Tremendous impact of substituent group on the extraction and selectivity to Am(III) over Eu(III) by diaryldithiophosphinic acids: experimental and DFT analysis

Comparative extraction of Am3+ over Eu3+ from nitric acid media by three dithiophosphinic acids (L) bearing different substituent aryl groups was studied. Within the experimental pH range, the distribution ratio of Am3+ and Eu3+ increases with the increase of pH due to deprotonation of the acidic extractant. Both the extraction and separation ability toward Am3+/Eu3+ by these extractants were affected drastically by the substituent aryl groups. The bis-phenyl substituted extractant shows almost no ability to extract and separate Am3+ from Eu3+, while bis(ortho-trifluoromethylphenyl) dithiophosphinic acid has the optimum extraction and separation capabilities. To help reveal the complexation and extraction behavior, the structure and bonding characters of neutral ML3 (M = Am or Eu) complexes as well as the biphasic extraction reaction were studied by density functional theory (DFT). The conduct-like screening model implicit solvation model was used to simulate the extraction reaction in the aqueous-toluene biphasic system. The calculated sequence of the Gibbs free energy difference of extraction (ΔΔG ext Am/Eu ) is consistent with the experimental results.