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

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.
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