Solvent extraction of heavy metals with macrocyclic ligands based on calix[4]arenes

Abstract

Ligand 1, a calix[4]arene-based compound with a crown-6 moiety, efficiently extracts the heavy metal ions Hg²⁺ and Pb²⁺ from weakly acidic solutions due to their compatible ion radius. The CO₂H groups are essential for the metal extraction, as seen from a comparison with the corresponding ester compound 2. Ligand 3 as a crown-5 is also capable of extracting smaller cations such as Zn²⁺ and Cd²⁺ from weakly acidic solutions. Compound 5, bearing four CO₂H groups, shows quantitative extraction for Hg²⁺ and Pb²⁺ at pH values above 2.5 and 2.8, respectively. Extraction takes place to some extent even if water-soluble organic complexing reagents are present in the aqueous phase, such as EDTA. Quantitative back-extraction is achieved by using 0.75 M HNO₃.