THE EXTRACTION BY N,N-DIALKYLAMIDES .2. EXTRACTION OF ACTINIDE CATIONS

N,N-dialkylamides are potential alternative extractants to tributylphosphate for the actinide separation in nuclear fuel reprocessing. Extraction mechanisms of U(VI), Th(IV) and Pu(IV) from nitric acid media are investigated for the amide DOBA(((C4H9) (C2H5) CHCH2)2 NC(O) C3H7) and DO(i)BA (((C4H9) (C2H5) CHCH2)2 NC(O) CH(CH3)2). For low aqueous acidities, amides are neutral extractants. Extraction stoichiometries of UO2(NO3)2(Amide)2 (Amide = DOBA or DO(i)BA), Pu(NO3)4(DOBA)2 have been established. Th(IV) is supposed to form a di- and a trisolvate in the inner-coordination sphere. For concentrated solvents, bilogarithmic dependences of distribution coefficients with ligand concentration point out non-integer slopes higher than those expected from stoichiometry. Such features, very often noted for amides, do not seem to be due to new chemical second-sphere species, which has never observed experimentally, but to polyoriented interactions between free ligands and complexes and may be considered as part of non-ideality. Amides behave as anionic extractants with increasing acidity. New species are formed, involving UO2(NO3)3- and Pu(NO3)6(2-) anions, with the protonated amide ligands in the outer-coordination sphere of metals.