Calix[n]arene-based uranophiles bearing hydroxamic groups on the lower rim (2n; n = 4 and 6) have been synthesized and the extractability (Ex%) and the selectivity towards uranyl ion (UO22+) estimated in a two-phase (water-chloroform) solvent-extraction system. Ex% for 2 4 and 26 increases from pH 2 and saturation is reached at around pH 5 where 100% extractability occurs. Since the pKa values for hydroxamic acids are 8-9, the apparent pKa shift caused by the UO22+-complexation amounts to 6-7 pK units. Extraction of UO22+ from aqueous carbonate solution established that 26 in the organic phase can compete efficiently with CO 32- ions in the aqueous phase for UO2 2+ whereas in 24 and a calixarene-based uranophile bearing six carboxy groups (16) UO22+ is reextracted to the aqueous phase. The difference indicates that 26, which has the hexacoordination geometry preorganized for the binding of UO 22+, is superior to 24 as a uranophile. We also found that the selectivity of 26 is superior to 16: the Ex% values for 26 is scarcely affected by the addition of competing metal cations (except Fe3+). These results show that 26 serves as an excellent UO22+-selective extraction reagent.
|Number of pages||4|
|Journal||Journal of the Chemical Society, Perkin Transactions 2|
|Publication status||Published - Jan 1 1991|
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