Heavy Metal Extractions from NaCl Brines to Pseudoprotic Ionic Liquids

Abstract

In liquid–liquid extractions, the aqueous phase from which heavy metal ions are extracted often contains a mixture of salt ions (brine). These additional salt ions may have an effect on the efficiency and selectivity of the extraction of a specific heavy metal ion and may cause a shift in extraction mechanism when the organic phase is an ionic liquid. This has been demonstrated in this study for the extraction of Co, Ni, or Zn chloride by three pseudoprotic ionic liquids (PPILs) in the absence or presence of NaCl up to 4 times the concentration in seawater. In most cases, the extraction efficiency was increased by the presence of NaCl. This increase resulted from a shift in equilibrium referred to as the van’t Hoff–le Chatelier effect. In all cases, also, sodium ions were extracted, but at low efficiencies. An increase in NaCl presence in the brine leads to a slightly higher sodium extraction efficiency, which can also be contributed to the van’t Hoff–le Chatelier effect. Zn ions were transferred to the PPILs by neutral extraction, but only without NaCl in the brine. In all other cases, the transfer of the metal ions occurred with increasing anion exchange between the chloride ions and the trihexylammonium octanoate and trioctylammonium benzoate anions if more NaCl was present. In the case of extraction of Co or Ni ions with trioctylammonium salicylate, a shift in mechanism from anion into cation exchange was observed with a change in NaCl concentration.