Reference : UMR6118-REMMAR-006
Workplace : RENNES
Date of publication : Monday, November 14, 2022
Type of Contract : FTC Scientist
Contract Period : 18 months
Expected date of employment : 6 February 2023
Proportion of work : Full time
Remuneration : 2600-3000
Desired level of education : PhD
Experience required : Indifferent
Scientific context. Accurate prediction of the biogeochemical behavior of trace elements (TE; e.g. As, Cr, U, Cu) in natural systems is of major concern because of the severe threats they cause to human health, aquatic life and the environment. Environmental TE transport, bioavailability and/or toxicity are known to be controlled by their speciation. However, TE speciation determination is hampered by the occurrence of colloids, which are ubiquitous, small (1-nm to 1-µm-size), highly heterogeneous and reactive organic, inorganic particles or organomineral assemblages towards TEs. Understanding and prediction the colloids-TE interaction is particularly challenging for redox sensitive TE, whose toxicity and biogeochemical behavior is primarily dictated by the oxidation state. Recent breakthroughs of our team demonstrated that the misapprehension of combined role of colloids and redox conditions is a major limitation for accurate prediction of on TE speciation. If the catalytic (kinetic) activity of colloids towards redox transformation of TE has been widely investigated, the thermodynamics of redox reactions at colloids-water interface must also be assessed to shed light on the complex redox behavior of TE (Marsac, R.; Banik, N. L.; Lützenkirchen, J.; Buda, R. A.; Kratz, J. V.; Marquardt, C. M. Modeling Plutonium Sorption to Kaolinite: Accounting for Redox Equilibria and the Stability of Surface Species. Chemical Geology 2015, 400, 1–10. https://doi.org/10.1016/j.chemgeo.2015.02.006)
Decription of the activities. This project will investigate the binding mechanisms of redox-sensitive elements, especially Cr(III/VI), Cu(0/I/II), Ce(III/IV) and U(IV/V/VI), with redox reactive colloids composed of MnO2 (birnessite) or Fe3O4 (Fe(II)Fe(III)2O4; magnetite). MnO2 is a powerful oxidizing agent for TE. Redox properties of magnetite can be tuned by partially oxidizing Fe(II) to Fe(III) from Fe(II)/Fe(III) = 0.5 (stoichiometric magnetite) to 0 (Fe2O3; maghemite) (Jungcharoen, P.; Pédrot, M.; Heberling, F.; Hanna, K.; Choueikani, F.; Catrouillet, C.; Dia, A.; Marsac, R. Prediction of Nanomagnetite Stoichiometry (Fe(II)/Fe(III)) under Contrasting pH and Redox Conditions. Environ. Sci.: Nano 2022, 9 (7), 2363–2371. https://doi.org/10.1039/D2EN00112H). Batch reaction experiments will be carried out under different physico-chemical conditions (EH, pH, presence of organic molecules, etc…) and the solutions will be analyzed after filtration by ICP-MS. All the experiments will be carried out under controlled atmosphere (PO2 < 1 ppm in a "glove box") to avoid any oxidation of the magnetites by atmospheric O2. Solid phase samples will be analyzed by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). A mechanistic TE-colloid binding model will be developed. It will account for the complexity of the colloid-water interfaces and electron transfer processes, by combining equilibrium surface complexation models with kinetic equations, on the basis of our recent modeling develpments (Zhou, L.; Cheng, W.; Marsac, R.; Boily, J.-F.; Hanna, K. Silicate Surface Coverage Controls Quinolone Transport in Saturated Porous Media. Journal of Colloid and Interface Science 2022, 607, 347–356. https://doi.org/10.1016/j.jcis.2021.08.142). This model will be implemented in popular and open geochemical codes (e.g. PHREEQC) widely used by a broad scientific community (e.g. chemists, (bio)geochemists, hydrogeologists, ecotoxicologists) to facilitate the dissemination of the results to both public and private sectors.
Scientific background of the candidate. To be successful, the candidate should be a (geo)chemist with strong background in aqueous solution chemistry, with knowledge on the solid-liquid interface. If possible, the candidate should have skills in analytical chemistry, spectroscopy and geochemical speciation modeling. He/She should demonstrate its capability to work in a team and have good communications skills in English (both oral and written). He/She will work in close collaboration with a PhD student and is expected to actively participate in the supervision of Master students.
Geosciences Rennes is a joint research lab between Rennes 1 University (UR1) and CNRS (French National Centre for Scientific Research). The Postdoc will join the “Nanoscale” research team (https://geosciences.univ-rennes1.fr/en/nanoscale).
- Rémi Marsac (CNRS researcher).
- Mathieu Pédrot (Ass. Prof. at UR1; head of the "Nanoscale team").
Eligibility criteria. This must be the first postdoctoral experience of the candidate, who should not have spent more than 18 months in France between the 19th of May 2019 and the beginning of the contract.
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