Redox Interaction between Selenite and Mackinawite in Cement Pore Water

In cement-rich radioactive waste repositories, mackinawite (FeS) forms at the steel corrosion interface within reinforced concrete and potentially retards the transport of redox-sensitive radionuclides (e.g., ⁷⁹Se) in porous cement media. Redox interactions between selenite and mackinawite under hyperalkaline conditions remain unclear and require further investigations. Here, using comprehensive characterization on both aqueous and solid speciation, we successfully monitored the whole interaction process between selenite and mackinawite under hyperalkaline conditions. The results show similar chemical environments for SeO₃^2- and S^2-/S^2- at the mackinawite-water interface, verifying an immediate reduction. After 192 h of reaction, SeO₃^2- was reduced to solid Se⁰ and SeS₂ species, accompanied by the oxidation of S^2-/S^2- to S₂O₃^2- and Fe(II) to Fe(III) in mackinawite. Aqueous speciation results showed that ∼99% of aqueous selenium was present as Se₄S nanoparticles due to the dissolution of Se from the solid. In parallel, ∼62% of S^2-/S^2- was released into the solution, with mackinawite transforming into magnetite, Fe(OH)₃ and FeS₂O₃⁺ complexed to Cl^- or OH^- species, and magnetite subsequently dispersed in the solution. This study provides valuable data about the retardation mechanisms of redox-sensitive radionuclides by soluble iron sulfides, which is critical to advance our understanding of reactive concrete barriers used in nuclear waste disposal systems.