Chemistry of the Extraction of La(NO3)3 and Ce(NO3)3 from Nitrate Solutions with Mixtures of Tri-n-Octylmethylammonium Nitrate and Tri-n-Butyl Phosphate in Toluene

The boundaries of the regions of the synergistic extraction of lanthanum nitrate and cerium(III) nitrate from low-acid nitrate solutions containing the salting-out agent NH4NO3 with 1.0 M isomolar mixtures of tri-n-octylmethylammonium nitrate and tri-n-butyl phosphate (TBP) in toluene were determined by the method of isomolar series. For the first time, the chemistry of the extraction in the determined regions of the synergistic effect is defined as a continuous sequence of the mixed complexes being extracted with the composition (R4N)n[Ln(NO3)3 + n ⋅ mTBP], where n = 1–3, m = 3–1 for lanthanum and n = 1–3, m = 4–1 for cerium(III), depending on the molar ratio between the extractants. The opposite changes in n and m in the mixed complexes with increasing concentration of the quaternary salt in the mixture is explained by the replacement of phosphoryl ligands of tri-n-butyl phosphate by nitrate ligands of tri-n-octylmethylammonium nitrate until their complete replacement from the coordination sphere of the lanthanide.