Assessing the bioavailability of the radionuclides technetium-99,
      selenium-79 and uranium-238 in contaminated soils using the diffusive
      gradients in thin-films (DGT) technique
Creators: name: Chapman, Alex
Publisher: name: Lancaster University
Year: 2018

description: The increasing global inventory of 99Tc, 79Se and 238U as components of
      nuclear waste, coupled with the legacy of contaminated sites, means that
      increasing attention has been centred on understanding the environmental
      behaviour of these radionuclides. Over the last 15 years, the technique of
      diffusive gradients in thin-films (DGT) has emerged as a promising tool
      for assessing bioavailability based on its demonstrated ability to
      successfully predict plant uptake for a range of metals and metalloids in
      soil (Zhang and Davison, 2015). It is therefore utilised in this thesis to
      address a substantial knowledge gap regarding the bioavailability and
      aging of Tc, Se and U in soils. Initially, the performance of a
      Chelex-ferrihydrite mixed binding layer (MBL) DGT for the novel
      simultaneous measurement of Se and U was investigated to validate its
      suitability for subsequent use. An assessment of the availability and
      aging of all three elements was centred on an 18-month laboratory
      incubation of a suite of spiked soils, throughout which time a series of
      DGT deployments were made. Lastly, the ability of DGT to predict ryegrass
      uptake of Tc across a range of soil types was investigated. The
      experimental work presented in this thesis reveals that the MBL DGT is an
      acceptable method for determining labile Se and U in soil, although at
      higher pH (&gt; 7) and with increasing concentrations of HCO3 - ,
      uptake of both elements was impaired. The availability and aging of Se and
      Tc within soil is governed by soil organic carbon, in addition to the Al
      and Fe oxide content for Se. Quantitatively, the aging of Tc could be best
      described by a pseudo-secondorder model, yet a natural exponential
      function provided the best fit for Se. U was found to be particularly
      resilient to aging within soils exposed to a fluctuating wet-dry moisture
      regime, where changes in pH were hypothesised to alter the solubility of
      key U binding phases, namely Fe and Al oxides and dissolved carbonate
      ligands. DGT was unable to reliably predict the extent of uptake of Tc
      within ryegrass across a range of soil types, although in situ deployments
      whilst the ryegrass was actively growing did yield encouraging results.
      Remedial efforts to reduce the soil-to-plant radionuclide transfer
      following a contamination event should look to utilise the complexing
      capacity of Corg in the case of Tc and Se, or prevent the exposure of the
      soil to wetting/drying events in the case of U. Where possible, DGT
      measurements of bioavailability within Tc-contaminated soils should be
      made in situ, although in a field setting this is logistically challenging
      and introduces significant uncertainties to the data. In practice, to
      reliably ascertain the bioavailability of Tc, it would seem that the best
      approach is to directly measure the accumulated activity within the plant.