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

Abstract

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, Se, Mo, and Sb) were employed as species probes to detect the Eh values imposed by steel (Fe) and steel corrosion products (magnetite/hematite, and magnetite/goethite couples) in cement pore water. Nuclides showed good sorption affinity, especially toward Fe, in decreasing K order for U > Sb > Se > Mo under both N and H atmospheres. The reduced nuclide species were identified as UO, UO, FeSe, FeSe, Se, Sb, and SbO, 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 and Fe-oxyhydroxides couples. This Eh value appears to be controlled by the nanocrystalline Fe(OH)/Fe(OH) or (Fe,Ca )(OH)/Fe(OH) 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.

Atomistic structure of alkali-silica reaction products refined from X-ray diffraction and micro X-ray absorption data

“…The comparative study of XANES is based on the fact that similar XANES spectra often indicate similar chemical environment, since a XANES spectrum contains abundant features that often alter due to a change of neighboring atoms or their coordination geometry [29]. This has been extensively demonstrated for cementitious systems [30][31][32][33][34]. Here, eight points of interests (POIs) were selected for XANES measurements along the veins of ASR products in ES1, ES2 and N1, in a way that the measured spots move from crystalline region to amorphous region, as shown in Figure 2.…”

Selenite Sorption on Hydrated CEM-V/A Cement in the Presence of Steel Corrosion Products: Redox vs Nonredox Sorption

“…A similar phenomenon was observed in the case of M/G (Figure 3c). In addition, the selenium sorbed species have been investigated in our previous work, [20] showing that at least ~50% sorbed Se(IV) was reduced under similar experimental conditions. The study indicated that both Fe(0) and magnetite still showed strong reducing ability towards Se(IV) in hyperalkaline CPW.…”

“…Fe 0 and its corrosion products can also immobilize Se(IV) through reductive precipitation and surface adsorption (inner-sphere complexation in most cases). [19,20] Furthermore, most CEM-V/A cement probably contains Fe-(oxyhydr)oxides and metal (bi)sulfides, which originate from the blended BFS and FA, and which are effective for RNs removal. As previously reported, under acidic to slightly alkaline conditions, Fe-(bi)sulfides (e.g., pyrite, [21] pyrrhotite, [22] greigite, [14,23] and mackinawite [24]) are able to efficiently immobilize Se(IV), resulting in Se(0), FeSe, and/or FeSe 2 reduction products.…”

“…In addition, comparing with the leaching kinetic experiment of HCEM-V (Figure S3),[MBS] in batch sorption experiments were generally lower probably due to the introduction of Se(IV) oxidant. In conclusion, introducing Fe-(oxyhydr)oxides into the hydrated ternary blended cement would not obviously enhance its uptake performance towards selenite, although Fe-(oxyhydr)oxides exhibited good affinity for selenite in hyperalkaline conditions via reductive immobilization and adsorption [20]. The lack of reactivity in the reactors with extra amounts of Fe-(oxyhydr)oxides could be due to the inherent existence of reducing and oxidizing Fe phases in blended cement [37,38].…”

Influence of Silica Coatings on Magnetite-Catalyzed Selenium Reduction

“…In reductive precipitation processes, Fe II -bearing minerals catalyze the reduction of Se IV forming predominantly Se 0 , but also Fe selenides (FeSe, and FeSe2) depending on the experimental conditions. 36,37,39,77,78 In adsorption processes, sorbed Se IV may form inner-sphere (FeSeO3) and outer-sphere (Na2SeO3 electrostatic sorption) complexes. 77,[79][80][81] Although Se IV is generally sorbed by bidentate inner-sphere complexation, 78 the exact Se IV solid species in our system are hard to describe by our XANES measurements.…”