The Missing Sink? Controls on Iodine Migration in the Geosphere

129I (t1/2 1.57 x 106 years) is a high yield fission product which has a complex and poorly understood biogeochemistry in the Earth’s subsurface. Some forms are potentially environmentally mobile such that return of 129I to the biosphere may ultimately challenge the Environmental Safety Case for the Geological Disposal Facility (GDF). Our hypothesis is that there are features of iodine biogeochemistry, specifically its biotransformation, and sorption on to secondary minerals in the vicinity of the GDF, which will retard its transport through the geosphere but are currently not captured in the Environmental Safety Case.
There will be three components in this project-
1. Iodine speciation. The first stage of this project will be to develop tools for the quantitative determination of different chemical forms of 129I. These will be based on High Performance Liquid Chromatography for inorganic and small molecular species, and on Size Exclusion Chromatography for larger organic species. Once proven, these separations will be coupled to ICP-MS for direct determination of iodine species. Initial experiments will use stable iodine, with 125I radiotracer where needed to aid development, before progressing to realistic mass concentrations of 129I. Chromatographic techniques will be complemented by synchrotron measurements as required (1).
2. Biotransformation of Iodine. Hydrogen will be in the vicinity of the GDF so, building on earlier work we have done (2), we will focus on hydrogen-driven microbial transformations of iodine, particularly the response of different chemical forms to progressive reduction through the NO3-, Fe(III), SO42- electron acceptor sequence, across the pH gradient (11 decreasing to circumneutral) expected in the vicinity of the GDF. The incorporation of iodine into microbial biomass will also be quantified. Initial experiments will use well characterised single organisms, progressing to mixed cultures, typical of subsurface communities and including fully characterised alkaliphiles. The structural and functional evolution of these communities will also be explored.
3. Mineral Sinks for Iodine. There is some evidence that minerals (e.g. illite, chlorite, saponite, montmorillonite) in the chemically disturbed zone and /or far field of a GDF may have some ability to sorb iodine species (4) but their significance is unclear. The affinity of microbial biomass and natural organic matter for iodine is better established (5). However, in both cases, the extent, reversibility and susceptibility to competition from major ions are yet to be explored fully, and this will be undertaken under conditions expected in the vicinity of a GDF in this element of the project.