Extraction efficiency of recent macrocyclic ligand for the separation of heavy metal ions by liquid-liquid extraction: A review

The separation of heavy metals is a priority for environmental protection. Liquid-liquid extraction is the most widely applied separation technique by industry for heavy metals in wastewater. The purpose of this article is to describe the different types, properties, and optimal conditions for macrocyclic ligands as a carrier that can be used to improve the efficiency of metal ion separation using the liquid-liquid extraction method. The research method used in this article was literature study, which is a review of articles related to the topic in the ten years publishing period (2011 - 2020). The macrocyclic ligands developed for liquid-liquid extraction were calixarene and its derivatives, crown ether and its derivatives, resorcinarenes and their derivatives, and other macrocyclic ligands. These ligands were developed for the extraction of heavy metals such as Ag(I), Hg(I), Hg(II), Pb(II), Cr(III), Cd(II), Cu(II), Co(II), Ni(II), and Zn(II). The ability of macrocyclic compounds to separate metal ions was greatly influenced by their functional groups. The sulfur atom in modified macrocyclic ligands was a functional group that acts as a soft base. Metal ions which are soft acids were more attracted to the sulfur groups so that the extraction efficiency of the modified macrocyclic compounds was better than the unmodified ones. In addition, the influence of pH, ligand concentration, type of solvent, and agitation time resulted the extraction efficiency in the range of 50 - 100%. Based on these results, it can be concluded that macrocyclic ligands have potential to increase the extraction efficiency of heavy metals in the liquid-liquid extraction method.