Selective extraction of metals from chloride solutions with the tetraoctylphosphonium oleate ionic liquid

The solvent extraction behavior of a series of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, In3+, La3+, Nd3+, Sm3+, Dy3+, Er3+, Yb3+) from an aqueous chloride feed solution by the nonfluorinated fatty acid-based ionic liquid (IL) tetraoctylphosphonium oleate [P8888][oleate] has been investigated as a function of the pH. The possibility to extract metal chlorides from an aqueous stream via the anion or cation of the hydrophobic, low-viscous water-saturated [P8888][oleate] IL has been exploited. [P8888][oleate] can be considered as a bifunctional or binary IL. At high pH values (pH > 5), all metals are extracted via the oleate anion, whereas some transition metals are extracted at high HCl concentrations and thus low pH values as anionic chloro complexes in combination with [P8888] cations. A difference of one pH unit is observed between the extraction curves (%E as a function of the pH) of the transition metals and those of the rare earth metals. Rare earths are not extracted at low pH values, whereas some transition metals (Fe, Mn, Co, Zn, Cu, In) are extracted. This makes [P8888][oleate] a promising extractant for the separation of transition metals from rare earths. It is also shown that this bulky and long-chained IL has a very low viscosity due to the uptake of water.