A long-term disposal of technetium-99 (99Tc) has been considered in a type of cementitious formulation, slag-based grout, at the U.S. Department of Energy, Savannah River Site, Aiken SC, U.S.A. Blast furnace slag, which contains S and Fe electron donors, has been used in a mixture with fly ash, and Portland cement to immobilize 99Tc(VII)O4−(aq) in low level radioactive waste via reductive precipitation reaction. However the long-term stability of Tc(IV) species is not clearly understood as oxygen gradually diffuses into the solid structure. In this study, aging effects of Tc speciation were investigated as a function of depth (<2.5 cm) in slag-based grout using X-ray absorption spectroscopy. All of Fe(II) in solids was oxidized to Fe(III) after 117d. However, elemental S, sulfide, and sulfoxide persists at the 0–8 mm depths even after 485d, suggesting the presence of a reduced zone below the surface few millimeters. Pertechnetate was successfully reduced to Tc(IV) after 29d. Distorted hydrolyzed Tc(IV) octahedral molecules were partially sulfidized and or polymerized at all depths (0–8 mm) and were stable in 485d aged sample. The results of this study suggest that variable S species contribute to stabilize the partially sulfidized Tc(IV) species in aged slag-based grout.
- Cementitious materials
ASJC Scopus subject areas
- Environmental Engineering
- Environmental Chemistry
- Waste Management and Disposal
- Health, Toxicology and Mutagenesis