Hydration–Complexation Balance in Eu(III)/Er(III) Extraction from Thiocyanate Media by Aliquat‑336

We report on the speciation of two lanthanides, Eu(III) and Er(III), in aqueous and organic thiocyanate solutions obtained by solvent extraction with a quaternary ammonium extractant (Aliquat-336.thiocyanate, AQ.SCN) in toluene. The connection between the metal extraction behavior and the metal speciation in the two phases is investigated by a combination of spectroscopic techniques including UV-vis, photoluminescence, and ATR-FTIR spectroscopy. Extraction studies show a higher extraction of Er(III) than Eu(III), with extractant dependency exponents of 2 and 1.5 respectively, indicating the formation of -1 to -2 charged anionic complexes in the organic phase. Spectroscopic studies indicate that the anionic complexes are not formed in the aqueous phase for either lanthanide, even at high NaSCN concentrations. Photoluminescence of Eu(III) in the organic phase shows the presence of multiple species with 3 and 5-6 thiocyanate anions in the inner coordination sphere corresponding to the non-integral exponents in extraction studies. ATR-FTIR spectra show differences in the CN stretching region of SCN- for the two lanthanides, corresponding to predominantly inner-sphere coordination of thiocyanate to Eu(III) and outer-sphere coordination to Er(III). Thus, while extraction studies show that the overall charge on the anionic species in the organic phase is greater for Er(III) than Eu(III), spectroscopic results reveal that the number of thiocyanate anions in the inner coordination sphere of Eu(III) is larger than that of Er(III). Thus, a balance of lanthanide hydration and complexation in the organic phase plays a major role in the extraction of lanthanides with AQ.SCN from thiocyanate solutions.