Spatial distribution, chemical state, solubility of rhenium in a reducing cement waste form: Implications for predicting technetium mobility in saltstone. (October 2017)

• Record Type: Journal Article
• Authors: Arai, Yuji; Meena, Amanda H.; Lenell, Brain; Powell, Brian A.; Kaplan, Daniel I.
• Is Part Of: Applied geochemistry. Volume 85:Part B(2017)
• Page Start: 180
• Page End: 187
• Publication Date: 2017-10
• Subjects: Rhenium -- Technetium -- Saltstone -- Immobilization -- Slag; Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2017.02.001
• Languages: English
• ISSNs: 0883-2927

Abstract

Abstract: Reducing cementitious materials (RCMs) are presently being developed for the long-term, subsurface disposal of low-level radioactive waste within the United States Department of Energy complex. Slag is included in the formulation of the RCM to facilitate the reductive precipitation of redox-sensitive radionuclides, particularly 99 Tc. Using Re(VII)O4 − as a chemical surrogate for Tc(VII)O4 −, depth sequence Re distribution, solubility, and chemical state were evaluated to provide a spatial measure of the impact of atmospheric O2 migration into field aged (<300 d) RCM monoliths. Total digestion analysis showed that total Re concentration increased with decreasing depth, indicating more Re was concentrated at the surface. A synchrotron based X-ray microprobe energy map also supports this finding. There was no clear depth dependent trend in Re solubility. Overall 30–60% of the total Re was soluble at the depth up to ∼3 cm. Partial reduction of Re(VII) to Re(IV) was also observed at these depths. These results suggest that long-term immobilization of Tc in RCM waste forms may be more complicated than previous thought and may require the consideration of non-uniform distribution of redox sensitive Tc in RCMs. Highlights: Saltstone Tc-99 immobilization technology is discussed. Spatial distribution and chemical state of Re were evaluated in 1yr-aged saltstone. Total digestion data suggested that Re was concentrated closer to the surface. Leaching studies indicated that only 30–60% of the Re could be readily leached. X-ray microprobe analysis supported the heterogeneous distribution of Re in saltstone.