A kinetic interpretation of the effect of extractant and acid on neodymium(iii) extraction by a diglycolamide

Abstract

With an increasing demand for rare earth elements in modern technologies, it is crucial to develop a full understanding of the purification process for lanthanides. Recent studies have focused heavily on the separation of lanthanides using solvent extraction with diglycolamide extractant ligands. These completely incinerable extractants, especially N,N,N′,N′-tetra(n-octyl)diglycolamide (TODGA), provide best-in-class thermodynamic separation for the early-middle lanthanides. Thermodynamic parameters are well characterized for lanthanide extraction from nitric acid media by TODGA, but there exists a lack of knowledge about the kinetics. We developed a greater understanding of the rates and mechanism of lanthanide extraction by TODGA using operando visible spectroscopy to monitor changes in a coordination-sensitive neodymium(III) absorbance band. Due to the dynamic interplay between equilibrium metal distribution and the rate of mass transfer, determining these parameters across a wide range of component concentrations revealed novel thermodynamic and kinetic findings. An activity-based treatment of the non-linear dependence of metal extraction on acidity allowed us to quantitatively identify the changes in speciation across the acidity range. Plateau and linear regions in the kinetic data are directly related to slopes in the equilibrium data, which, together, provide the reaction orders with respect to each component. Extraction and stripping rate laws follow the interfacial two-step consecutive reactions mechanism, where adsorption and desorption of neodymium(III) complexes at the interface govern the rate limiting steps, while TODGA, nitrate anions, and nitric acid adducts associate and dissociate in fast equilibria.