Sorption of Pu(IV) and Tc(IV) on concrete and mortar and effect of the complexation by isosaccharinic acid.
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Sorption of Pu(IV) and Tc(IV) on concrete and mortar and effect of the complexation by isosaccharinic acid. Rojo H., García-Gutiérrez M., Missana T., and Galán H. Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. CIEMAT. Av. Complutense, 40. 28040, Madrid, Spain. ABSTRACT Batch sorption experiments were carried out, in a glove box under N2 atmosphere, for analysing the effect of increasing ISA (isosaccharinic acid) concentrations on sorption of Tc(IV) and Pu(IV) onto previously characterised Spanish concrete and mortar used in a radioactive waste repository. Results clearly showed that concentrations of ISA around 10-4–10-3 M are able to decrease the distribution coefficients (Rd) of the tetravalent elements analysed, both on concrete and mortar. The decrease in Rd experimentally observed was approximately 2 orders of magnitude for technetium and between 3 and 5 orders of magnitude for plutonium. INTRODUCTION Concrete and mortar are materials widely used for the immobilization of the waste in low and intermediate level nuclear waste (LILW) repositories. These materials determine the chemical and physical properties of the repository near-field for a long period of time, and provide the system with alkaline conditions, which favour radionuclide immobilization by sorption and low solubility, under which tetravalent actinides are very stable. That is the case of Pu(IV) and Tc(IV) used in this study as representative tetravalent radionuclides. In LILW repositories the presence of organic compounds as, for example, paper, wood, plastic and cloth is important. Isosaccharinic acid (ISA) is the main degradation product of these cellulosic materials under alkaline conditions, and it behaves as an organic ligand. It is important to analyse its behaviour in the frame of radioactive waste repositories, because, its presence may significantly increase the mobility of radionuclides. The main objective of this work is to study the effect of ISA on Pu(IV) and Tc(IV) sorption, as well as the stability of the Pu(IV) – ISA and Tc(IV) – ISA complexes forming under the chemical conditions generated by concrete and mortar used in the LILW Spanish repository of El Cabril (Córdoba, Spain). EXPERIMENTAL METHODOLOGY All sorption experiments, the preparation of synthetic pore water, and the reduction of Tc(VII) to Tc(IV) were carried out in a glove box under controlled N2 atmosphere (O2, CO2 < 5 ppm). The concrete used in this study was obtained from the hydration of Portland cement sulphate resistant type I (CEM I 42.5R/SR) mixed with sand and gravel, and the mortar was obtained from Portland cement type IV/B sulfate resistant (CEM IV/B 32.5/SR/BC) mixed with sand. Both materials were ground to a size < 1mm, and sieved (ASTM Nº 18). SEM, XRD and FTIR were the techniques employed to characterize these materials.
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For each material, the corresponding synthetic pore water was obtained by contacting them with Milli-Q water previously boiled to minimize its CO2 content. The solid to liquid relation to prepare this pore
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