Chapter 7: Ionic Liquids for Metal Extraction from Aqueous Matrices

Ionic Liquids (ILs), in particular, so-called “task specific” ones (TSILs) have been designed and used successfully for the extraction of metals from aqueous matrices during the last few years. Extraction mechanisms are not fully understood for most cases, but they include neutral extraction, neutral co-extraction, ion exchange, or a combination of either. Numerous factors have been identified to influence the extraction. Amongst them are competing ligands, for example, humic substances or inorganic anions (e.g. carbonate), which reduce the extraction efficacy for many metals. In contrast, the presence of Cl⁻, for example, may enhance extraction under certain conditions significantly. TSILs may be considered tailorable and promising candidates for certain applications such as the selective extraction of metallic species, yet one limiting factor for a broader use is their partial solution into the aquatic phase, the so-called leaching. Many of the constituents of ILs must be considered toxic towards aquatic biota. Therefore, one of the aims for research must include the reduction of leaching, which may be achieved by using more hydrophobic components, or the immobilization of ILs on/in carrier matrices. The latter has been used successfully already in hollow fiber systems, which showed a reduced leaching and a quantifiable uptake of metals over time, making this method useful for analytical purposes. Many questions have not been answered yet, and ILs still offer a wide field for innovative research pointing in the direction of greener applications in chemistry.