Reduction of Se(IV), Se(VI) and Re(VII) by Fe reactive phases present in CEMs and COx

The retention including sorption and redox transformations of radionuclides (RNs) (e.g., ⁷⁹Se and ⁹⁹Tc) by multiple barriers is critical for assessing the risks of a nuclear waste repository. Both cementitious materials and the embedded steel corrosion products e.g., magnetite, mackinawite and pyrite, keep the reinforced cementitious structures under hyperalkaline and chemically reducing conditions. A precise determination of the redox potential (Eh) of the system is vital to assess the retention of redox-sensitive RNs. Furthermore, pyrite, magnetite and mackinawite as the naturally present redox-active Fe phases have the potential for the retention of Se and Tc leakage in nuclear waste disposal systems and enriched Se contamination. In my PhD thesis, redox interactions between selenite and mackinawite in cement pore water (CPW) was investigated by a comprehensive characterization on both aqueous and solid speciation. X-ray Absorption Spectroscopy (XAS) reveals that Se(IV) was reduced to a mixture of Se(0) and SeS₂, accompanied by the oxidation of S^2-/Sn^2- to S₂O₃^2- and Fe(II) to Fe(III) in mackinawite identified by X-ray photoelectron spectra (XPS). However, ∼99% of aqueous selenium was present as Se₄S nano-particles confirmed by transmission electron microscopy (TEM) via the dissolution of Se from the solid. In parallel, ∼62% of S^2-/Sn^2- was released into the solution, with mackinawite transforming to magnetite, Fe(OH)₃ and FeS₂O₃⁺ complexed to Cl^- or OH^- species. Furthermore, Se (VI) were used as a molecular probe to explore redox interactions by mackinawite, pyrite and magnetite in CPW and the Eh values of the systems were obtained by using the Nernst equation based on chemical/spectroscopic measurements. Remarkably, the measured Eh value around -0.388 V of Se(VI) reaction with mackinawite in CPW system was mainly controlled by the FeO(OH)/FeS couple. In the system of Se(VI) reaction with magnetite in CPW, the measured Eh was around -0.536 V probably determined by the Fe₃O₄/Fe²⁺ couple. It seems that the Eh value (-0.350 V) of pyrite reaction with Se(VI) in CPW system was imposed by the Fe(OH)₃/Fe(OH)₃^- couple. In addition, the reduction of Se(VI) and Re(VII), as the chemical surrogate for Tc(VII), by mackinawite, magnetite and pyrite has been investigated by XAS and S/TEM in 0.1 M NaCl solutions under air, N₂ and H₂ atmospheres. The results show that Se(VI) could be reduced to Se(-II, -I and 0) and three different morphologies and structures of Se(0) formed: trigonal γ-Se on magnetite, monoclinic β-Se on mackinawite and amorphous Se on pyrite. Also, the retention mechanisms of Re(VII) follows non-redox complexation and reduction to ReO₃ under air atmosphere, but reductive precipitation, i.e., Re(IV), under N₂ and H₂ atmospheres. The Eh of the suspensions, effected by dissolved aqueous O₂ and H₂, have an impact for the Se and Re retention. Besides, Aqueous analysis shows that Se(VI) uptake was not significant with quite the same value (K_d = 4∼5 mL/g) on Callovo-Oxfordian in both N₂ and H₂ conditions. Se(VI) was adsorbed on nature pyrite, and reduced to Se(0) directly under N₂ condition, but stepwise reduction of Se(VI) to Se(IV) and then to Se(0) by natural pyrite was observed under H₂ condition. H₂ could dissociate into H^- on pyrite (210) surface and H^- probably favor the reduction of Se(VI). The above results obtained could provide valuable data for the safety assessment of nuclear waste disposal and a better understanding for the reductive precipitation of enriched Se contamination in natural confinement. (author)