Prediction of the Weathering Properties of Minerals Based on the Ion-Ion Interaction Potential
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Prediction of the Weathering Properties of Minerals Based on the Ion-Ion Interaction Potential Slavica Raicevic1, Judith V. Wright2, Jasmina Vujic3 and James L. Conca4 1 Institute of Nuclear Sciences VINCA, Radiation and Environmental Protection Laboratory, P.O. Box 522, 11001 Belgrade, Serbia and Montenegro, [email protected] 2 PIMS NW, Inc., 201 North Edison, Suite 226, Richland, WA 99336, U.S.A. [email protected] 3 University of California, Department of Nuclear Engineering, 4105 Etcheverry Hall, Berkeley, CA 94720, U.S.A. [email protected] 4 Los Alamos National Laboratory, 115 North Main Street, Carlsbad, NM 88220, U.S.A. [email protected] ABSTRACT Selection of solid matrix for immobilization of nuclear waste and of geological sites for its disposal represents a complex and consuming task. Previously, we demonstrated that the ion-ion interaction potential (IIIP), representing the main term of the cohesive energy, can be used for estimation of stability of solid matrix/impurity systems. Here we demonstrated a strong correlation between the weathering properties of minerals and their IIIP. Based on these results we proposed a simple theoretical criterion which can be used for assessment of the long term stability of solid matrix for immobilization of radioactive and toxic metals. This criterion was applied in analysis of stability of autunites and some possible practical consequences of results obtained for immobilization of uranium by apatite are discussed. INTRODUCTION In all geological scenarios the effectiveness of long-term isolation of nuclear waste relates to the possibility that at some time in the future, water may reach the waste and dissolve radionuclides in unanticipated quantities. For this reason, the weathering properties of solid matrices used for immobilization of nuclear waste, and which make up the geological environment for its disposal, are of major interest. Igneous and metamorphic minerals are generally formed at high temperatures, high pressures or both compared to the Earth’s surface. The term for the collection of processes by which these minerals re-equilibrate and readjust to surface conditions is weathering, referring to the low-temperature reactions between water and minerals in soils and rocks. One of the factors that controls the rate at which minerals weather is their relative stability at the Earth’s surface. Goldich proposed that the weathering rates of minerals follow the sequence, similar to the original Bowen’s reaction series, whereby minerals such as olivine and Ca-feldspar weather at the fastest rates, and minerals such as muscovite and quartz weather at the slowest rates [1]. Unfortunately, comparisons between mineral weathering rates based on laboratory and field data commonly reveal large discrepancies which limit our ability to correctly predict the long-term stability of the solid matrix. It has been shown that the lattice energy and the Gibbs free energy represent suitable parameters for estimation of the solubility product (logKsp) of minerals [2]. Un
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