Mechanistic insights into the roles of copper in enhanced peroxymonosulfate activation for tetracycline degradation by copper ferrite composite catalyst

Tetracycline (TC) pollution has attracted great attention due to its wide use, non-biodegradability and potential carcinogenicity. Bimetallic catalysts/peroxymonosulfate (PMS) systems have been considered as powerful technologies for TC degradation. Although the excellent catalytic activity of copper ferrite (CuFe₂O₄) composite oxides for PMS has been already elucidated, the roles of increased Cu in the degradation especially from non-radical pathways are not clarified clearly. In this study, we prepared a series of CuO/CuFe₂O₄/Fe₂O₃ (CCF) catalysts to confirm the key role of molar ratios of Cu: Fe (ranging from 1:2 to 2:1) in the enhanced TC degradation. The optimized CCF-3 catalyst with the Cu: Fe molar ratio of 2:1 showed the best degradation efficiency up to 95.5 % and the fastest apparent degradation rate of 0.1350 min⁻¹ towards TC. In addition, the CCF-3/PMS system showed excellent adaptability and good stability in TC degradation. Importantly, a novel molecular-scale mechanism was proposed. Both radical (SO₄^∙-, ∙OH, O₂^{∙ –}) and non-radical (¹O₂, direct electron transfer) pathways were associated with the CCF-3/PMS catalytic system, where ¹O₂ played the dominant roles. The increased Cu significantly boosted the non-radical pathways via promoting the formation of oxygen vacancies (O_v) and its further conversion to ¹O₂, and facilitating electron transfer between bimetallic Cu/Fe interface. This work provides a deep insight into the formation and reaction mechanisms of non-radical species for CCF/PMS systems, and a novel foundation for the design of CuFe₂O₄ composite catalysts.