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Mechanism of Europium Retention by Calcium Silicate Hydrates: An EXAFS Study

M.L. Schlegel 1 I. Pointeau 2 N. Coreau 2 Pascal E. Reiller 3 
1 LRSI - Laboratoire de Réactivité des Surfaces et des Interfaces
DPC - Département de Physico-Chimie : DEN/DPC
2 L3MR - Laboratoire de Mesures et Modélisation de la Migration des Radionucléides
SECR - Service d'Etudes du Comportement des Radionucléides : DEN/DPC
3 LSRM - Laboratoire de Spéciation des Radionucléides et des Molécules
DPC - Département de Physico-Chimie : DEN/DPC
Abstract : The uptake of Eu by calcium silicate hydrate (C-S-H) phases as a function of Eu/sorbate ratio (from 37 to 450 µmol/g C-S-H), C-S-H Ca/Si mole ratio (1.3, 1.0, and 0.7), and initial supersaturating conditions was probed by solution kinetics experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy, to shed light on the retention mechanism of trivalent radionuclides under waste repository conditions. The rates of Eu (9.7 x 10-10 M) uptake in C-S-H suspensions and in solutions at equilibrium with C-S-H were rapid. Uptake of more than 90% of dissolved Eu was generally observed within 15 min. Europium LIII-edge EXAFS spectra collected on samples of Eu sorbed on, or coprecipitated in, C-S-H differed from that of Eu(OH)3(s) expected to precipitate under the pH conditions of C-S-H waters, ruling out compelling precipitation of pure hydroxide phases. Fourier transforms for EXAFS spectra for Eu in sorption/coprecipitation samples displayed comparable features at distances typical of neighboring cationic shells, pointing to similar crystallochemical environments. Optimal spectral simulations were obtained by assuming the presence of Si, Si/Ca, and Ca cationic shells surrounding Eu at distances of 3.2, 3.7-3.8, and 3.8-3.9 Å, respectively. The nearly continuous distribution of (Si, Ca) backscattering shells parallels the distribution in Ca-(Ca, Si) interatomic distances in structural models of C-S-H. Discernible effects of experimental parameters on the Eu local environment were observed by comparison of Fourier transforms, but could not be confirmed by EXAFS quantitative analysis. These results indicate that sorbed or coprecipitated Eu is located at Ca structural sites in a C-S-H-like environment. Kinetics and spectroscopic results are consistent with either Eu diffusion within C-S-H particles or precipitation of Eu with Ca and Si creating a C-S-H-like solid phase.
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Submitted on : Monday, August 18, 2008 - 2:33:14 PM
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M.L. Schlegel, I. Pointeau, N. Coreau, Pascal E. Reiller. Mechanism of Europium Retention by Calcium Silicate Hydrates: An EXAFS Study. Environmental Science and Technology, American Chemical Society, 2004, 38 (16), pp.4423-4431. ⟨10.1021/es0498989⟩. ⟨cea-00311490⟩



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