Masters Theses

Date of Award

12-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

R. Bruce Robinson

Committee Members

Gregory Reed, Wayne Davis

Abstract

The fission product cesium-137, represents an environmental concern to U. S. Department of Energy and nuclear industry. Cesium- 137 though present in trace concentrations in various tanks, waste streams, soil and sediments, accounts for a major fraction of radioactivity. With regard to environmental, safety and, health issues, there is a need to remove cesium-137 isotope from various media. High concentrations of other metal ions especially sodium present in these waste often pose a problem in cesium removal. The solvent extraction process has been previously used and is a suitable method for removal of metals from radioactive waste. Crown ethers, which are known to selectively form complexes with alkali and alkaline earth metals [Pedersen, 1967, 1970], are potential solvent extractants for metal removal, Crown ethers of 21-crown-7 family have been found previously to selectively form complexes with cesium [Deng et al., 1994,1995]. Three crown ethers of 2-crown-7 family viz., dibenzo-21-crown-7, bis(tert-butylbenzo)21-crown-7 and dicyclohexano-21-crown-7 were tested as cesium extractants in this research. Cesium was extracted from a nitrate solution also containing a high concentration of sodium, Cesium nitrate concentration was varied from 4.00X10-3 to 4.00X10-1 M and sodium nitrate concentration was fixed at 3.00 M for all the tests. Crown ether concentration was fixed at 0.025 M. Two diluents, n-octanol and 1,2-dichloroethane were tested. Distribution coefficients in the range of 6.20X10-3 - 6.12X10-2, loadings in the range of 0.22 - 21.2% and selectivities in the range of 2.80 - 107.41 were exhibited by the crown ethers tested. The cesium distribution coefficients, loadings and selectivities were very low for practical applications. Distribution coefficients. loadings and selectivites were found to increase with decreasing initial aqueous cesium concentrations, Since real wastes have cesium concentration in the range of 10-5 - 10-3 m, lower than those tested in this research, these trends indicate better performance by the crown ethers for real waste. Cesium and sodium extraction data were modeled by the computer program SXLSQI. Cesium and sodium were found to be extracted as 1:1 complexes (metal : crown ether molecule) with all the crown ethers tested. Using extraction equilibria found by the model, cesium extraction by crown ethers at initial aqueous phase other than those tested cesium concentrations can be estimated. Since cesium extraction was low for practical applications, various synergistic combinations of organic acids and crown ethers were also tested to enhance cesium extraction. Organic phase consisted of 0.025 M of each extractant (crown ether and organic acid) . Initial cesium and sodium concentrations were fixed in the aqueous phase. Aqueous phases at three pHs, 10, 12 and, 14, were tested. These synergistic combinations were found to be pH dependent, exhibiting increasing extraction with increase in pH. Very little synergism was observed in most cases. Antagonism was also observed in some cases. Dodecylphenol-crown ether combinations exhibited maximum synergism in cesium extraction in this research. Synergistic factors as high as 63.2 were obtained, With HMNA-crown ether combination in toluene cesium loadings as high as 114.23% was obtained (loadings in this case were calculated only on the basis of organic acid concentration only while total extractant concentration was 0.05M thus resulting greater than 100%). Cesium distribution coefficients, loadings and selectivites were still too low for practical applications.

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