The overall objective of this work is to
improve the tools designed to describe and predict the transfer of
radionuclides in the soil / soil solution / plant root system. By
conducting a critical analysis of existing numerical models, the
objective is to develop a generic model able to better account for these
transfers in the case of Cs. The first part of the work was devoted to
the analysis of the models available in the literature to describe the
cesium adsorption on clay minerals. The last is considered as the
process that mainly controls the environmental availability of this
element in the soils. This analysis enabled us to propose a new
mechanistic model combining two adsorption models which combines: (i) a
surface complexation approach to take into account the competition
between Cs and other cations as well as the influence of the ionic
strength and pH of the solution, on hydroxyl sites with variable charges
(frayed edge); and (ii) cation exchange approaches to simulate the
adsorption of cations on permanent negatively charged sites of planar
surfaces of clay minerals. Our minimalist approaches, referred to as the
“1-pK DL/IE” has been tested in order to model the adsorption of Cs on
(i) three reference clay minerals (illite, montmorillonite and
kaolinite) and (ii) several natural clay materials, in a wide range of
Cs concentrations and physicochemical conditions. This work allowed to
validate the 1-pK DL/IE model and to demonstrate that it constitutes a
major advantage over the various existing models, as it takes account
for variable levels of Cs interactions with these clayey substrates
without prior adjustment of the parameters.
The
second part of the work was devoted, (i) to the performing of a series
of experiments, carried out in controlled environments on dynamic
systems (flow reactor, Rhizotests coupling soil, solution and plant) and
(ii) modeling the (bio) availability of Cs in these systems. These
experiments were performed on a natural soil (Auzeville, France),
containing clay minerals, placed in different physicochemical
environments. Following these tests, the observed interactions between
solid and solution were correctly reproduced with the 1-pK DL/IE model
taking into account only the clay fraction of the soil. These
simulations were also compared with simulations obtained using a simpler
model (Kd) or a model allowing to estimate the impact of the processes
limited by their kinetics on the interactions between solid and solution
(E-K approach). Finally, the development of a numerical tool for
coupling the description of geochemical interactions with transfer to
the plant (Michaelis-Menten approach) allowed to reproduce adequately
the trials carried out in Rhizotests coupling soil, solution and plant,
and to better characterize of the Cs fraction available for plants.
