Immobilization and long-term evolution of selenate in Portland cement
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Immobilization and long-term evolution of selenate in Portland cement Isabel Rojo1, Mireia Grivé2, Miquel Rovira1, Olga Riba2, David García2, Cristina Domènech2, Joan de Pablo1,3 1 CTM Centre Tecnològic, Av. de les Bases de Manresa 1, 08242 Manresa, Spain 2 Amphos 21, Passeig de Garcia i Fària 49-51, 1-1, 08019 Barcelona, Spain 3 Chemical Engineering Department, Universitat Politècnica de Catalunya UPC, Av. Diagonal 647 H4, 08028 Barcelona, Spain
ABSTRACT The long-term selenate uptake capacity of leached cement was studied by means of a replenishment batch experiment with cement pore water (CPW) doped with selenate. The corresponding blank experiment (without Se) was also done. The systems were studied for 31 cycles (18-days each cycle) to understand the long-term selenate immobilization in leached cement. Results showed that the retention capacity of leached cement exponentially decreases with cycle evolution. Precipitation of ettringite, identified by SEM and XRD, occurred along the experiments. The characterization of the cement solid phases indicated that selenate was only retained in the precipitated ettringite. Experimental data have been successfully modeled by assuming that selenate incorporates into the precipitating ettringite. Precipitation of ettringite is controlled by the kinetic dissolution of the initially present monocarboaluminate.
INTRODUCTION Cements play an important role in repositories designed for the safe disposal of radioactive wastes as they act as a chemical barrier for the retention of radionuclides. Selenium oxyanions are of particular interest because, in nuclear waste management, selenium is high priority in the safety assessment due to their enhanced mobility in alkaline environments [1]. Several studies have dealt with the retention of selenium oxyanions in cementitious systems. Selenate, contrary to selenite, forms weakly bonded outer-sphere complexes and presents low affinity for oxide surfaces [2]. The replacement of sulfate by selenate into channels and interlayers in the lattice of ettringite and monosulfate (AFm) has been considered to be the most efficient retention process of selenate [2,3,4]. The objectives of this work are: to study the capacity of cement to uptake selenate in longterm periods, and to study the possibility of Se being incorporated in secondary ettringite in a system where Se-doped cement pore water is contacted with cement. In order to simulate longterm behavior a degraded cement and a cement pore water in equilibrium with it were prepared.
EXPERIMENTAL DETAILS Two batch experiments were performed by suspending 0.5 g leached cement in 10 ml of CPW (Table I) one with 1.26 μM Se-doped CPW and the other one without Se (blank
experiment). The leached cement was prepared as detailed elsewhere [5] and had a specific surface area of 10.97 ± 0.01 m2/g (BET-N2 method). Manipulations were performed at atmospheric conditions at 25r 1 ºC. After 18-day equilibration time (1 cycle) on a rotary shaker, the solid was isolated by centrifugation and the
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