Thermodynamic Study on the Synergistic Mixture of Bis(chlorophenyl)dithiophosphinic Acid and Tris(2-ethylhexyl)phosphate for Separation of Actinides(III) from Lanthanides(III)

Abstract

The extraction of Am(III), Cm(III), and nearly the whole lanthanide(III) series from dilute nitric acid solutions into the synergistic mixture composed of bis(chlorophenyl)dithiophosphinic acid [(ClPh)₂PSSH] and tris(2‐ethylhexyl)phosphate (TEHP) as synergist was studied. The extractions were investigated as a function of the aqueous concentrations of nitric acid, (ClPh)₂PSSH, and TEHP. Am(III)/Ln(III) separation factors of over 3,000 and also surprisingly high Am(III)/Cm(III) separation factors of approximately 8 were achieved. Using NMR, the aggregation of (ClPh)₂PSSH has been determined, and its effect on the extraction has been considered. Treatment of distribution data by slope analysis suggests that the extracted An(III) and Ln(III) complexes have the composition ML₃(Syn)_xorg, where HL=(ClPh)₂PSSH and Syn=neutral synergist TEHP. Furthermore, the thermodynamic parameters ΔH ⁰, ΔS ⁰, and ΔG ⁰ of the extraction have been determined in the temperature range between 10 and 45°C.

Keywords:

• Americium
• curium
• lanthanides
• extraction
• aggregation
• thermodynamics
• actinides
• equilibria
• synergism
• stoichiometry

Acknowledgments

The authors are grateful for financial support from the Commission of the European Union under the PARTNEW project, Contract FIKW‐CT‐2000‐00087.3 We would like to thank H. Vijgen for performing the ICP‐MS measurements and Dr S. Maischak for the alpha‐measurements.