Isolated copper ions and surface hydroxyl groups as a function of non-redox metals to modulate the reactivity and persulfate activation mechanism of spinel oxides

In advance oxidation processes (AOPs), modulating the activation mechanism from free radical to the non-radical pathway is the most attractive strategy for the treatment of recalcitrant pollutants. Herein, we achieved this goal after modifying the spinel CuFe₂O₄ catalyst by introducing non-redox metals (M = Mg, Ca, Ba, and Zn) as CuMFe₂O₄. The modified catalysts were evaluated for the removal of acetaminophen (ACE) using the persulfate (PS) activated system. The results revealed that the non-redox metals not only enhanced the catalytic activity of CuFe₂O₄ in the order of Mg > Zn > Ca > Ba but also switched the original radical pathway of CuFe₂O₄/PS system to the non-radical one (i.e., direct electron transfer path) of CuMgFe₂O₄/PS system. The critical role of non-redox metals to modulate the reaction mechanism was studied in detail using the extensive radical scavengers, EPR analysis, target pollutants selectivity, electrochemical studies, decomposition of oxidants, and identification of degradation products. Additionally, various experiments and characterizations, including XRD, EPR, H₂-TPR, XPS, and FTIR, showed that the changes in the activation mechanism and catalytic activity (6–8 folds higher in the case of CuMgFe₂O₄) were related to the generation of a large amount of surface-bound isolated Cu²⁺ ions and abundant surface hydroxyl groups. The non-radical pathway of the CuMgFe₂O₄/PS system showed less susceptibility to the changing solution pH, excessive amount of humic acid (HA) or anions, and complete recyclability, and thus demonstrating good practical utility for wastewater. This study provides deep insight into the intrinsic role of non-redox metals to modulate both the activation mechanism and catalytic properties of the catalyst and to design new catalysts for the persulfate-based advance oxidation process.