Cesium and Lead Uptake by CSH Phases of Hydrated Cement
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Cesium and Lead Uptake by CSH Phases of Hydrated Cement I. Pointeau1,3, N. Marmier1,*, F. Fromage1, M. Fedoroff2 and E. Giffaut3 1 Groupement de Recherche en Chimie Inorganique, Université de Reims, GRECI BP1039, 51687 REIMS Cedex, France. 2 Centre d’Etude de Chimie Métallurgique, Centre National de la Recherche Scientifique, 15 rue Georges Urbain, 94407 Vitry-sur-Seine, France 3 Agence Nationale pour la gestion des Déchets Radioactifs, 1/7 rue Jean-Monnet, 92298 Châtenay-Malabry Cedex Email: [email protected] ABSTRACT As Kd from radwaste elements in concrete systems show a wide range of values, a modelling of cesium and lead immobilization in Calcium Silicate Hydrate (CSH, xCaO.SiO2.H2O, with 0.7 (aged cements)Si(Q2B)OH, and silanol with a sorbed labil calcium layer. Materials and method Figure 1. Silanol sites & layer structure of
Cement phases
Three CSH (Ca/Si = 0.83, 1.00 and 1.25) have been obtained by shaking CaO, and fumed silica powders in the stoichiometric ratio for one month, in demineralized decarbonated water (liquid/solid mass ratio = 20). The pH, [Ca] and [Si] of each WCSH are given in Table 1. Table 1. Characterization of CSH solids and composition of WCSH
CSH 1.25 CSH 1.00 CSH 0.83 29
pH
[Ca2+]WCSH
[Si]WCSH
mol Ca / g CSH
mol Si / g CSH
mol Q1/ g CSH
mol Q2B / g CSH
12.2 12.0 11.5
1.25 10-2 5.9 10-3 1.6 10-3
1.5 10-5 2.3 10-5 1.7 10-3
8.0 10-3 3.8 10-3 3.4 10-3
6.4 10-3 3.7 10-3 4.1 10-3
4.9 10-3 1.6 10-3 8.2 10-4
6.4 10-4 8.7 10-4 1.0 10-3
Si MAS/NMR measurements have been carried out on the three CSH phases, and >Si(Q1)OH and >Si(Q2B)OH site densities have been extracted (see Table 1).
The surface area is measured as about 30 m2.g-1 with BET N2 adsorption and about 290 m2.g-1 with BET H2O adsorption. The last value gives an average site density of 10 sites.nm-2. Sorption experiments Batch experiments have been performed by shaking during 1 day at room temperature: 50-100 mg of CSH in 10 mL of a 10-6 to 10-3 mol.L-1 CsOH solution in WCSH, and 2-8 mg of CSH in 10 mL of a 10-6 to 3.10-3 mol.L-1 Pb(NO3)2 solution in WCSH. NaNO3 salt can be added in the batch, in the studying of ionic strength (latter on called I) effects. Before filtration, the pH has been measured and after filtration (polyamide 0.2 µm filter), the cesium and lead concentrations have been measured in the filtrate by Atomic Absorption Spectrometry (AAS). Polycrystal batches have been used for the sorption experiments, and a control experiment, carried out without CSH, has showed that cesium and lead are not sorbed on the batch walls. The pH and [Ca2+] have been controlled and both parameters remained constant in all the range of the isotherms. Experimental results Cesium Three parameters are varied in batch experiments: Ca/Si ratio, I and the liquid/solid ratio (mL.g-1, later on called l/s). The cesium behavior is usual: the sorption decreases with the increase of Ca/Si ratio and the increase of I. The decrease of the sorbed cesium rate, when [Cs+]ini increases, shows the saturation of one surface site.
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