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Enhancing Nuclear Safety


Research

Thesis viva

Dynamic modeling of the (bio) availability of radionuclides in soils: a comparative model-experiment approach applied to the transfer of Cs (I) in the rhizosphere


Mohamed Amine Cherif will defend his thesis

on Monday 18th December 2017 at 2:00 pm

at CEREGE amphitheater

Avenue Louis Philibert

13090 Aix-en-Provence (France)



Jury


Anna RIGOL, University of Barcelone

Gilles MONTAVON, CNRS

Catherine KELLER, University Aix-Marseille

Laurent CHARLET, University Grenoble-Alpes

Laureline FEVRIER, IRSN

Frederic GERARD, INRA

Olivier BILDSTEIN, CEA

 


Abstract (in French)

 

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.



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