Mxene-decorated spinel oxides as innovative activators of peroxymonosulfate for degradation of caffeine in WWTP effluents: Insights into mechanisms

In the frame of the environmental issues related to the efficiency of wastewaters treatment, the generation of advanced oxidation processes (AOPs) by 2D materials appears one of the most promising solutions. In this study, a novel catalytic system for peroxymonosulfate activation (PMS) was designed based on MXene (Ti₃C₂T_x) decorated with spinel oxides Co₃O₄, Fe₃O₄ and CoFe₂O₄ catalysts. Their efficiency in caffeine (CAF) degradation via PMS activation was assessed. The insertion of spinel oxides inside the multilayer structure of MXene along with their uniform surface decoration was demonstrated by SEM and TEM analyses and it also avoided the aggregation of the magnetic particles, thus increasing their efficiency. Among the different catalysts, the MXene/CoFe₂O₄ (MXCF) stood out as the most effective, mainly due to the Fe and Co redox cycles. The complete degradation of CAF was achieved in the dark within 10 min at natural pH using 0.2 g/L of MXCF and 0.5 mM of PMS. The novelty of current study lies in the efficient activation of PMS by, for the first time, MXCF in the dark along with mechanistic elucidation of PMS activation. The important role of Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺ redox cycles alongside surface bound functional groups were highlighted. Radical scavenging and EPR experiments confirmed ^•OH and ¹O₂ as the main ROS involved in the CAF degradation. The CAF degradation pathways pointed to hydroxylation and imidazole ring opening mechanisms and MXCF catalyst also exhibited high efficiency in the degradation of sulfamethoxazole and phenol via PMS activation. To further highlight the relevance of the obtained results, treatment of tertiary effluents of wastewaters treatment plant (WWTP) in Bratislava contaminated by CAF exhibited a complete pollutant degradation after 3 h by supplying 0.2 g/L of catalyst and 2 mM PMS in the dark.