Applicability of a Calixarene-Crown Compound for the Removal of Cesium from Alkaline Tank Waste

In terms of extractive strength and selectivity over sodium, calix[4]arene-bis-(2,3-naphtho-crown-6) exhibits superior properties in comparison with simple crown ethers for separating cesium from an alkaline solution simulating a Hanford tank supernatant waste. The calixarene was studied at 0.01 M in four diluents. In two cases a cesium distribution coefficient near unity was obtained. Successive water washes sufficed to strip the loaded solvents. In addition, selectivity ratios for cesium over sodium exceeded 10⁴ for all of the diluents. By contrast, bis(tert-butylbenzo)-21-crown-7 gave distribution coefficients an average of 200 times lower under identical conditions; its selectivity ratio for cesium over sodium was at best 200 in one of the four diluents tried. Although Cs/Na selectivity in the case of the calixarene is so high as to make Na⁺ ion extraction inconsequential, Cs/K selectivity lay in the lower range 122-935. Owing to the relatively high concentration of K⁺ ion in the waste, strong competition due to K⁺ ion extraction resulting in high loading of the calixarene was observed to limit the effectiveness of cesium extraction and stripping. For both the calixarene and crown ether, extraction strength correlates with the empirical solvent polarity parameters DP* and E_T.