Valence-dependent activation mechanisms of persulfates by copper foam for tetracycline degradation

Copper-based materials-mediated persulfate activation for pollutant degradation has attracted great interest, with copper species playing a crucial role in generating reactive species. However, the mechanism behind copper valence transformation during the catalytic activation of different persulfates, peroxydisulfate (PDS) and peroxymonosulfate (PMS), remains poorly understood. In this study, commercial copper foam (CF) was used as an activator to evaluate the degradation of tetracycline (TC) through different activation pathways, comparing both PDS and PMS systems. The experimental results show that the PMS/CF system achieved a significantly higher TC degradation rate than the PDS/CF system. Quenching experiments, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and copper species analysis confirmed that the PMS/CF system primarily follows a hydroxyl radical (•OH)-dominated activation mechanism at the Cu⁰/Cu⁺ active interface, while the PDS/CF system operates through a non-radical activation pathway mediated by singlet oxygen (¹O₂), alongside contributions from superoxide (O₂^•-) and •OH radicals at the Cu⁺/Cu²⁺ interface. Additionally, this study offers a detailed comparison of the degradation pathways and toxicity of TC degradation products in both PMS/CF and PDS/CF systems, considering the impact of environmental factors. This research provides valuable insights into the role of different copper species (Cu⁰/Cu⁺/Cu²⁺) in persulfate activation, guiding further studies on developing Cu-based catalysts for advanced oxidation processes (AOPs) applications.