Departamento de Medioambiente, Avenida Complutense 22, 28040 Madrid (SPAIN) ABSTRACT Adsorption of uranium onto a Spanish smectite was studied, analyzing the effects of the most important parameters such as pH, ionic strength, radionuclide concentration and solid to liquid ratio. Batch sorption studies, in anoxic conditions under N2 atmosphere, were carried out on the bentonite previously purified and converted into the homoionic Na-form. In the sorption edges, two regions could be clearly distinguished. At pH lower than 5, sorption depended strongly on the ionic strength, possibly indicating the predominance of the uranyl ionic exchange process. At higher pH, sorption did not depend on the ionic strength but only on pH. The sorption behavior in this region suggested the predominance of a surface complexation mechanism. Sorption isotherms showed a non linear behavior in the concentration range used. Sorption data were interpreted using a non electrostatic standard model combining surface complexation, with the weak and strong SOH sites of the clay, and ionic exchange. The acid – base properties of the weak SOH sites were determined by potentiometric titrations. The model used was able to reproduce, in a very satisfactory way, all the data in a wide range of experimental conditions. INTRODUCTION The sorption and transport properties of radionuclides in the near- and far-field barriers of a deep geological radioactive waste repository are amongst the principal aspects to be evaluated for the performance assessment (PA) of such disposal systems. The study of the clay materials is of particular importance since the backfill material is compacted clay in most designs and also because argillaceous formations are particularly suitable as host rock formations. In these systems, the transport of radionuclides is mainly a diffusion-controlled process retarded by sorption. For the description of sorption processes, PA models use the Kd-approach, which lumps together all the solid/water interactions. A mechanistic approach to sorption processes, which is based on a thermodynamic description of the radionuclide/solid interactions, is more appropriate to predict sorption behavior under variable conditions, and its implementation in performance assessment calculations would be of great interest instead of the usual “Kdapproach”. For this reason, it is important to analyze in detail radionuclide sorption processes in clayey materials and to use appropriate mechanistic models for their description. The Spanish FEBEX bentonite has been used over the last decade in different international research projects (FEBEX I and II, CRR, CFM, FUNMIG) as reference backfill material and its sorption properties were analyzed. Previous works studied the sorption, by a mechanistic approach, of Cs [1], Eu [2], Se [3] and divalent elements as Sr, Co and Ca [4]. No uranium sorption data
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