New insights into the mechanisms of tartaric acid enhancing homogeneous and heterogeneous copper-catalyzed Fenton-like systems.

The specific roles of tartaric acid (TA), as an eco-friendly ligand, in homogeneous and heterogeneous copper-catalyzed systems were systematically revealed and new mechanisms of TA enhancing the three Fenton-like processes were proposed to provide a theoretical significance in overcoming the deficiency of conventional Fenton processes. The results identified hydroxyl radical (•OH) as the main species responsible for the simultaneous decomposition of TA and metronidazole (MNZ) according to TOC removal. The ESR technique was used to detect superoxide radicals (•O), carbon-centered radical (•R) and hydrogen radical (•H) in the Cu/TA/HO system, which contributed to the acceleration of the Cu/Cu redox cycle. The enhancing effect of TA on the homogeneous process was ascribed to the formation of a soluble complex with Cu, which favored the pH range extension, Cu oxidation, and radical generation. Moreover, the adsorption of TA on the catalysts surface promoted the consumption of HO, inducing •OH generation. The formed surface complex (≡Cu-TA) also accelerated the regeneration of ≡Cu, which was confirmed by density functional theory (DFT) calculation and surface characterization analysis (SEM, XRD, and XPS). The possible degradation pathways of MNZ in TA-modified Fenton-like system were also clarified.