Implications for Heavy Metal Extractions from Hyper Saline Brines with [NTf2]- Ionic Liquids: Performance, Solubility, and Cost

The bis(trifluoromethanesulfonyl)imide anion, [NTf2](-), can be paired with organic cations to give hydrophobic ionic liquids (ILs) that form secondary phases with water. These ILs are often identified as green solvents and considered as replacements for traditional organic solvents in chemical processes, i.e., aqueous biphasic extractions. Here, we consider a range of hydrophobic [NTf2](-) ILs as extraction phases with hypersaline brines for heavy metal remediation. Extraction experiments were complicated by the partial solubility of the hydrophobic ILs, and ion chromatography was used to quantify the anion and cation losses to the aqueous phase. Although IL leaching was lower in hypersaline brine than in water (i.e., salting-out), IL losses were significant at relatively low volume ratios (Vaq/VIL) for short-chain and functional ILs. IL purity was also affected by cation exchange; more organic cations were lost to the aqueous phase than [NTf2](-) anions. Solvent replenishment costs were extremely high due to loss to the aqueous phase and high IL prices. New separation technologies will be required if these ILs are to be used industrially; recovery is unlikely to offset the cost with current separation methodologies.