Batch experiments have been used to assess the sorption properties of a potential repository backfill, NRVB (Nirex Reference Vault Backfill). In this study, UO22+, Eu3+, Am3+ and Th4+ have been used as model radionuclides and ethylenediaminetetraacetic acid (EDTA), isosaccharinic acid (ISA) and humic acid (HA) as competing ligands. The NRVB is an effective scavenger of all radionuclides, with the majority sorbed within minutes. Ultrafiltration showed that for solutions of U in contact with NRVB, for the small amount of U remaining in solution, nearly all (79 %) was present as clusters or colloidal material in a very narrow and relatively small size range (0.9 - 1.4 nm); for Eu (> 94 %) is attached to large NRVB derived colloids or particulates; for Th (82 %) is present in the true solution; whilst for Am 58 % is in the true solution.High concentrations of EDTA (>0.01 M) were able to reduce the extent of sorption at apparent equilibrium for all metal ions. ISA was very effective as a competing ligand for all metal ions, generally at a lower concentration than EDTA in equivalent systems. Humic acid was found to be ineffective as a competing ligand at any realistic concentration. In all systems, there was evidence of significant irreversibility, with concentrations of EDTA and ISA that were able to prevent sorption unable to remove radionuclides from contaminated NRVB. For the uranyl systems, luminescence spectroscopy was used to analyse the mechanism of sorption. For CSH (calcium silicate hydrate), the spectra were consistent with surface complexation, followed by some degree of incorporation. For NRVB, the spectrum was dominated by a feature that was similar to uranyl sorbed to CSH as a surface complex and/or incorporated into the structure. There was also a minor component that was assigned as a calcium uranate like surface precipitate.The sorption data were fitted with a simple surface complexation model, which had a single surface binding site. The modelling showed that the uptake of all radionuclides is consistent with surface complexation or surface precipitation. The model was less effective at predicting the effects of the competing ligands on sorption. Thermodynamic speciation and surface complexation modelling were able to explain the behaviour in the systems qualitatively, but cannot be used to predict sorption absolutely.
|Date of Award
|1 Aug 2014
- The University of Manchester
|Nicholas Bryan (Supervisor)
- Nirex Reference Vault Backfill (NRVB)