Ammonium-activated peracetic acid for enhanced tetracycline degradation: Performance and mechanism

Water matrices commonly coexist with the target contaminants in peracetic acid (PAA) oxidation processes, affecting PAA application and pollutant removal. Particularly, in pharmaceutical wastewater, ammonium and tetracycline (TTC) often coexist. However, the influence of components in water matrices (e.g., ammonium) on TTC degradation during PAA oxidation remains poorly understood. Therefore, the effects of ammonium, phosphate, borate, and carbonate under varying pH conditions were systematically studied. Notably, ammonium was firstly reported to significantly facilitate TTC degradation by PAA in this study. With the presence of ammonium, the pseudo-first-order reaction rate constants (k) of TTC (0.128-0.314 min^-1 across different pH conditions) within 5 min were substantially higher than those in other systems. Radical quenching experiments and electron paramagnetic resonance (EPR) analyses identified reactive nitrogen species (RNS) (i.e., peroxynitrite (ONOO^-), NO^• and NO₂^•) as the active oxidants, rather than reactive oxygen species or organic radicals. Density functional theory (DFT) calculations further demonstrated that the distributions of TTC, PAA, and ammonium species collectively affected TTC degradation, and ammonium lowered the lowest unoccupied molecular orbital (LUMO) energy of PAA species (i.e., PAA and PAA^-), which in turn facilitated the direct oxidation of TTC by PAA through enhancing PAA reactivity. These findings advance the understanding of TTC removal by PAA in wastewater containing ammonium, and highlight important implications for PAA application in the treatment of wastewater containing ammonium and tetracycline co-contamination.