TERRAPUB Geochemical Journal
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Geochemical Journal, Vol. 43 (No. 6), pp. e37-e42, 2009

EXPRESS LETTER

Modeling the competition between solid solution formation and cation exchange on the retardation of aqueous radium in an idealized bentonite column

Haibing Shao,1,3 Dmitrii A. Kulik,2 Urs Berner,2 Georg Kosakowski2,4 and Olaf Kolditz1,3

1Department of Environmental Informatics, UFZ-Helmholtz Centre for Environmental Research, Permoserstr. 15, 04318 Leipzig, Germany
2Department of Nuclear Energy and Safety Research (NES), Waste Management Laboratory (LES), Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
3Applied Environmental System Analysis, Technische Universitaet Dresden, Helmholtzstraße 10, D-01069 Dresden, Germany
4University of Tübingen, Sigwartstraße 10, D-72076 Tübingen, Germany

(Received August 3, 2009; Accepted November 12, 2009; Online published November 25, 2009)

Abstract: Clays and clay rocks are considered viable geotechnical barriers in radioactive waste disposal. One reason for this is the propensity for cation exchange reactions in clay minerals to retard the migration of radionuclides. Although another retardation mechanism, namely the incorporation of radionuclides into sulfate or carbonate solid solutions, has been known for a long time, only recently has it been examined systematically. In this work, we investigate the competitive effect of both mechanisms on the transport of radium (Ra) in the near-field of a low- and intermediate level nuclear waste repository. In our idealized geochemical model, numerical simulations show that barium (Ba) and strontium (Sr) needed for Ra sulfate solid solutions also partition to the cation exchange sites of montmorillonite (Mont), which is the major mineral constituent of bentonite that is used for tunnel backfill. At high Mont content, most Ra tends to attach to Mont, while incorporation of Ra in sulfate solid solutions is more important at low Mont content. To explore the effect of the Mont content on the transport of radium, a multi-component reactive transport model was developed and implemented in the scientific software OpenGeoSys-GEM. It was found that a decrease of fixation capacity due to low Mont content is compensated by the formation of solid solutions and that the migration distance of aqueous Ra is similar at different Mont/water ratios.
Key words: reactive transport, solid solutions, cation exchange, bentonite, radioactive waste repository, montmorillonite, OpenGeoSys, GEM-Selector


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