The chemistry of technetium in cement waste forms has been studied with x-ray absorption fine structure (XAFS) spectroscopy. Using the Tc K-edge x-ray absorption near-edge structure (XANES) as a probe of the technetium speciation, our results show that partial reduction of the pertechnetate ion, Tc04-, takes place in the presence of the cement additive, blast furnace slag (BFS). The addition of the reducing agents FeS, Na2S, and NaH2P02 produces more extensive reduction of Tc04-, while the compounds FeO, Fe304, and Mn304 are observed to be unreactive. The extended x-ray absorption fine structure (EXAFS) data for the BFS, Na2S, and FeS treated cements indicate the presence of Tc clusters possessing first shell S coordination. For the Na2S and FeS additives, Te-Te interactions are detected in the EXAFS demonstrating an extended structure similar to that ofTcS2. The EXAFS spectrum of the NaH2P02 treated cement reveals Te-O and Te-Te interactions that resemble those found in the structure ofTc02.

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Approximately 400 × 106 liters of low-level alkaline salt solution will be treated at the Savannah River Plant (SRP) Defense Waste Processing Facility (DWPF) prior to disposal in concrete vaults at SRP. Treatment involves removal of Ca+and Sr+2 followed by solidification and stabilization of potential contaminants in saltstone, a hydrated ceramic waste form.Chromium, technetium, and nitrate releases from saltstone can be significantly reduced by substituting hydraulic blast furnace slag for portland cement in the formulation designs. Slag-based mixes are also compatible with Class F fly ash used in saltstone as a functional extender to control heat of hydration and reduce permeability. (Class F fly ash is locally available at SRP.)A monolithic waste form is produced by the hydration of the slag and fly ash. Soluble ion release (NO3−) is controlled by the saltstone microstructure (bulk porosity and pore size and connectivity). Chromium and technetium are 103−104 times less leachable from slag mixes than from cement-based waste forms. Results suggest that chemical stabilization rather than physical entrapment, as in the case of nitrate is responsible for this improved leaching. Reduction of Cr+6 and Tc+7 to Cr+3 and Tc+4 by ferrous iron or Mn+2 in the slag and subsequent precipitation of the relatively insoluble phses Cr(OH)3 and TcO2 are proposed as the stabilizing reactions.

URS Washington Group, Quality and Testing Division management and technical personnel provided use of the SRS Civil Engineering Laboratory and test equipment, and technical expertise and input that was essential for performing the tests and obtaining the results in this report. W. Pope, Jr., W. B. Mhyre, and J. T. Waymer, URS, are recognized as key contributors to this project.