Extraction mechanism of U(VI) ions by N,N-dialkylamides

N,N dialkylamides are good extractants of UO22+ ions from the nitric solutions obtained on dissolution of nuclear irradiated fuels. The extraction mechanism of U(VI) ions proved to be rather complex. Two species were identified by spectrophotometry in the organic phase: UO2(NO3)2L2 (1) and UO2(NO3)3HL (2), L standing for the amide molecule. The complex (1) is typical for neutral molecules extractants. However, when log D(U(VI)) is plotted against log C (amide), the slope of the straight line obtained is greater than 2, which suggests that amide (1) interactions take place in the 2sup(nd) coordination sphere of U(VI) ions. The crystal structure of (1) where L is the N,N-di-n-butyldodecanamide show that Van der Waals interactions occur between the dodecyl chains of two (1) molecules. The complex (2) is characteristic for anionic extractants. For the amide molecules studied, it takes place in very acidic media. In order to investigate the nature of the UO2(NO3)3HL complex, the infra-red shift of νc=0 vibration were compared in the following compounds: free amide, HNO3-L, phenol L, UO2(NO3)3HL. The results are discussed. 14 refs