Residence time effects on technetium reduction in slag-based cementitious materials

Area of Science:

• Geochemistry
• Materials Science
• Environmental Science

Background:

• Technetium-99 (⁹⁹Tc) immobilization is crucial for low-level radioactive waste disposal.
• Slag-based grout, utilizing blast furnace slag, fly ash, and Portland cement, is a potential matrix for ⁹⁹Tc.
• Long-term stability of reduced technetium (Tc(IV)) species in grout under oxidizing conditions is not well understood.

Purpose of the Study:

• To investigate the aging effects on technetium speciation within slag-based grout.
• To assess the long-term stability of immobilized ⁹⁹Tc(VII)O₄^-(aq) as it transforms into Tc(IV) species.
• To understand the role of sulfur and iron species in stabilizing Tc(IV) over time.

Main Methods:

• X-ray absorption spectroscopy (XAS) was employed to analyze Tc speciation.
• Samples were analyzed at various depths (<2.5cm) and aging times (up to 485 days).
• Changes in iron (Fe) and sulfur (S) speciation were correlated with Tc stability.

Main Results:

• Iron(II) in the grout was fully oxidized to Iron(III) within 117 days.
• Elemental sulfur, sulfide, and sulfoxide persisted in the grout up to 8mm depth after 485 days, indicating a reduced zone.
• Pertechnetate (⁹⁹Tc(VII)O₄^-) was reduced to Tc(IV) within 29 days.
• Distorted, hydrolyzed Tc(IV) octahedral molecules were found to be partially sulfidized and/or polymerized.
• These Tc(IV) species remained stable in samples aged for 485 days.

Conclusions:

• Variable sulfur species play a key role in stabilizing partially sulfidized Tc(IV) species within aged slag-based grout.
• The persistence of reduced sulfur species contributes to maintaining a reductive environment.
• Slag-based grout demonstrates potential for long-term immobilization of ⁹⁹Tc in radioactive waste.