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

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.