Enhanced activation of sulfite by the iron-citrate complexes under NUV irradiation for metronidazole degradation: Kinetic, degradation mechanism and efficiency in real water

In the present study, a novel approach for the elimination of emerging pollutants through an advanced oxidation process has been developed by combining near ultraviolet (NUV) irradiation with an iron-citrate complex (Fe(III)-Cit) to overcome the defects of the traditional Fe(III)/sulfite system and enhance the activation of sulfite in a neutral aqueous solution. The performance of this system was studied using metronidazole (MTZ) as the target pollutant, and the impact of operational conditions such as initial pH, Fe(III)-Cit complex concentration, sulfite dose, metronidazole concentration and dissolved oxygen were studied. The rate constant of MTZ degradation at natural pH (7.0 ± 0.2) by the Fe(III)-Cit/sulfite/NUV system is over 12.72-times and 13.25-times higher than the sulfite/NUV and Fe(III)-Cit/NUV individual binary systems. The effectiveness of the system was further examined in various real water matrices. The latter proved to be effective in the following order: groundwater > ultra-pure water > > seawater, revealing that it was very effective in the former. Chemical probe experiments indicated that sulfate radicals and hydroxyl radicals were the principal reactive oxygen species involved in the photodegradation of MTZ. Additionally, dissolved oxygen played a crucial role in the reaction. MTZ degradation products were determined by HPLC/MS, leading to the identification of nine different by-products and the proposal of possible degradation pathways. Although further studies are required to better understand this system, this work paves the way for the application of activated sulfite-based processes for environmental remediation without pH adjustment, offering an alternative to the use of conventional oxidants.