Thesis proposals
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M2 Internship: Can cerium redox speciation be predicted in the environment ?
Summary:
Rare earth elements (REE), or lanthanides, form a group of 15 chemical elements (from La to Lu). REE occur naturally in water as a result of weathering processes at the Earth's surface. They occur predominantly in the +III oxidation state. However, cerium (Ce), the most abundant REE in the Earth's crust and used for many applications (technical, medical, catalytic...), can oxidize to Ce(IV) under the oxidizing conditions found at the Earth's surface and, as a result of human activities, contaminates the natural environment in the form of CeO2(s) nanoparticles.1-3 This oxidation can occur via an adsorption-oxidation mechanism of Ce(III) on the surface of some solid phases, such as manganese (Mn) or iron (Fe) oxides. Ce(IV) is preferentially removed from solution by Ce4+ adsorption or CeO2(s) precipitation compared to other REE(III), which generally results in the development of a positive Ce anomaly on the solid surface (enrichment of Ce compared to other REE on the solid surface) and a negative anomaly in solution. This feature can be used, for example, as a proxi of redox conditions in paleoenvironments or to trace water reservoirs.
However, the mechanisms involved in the redox transformations of Ce are not sufficiently elucidated to develop a predictive model of its speciation in the natural environment. For example:
1) Unlike the Mn2+/Mn(IV)-oxide couple, the redox potentials of the various Fe2+/Fe(III)-oxide couple are lower than that of Ce3+/CeO2(s). However, oxidation of Ce(III) to Ce(IV) has often been reported on Fe(III) oxides.3
2) Synchrotron X-ray absorption spectroscopy (XAS) results have shown the oxidation of Ce(III) to Ce(IV) and the formation of CeO2 nanoparticles in the presence of organic matter (OM), which is better known for its reducing than oxidizing capacity (recent unpublished data).
The aim of this internship is to elucidate the mechanisms controlling cerium speciation under natural conditions, by developing a methodology to identify Ce(III) and Ce(IV) complexation reactions on the surface of inorganic particles (Fe/Mn/Al oxides, clays, etc.... alone or in mixtures) or by OM, as well as the precipitation of nano-CeO2.
Experiments will be carried out under various physico-chemical conditions, part of which will be under a controlled atmosphere (PO2 < 10 ppm in a "glove box") in order to control redox conditions in the absence of atmospheric O2, and thus better constrain the mechanisms. The methodology may combine theoretical and analytical approaches, including:
1) UV-vis absorption or fluorescence spectroscopy, previously used to determine REE-MO complexation constants.4
2) Electron microscopy (SEM, TEM or STEM), with detectors in the X-ray energy range (EDX, EELS, etc.) to determine the spatial distribution of Ce and the formation of nanoparticles.1
3) Synchrotron XAS spectroscopy to determine Ce redox speciation.3 A beamtine request is currently being evaluated.
4) Modeling the speciation of Ce.2 R. Marsac will provide 20 hours of training at the beginning of the internship.
Our team is looking for an excellent student, with the desire and ability to carry out a PhD thesis in our team, and an environmental (geo)chemist profile with solid skills in solution chemistry and analytical chemistry. The candidate should have excellent team spirit and a good level of English (written and spoken).
