Solvent extraction technique has been widely used in mining and refinement of rare earths. Although all rare earths are chemically similar, the extraction efficiency increases as a function of atomic number. Combining MC simulation and X-ray fluorescence measurements at NSF’s ChemMatCARS, researchers from Northwestern University and The University of Chicago revealed the electrostatic origin of this selectivity.

Rare earths, which are fundamental components of modern technologies, are often extracted from aqueous solutions using surfactants at oil-water interfaces. Heavier lanthanides are more easily extracted, even though all lanthanides are chemically very similar. Using x-ray fluorescence measurements and theoretical arguments, we show that there is a sharp bulk-concentration-dependent transition in the interfacial adsorption of cations from aqueous solutions containing Er3+ or Nd3+ in contact with a floating monolayer. The threshold bulk concentration of erbium (Z=68) is an order of magnitude lower than that of neodymium (Z=60), and erbium is preferentially adsorbed when the solution contains both ions. This implies that elemental selectivity during separation originates at the surfactant interface. Electrostatic effects arising from the interface dielectric mismatch, ionic correlations, and sizes of the ions explain the sharp adsorption curve and selectivity.


Mitchell Miller1, Yihao Liang2, Honghao Li1,2, Miaoqi Chu1, Sangjun Yoo1, Wei Bu3, Monica Olvera de la Cruz1,2, and Pulak Dutta1

1Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA

2Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA

3Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, USA


Phys. Rev. Lett. 122, 058001, 2019