Cellulose, proteins, starch and simple carbohydrates molecules control the hydrogen exchange capacity of bio-indicators and foodstuffs
Introduction
At the present time, tritium is one of the main radionuclides released into the environment at nuclear installations. According to current forecasts, these release rates are expected to rise due to the planned development of nuclear power plants and their fuel management methods, as well as to new tritium emitting facilities, such as the International Thermonuclear Experimental Reactor (ITER) and the Evolutionary Power Reactor (EPR). Understanding the behavior of tritium in the environment is therefore an ongoing societal issue, in recognizing this behavior to be directly related to the chemical forms of tritium, i.e. tritium speciation (ASN, 2010; IRSN, 2017). In environmental matrices, tritium is found in the form of Tissue-Free Water Tritium (TFWT) and Organically Bound Tritium (OBT) after the integration of tritiated water (HTO) during photosynthesis and metabolic processes (Diabaté and Strack, 1993; Pointurier et al., 2003). Over the last decade, a focus on monitoring OBT has become a major concern in many countries for both public and regulatory assurance (Kim et al., 2013; Péron et al., 2016; Baglan et al., 2018). OBT is typically differentiated into two pools: the exchangeable pool (E-OBT), which equilibrates with the surrounding atmosphere; and the non-exchangeable pool (NE-OBT), which is experimentally inert and remains in organic matter until its degradation (Sepall and Mason, 1961; Kim et al., 2013). The latter is directly representative of the amount of tritium released into the environment during growth of the biological organism; the interest of its study lies in the ability to conduct retrospective studies of tritium release into the environment.
From an analytical standpoint, it is widely assumed that the E-OBT fraction corresponds to the tritium bound to heteroatoms, while the NE-OBT fraction corresponds to the tritium covalently bound to carbon (Mann, 1971; Kim et al., 2013). Based on the molecular model, it is then possible to assign to each organic molecule a theoretical exchangeable parameter from the hydrogen bound to the heteroatom pool versus total hydrogen atoms. However, this description has been challenged since previous studies highlighted major limitations of E-OBT accessibility in environmental matrices (Sepall and Mason, 1961; Baumgartner and Donhaerl, 2004; Péron et al., 2018) when comparing the theoretical parameter to an experimentally determined parameter (Feng et al., 1993; Péron et al., 2018). Molecular conformation is thus supposedly responsible for the loss of exchange capacities from a part of the theoretically exchangeable hydrogen positions, hence the designation buried tritium (BT) (Baumgartner and Donhaerl, 2004). The IAEA (International Atomic Energy Agency) has therefore suggested in its EMRAS program (EMRAS, 2010) that the NE-OBT fraction must be defined as both covalently carbon bound tritium atoms and buried tritium atoms (Kim et al., 2008, 2013). This issue is still undergoing heated debate and requires further investigation.
The present work is therefore aimed at providing insight into tritium speciation and distribution on environmental matrices by means of evaluating the impact of molecular composition and arrangement on OBT behavior. To this end, it is being proposed herein to: (i) compare and parameters and, through reliance on the knowledge of molecular structures, grasp the origin of the buried tritium form, (ii) define the exchangeable capacity of environmental matrices compared to their main constituent, and (iii) assess the NE-OBT distribution with respect to both structural and exchangeable aspects.
Starch, cellulose, proteins and simple carbohydrates are among the most widespread biomolecules of the biomass on earth and may be found under diverse molecular structures in environmental matrices (Sun and Cheng, 2002; Hopkins, 2003; Sakintuna et al., 2003). Two food chain matrices, i.e. wheat grains and apples, plus a bio-indicator, water-milfoil, presenting initial anthropogenic OBT activities, have been selected while their main constituents were effectively extracted in order to represent the corresponding biomolecule types and undergo exchangeability assessments.
To access the exchangeable parameter, an original methodology based on isotopic exchange under a soft path regime has been previously developed (Péron et al., 2018) and subsequently validated for exchange capacity investigations in carbohydrate molecules. Knowledge of this parameter thus yields insight into the true nature of the exchangeable hydrogen pool within a studied environmental matrix and serves to improve our understanding of OBT speciation (Nivesse et al., 2020).
Section snippets
Reagents and chemicals
Tritium solutions were prepared at the Subatech Laboratory using a certified and calibrated source and a low-level tritium water source “Eau des Abatilles” (whose HTO activity lies significantly below 0.2 Bq.L−1 (Fourré et al., 2014)).
All reagents were purchased from Fisher Scientific International and met at least ACS reagent grade (i.e. match or exceed the specifications established by the American Chemical Society). Ultrapure water (18.2 MΩ cm resistivity at 25° and < 5 μg.L−1 TOC) obtained
Molecular model-based theoretical exchangeable parameter ()
For each studied matrix, a theoretical exchangeable parameter () was calculated from the molecular model of its main components and respective experimentally determined composition in % by weight. The main essential components analyzed along with their associated and distributions in % by weight in the A-1, A-2, B-1, B-2, C-1 and C-2 matrices are listed in Table 2. The distribution in % by weight of starch (A-2) in wheat grains (A-1) was directly measured at 75.3 ± 3.7% of
The impact of molecular structure on hydrogen exchangeability
Among all the matrices studied, only wheat grains (Matrix A-1) and their extracted starch (Matrix A-2) presented both isotopic exchangeable parameter results ( = 31.0 ± 1.0% and = 31.1 ± 1.0%, respectively) similar to their calculated theoretical exchangeable parameters ( = 30.0% and = 31.6%, respectively). Wheat grains are mainly composed of starch, a macromolecule comprising 25% amylose (α(1 → 4) bound glucose molecules) and 75% amylopectin (α(1 → 4) bound
Conclusion
The impact of molecular structure and conformation on tritium behavior has been highlighted and evaluated in both food chain and bio-indicator matrices, as well as in widespread biomolecules contained in the Earth’s biomass. Theoretical parameters were compared to experimentally determined parameters to understand OBT speciation of studied matrices in respect to their molecular constituent. Complex 3D molecular structures of cellulose, proteins and simple carbohydrates have been
Credit author statement
A-L Nivesse: Investigation, Validation, Writing - Original Draft, Writing - Review & Editing, Visualization. N. Baglan: Conceptualization, Methodology, Resources, Writing - Review & Editing, Supervision. G. Montavon: Supervision. G. Granger: Investigation, Validation. O. Péron: Conceptualization, Methodology, Resources, Writing - Review & Editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financed by the CEA Research Center, Subatech Laboratory, France’s Loire Valley Regional Council (under the POLLUSOLS OSUNA Project) and the EDF utility company. The authors would also like to thank the members of the Environmental Laboratory at the Cernavodă Nuclear Power Plant for providing the apple matrix and Dr. Gurvan Rousseau from SMART Nantes laboratories for supervising water-milfoil matrix sampling.
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