Revisiting the synergistic oxidation of peracetic acid and permanganate (VII) towards micropollutants: The enhanced electron transfer mechanism of reactive manganese species

Synergistic actions of peroxides and high-valent metals have garnered increasing attentions in wastewater treatment. However, how peroxides interact with the reactive metal species to enhance the reactivity remains unclear. Herein, we report the synergistic oxidation of peracetic acid (PAA) and permanganate(VII) towards micropollutants, and revisit the underlying mechanism. The PAA-Mn(VII) system showed remarkable efficiency with a 28-fold enhancement on sulfamethoxazole (SMX) degradation compared to Mn(VII) alone. Extensive quenching experiments and electron spin resonance (ESR) analysis revealed the generation of unexpected Mn(V) and Mn(VI) beyond Mn(III) in the PAA-Mn(VII) system. The utilization efficiency of Mn intermediates was quantified using 2,2 '-azino-bis(3-ethylbenzothiazoline)-6-sulfonate '-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), and the results indicated that PAA could enhance the electron transfer efficiency of reactive manganese (Mn) species, thus accelerating the micropollutant degradation. Density functional theory (DFT) calculations showed that Mn intermediates could coordinate to the O1 of PAA with a low energy gap, enhancing the oxidation capacity and stability of Mn intermediates. A kinetic model based on first principles was established to simulate the time-dependent concentration profiles of the PAA-Mn complexes and quantify the contributions of the PAA-Mn(III) complex (50.8 to 59.3 %) and the PAA-Mn(V/VI) complex (40.7 to 49.2 %). The PAA-Mn(VII) system was resistant to the interference from complex matrix components (e.g., chloride and humic acid), leading to the high efficiency in real wastewater. This work provides new insights into the interaction of PAA with reactive manganese species for accelerated oxidation of micropollutants, facilitating its application in wastewater treatment.