Enhanced photocatalytic degradation of metronidazole using novel Cu(II) gallic acid mixed ligand complexes with diverse co-ligands

Pharmaceutical contaminants such as metronidazole (MNZ) represent a significant risk to aquatic environments due to their high water solubility, toxicity, and resistance to biodegradation. In this study, a novel series of Cu(II) based mixed ligand complexes incorporating gallic acid (GAH) with co-ligands; 8-hydroxyquinoline (8HQ), pyridine (Py), and anthranilic acid (AAH) were synthesized and characterized using Fourier transform infrared (FT-IR), UV-Vis spectroscopy, magnetic moment analysis, molar conductivity, powder X-ray diffraction (PXRD), and thermogravimetric analysis/derivative thermogravimetry (TGA/DTG). The novelty of these complexes lies in the strategic integration of GAH with co-ligands of different donor types (N,O–, N–, and N,O–functionalities), creating unprecedented coordination environments around Cu(II) that enable systematic modulation of electronic and photocatalytic properties. The complexes were discovered to be crystalline, thermally stable, and non-electrolytic, with distorted tetrahedral or octahedral geometries. Their photocatalytic performance was evaluated by the degradation of MNZ in aqueous media under UV light. Parameters such as irradiation period, pH, initial MNZ concentration, catalyst dosage, and H₂O₂ addition were investigated. Among the synthesized catalysts, the mixed ligand complexes showed superior photocatalytic activity compared to the binary [Cu(GA)₂].1.5H 2 O complex, attaining degradation efficiencies up to 93.24%. The enhanced activity was attributed to improved structural features, efficient charge separation and favorable redox behavior. The photocatalytic process followed pseudo-first-order kinetics, and the catalysts remained stability and reusability during several cycles. This work demonstrates the potential of GAH based mixed ligand complexes as sustainable and effective photocatalysts for the treatment of pharmaceutical wastewater.