# Tritium speciation in environmental matrices by isotopic exchange

Anne-Laure Nivesse, Nicolas Baglan, Gilles F Montavon, Olivier Peron

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Anne-Laure Nivesse, Nicolas Baglan, Gilles F Montavon, Olivier Peron. Tritium speciation in environmental matrices by isotopic exchange. MIGRATION 2019: 17th International Conference on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, Sep 2019, Kyoto, Japan. in2p3-02291303

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Submitted on 8 Jul 2021

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Distributed under a Creative Commons Attribution 4.0 International License
# Tritium speciation in environmental matrices by isotopic exchange

A-L. NIVESSE¹,²*, N. BAGLAN³, G. MONTAVON¹, O. PÉRON¹

¹ Laboratoire Subatech, Groupe Radiochimie, UMR 6457, IN2P3/CNRS/IMT Atlantique/Université de Nantes, 4 rue Alfred Kastler, BP20722, 44307 Nantes Cedex 3, France (*nivesse@subatech.in2p3.fr)

² CEA, DAM, DIF, F-91297 Arpajon, France

³ CEA, DIF, DRF, JACOB, IRCM, SREIT, LRT, F-91297 Arpajon, France

## Background

Tritium (³H) the radioactive isotope of hydrogen is a beta emitter with a half-life of 12.312 (25) years [1]. It can integrate organics molecules of living organisms by following the water cycle and form the organically bound tritium fraction (OBT). In 2017, the IRSN published a report on the update of knowledge on tritium in the environment [2], taking up some of the questions that have been waiting to be answered since the publication of the Livre Blanc du Tritium by ASN in 2010 [3]. Among these, those concerning the behavior, fate and speciation of this radionuclide in the environment are still relevant. The existence of two forms of OBT is commonly accepted: a non-exchangeable fraction (NE-OBT) and an exchangeable fraction (E-OBT) with the near environment. However, there is no consensus on their definition, therefore several one can be found in the literature.

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**OBJECTIVE** : The main goal is to improve the global understanding of tritium exchange mechanisms in environmental matrices, to validate the E-OBT and NE-OBT information and to develop the knowledge about tritium migration processes in the environment.

## Experimental approach

### Isotopic exchange « hard way »

In a tritium free water bath, tritium atoms in the exchangeable position are exchanged with the hydrogen atoms of the bath : E-OBT fraction is removed from the sample. E-OBT is measured in the exchange bath and NE-OBT remaining in the sample after combustion [4].

<figure><img src="image_5.png" /></figure>

The method leads to a potential solubilisation of a part of the sample. A proportion of the NE-OBT fraction may contribute to the E-OBT fraction measurement, which can induce an analytical bias. Solubilisation is currently estimated by elemental CHNS-O analysis of the sample before and after exchange.

### Isotopic exchange « soft way »

The samples are exposed to different tritiated atmospheres in order to set up a tritium vapor phase line, with controlled and stable temperature and relative humidity parameters. This method makes it possible to determine the fraction of exchangeable hydrogen ($\alpha_{iso}$) in the sample [5].

<figure><img src="image_6.png" /></figure>

$$ \left(\frac{T}{H}\right)_{\text{OBT}} = \alpha \times \left(\frac{T}{H}\right)_{\text{E-OBT}} + (1-\alpha) \times \left(\frac{T}{H}\right)_{\text{NE-OBT}} $$ $$ \left(\frac{T}{H}\right)_{\text{NE-OBT}} = \frac{\left[\left(\frac{T}{H}\right)_{\text{OBT}} - \alpha \times \left(\frac{T}{H}\right)_{\text{liq.}}\right]}{1-\alpha} $$

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

→ The ($\alpha_{iso}$) parameter determination highlights the BT form in a matrix by confrontation with the ($\alpha_{theoretical}$) parameter obtained from molecular models of the matrix constituents.

## Results and discussion

### 1. Humic substances :

Humic substances (HS) are part of the soil organic matter. Their molecular structures complexity favors the presence of BT [6].

**Isotopic exchange**

<figure><img src="image_9.png" /></figure>

**HS functional groups characterizations and contributions**

<table><thead><tr><th></th><th>Method</th><th>HS-Li</th><th>HS-Le</th></tr></thead><tbody><tr><td>α<sub>total acidity</sub></td><td>Ba(OH)<sub>2</sub> method</td><td>14.3 ± 2.0 %</td><td>14.5 ± 1.3 %</td></tr><tr><td>α<sub>carboxyl</sub></td><td>Ca(OAc)<sub>2</sub> method</td><td>0.14 ± 0.07 %</td><td>0.56 ± 0.03 %</td></tr><tr><td>α<sub>total OH</sub></td><td>ISO14900:2017 method</td><td>15 ± 7 %</td><td>15 ± 6 %</td></tr><tr><td>α<sub>depro</sub></td><td>Calculated</td><td>15 ± 11 %</td><td>16 ± 7 %</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>Stevenson (1982), $\alpha_{th}$ = 42 %</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Schulten and Schnitzer (1993), $\alpha_{th}$ = 13-16 %</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Andreux et al. (1994), $\alpha_{th}$ = 22-30 %</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Richard (2002), $\alpha_{th}$ = 19 %</figcaption></figure>

**Conclusion:** Humic substances (HS) isotopic exchange demonstrates a higher exchangeable capacity than characterizations of functional groups, pointing out the limitations of the standard methods in evaluating accessibility, reactivity and migration of the hydrogen element in soils.

### 2. Myriophyllum Spicatum (La Loire, France) and wheat carbohydrates (OBT WG) :

<table><thead><tr><th rowspan="2"></th><th colspan="2">Previous works [5]</th><th colspan="2">This work</th></tr><tr><th>Wheat grains (OBT WG)</th><th>Alpha and microcristalline cellulose</th><th>Carbohydrates* (wheat grains OBT WG)</th><th>Myriophyllum Spicatum (La Loire, France)</th></tr></thead><tbody><tr><td>(α<sub>th</sub>)</td><td>30 %</td><td>30 %</td><td>31.6 %</td><td>30 %</td></tr><tr><td>(α<sub>iso</sub>)</td><td>31 ± 1 %</td><td>21 ± 1 % and 13 ± 1 %</td><td>31.1 ± 1.0 %</td><td>26.4 ± 0.5 %</td></tr><tr><td>(α<sub>th</sub>) vs (α<sub>iso</sub>)</td><td>✓</td><td>≠</td><td>✓</td><td>≠</td></tr></tbody></table>

<figure><img src="image_14.png" /><figcaption>Molecular model of starch</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Molecular model of cellulose</figcaption></figure>

* Wheat carbohydrates :

- 85 % of starch ($\alpha_{th}$ = 30 %)

- 15 % of maltose ($\alpha_{th}$ = 36.4 %)

**Conclusions:**

- Exchangeable capacity model based on major constituent is validated for cereals matrices type.

- Molecular structure and conformation are responsible for the buried tritium (BT) form.

<table><thead><tr><th></th><th>Wheat grains (OBT WG)</th><th>Alpha and microcristalline cellulose</th><th>Carbohydrates* (wheat grains OBT WG)</th><th>Myriophyllum Spicatum (La Loire, France)</th></tr></thead><tbody><tr><td>NE-OBT (Bq.L<sup>-1</sup>) « soft way »</td><td>30.3 ± 2.6</td><td>-</td><td>24.3 ± 2.1</td><td>26.5 ± 2.3</td></tr><tr><td>NE-OBT (Bq.L<sup>-1</sup>) « hard way »</td><td>33.1 ± 2.6</td><td>-</td><td>32.7 ± 2.5</td><td>33.7 ± 3.1</td></tr><tr><td>NE-OBT deviation</td><td>8.5 ± 1.0 %</td><td>-</td><td>25.8 ± 2.9 %</td><td>21.3 ± 2.7 %</td></tr></tbody></table>

**Conclusion:** Analytical bias leaded by « hard way » isotopic exchange is verified according to the solubility properties of the studied matrix.

## Perspectives

<figure><img src="image_16.png" /></figure>

* **Analytical concerns:** Apple and simple carbohydrates (OBT WG)

<figure><img src="image_17.png" /></figure>

* **Speciation concerns:** Evolution of the ($\alpha_{iso}$) parameter depending on the degree of crystallinity of cellulose and environmental matrices applications.

## References

[1] LNHB, Mini Table de Radionucléides 2015, EDP Sciences, Les Ulis, 2015.

[2] IRSN, 2017, Actualisation des connaissances acquises sur le tritium dans l'environnement. IRSN/PRP-ENV/SERIS 2017-00004

[3] ASN, 2010, Livre Blanc du Tritium.

[4] Baglan, N., et al. (2010). Métrologie du tritium dans différentes matrices: cas du tritium organiquement lié (TOL). Radioprotection, 45(3), 369-390.

[5] Péron, O., et al. (2018). Towards speciation of organically bound tritium and deuterium: Quantification of non-exchangeable forms in carbohydrate molecules. Chemosphere 196 120-128.

[6] Pansu, M., & Gautheyrou, J. (2007). Handbook of soil analysis: mineralogical, organic and inorganic methods. Springer Science & Business Media.