Method development for evaluating the redox state of Callovo-Oxfordian clayrock and synthetic montmorillonite for nuclear waste management

• Three reduction methods were studied with 57Fe Mössbauer spectrometry on Fe-clays.
• Reduction with H2(g) in dried condition shows intermediate hyperfine parameters.
• A complex mechanism occurs for structural Fe(III) reduction in this condition.
• H2(g) is a powerful reductive reagent as sodium dithionite in our study.
• The clayrock redox and confinement properties will only be slightly affected by hydrogen gas.

Understanding the redox characteristic of the host geological layer is vital for radioactive waste management. In order to predict the radionuclides behavior during their release it must be thoroughly evaluated. This redox property could be affected by hydrogen gas which arises from the anaerobic corrosion of the stainless steel container. In this study, reduction methods using hydrogen gas or sodium dithionite as a reductive agent were tested on reference synthetic montmorillonites with various Fe(III) contents. The reduced samples were systematically studied with 57Fe transmission Mössbauer spectrometry. After reduction with H2(g) in dry conditions, the Mössbauer spectra are characterized by hyperfine parameters located between those for Fe(III) and Fe(II), compared to reduction in water suspension with Na2S2O4(aq) and H2(g) which gives standard Fe(II) hyperfine parameters. The former results with dry H2(g) highlight an incomplete reduction and the possibility to have a Fe(III)–Fe(II) system with one-electron sharing. A natural clayrock sample, the Callovo-Oxfordian (COx), was also considered. The results above allowed its reduction to be evaluated. Much attention has also been focused on the modeling of the hyperfine spectra because of COx structure complexity. In addition, a hyperfine parameter data base was developed for a variety of Fe components based on an extensive literature review. This database provides additional statistical order to the study. This study highlights also that Mössbauer spectrometry remains a useful and non-destructive method to determine the reduction process and the reduction capacity of a reactant. Therefore we could estimate the redox property of the rock to evaluate and predict radionucleide behavior for nuclear waste management.