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 (UVI, SeIV, MoVI, and SbV) were employed as species probes to detect the Eh values imposed by steel (Fe0) and steel corrosion products (magnetite/hematite, and magnetite/goethite couples) in cement pore water. Nuclides showed good sorption affinity, especially toward Fe0, in decreasing Kdorder for U > Sb > Se > Mo under both N2and H2atmospheres. The reduced nuclide species were identified as UO2, U4O9, FeSe, FeSe2, Se0, Sb0, and Sb2O3, 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 Fe0and Fe-oxyhydroxides couples. This Eh value appears to be controlled by the nanocrystalline Fe(OH)2/Fe(OH)3or (Fe1–x,Cax)(OH)2/Fe(OH)3couple, 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.