Modelling of Pu Sorption onto the Surface of Goethite and Magnetite as Steel Corrosion Products
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Modelling of Pu Sorption onto the Surface of Goethite and Magnetite as Steel Corrosion Products Lara Duro1, Tiziana Missana2, Sonia Ripoll1, Mireia Grivé1 and Jordi Bruno1 1 Enviros Spain - Pg. de Rubà 29-31, 08197 Valldoreix (Spain) 2 Ciemat -Avda. Complutense 22, 28040 Madrid (Spain) ABSTRACT We have studied the surface interaction between tetravalent plutonium and two different steel corrosion products: magnetite, which is the final product of the anoxic corrosion of steel, and goethite, which exemplifies a further oxidation of the steel surface due to the presence of active oxidants in the system. The pH of the experiments has been varied from 2 to 10 and the ionic strength from 0.001 to 0.1 M NaClO4. All the sorption experiments were carried out under N2 atmosphere. No significant effect of ionic strength was observed under the conditions studied. The pHpcz of the solids (7.78 for goethite and 6.95 for magnetite) was determined by modelling potentiometric titration data. The results of the experiments show that the sorption edge of plutonium occurs between pH 3 and 4 when using goethite as a sorbing surface and between pH 4 and 5 when magnetite is used. We have modelled the sorption data by using a simple surface complexation approach with no electrostatic term. The model used involves a reduction process of Pu(IV) to Pu(III) in the presence of magnetite, which can be explained by the interaction of the actinide with the ferrous iron present in the solid. In the case of the experiments conducted with goethite, this reduction process is not possible and, therefore, in the model we have included the sorption of tetravalent Pu. INTRODUCTION The role of the corrosion products of the steel canister intended to be used in a HLNW repository in retarding the migration of radionuclides is a matter of concern for the performance assessment of the repository. In this context, several publications have dealt with the effect of these materials in the retention of actinides, mainly uranium, via sorption at their surface [1]. In this work, we present part of the results obtained from an experimental program aimed at assessing the sorption of another actinide of special relevance for performance assessment, as is plutonium. There are very few studies in the literature dealing with the sorption of tetravalent actinides onto mineral surfaces. One of the reasons of this lack of experimental data is the low concentration of radionuclide that must be used to prevent the precipitation of solid phases. Particularly, in the case of plutonium, the precipitation of the amorphous hydroxide can occur for aqueous concentrations of the actinide over 10-8 M and, therefore, the initial concentration in the experiments must be kept below this value. The understanding of the sorption of plutonium onto the surface of steel corrosion products has, furthermore, an additional complication: the likely reduction of Pu(IV) to Pu(III) onto the surface of ferrous iron bearing solids, such as is magnetite (Fe3O4). Magnetite has proven to be a good reduct
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