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

Mössbauer spectrometry insights into the redox reactivity of Fe-bearing phases in the environment

“…1). Similarly such sample recorded at 77 K, then at 300 K, then again at 77 K showed different clay mineral redox signatures because of an incomplete sealing of the material and its contact with atmospheric O 2 during the 24 h measurement at 300 K. In contrast, Mössbauer measurements carried out on COx and Fe III -bearing synthetic Montmorillonite samples after reaction with H 2 evidenced a very limited or a lack of reduction up to 120 °C [91,111]. These observations helped to realize (i) how difficult it is to work in surface laboratories with clay rock samples having redox properties truly representative of in situ conditions, and (ii) that an understanding of hysteretic processes is needed to predict the evolution of clayey barriers in repository systems with regards to redox conditions.…”

“…In experiments in which natural clay minerals were exposed to H 2 , 57 Fe Mössbauer spectrometry hyperfine parameters were different before and after reaction with H 2 (g) in dry condition, indicative of the formation of a Fe III -Fe II partially reduced composite system within the clay structure [111].…”

“…Comparison of Fe oxidation and Se reduction rates using Mössbauer spectrometry and X-ray Adsorption Near-Edge Spectroscopy (XANES) evidenced a lag time between the two processes, leading to the conclusion of a temporary storage of either e − in the clay layer or H 2 as intermediate reducing species [118]. Sorption of H 2 (g) in clay interlayer has been since documented by combining Mössbauer spectroscopy and quasi-elastic neutron scattering (QENS) and shown to be highly dependent for Montmorillonite on water activity [91,111,121,122], but not for other clays such as sudoite [123]. Physisorption of H 2 has been shown to occur within laponite clay interlayer by hopping from an exchangeable cation to the next [124].…”