Neptunium and technetium behavior in geological systems

Technetium, if present in groundwaters as the TcO4- ion, will not adsorb significantly on the minerals normally encountered in geological formations. Neptunium(V) has been reported to adsorb to some extent, but reported sorption ratios are sometimes as low as 0.5 L/kg. A number of investigators have suggested that if Tc(VII) and Np(V) are reduced, then the expected lower solubilities of the lower valence states might significantly reduce the potential rate of migration of these nuclides. There is also evidence that the lower valence forms of these elements may tend to be more highly adsorbed on geological components than the oxidized valence states. Thus, performance of these two nuclides in high level waste repositories may be dependent on whether reduction to a lower valence state actually occurs under repository geochemical conditions and whether the solubilities of the reduced species are as low and sorption as high as expected