Next Generation Extractants for Cesium Separation from High-Level Waste
- ORNL
- Pacific Northwest National Laboratory (PNNL)
- Texas Tech University, Lubbock
- University of Texas
This project seeks a fundamental understanding and major improvement in cesium separation from high-level waste by cesium-selective calixcrown extractants. Systems of particular interest involve novel solvent-extraction systems containing specific members of the calix[4]arene-crown-6 family, alcohol solvating agents, and alkylamines. Questions being addressed bear upon cesium binding strength, extraction selectivity, cesium stripping, and extractant solubility. Enhanced properties in this regard will specifically benefit applied projects funded by the USDOE Office of Environmental Management to clean up sites such as the Savannah River Site (SRS), Hanford, and the Idaho National Environmental and Engineering Laboratory. The most direct beneficiary will be the SRS Salt Processing Project, which has recently identified the Caustic-Side Solvent Extraction (CSSX) process employing a calixcrown as its preferred technology for cesium removal from SRS high-level tank waste. Disposal of high-level waste is horrendously expensive, in large part because the actual radioactive matter in underground waste tanks at various USDOE sites has been diluted over 1000-fold by ordinary inorganic chemicals. To vitrify the entire mass of the high-level waste would be prohibitively expensive. Accordingly, an urgent need has arisen for technologies to remove radionuclides such as {sup 137}Cs from the high-level waste so that the bulk of it may be diverted to cheaper low-level waste forms and cheaper storage. To address this need in part, chemical research at Oak Ridge National Laboratory (ORNL) has focused on calixcrown extractants, molecules that combine a crown ether with a calixarene. This hybrid possesses a cavity that is highly complementary for the Cs{sup +} ion vs. the Na+ ion, making it possible to cleanly separate cesium from wastes that contain 10,000- to 1,000,000-fold higher concentrations of sodium. Previous EMSP results in Project 55087 elucidated the underlying extraction equilibria in cesium nitrate extraction by the calixcrown used in the CSSX process, calix[4]arene-bis(t-octylbenzo-crown-6), designated here as BOBCalixC6 (see structure). This understanding led to key improvements in the development of the CSSX process under the EM Efficient Separations and Crosscutting Program, entailing a method to back-extract or 'strip' cesium from the calixcrown subsequent to cesium extraction from waste. Having this stripping method allowed the cesium to be concentrated in a relatively pure aqueous stream and the extractant to be regenerated for recycle. Closing the cycle then made possible the design of a process flowsheet and successful demonstration through collaboration with Argonne National Laboratory and Savannah River Technology Center under funding from the USDOE Office of Project Completion and Tanks Focus Area. Despite these successes, the CSSX process represents young technology that can benefit substantially from further fundamental inquiry. First, reversibility of the process (stripping efficiency) still presents the greatest potential for problems and the greatest potential for improvement. Second, although the calixcrown extractants for cesium are two orders of magnitude stronger than the next best simple crown ether, a minor fraction of the extractant capacity is utilized. Third, potassium competes significantly with cesium for the calixcrown binding site, an important issue in dealing with Hanford wastes having potassium concentrations as high as 1 M. Fourth, the calixcrown solubility needs to be improved. And finally, the mechanism of extraction must be understood in detail to provide the base of knowledge from which further development of the technology can be rationally made.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 944590
- Country of Publication:
- United States
- Language:
- English
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Next Generation Extractants for Cesium Separation from High-Level Waste: From Fundamental Concepts to Site Implementation
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