presence of very high concentrations of hydrogen peroxide was evaluated. The results show that in this case, occurring under very intense radiation fields causing radiolysis of most of the water present, studtite is by far the most stable phase within the full range of temperature studied.

No abstract

The full crystal structure of becquerelite mineral phase was successfully determined using theoretical solid-state methods for the first time. Additionally, a complete study of its thermodynamic and mechanical properties and stability is reported.

The thermodynamic properties of schoepite and metaschoepite were obtained by means of theoretical solid-state methods as a function of temperature. Since the values of these properties for schoepite have not been measured experimentally, they were predicted. The computed thermodynamic functions of metaschoepite were in excellent agreement with the experimental information. These functions were used to obtain the thermodynamic properties of formation of these materials from the corresponding elements. The calculated Gibbs free energy of formation of metaschoepite was shown to be very reliable and differ from the experimental value at 800 K by only 2.0%. Besides, it extends the range of temperature in which this property is known to 0−1000 K. Then, these properties were combined with those of other important uranyl-containing materials to study the reactions of formation of schoepite and metaschoepite from uranium trioxide and the reactions of transformation of these materials into dehydrated schoepite, rutherfordine, and soddyite. Schoepite becomes unstable with respect to uranium trioxide for temperatures higher than 110 °C (383 ± 27 K) and its dehydration occurs at 64 °C (337 ± 44 K). The corresponding values of these temperatures for metaschoepite are 82 °C (355 ± 6 K) and 5 °C (278 ± 9 K), respectively. Under hydrogen peroxide free conditions, schoepite and metaschoepite were found to be less stable than rutherfordine and soddyite. The thermodynamic stability of schoepite with respect to metastudtite and studtite was then studied under different conditions of temperature and concentrations of hydrogen peroxide. Schoepite and metaschoepite have very similar thermodynamic stabilities, the first being slightly more stable than the second one. The availability of the thermodynamic properties of these minerals allowed to determine their relative thermodynamic stability with respect to a rich subset of the most relevant secondary phases resulting from corrosion of spent nuclear fuel. Schoepite and metaschoepite were found to be the first and second most stable phases under intermediate hydrogen peroxide concentrations and the second and third most stable phases under high concentrations of hydrogen peroxide, respectively.

No abstract

No abstract

Cesium adsorption onto Illite has been widely studied, because this clay is especially relevant for Cs migration-retention in the environment. The objective of this study is to analyze how Cs adsorption onto Illite is affected by structural changes produced by the presence of different exchangeable cations--and specifically interlayer collapse. Cs sorption isotherms were carried out with Illite previously exchanged with Na, K, or Ca, at a broad enough range of ionic strength, for the determination of the possible affect of the electrolyte on the structure of Illite. In the presence of Ca, the maximum sorbed Cs was unexpectedly high (900 mequiv · kg(-1)) given the cationic exchange capacity commonly accepted for Illite (near 200 mequiv · kg(-1)). This was explained by the expansion of Illite layers (decollapse) induced by large hydrated cations such as Ca(2+) that may facilitate cation uptake--especially Cs(+), which is a highly selective cation. In the presence of Ca (and most probably of other divalent cations), Cs accessibility to exchange positions is increased. Both experimental evidence and the modeling of Cs sorption onto Illite supported the hypothesis of decollapse. Our results demonstrate the requirement of accounting for Illite decollapse especially for high Cs loadings, because of the potential prediction errors for its migration. Ignoring the Illite decollapse could lead the biased estimation of selectivity coefficients and consequently the erroneous prediction of sorption/migration behavior of Cs, and possibly other contaminants, in the environment.

The retention behavior of three toxic chemicals, As, Cr and B, was investigated for an outcropping rock formation, the Albian Tégulines Clay (France, Aube). At a shallow depth, Tégulines Clay is affected by weathering processes leading to contrasted geochemical conditions with depth. One of the main features of the weathering is the occurrence of a redox transition zone near the surface. Batch sorption experiments of As(V), As(III), Cr(VI) and B were performed on samples collected at two depths representative either of oxidized or reduced mineral assemblages. Batch sorption experiments highlighted a distinct behavior between As, Cr and B oxyanions. Cr(VI) retention behavior was dominated by redox phenomena, notably its reduction to Cr(III). The in-situ redox state of the Tégulines Clay samples has a significant effect on Cr retention. On the contrary, As(V) reduction into As(III) is moderate and its retention slightly affected by the in-situ redox state of the Tégulines Clay. As(V) retention is higher than As(III) retention in agreement with literature data. B retention is strongly related to its natural abundance in the Tégulines clay samples. Distribution coefficient of B corrected from its natural content is expected to be very low for in-situ conditions. Finally, the retention and mobility of these oxyanions were affected by clay mineralogy, natural abundance, and reducing capacity of the Tegulines Clay.