Enhanced Separation of Am(III) and Cm(III) from Nitrate Solution by Bis(isobutylphenyl)dithiophosphinic Acid

This work reports a novel dithiophosphinic acid extractant, bis(isobutylphenyl)dithiophosphinic acid (HL), for the mutual separation of Am(III) and Cm(III) and for the separation of trivalent actinides (An(III)) from lanthanides (Ln(III)). The compound was successfully synthesized and structurally confirmed by 1H and 31P NMR spectroscopy. Focusing specifically on Am(III) and Cm(III), the extraction behavior was systematically investigated as a function of pH, ligand concentration, nitrate concentration, and temperature. Compared with the conventional extractant, bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HL301), HL exhibits stronger extraction efficiency (pH1/2 = 3.39 for Am(III) and 3.64 for Cm(III)) and a notably improved separation factor for Am(III) over Cm(III) (SFAm(III)/Cm(III) = 4.8), while retaining excellent separation ability for An(III) from Ln(III). The extraction proceeds via a cation-exchange mechanism, yielding a 1:3 metal-extractant complex with the release of three protons. Increasing nitrate concentration suppresses extraction due to the competition for metal ion between NO3− in the aqueous phase and the extractant. The extraction reaction is endothermic with negative entropy changes, exhibiting ΔH° values of 24.06 kJ·mol−1 for Am(III) and 27.12 kJ·mol−1 for Cm(III) at 298.15 K, along with ΔS° values of −90.63 J·mol−1·K−1 and −89.11 J·mol−1·K−1, respectively. This work offers a promising extractant for the separation of Am(III) from Cm(III) and An(III) from Ln(III), and mechanistic insights into coordination–selectivity relationships involving soft sulfur donors.