Removing the “ heavy mineral effect ” to obtain a new Pb isotopic value for the upper continental crust

Cesium-137 (half-life, $30 years) is an important fission product from the irradiation of uranium-based fuels and it has been released in the past to soils and waters as a result of nuclear accidents or weapon testing. Radiocesium is particularly relevant from an environmental point of view because it always exists as the monovalent cation Cs+, which presents very high solubility. Cesium migration in the environment is mainly controlled by sorption onto mineral surfaces; in particular, it is rather strongly adsorbed onto clays by ionic exchange. Thus, with the objective of designing a geochemical reactive barrier to treat Cs-137 pollution in a salt marsh zone, several natural clayrocks were analysed as possible sorbent. The composition of the rocks was variable and within the clayey fraction the mayor clay minerals were illite, smectite and kaolinite. The high salinity of natural waters presents in the zone obliged the understanding and quantification of the the effects of competitive ions, which may hinder cesium adsorption. Furthermore the water chemical composition variability leads to significant differences in distribution coefficients (Kd). The semi-empiric approach in the analysis of experimental data, above all when sorption is non-linear (as in the case of Cs) is unsatisfactory. Thus, the development of sorption models, validated by experimental data, obtained under conditions representative of the site is needed. A multisite cation exchange model, considering both the solid (mixed clays) and water composition was used to successfully explain cesium sorption in the analysed natural clayrocks and the effects of highly competitive ions present in the aqueous phase, in particular K+, NH4 and Na+. Removing the “heavy mineral effect” to obtain a new Pb isotopic value for the upper continental crust M. GARÇON1*, C. CHAUVEL1, C. FRANCE-LANORD2, M. LIMONTA3 AND E. GARZANTI3 1ISTerre, Université de Grenoble, France 2CRPG-CNRS, Vandoeuvre-lès-Nancy, France 3Laboratorio di Petrografia del Sedimentario, Università di Milano-Bicocca, Italy (*correspondence: marion.garcon@ujf-grenoble.fr)