exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

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Multiscale study of mineral surfaces/actinides interactions IPNO Participation: O. Félix, S. Lectez, T.T.H. Pham, K. Sebarri, Y. Zhao, R. Drot, F. Mercier-Bion, J. Roques, E. Simoni, V. Sladkov Collaborations : LCPME (Nancy), LECIME (Chime ParisTech), LPS (CEA Saclay), EDF-R&D (Moretsur-Loing) and PECSA (Jussieu). Les interactions entre les actinides et les minéraux présents dans les sols sont en grande partie responsables de la migration/rétention de ces radiotoxiques dans la géosphère. Compte tenu des multiples paramètres intervenant dans ces processus, il est fondamental non seulement de rassembler des données macroscopiques mais également de conduire des investigations à l’échelle moléculaire afin de pouvoir interpréter les résultats obtenus, le plus rigoureusement possible. Parmi ces paramètres, la température et la matière organique n’ont pour l’instant été que peu pris en compte. Le travail présenté ici concerne donc l’étude des mécanismes d’interaction de l’ion uranyle avec la silice en présence d’ions acétate et oxalate à température ambiante et les effets de la température (20 à 90°C) sur la rétention de l’uranium par un solide phosphaté ainsi que les mesures des variations d’enthalpies et d’entropies associées. De plus, des calculs de dynamique moléculaire ab-initio, permettant de prendre en compte la température et le solvant, ont été réalisés sur le système UO 2 2+ /gibbsite. Effect of the organic matter Organic compounds present in natural media (cellulose degradation products, humic substances,...) influence the migration of (radio)toxic ions in the environment: it may increase their mobility if the organic-ion complexes stay in aqueous solution or decrease their migration if these complexes have a higher affinity with the mineral surface than that of the free ion. Much of the studies relative to the effect of the organic matter on ions retention onto mineral surfaces imply humic substances. Our approach is to study the role of simple organic molecules (short-chain mono or dicarboxylic acids) in the sorption of the uranyl cation onto silica surfaces (slices and powders). Sorption edges as a function of pH are obtained by -liquid scintillation and capillary zone electrophoresis. Spectroscopic techniques are used to identify the surface complexes: Time-Resolved Laserinduced Fluorescence Spectroscopy (TRLFS), X- ray Photoelectron Spectroscopy (XPS), Nuclear Microprobe Analysis (NMA) and ATR-FTIR. The results obtained from combination of the thermodynamical and spectroscopic approaches show a different behaviour between the different organic molecules in terms of the retention of uranyl onto silica (sorption percentages and species formed at the solid/solution interface). Indeed, the dicarboxylic acids, oxalate and malate, decrease the quantity of uranium sorbed onto silica and only for oxalate, all the spectroscopic techniques evidence the presence of surface ternary complexes, as illustrated by the NMA results given in the Figure 1.The observations concerning the effect of acetate on the U(VI) retention showed no difference in the sorption percentages of U(VI) and organic matter between the binary (silica-acetate, silica-uranyl) and ternary systems (silica-acetate-uranyl) and the same chemical environment of U(VI) suggesting that the uranyl-acetate complexes stay in the aqueous solution rather than sorbing onto the silica. All these results indicate that the functional group is not the only parameter occurring in the behaviour of carboxylic molecules. The number of carboxylic group, the spacing between them and the length of the carbon chain play also a role in the retention of uranyl ions onto the surface in presence of carboxylic acids. Fig. 1: Nuclear Microprobe spectra (backscattering of incident protons of 1.725 MeV) of silica slides after contact of 5 days with an oxalate+uranyl solution. Beam parameters: diameter of 10 µm, current of 200 pA. The presence of surface ternary complexes is evidenced via the correlation between the C and U signals. Temperature effects The temperature effects on the U(VI) sorption onto a lanthanum phosphate compound (LaPO 4 ) and magnetite (Fe 3 O 4 ) are investigated. A several steps study is required to properly determined the retention mechanisms and the temperatureinduced modifications in the sorption processes. In a first time, the intrinsic surface properties of the solids under study are investigated for different temperatures (ranged from 20°C to 95°C). Mass and potentiometric titrations together with microcalorimetry measurements are carried out as a 127
function of temperature. The recent results allow one to demonstrate that the monazite’s (LaPO 4 ) point of zero charge remains constant whatever the temperature in a non-complexing medium such as NaClO 4 (pH PZC = 1.8). Moreover, microcalorimetry measurements show that the surface hydration process is exothermic, close to -0.4 J/g at 30°C but it is found to depend on the temperature. Then, further study are in progress to precisely determine the variation in the enthalpy of hydration versus temperature in order to calculated the rC p associated to the reaction. In a second step, the retention of U(VI) by the hydrated surface can be investigated. A spectroscopic study is carried out by using Laser-Induced Fluorescence (LIFS), X- ray Absorption Spectroscopy (XAS) and vibrational spectroscopies (Infrared and Raman) which allows one to experimentally determine all the components involved in the sorption process. The first results obtained for lanthanum phosphate show that U(VI) is sorbed on two types of surface sites whatever the temperature. Nevertheless, the reactivity of both sites is found to be temperaturedepend since the relative proportions of the resulting surface complexes strongly change as a function of temperature. Additionally, microcalorimetric measurements are performed in order to measure the heat of sorption. The U(VI) retention by lanthanum monophosphate is an endothermic process with an associated heat of 0.3 J/g. The extensive study of these systems is under progress and a global understanding of the temperature effects on the U(VI) retention mechanisms by oxide-like compounds should be achieved in a near future. Then, the results will allow one to check the validity of the retention mechanisms determined at 25°C (most of the studies found in the literature) for higher temperatures (up to 150°C). culations which take place in static and vacuum conditions at T=0 K. In a first part, a (001) gibbsite face model composed of two sheets, where the deepest one was frozen to atomic bulk positions, was optimized. Geometrical relaxations clearly demonstrated the validity of this surface model to mimic the real system. In a second part, water interactions with the optimized surface model were investigated. Two adsorption modes were identified in agreement with experimental results. The first one takes place through a water molecule hydrogen and a surface oxygen (Fig. 3-a) while in the second adsorption mode, a surface hydrogen interacts with a water molecule oxygen (Fig. 3-b). The first adsorption mode was calculated as the most stable one. Gibbsite surface model Solvent 2.12 Å 1.85 Å O Al H Fig. 3: (001) Gibbsite face hydration. 2.89 Å 2.64 Å (b) Finally, the uranyl interaction with the hydrated (001) gibbsite face was simulated. Two types of adsorption mechanisms were considered: the outer sphere mechanism (Fig. 4-a) as well as the inner sphere one (Fig. 4-b). (a) Fig 2: Example of thermograms obtained for La- PO 4 hydration (top) and U(VI) sorption onto LaPO 4 (bottom). Theoretical calculations In order to investigate, at the solid/liquid interface, the uranyl ion behaviour with the (001) gibbsite face, we have used an Ab-initio Molecular dynamics approach as implemented in the CPMD code (Car-Parrinello Molecular Dynamics). With this original methodology, temperature, solvent and dynamic effects are explicitly taken into account in simulations which is innovator relative to usual cal- Fig. 4: Uranyl adsorption on the gibbsite face. (a) outer sphere and (b) inner sphere mechanism. Both adsorption modes were calculated as possible. The second one (inner sphere mechanism) is the most stable because of strong covalent interactions. However, even though it is the most stable one, the needed activation energy (E a ) to reach it, is not known. Therefore, in order to evaluate the gibbsite retention capability towards uranyl migration, current work are devoted to evaluate E a. This value, will be used to predict if at a given temperature the energy barrier will be able to be crossed. 128
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EXOTIC NUCLEI STRUCTURE AND REACTIO
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in the spectra by analyzing their t
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were taken from ref. [5]. Concernin
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keV -ray with the particles and the
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the 605.9 keV line of the 74 Zn 2+
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Figure 2 a n d analysis. Figure 2 s
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ing particles was thus obtained by
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First p-p p collisions in the ALICE
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Quarkonia physics with ALICE IPNO P
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NA50 updated puzzle IPNO Participat
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Status of the HADES experiment (I):
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Status of the HADES experiments (II
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Status of the HADES experiment (III
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Hadron physics in pbar-p p annihila
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Hadron Electrodynamics IPNO Partici
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G0 experiment at Jefferson Lab. IPN
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GEp-III and GEp-2 at Jefferson Lab
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Exotic low mass narrow baryons from
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The PVA4 parity violation experimen
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Etude des Distributions de Partons
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Latest Results from GRAAL IPNO Part
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Persistence of the Polarization in
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The northern site of the Pierre Aug
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The Pierre Auger southern Observato
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In figure 4 we plot the differentia
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THEORETICAL PHYSICS Research in the
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transfer reaction, the 34 Si(d, 3 H
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Collective excitations with SKYRME
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Pairing and nuclear incompressibili
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Pairing properties in nuclei and in
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Evaporation-residue residue cross s
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Extending the VMI model: normal and
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Alpha particle condensation in nucl
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