The impact of phase modification on the equilibrium and kinetics of lanthanide(iii) extraction from nitric acid media by TODGA.

Organic phase modification is a commonly implemented strategy to avoid third phase formation in f-element solvent extraction. For trivalent lanthanide extraction by neutral extractants in alkane diluents, such as N,N,N',N'-tetra(n-octyl)diglycolamide (TODGA) in n-dodecane, the addition of a phase modifier is necessary to operate at high organic acid and metal loadings. Phase modifiers such as tri(n-butyl)phosphate (TBP) and N,N-di(n-hexyl)octanamide (DHOA) have been proven to be effective at preventing organic phase splitting well beyond the critical concentrations that limit the TODGA/n-dodecane phase without any modifiers. While the extraction capacity and loading limits of lanthanide extraction from nitric acid media by TODGA in alkane diluents have been evaluated for DHOA- and TBP-modified phases, the fundamental bulk equilibrium and kinetic mass transfer processes have not been well characterized for these systems. Solvent extraction and operando absorption spectroscopy were used to evaluate the impacts of DHOA and TBP on the equilibrium complexation and the reaction orders with respect to the extractant and acid. Compared to the unmodified organic phase, we determined that DHOA has very little effect on both the metal complexation and the interfacial kinetic mechanism. DHOA appears to act as a cosolvent, as phase modifiers are traditionally intended. In contrast, the addition of TBP to TODGA in n-dodecane altered the bulk TODGA : lanthanide(iii) speciation and distinctly changed the transition state complex during reversible phase transfer of the cation between the aqueous phase and the organic phase. These results are novel and provide significant insight into the fundamental parameters defining this relevant f-element extraction system.