XANES-Based Determination of Redox Potentials Imposed by Steel Corrosion Products in Cement-Based Media

The redox potential (Eh) in a cementitious nuclear waste repository is critical to the retardation behavior of redox-sensitive radionuclides (RNs), and largely controlled by embedded steel corrosion but hard to be determined experimentally. Here, we propose an innovative Eh determination method based on chemical/spectroscopic measurements. Oxidized nuclides (U VI , Se IV , Mo VI , and Sb V ) were employed as species probes to detect the Eh values imposed by steel (Fe 0 ) and steel corrosion products (magnetite/hematite, and magnetite/goethite couples) in cement pore water. Nuclides showed good sorption affinity, especially toward Fe 0 , in decreasing K d order for U > Sb > Se > Mo under both N 2 and H 2 atmospheres. The reduced nuclide species were identified as UO 2 , U 4 O 9 , FeSe, FeSe 2 , Se 0 , Sb 0 , and Sb 2 O 3 , but no redox transformation occurred for Mo. Eh values were obtained by using the Nernst equation. Remarkably, their values fell in a small range centered around -456 mV at pH ∼ 13.5 for both Fe 0 and Fe-oxyhydroxides couples. This Eh value appears to be controlled by the nanocrystalline Fe(OH) 2 /Fe(OH) 3 or (Fe 1- x ,Ca x )(OH) 2 /Fe(OH) 3 couple, whose presence was confirmed by pair distribution function analyses. This approach could pave the way for describing the Eh gradient in reinforced concrete where traditional Eh measurements are not feasible.