Synergistic Extraction of Dysprosium and Aggregate Formation in Solvent Extraction Systems Combining TBP and HDBP

Abstract

During treatment of nuclear fuel in the Plutonium/URanium EXtraction (PUREX) process, the extractant tri-n-butyl phosphate (TBP) is known to degrade to dibutylphosphoric acid (HDBP), which increases the extraction of metal ions, thereby inhibiting their stripping from the organic phase. To better understand this phenomenon, we investigated how mixtures of TBP and HDBP influenced the extraction of metal, nitric acid, and water, and correlated the results to aggregated structures in the organic phase. The mole ratios of TBP-HDBP mixtures had a non-linear effect on the extraction of Dy³⁺ and water from 0.2 M HNO₃, indicating synergism. In 2 M HNO₃, the TBP:HDBP mole ratio had a more linear relation to Dy³⁺ and water extraction, so the synergistic effect was less pronounced than in the low acid system. The extraction of nitric acid showed no synergistic effect and follows closely what would be expected in a system using TBP only. The small-angle X-ray scattering (SAXS) data of the 0.2 M acid system showed maximum contrast at a TBP:HDBP mole ratio of 0.25, so that the synergistic mixture is also the most aggregated at 0.2 M acid. The 2 M acid system also showed that the mixed system is more aggregated than the end members, although this does not result in peak extraction. Previous studies of synergistic extraction of metal cations explain the enhanced extraction by increased dehydration of the metal ion. Although our data do not rule out the formation of mixed complexes according to the classical mechanism of synergism, our evidence of increased water extraction and aggregate formation in systems combining TBP and HDBP are complementary to the metal-centric dehydration aspects of the process. The findings in this study give insights into the complex chemistry of solvent extraction, providing a possible link between formation of aggregates in the organic phase and synergistic extraction.

Keywords:

• Synergism
• aggregation
• reverse micelles
• nuclear fuel separation
• lanthanides
• Small-Angle X-ray Scattering (SAXS)

ACKNOWLEDGMENTS

The authors wish to thank the U.S. Department of Energy through the Nuclear Energy University Program, NEUP Contract No. 120569 and DE-NE0000156, for financial support for the experiments and for the HPGe detector, respectively. The IC measurements were carried out in the Metrohm Lab at UC Irvine. The work at Argonne and the use of the Advanced Photon Source are supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences, under contract No DE-AC02-06CH11357. The authors wish to thank Dr. Renato Chiarizia for valuable discussions regarding the theoretical calculations of acid uptake and selection of stability constants.

Notes

¹The parameter, q, used to calculate the activity coefficients was not given for Dy(NO₃)₃ in this work but was estimated to a value of 1.4 observing trends in the values of Ln(Cl)₃ and comparing Cr(NO₃)₃ vs. Cr(Cl)₃.