Fig. 1. k3-weighted (A) and Fourier-transformed k3-weighted (B) EXAFS data in k space of the Nd (upper) or Gd (bottom) L3-edge in the precipitate collected from the 0.1 mM NdCl3 or GdCl3 aqueous solution (pH =3.0)/0.2 M HDEHP-dodecane solution; Blue lines represent experimental data; red dash lines represent the model fit. (C) Proposed structure of rare earth ion-HDHEP complex.
Critical chemical elements, which include 17 rare-earth elements, 6 platinum-group elements, and 13 other assorted elements, are vital components of modern engineering applications, which can range from clean-energy production and communications to computing.
Obtaining critical elements on an industrially-useful scale via the liquid–liquid chemical extraction technique has been widely used to mine, recycle, separate, and purify these important elements. However, when using this extraction technique in two-phase aqueous/organic systems, third phases and precipitates can form that adversely impact the end-result quality and efficiency of the process.
To date, studies seeking to ameliorate these negative results have focused on the organic phase of the process. But in this study, researchers concentrated on the aqueous phase in order to understand how precipitation can occur and interfere with the desirable shift of rare-earth elements from the aqueous phase to the desired end-result organic phase, yielding critical elements with greater separation and purity. The research team comprised members from the University of Illinois at Chicago, The University of Chicago, and Argonne National Laboratory, and included Pan Sun, first author on the journal article describing this study. Sun is a postdoctoral student funded by NSF’s ChemMatCARS working under the guidance of Wei Bu, manager of the Liquid Surface/Interface X-ray Scattering program at ChemMatCARS.
A variety of research techniques were used in this study, including microscopy, ex situ and in situ measurement of kinetics, Fourier-transform infrared spectroscopy, and x-ray absorption fine structure (XAFS) analysis utilizing the 12-BM-B bending magnet x-ray beamline managed by the Argonne X-ray Science Division at the U.S. Department of Energy’s Argonne Advanced Photon Source.
The organo-phosphoric acid extractant bis(2-ethylhexyl) phosphoric acid (HDEHP) dissolved in n-dodecane in contact with aqueous solutions of metal chlorides that contain mostly trivalent metalsbiphasic solvent extraction system was perfect for this study because of its role as an industrial extractant for the purification of rare earth elements and the reprocessing and recycling of nuclear fuel, one of the rare-earth applications.
The team’s results reveal the importance of water-soluble extractants in the extraction process, even at lower solubility levels. This was revealed by precipitation of HDEHP together with metal ions in the aqueous phase, which supports the transfer of HDEHP from the organic to the aqueous phase. Competitive exchange of HDEHP and metal ions among these three phases—organic, aqueous, and precipitate—alters the traditional view of extraction kinetics.
These results provide new, critical information about the way precipitation may occur in the aqueous phase and compete kinetically with the intended conversion of aqueous rare-earth elements to the organic phase, which holds great promise for improving the separation and purification of critical elements.
See: Pan Sun1,2,4*, Xiao-Min Lin3, Mrinal K. Bera2, Binhua Lin2, Dongchen Ying1, Tieyan Chang2, Wei Bu2**, and Mark L. Schlossman1***, “Metastable precipitation and ion–extractant transport in liquid–liquid separations of trivalent elements,” Proc. Natl. Acad. Sci., 121 (13), e2315584121 (2024).
Author affiliations: 1University of Illinois at Chicago, 2The University of Chicago, 3Argonne National Laboratory 4Present Address: Chemical Sciences Division, Oak Ridge National Laboratory
Correspondence: *psun20@uic.edu, **weibu@uchicago.edu, ***schloss@uic.edu
This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Separations Program under Award Number DE-SC0018200 to M.L.S. and from the NSF CHE-1834750 to NSF’s ChemMatCARS. We thank Artem Gelis (University of Nevada at Las Vegas) for purification of HDEHP, Benjamin J. Reinhart for assistance at Sector 12 of the APS, M. Alex Brown (Argonne National Laboratory) for advice on fitting EXAFS data, and Snezhana Abarzhi for a discussion on fluid instabilities. ICP-OES measurements were performed at the Center for Nanoscale Materials, Argonne National Laboratory. Use of the APS and the Center for Nanoscale Materials, both Office of Science User Facilities operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE- AC02- 06CH11357.
For information on liquid surface/interface studies at NSF’s ChemMatCARS, contact:
Wei Bu
(630) 252-0470
weibu@uchicago.edu