The influence of amino acid side chain structure on the leaching behavior of copper from printed circuit boards

This study evaluated the potential of 15 biological amino acids as eco-friendly lixiviants for copper (Cu) extraction from printed circuit boards (PCBs) under alkaline conditions. The Cu leaching behaviors, complexation mechanisms, and the effect of temperature were examined. The amino acids were classified by their Cu leaching efficiencies: Type-I (Gly, Val, Ala, Arg, His, Lys), Type-II (Pro, Glu, Asp, Thr, Ser), and Type-III (Asn, Gln, Met, Cys). Type-I amino acids achieved >90 % Cu leaching, while Type-II showed moderate dissolution. Type-III formed solid Cu-amino acid complexes as confirmed by XRD and SEM analyses. Increasing temperatures enhanced Cu leaching in both Type-I and Type-II, particularly during the initial stages. Minimal dissolution of aluminum (Al) and iron (Fe) was observed at pH 10-12 and Eh 0-200 mV, allowing for selective Cu dissolution in liquid or solid phases. Aliphatic and polar positively charged amino acids were the most effective due to their ability to form stable bi- and/or tridentate complexes, while carboxyl- and alcohol-containing side chains hindered Cu dissolution. The precipitation of solid Cu-amino acid complexes was attributed to amide and sulfur-containing side chains, as nitrogen, oxygen, and sulfur can form strong coordination bonds with Cu. This is the first study to evaluate the role of amino acid side chain structures in Cu leaching from e-waste and highlights the potential of less commonly studied amino acids in hydrometallurgical applications.