Animé par Ophélie Bugnon
jeudi 18 avril 2019 à 14:00
Amphi G. Besse
- Nivesse1),2)*, N. Baglan2), G. Montavon1), O. Péron1)
1)Laboratoire Subatech, Groupe Radiochimie, UMR 6457, IN2P3/CNRS/IMT Atlantique/Université́ de Nantes, 4 rue Alfred Kastler, BP20722, 44307 Nantes Cedex 3, France
2)CEA, DAM, DIF, F-91297 Arpajon, France
Tritium is the radioactive isotope of hydrogen and can therefore integrate organics molecules of living organisms to form the organically bound tritium fraction (OBT) . OBT is usually distinguished into two forms: a non-exchangeable fraction (NE-OBT) and an exchangeable fraction (E-OBT) with the near environment . However, there is no consensus on their definition and several variations can be found in the literature, differentiating them from an analytical, structural or kinetic point of view. The main interest of the NE-OBT study related to its non-exchangeability capacity is to enable retrospective studies of tritium releases in the environment.
NE-OBT analysis involves an isotopic exchange step to remove E-OBT from a sample. The current method, where the dehydrated sample is covered with aqueous water, has the disadvantage of causing the dissolution of a part of the organic compounds of the sample, which can lead to an analytical bias . A new method has been developed to overcome this potential solubilisation. It consists of a tritium labelling line, with a controlled and stable ratio (T/H), which allows the determination of the exchangeable hydrogen fraction (α) and the TOL-NE activity’s calculation using equation (1) , .
(T/H)OBT = a x (T/H)E-OBT + (1-a) x (T/H)NE-OBT (1)
Following previous work on carbohydrate molecules , it is accepted that a theoretical (α) calculated from the molecular formula may be different from the experimental obtained one. According to this, investigations are conducted on a similar matrix, starch extracted from tritium-labelled wheat grains, to study the exchangeability parameter (α) and the NE-OBT and E-OBT recovery in this main component of wheat grains. Thereby, isotopic exchange studies are conducted on bio-indicators such as humic substances and obtained (α) parameters are related to the mobility of hydrogen in functional groups of interest for the transfer of metals in soils. Afterwards, the (α) parameter of Myriophyllum Spicatum, an aquatic macrophyte from La Loire (France), samples downstream of Dampierre nuclear power plant with initially 3.4 ±0.1 Bq.kg-1 of fresh material of OBT activity concentration is also highlighted.
The main aim of this work is to improve the global understanding of tritium exchange mechanisms in environmental matrices, to validate the E-OBT and NE-OBT information determined on the tritium labelling line and to develop the knowledge on migration processes of tritium in the environment.
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Astatine (At, Z=85) is a very rare radioelement belonging to the halogen group in the periodic table, below iodine.  All astatine isotopes are short-lived, the longest-lived one has a half-life time of 8.1 h. For research purpose, small quantities of astatine can be artificially produced (by a cyclotron). Actually, one may work with astatine only at ultra-trace concentrations (typically between 10-11 and 10-15 mol/L), which makes spectroscopic tools inapplicable to probe astatine molecular structures. Due to these facts, the basic chemistry of astatine has remained limited. One of the astatine isotopes, At-211, is considered as a promising candidate for targeted alpha-immunotherapy due to its suitable physical properties (half-life time of 7.2 h and 100% alpha emitter). A prerequisite is to label At-211 in a stable manner to a carrier-targeting agent, which requires knowledge on astatine speciation and reactivity. In this context, a project aiming at exploring the chemistry of astatine has started in Nantes in 2002 and my Ph.D. thesis is a part of it.
The halogen elements (I, Br, Cl) are known to be able to form halogen bonds. Recently, the very first halogen bonds involving astatine has been evidenced, that is between the astatine monoiodide (AtI) and nine Lewis bases. The first part of my Ph.D. thesis aims at extending the previous halogen-bond scale. So far, the interactions between AtI and eight other Lewis bases have been investigated by liquid/liquid competition method and a new record (i.e. the strongest halogen bond involving astatine) was found.
Moreover, the speciation of astatine in aqueous solution is of fundamental interest for chemists. It is generally assumed that astatide (At-) is the abundant species in basic media. However, a new anionic species, AtO(OH)2-, has been recently proved to predominate in basic and non-reductive conditions. The second part of my thesis is thus to properly characterize the astatine species and clarify their predominant domains in basic aqueous conditions.
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