An X-ray absorption study of the fate of technetium in reduced and reoxidised sediments and mineral phases

Technetium is a long lived (2.13 × 105 a), beta emitting radionuclide which is a groundwater contaminant at a number of nuclear facilities throughout the world. Its environmental behaviour is primarily governed by its redox state. Under oxic conditions it forms the highly soluble pertechnetate (TcO4-) ion; under reducing conditions it forms the poorly soluble, reduced forms of Tc, particularly the Tc(IV) ion which is expected to precipitate as hydrous TcO2 above its solubility limit (10−9 mol l−1 at ∼pH 7) or to be strongly sorbed to mineral surfaces at lower concentrations. Thus the redox cycling behaviour of Tc is predicted to be key to its environmental behaviour in the natural and engineered environment. Here the results of a series of X-ray absorption spectroscopy (XAS) experiments which examine the oxidation state and coordination environment of Tc in a range of estuarine, aquifer and freshwater sediment suspensions, and in an environmentally relevant amorphous Fe(II) phase under both reduced and reoxidised biogeochemical conditions are presented. In reduced sediments and the amorphous Fe(II) phase prior to reoxidation, XAS results show that Tc was retained as hydrous TcO2-like phases across all samples. Under air reoxidation, experiments showed significant (up to 80%) remobilisation of Tc to solution as TcO4-. In pre-reduced freshwater sediments, aquifer sediments and the amorphous Fe(II) phase oxidised with air, XAS indicated that Tc remained associated with the solids as hydrous TcO2-like phases. By contrast, in air reoxidised estuarine sediment XAS analysis suggested that both hydrous TcO2-like phases and TcO4- were retained within the sediment. Finally, when microbially-mediated NO3 reoxidation occurred in estuarine and aquifer sediment slurries, experiments showed comparatively low (<8%) remobilisation of Tc from solids over similar timescales to air reoxidation experiments, whilst XAS again showed that both hydrous TcO2-like phases and TcO4- were retained within the sediment. By contrast, in the amorphous Fe(II) phase, although NO3 reoxidation again led to low (<4%) remobilisation of Tc from solids, XAS analysis showed that Tc was retained as hydrous TcO2-like phases alone. These results are discussed in the context of the redox cycling behaviour of Tc in the natural and engineered environment.