Implications of recently derived thermodynamic data and specific ionic interaction theory parameters for (Mg/Ca)nUO2(CO3)3(4-2n)- complexes on the predominance of the Mg2+-Ca2+-UO22+-OH--CO32- systems, and application to natural and legacy-mine waters.

The formation of alkaline earth(II)triscarbonatouranyl(VI) (AenUO2(CO3)3(4-2n)-) species that have been evidenced both in laboratory and in-field studies, is important from slightly acidic pH up to near degraded cementitious in carbonated waters. They are also showing distinctive luminescence properties with a hypsochromic shift relative to UO22+. The conditions of pH, activities of alkaline earth(II) free ions (mostly Mg2+ and Ca2+) and carbonate ions (HCO3-) can be predicted from the thermodynamic functions and constants. The predictive validity of the activity of major alkaline ions (mostly Na+) is determined from the models used to describe the ionic strength comportment of these species, particularly using coefficients from the specific ion interaction theory (SIT). The stability domains of these species are better defined as a function of the activity of the constituents, and applied to natural waters. In this work, using recently obtained complete thermodynamic data and SIT coefficients, we will draw the stability domains of the AenUO2(CO3)3(4-2n)- species in combinations of activities of H+, HCO3-, Mg2+, Ca2+, and Na+ for a wide selection of water compositions from the literature. Water samples were collected near a French mining legacy-site (Site du Bosc, Lodève, France). After determining the major ion compositions, we will verify that the luminescence signal of uranium is in agreement with the predicted speciation in the stability domains.

Measurement and modeling of the surface potential evolution of hydrated cement pastes as a function of degradation.

Hydrated cement pastes (HCP) have a high affinity with a lot of (radio)toxic products and can be used as waste confining materials. In cementitious media, elements are removed from solution via (co)precipitation reactions or via sorption/diffusion mechanisms as surface complexation equilibria. In this study, to improve the knowledge of the surface charge evolution vs the degradation of the HCP particles, two cements have been studied: CEM-I (ordinary Portland cement, OPC) and CEM-V (blast furnace slag and fly ash added to OPC). Zeta potential measurements showed that two isoelectric points exist vs HCP leaching, i.e., pH. Zeta potential increases from -17 to +20 mV for pH 13.3 to pH 12.65 (fresh HCP states) and decreases from 20 to -8 mV for pH 12.65 to 11 (degraded HCP states). The use of a simple surface complexation model of C-S-H, limited in comparison with the structural modeling of C-S-H in literature, allows a good prediction of the surface potential evolution of both HCP. Using this operational modeling, the surface charge is controlled by the deprotonation of surface sites (>SO(-)) and by the sorption of calcium (>SOCa(+)), which brings in addition a positive charge. The calcium concentration is controlled by portlandite or calcium silicate hydrate (C-S-H) solubilities.

Mechanism of europium retention by calcium silicate hydrates: an EXAFS study.

The uptake of Eu by calcium silicate hydrate (C-S-H) phases as a function of Eu/sorbate ratio (from 37 to 450 micromol g(-1) 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 A, 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.