Author:

Keywords:

technetium, pyrogallol, EXAFS

Abstract:

The redox-sensitive fission product technetium-99 (Tc) is of great interest in nuclear waste disposal studies because of its potential for contaminating the geosphere due to its very long half-life (2.13105 year) and high mobility under oxidising conditions, where technetium forms pertechnetate (TcO4-). Under suitable reducing conditions, e.g. in the presence of an iron(II) containing solid phase which can act as an electrondonor, the solubility can be limited by the reduction of pertechnetate followed by the formation of a surface precipitate [1]. However, by association with mobile humic substances (HS) or other associating/complexing species, the solubility of reduced Tc species may be drastically enhanced [2]. The elucidation of the identity and geometrical structure of the species causing this enhanced solubility often remains a difficult issue. EXAFS/XANES analysis can be very helpful to determine the molecular surroundings of reduced Tc organic complexes or Tc(IV) colloid-HS-associations. The interpretation of the EXAFS spectra is however not a straightforward process due to the strong influence of multiple scattering paths on the spectra.. Contrary to single scattering analysis which can be easily performed on the experimental EXAFS spectra, multiple scattering analysis can only be done, based on good approximations of the possible geometrical structures of the species under consideration. As the structure of humic substances is not exactly known and would be too complex for modelling, pyrogallol was used as a model compound for phenolic functional groups in natural organic matter. Detailed multiple scattering analysis of EXAFS data of Tc(IV)/pyrogallol solutions based on DFT modelled reference structures revealed the existence of a stable Tc-pyrogallol complex which is readily formed at pH 11 upon reduction of pertechnetate by hydrazine or dithionite in presence of pyrogallol. This initial complex serves as precursor for a pH dependent series of pyrogallol complexes exhibiting a reasonable stability towards technetium colloid or precipitate formation when lowering the pH from 11 to 2 after synthesis. The occurrence of stable, readily formed Tc-complexes with a humic substance model compound potentially changes current knowledge about Tc(IV) behaviour in natural systems containing free organic matter. This observation is an indication for an extra competition between the formation of immobile Tc(IV) precipitates upon reduction of TcO4- or potentially mobile organic Tc(IV) complexes and previously discovered Tc(IV) eigencolloids stabilised by dissolved HS through colloid-colloid interactions [3, 4]. As the slow reduction of TcO4- in the presence of phenolic functional groups at high pH is expected to occur during storage lifetime this competition could have important implications for the current knowledge of the migration behaviour of Tc. 1. Cui, D.Q. and T.E. Eriksen, Reduction of pertechnetate in solution by heterogeneous electron transfer from Fe(II)-containing geological material. Environmental Science & Technology, 1996. 30(7): p. 2263-2269. 2. Maes, A., et al., Quantification of the interaction of Tc with dissolved boom clay humic substances. Environmental Science & Technology, 2003. 37(4): p. 747-753. 3. Maes, A., et al., Evidence for the Formation of Technetium Colloids in Humic Substances by X-Ray Absorption Spectroscopy. Environmental Science & Technology, 2004. 38(7): p. 2044-2051. 4. Geraedts, K., et al., Evidence for the existence of Tc(IV) - humic substance species by X-ray absorption near-edge spectroscopy. Radiochimica Acta, 2002. 90(12): p. 879-884.