Synergic effects in the solvent extraction of the actinides—III tetravalent actinides

Abstract

By using 230Th, 237Np, and 239Pu as tracers, the solvent extraction of the tetravalent actinides from nitric acid into mixtures of 1,1,1-trifluoro-3-2′-thenoylacetone (TTA) with tri- n-butyl phosphate (TBP) or tri-n-butylphosphine oxide (TBPO) in cyclohexane has been studied. The extraction coefficients have been found to be larger for certain mixtures than for either component alone. With thorium and TTA, the addition of TBP effects the displacement of some of the chelated molecules and a mixture of ThX 4, ThX 3(NO 3)·TBP, and ThX 2(NO 3) 2·2TBP (where HX = TTA) is co-extracted. Unlike the systems hitherto studied the anion of the mineral acid in the aqueous phase now participates in the composition of the extractable species. With mixtures of TTA and TBPO the displacement of the chelated TTA is even greater and the extractable species ThX(NO 3) 3·TBPO must be involved. With mixtures of TTA and TBP the extraction of Pu(IV) and Np(IV) appears to involve only the species MX 4 and MX 3(NO 3)·TBP (M = Np or Pu). With mixtures of TTA and TBPO, however, the species extracted are MX 2(NO 3) 2·2TBPO with MX 3(NO 3)·TBPO and MX 4. A number of equilibrium constants have been calculated and their significance is discussed. Synergic effects among the tetravalent actinides appear to depend upon their ability to achieve a co-ordination number of eight.