EXAFS studies of cesium complexation by dibenzo-crown ethers in tri-n-butyl phosphate

Cesium K- and L(3)-edge EXAFS (Extended X-ray Absorption Fine Structure) have been evaluated as means of probing the interaction of cesium ions with several dibenzo-crown ethers in tri-n-butyl phosphate (TBP) under conditions proposed for the extraction of radiocesium from acidic nuclear wastes. K-edge EXAFS data are shown to be less satisfactory than the corresponding L(3)-edge data, a result of the difficulties inherent in the use of high X-ray energies, the weak spectral contributions of carbon and oxygen at long distances, the disorder characteristic of solution systems, and the low cesium concentrations present. Despite these difficulties, the EXAFS data yield information on the solution structure of the cesium-crown ether complexes that complements results obtained from liquid-liquid extraction studies. Specifically, the coordination of cesium in TBP is shown to differ from that observed in the same solvent upon addition of a dibenzo-crown ether. In addition, cesium coordination by the crown ethers is shown to be unaffected by the presence of water or the identity of the counterion. Finally, the average Cs-O bond length is shown to decrease from 3.23(3) Angstrom in the cesium di-(tert-butylbenzo)-18-crown-6 complex to 3.01(4) Angstrom in the corresponding 21-crown-7 and 24-crown-8 compounds, a result consistent with extraction studies indicating 1:1 complex formation in the 21-crown-7 and 24-crown-8 systems and a mixture of 1:1 and 1:2 complexes for the 18-crown-6.