Electrochemical and Spectroscopic Study of Eu^III and Eu^II Coordination in the 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid

Separation processes based on room temperature ionic liquids (RTIL) and electrochemical refining are promising strategies for the recovery of lanthanides from primary ores and electronic waste. However, they require the speciation of dissolved elements to be known with accuracy. In the present study, Eu coordination and Eu^III /Eu^II electrochemical behavior as a function of water content in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf₂ ]) was investigated using UV-visible spectrophotometry, time-resolved laser fluorescence spectroscopy, electrochemistry, and X-ray absorption spectroscopy. In situ measurements were performed in spectroelectrochemical cells. Under anhydrous conditions, Eu^III and Eu^II were complexed by NTf₂ , forming Eu-O and Eu-(N,O) bonds with the anion sulfoxide function and N atoms, respectively. This complexation resulted in a greater stability of Eu^II , and in quasi-reversible oxidation-reduction with an E₀ ' potential of 0.18 V versus the ferrocenium/ferrocene (Fc⁺ /Fc) couple. Upon increasing water content, progressive incorporation of water in the Eu^III coordination sphere occurred. This led to reversible oxidation-reduction reactions, but also to a decrease in stability of the +II oxidation state (E₀ '=-0.45 V vs. Fc⁺ /Fc in RTIL containing 1300 mm water).