Sulfite activated by in situ preparation of zero-valent iron agar composite membrane for the removal of metronidazole: Synergistic effect of radical and nonradical pathways

[Display omitted] •A novel nZVI/AM composite membrane were developed for removal of MNZ.•The mechanism of nZVI/AM/S(Ⅳ) was explored.•The formation and action mechanism of the nonradical pathway are discussed.•The degradation pathways and ecotoxicity of the intermediates of MNZ were proposed. The nanoscale zero-valent iron/sulfite(nZVI/S(IV)) system showed great potential for rapid removal of organic matter in water. In this study, it was optimized to construct the nZVI/agar composite membrane (AM)/S(IV) system to improve the stability and recoverability of ZVI for metronidazole (MNZ) removal. Under the optimal test conditions, the degradation rate of MNZ reached over 99 % and was mineralized. Through a series of experimental validation and technical tests, it was obtained that Fe2+ released from nZVI/AM in situ was the main active site, and SO4•-, •OH and 1O2 acted synergistically to degrade MNZ, and the generation pathway of 1O2 was demonstrated. At the same time, this study discussed the degradation process, toxicity changes of MNZ and catalyst stability in the nZVI/AM/S(IV) system.