Caffeine degradation using peroxydisulfate and peroxymonosulfate in the presence of Mn2O3. Efficiency, reactive species formation and application in sewage treatment plant water

The crucial role of manganese(III) species in the oxidative and catalytic reactivity of manganese oxides have been widely reported. However, the direct activation of radical precursors by Mn(III) oxides is still a matter of discussion. In this work, the ability of Mn(III)-bearing oxide (Mn2O3) for activation of peroxydisulfate (PS) and peroxymonosulfate (PMS) was evaluated and compared using caffeine (CAF) as a model pollutant. Complete CAF removal was achieved with 5 mM of PS or PMS after 8 h of reaction time, whereas 48% of mineralization was obtained after 70 h of reaction when PMS was used as oxidant. Electron paramagnetic resonance spin trapping (EPR) and radical scavenging experiments showed that the sulfate radical (SO4 center dot-) was the dominant radical in the Mn2O3/PMS system. In comparison, a non-radical mechanism such as Mn2O3 surface-activated PS promoted the removal of CAF in the system of Mn2O3/PS. Liquid chromatography coupled to mass spectrometry analyses enabled the detection of main degradation products and the proposition of the CAF degradation pathways. Investigations in the real sewage treatment plant water showed that the decrease of CAF removal observed in wastewater as compared to pure water can be alleviated by increasing the Mn2O3 dosage. These findings highlight the great ability of Mn-species in PS or PMS activation in different water matrices, which have important implications for the development of novel wastewater treatment technologies.