Novel Process Employs Crown Ethers for Removing Technetium
from Alkaline Waste Solutions

A combination of fundamental principles and process development has led to a remarkable method for extracting heptavalent technetium (Tc) from alkaline nuclear-waste solutions by the use of crown ethers. Technetium is a long-lived radioactive fission product found in wastes stored at several DOE sites. Almost two metric tons of Tc are stored in underground tanks at the Hanford site, where it is planned to separate the Tc from the 55 million gallons of other waste components in the tanks. In the long term, Tc represents a concern owing to its environmental mobility in the form of pertechnetate anion and likely health risk for hundreds of thousands of years to come. Technetium also represents a short-term concern owing to its volatility, which causes difficulties in vitrification processes proposed for nuclear waste. Research in the Chemical Separations Group of the Chemical and Analytical Sciences Division at ORNL has recently suggested extractive methods that could be used to remove technetium from the highly salted wastes stored in the tanks.

Although it had been well known that crown ethers possess the ability to efficiently extract sodium salts by binding the sodium ion, a key question of interest at ORNL concerns what factors determine which salt would be extracted selectively from a mixture of sodium salts. A fundamental understanding of the thermodynamics of such systems in fact led to the prediction that sodium pertechnetate could be selectively separated from the Hanford waste. In subsequent process development, this prediction was validated through invention of the SRTALK process. No pre-treatment of the waste solution is necessary, and the technetium can be recovered using a safe and inexpensive stripping process, regenerating the crown ether for many more cycles with minimal generation of secondary waste. Engineering tests with a waste simulant in a cascade of centrifugal contactors by collaborators at Argonne National Laboratory gave 89% removal of Tc from the waste, meeting the experimental goal. Remarkably, the tests gave a product stream concentrated 10-fold in practically pure sodium pertechnetate. Considering that the source of the recovered Tc would be a horrendously toxic and complex waste, the remarkable expected purity of the Tc product would make for an ideal feed for production of final waste forms for final disposal, with expected major cost savings. On the other hand, the ability to recover a pure product gives one reason to pause to consider whether it would be most desirable to bury it or to find uses for it!

A chemical depiction of SRTALK is shown above. The waste is a mixture of salts concentrated in sodium, potassium, hydroxide, nitrate, nitrite, and carbonate, but with a trace of radioactive contaminants such as ⁹⁹Tc. Most of the Tc is in the form of the negatively charged pertechnetate ion, which has the formula TcO₄^-. The crown ether complexes with sodium ions (Na⁺) as shown but can also complex with potassium ions (K⁺). The transfer of either of these metal ions into the solvent by the crown ether must also be accompanied by a negatively charged ion. Among the most easily transferred negative ion is pertechnetate with a selectivity over nitrate on the order of a thousand to one. When the solvent is contacted with water, the sodium pertechnetate may be released into the water, regenerating the crown ether for further extraction cycles. The process is described by a 1995 patent and in numerous publications. The governing fundamental principles are described in a series of papers from the early 1990s continuing to the present. In 1999 the development of the SRTALK process was recognized by a Lockheed Martin Technical Accomplishment Award. To learn more about our SRTALK process, please visit our reports page.