Oxidation of tetracycline hydrochloride by peroxomonosulfate and peroxodisulfate on a ZnNi@NC carbonaceous catalyst: Role of non-radical species and mediated or direct electron transfer mechanisms

This study investigated the activation pathways of peroxomonosulfate (PMS) and peroxodisulfate (PDS) on a metal-based carbon catalyst (ZnNi@NC) for the degradation of tetracycline hydrochloride (TC). ZnNi@NC was synthesized via pyrolysis of metal-organic frameworks (MOFs), which yielded Ni species encapsulated in a Ni3ZnC0.7 layer and N,O-doped graphitic carbon nanotubes. The catalytic performance of ZnNi@NC in PMS and PDS activation for TC degradation was found to be excellent. The ZnNi@C/PMS and ZnNi@C/PDS systems successfully degraded 13 mg of TC within 100 min. We complemented these results with quenching experiments and utilized characterization techniques, such as electron paramagnetic resonance (EPR) and Fourier transform infrared spectroscopy (FTIR), to verify the activation of PMS and PDS, leading to the production of singlet oxygen and the formation of a surface-active complex. Additionally, our research demonstrated that ZnNi@NC facilitated electron transfer between surface-active PDS and adsorbed TC, as confirmed through electrochemical analytical characterization. Further insights from scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) revealed that active sites, including graphitic N and pyrrolidinic N species, played a pivotal role in singlet oxygen generation. Specifically, PMS activated through a direct electron transfer pathway, while PDS employed an indirect electron transfer pathway. Furthermore, the study emphasized that the N,O-doped graphitic carbon framework played a key role in the formation of surface-activated PMS/PDS. In conclusion, this work provides novel insights into the distinct functionalities of Ni-based carbon catalysts in persulfate activation.