Saturday, 15 July 2006

In Situ Study of the Biochemical Cycle of Iodine in a Peat Bog.

Sophie Maillant1, Genevieve Villemin1, Marsha Sheppard2, and Elisabeth Leclerc-Cessac3. (1) Laboratoire Sols et Environnement UMR 1120 INPL-INRA, 2 av de la Foret de Haye, Vandoeuvre-lès-Nancy,, 54500, France, (2) ECOMatters Inc., 24 Aberdeen Ave., Pinawa, MB R0E1L0, Canada, (3) Andra, Direction Scientifique, Service Transferts, 1-7 rue Jean Monnet, Châtenay-Malabry, 92298, France

Iodine-129 along with its sister halogen Cl-36, are the two most significant radionuclides contributing to the human dose from recent safety assessments of deep or near-surface geological disposal of nuclear waste (SKB, 1999; Andra, 2005). Since iodine is generally present in environments as an anion (either iodide or iodate), it is soluble in groundwater and migrates relatively quickly through rock and overburden to low-lying landscape positions, such as wetlands. Iodine is generally retained by the organic matter of soils, and peat bogs combine both characteristics: they are often located at low-lying landscape positions and are very rich in organic matter. Therefore iodine is likely to accumulate in peat bogs. As iodine in peat is available for plant uptake, natural bog plants are likely to transfer iodine from the soil to natural food-chains and the use of peat in food production and other activities could transfer iodine directly to humans (Sheppard et al., 1989a). The fate of stable iodine, after a simulated localized input, was monitored in a peat bog under natural conditions (Sheppard et al., 1989b) to characterise the transfer of iodine in the groundwater-plant continuum. Since then, iodine has accumulated at the surface and has been fixed onto or within the peat material. Microscopic observations, using Transmission Electronic Microscope (TEM) and Energy Dispersive X-ray spectroscopy (EDXS) have shown the association of iodine with polyphenolic structures (PS) of humified plant cells (Maillant et al., submitted). Furthermore, amongst the plant species of the bog, sedges had the highest concentrations of iodine (up to 384 mg L-1). Therefore we suggest two processes that would lead to the association of iodine with the PS. First, this association could occur during the formation of the PS within the cells, i.e. during the senescence of the leaves. Second, it could be related to the variations of the redox potential due to the variations of the groundwater level as these variations favour the activity of the microorganisms, which in turn are able to link iodine with organic molecules. Here we present the results of the investigation for the first hypothesis. We have collected leafy tissues of various plant species at several stages of senescence, including leaves contained in the litter of the bog. These tissues were treated and analysed with TEM and EDXS to identify the possible association of iodine with cell structures. Present results indicate that PS have different structures depending on the species considered and that in the sedges the PS are associated with plasts remnants. Analyses of the iodine content are currently under process.

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