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The works performed within RTDC-1 permitted to ascertain some of the basic data on actinide
speciation in carbonate, sulphate and phosphate systems, and to tackle the difficult silicate system.
The retention models were compared and benchmarked; an inter-comparison exercise on data acquisition
was also proposed. It is appearing that the modelling of retention of radionuclides by clay
systems is robust as it permits to perform bottom-up modelling of natural samples from laboratory
determination. The modelling on oxides depends largely on the surface state and thermodynamic
stability of the phases. It seems necessary to account for the electrostatic properties of oxides. The
influence of organic molecules on radionuclide retention depends mainly on their origins and interactions
with the minerals. In a general manner, the anthropogenic molecules induce a decrease of
the retention of radionuclides on clays. Synergistic sorption can nevertheless happen at the surface
of oxides. For natural organic matter (NOM), the interaction with mineral has to be weak to apply
linear additive models. Otherwise, a change in the ‘free energy’ of the system must be accounted.
The identification of processes of natural organic layer coating on clays is still not clear. The solid
solution formation has been clearly linked to adsorption. A thermodynamic expression considering
the main component as the solvent can be proposed. A limiting quantity of actinides in natural calcite,
as well as the structure of these solid solutions, has been evidenced. The incorporation of Se to
pyrite is driven by redox processes.
The main results will only be recalled and will not be developed. Readers can refer to the published
works in references. Activity reports are also available in the annual proceedings of the Annual
Workshops [1-3].
2. Results
2.1 Work Package 1: Ionic species/speciation, processes determining physico-chemical conditions
and generation of missing thermodynamic data
The thermodynamic data or thermodynamic constant on complexation of curium (III) in carbonate
solutions at variable temperature, of sulphate complexation of lanthanides (III) and uranium (VI)
[4-6], and phosphate complexation of thorium (IV) was determined [2]. Common work on silicate
complexation of actinides (III) and (IV) were also undergone in the French Commissariat à
l’Énergie atomique and the Swedish Chalmers Technical University.
2.2 Work Package 2 - Ion exchange and surface complexation
The sorption data of radionuclides at different redox state – i.e., Se(IV), Co(II), Ni(II), Eu(III),
Cm(III), Y(III), U(VI), and Th(IV) –, on model oxides and clays – i.e., �-Fe2O3, goethite, -Al2O3,
gibbstite, smectite, illite – [1, 2, 7-9], as well as on actual minerals – i.e., biotite, granodiorite,
montmorillonite, Opalinus clays, Boom clay –, was studied [2]. The different sorption modelling
were used or benchmarked [2, 3]. The combination and confrontation of modelling and spectroscopic
data was also performed [9] as well as the influence of competing reaction [10]
2.3.1 Work Package 3 - Influence of organics on the retention of radionuclides by minerals
This work package is devoted to the influence of organics, either natural or anthropogenic on the
sorption of radionuclides.
The sorption of cellulose degradation product was characterised in far field conditions and was
shown to be very weak compared to natural organics [11-13]. The influence of the main organic
component in nature, i.e. humic substances, was considered [11, 12]. Either a purely additive model
[13-15], or a charge distribution model [16-19] were tested. The influence of the kinetic of humic
complexation was also accounted in transport model [20]. Spectroscopic modification of actinides,
chemical environments in humic-clay systems were also observed [21].
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