Reaction mechanism, degradation pathway and toxicity assessment of NH4+ enhanced potassium ferrate removal of levofloxacin

Abstract

Considering that ammonium ion (NH4+) was widely distributed in natural water and many sewage, it was of great practical significance to study NH4+ enhanced potassium ferrate (Fe(VI)) to rapid removal of levofloxacin (LEV) from water. In this experiment, firstly, the reaction kinetic of NH4+ enhanced Fe(VI) removal of LEV was studied, and the results showed that the enhanced removal reaction conformed to the second-order kinetic equation. When pH ≤ 9, the second-order rate constant kapp was linearly related to NH4+ dosage, but when pH= 10, the above linear relationship was broken, and the slope of the fitting line would change toward a smaller trend. Subsequently, tert-butanol (TBA) and Methyl phenyl sulfoxide (PMSO) were used as probes for·OH and high-valent iron-based intermediate (Fe(V)/Fe(IV)) respectively, which further revealed the reaction mechanism that NH4+ enhanced Fe(VI) to remove LEV as follows: NH4+ could promote Fe(VI) to produce a large amount of Fe(V)/Fe(IV), thus accelerating the degradation of LEV. In addition, 10 degradation intermediates of LEV were identified by quadrupole time-of-flight tandem ultra-performance liquid chromatography mass spectrometer (Q-TOF LC-MS), and then combined with density functional theory (DFT) calculation, two possible degradation pathways were inferred. Through the luminescence inhibition experiment of Vibrio fischeri and the toxicity evaluation of ECOSAR, it could be concluded that in the process of NH4+ enhancing Fe(VI) to remove LEV, the toxicity of the solution showed a trend of increasing first and then decreasing, and its toxicity was effectively improved and reduced after full oxidation. Finally, the experiments in actual water showed that it was feasible to use NH4+ in actual water to enhance Fe(VI) for rapid removal of LEV.