Extraction of Trivalent Lanthanides and Americium by Tri-n-octylphosphine Oxide from Ammonium Thiocyanate Media

Abstract

An investigation of the solvent extraction of trivalent lanthanides and Am³⁺ from ammonium-thiocyanate media by tri(n-octyl)phosphine oxide (TOPO) in toluene has been completed. This system is of interest both for its potential as a means of separating transplutonium actinides from fission-product lanthanides and for inherent interest in thiocyanate-based solvent extraction systems. Partitioning was monitored using radiotracer techniques where appropriate, and ICP-OES or ICP-MS for others. The extraction behavior of all members of the lanthanide series (except for Pm) plus Y have been investigated. Conditional enthalpies (all exothermic) were determined (for selected systems) from the temperature dependence of the extraction reaction. A comparison with nitrate media shows higher extractive power of TOPO in contact with thiocyanate media, arising at least in part from the lower heat of the phase transfer of thiocyanate (relative to nitrate). The moderate tendency of HSCN to partition into the extractant phase has been profiled. Slope analysis indicates that TOPO solvation decreases from four (M(SCN)₃TOPO₄) for the light members of the series to three (or less) for the heavy lanthanide ions; Am³⁺ is extracted with four TOPO molecules. Despite the decrease in Ln:TOPO stoichiometry across the series, extraction is generally flat for the light lanthanides and increases from Gd³⁺ to Lu³⁺. The extraction of Am³⁺ from mildly-acidic ammonium-thiocyanate media was found to be at least 10 times stronger than that of the lanthanides between La³⁺ and Gd³⁺.

Keywords:

• Lanthanide
• americium
• solvent extraction
• tri(n-octyl)phosphine oxide
• ammonium thiocyanate
• isothiocyanic acid

ACKNOWLEDGMENTS

This work was performed under the auspices of the Department of Energy, Division of Nuclear Energy Science and Technology, University Nuclear Energy Research Initiative at Washington State University under contract number DE-FC07-05ID14643. C. Badajoz completed a portion of this work as an intern at WSU as part of the fulfillment of a M.Sc. degree at ENSCP-Paris Tech. The authors would also like to acknowledge Charles M. Knaack for his invaluable assistance with the ICP-MS analysis.

Notes

ⁱ In the americium experiments, the final term (– ∑(x·[M(SCN)₃·TOPO _x ]_org) can be neglected, as [M]_t <10⁻⁷ M; in the lanthanide experiments, [M]_t in some cases is within 10% of the [TOPO]; hence, the correction must be considered. The adjustment of [TOPO]_{org, f} due to this final term in the lanthanide experiments was never greater than 3%.