Efficient removal of 3,6-dichlorocarbazole with Fe⁰-activated peroxymonosulfate: performance, intermediates and mechanism

Nowadays, polyhalogenated carbazoles (PHCZs) are a major pollutant that has recently sparked widespread concern. In this work, peroxymonosulfate (PMS) was activated by zero valent iron (Fe-0) to remove 3,6-dichlorocarbazole (3,6-CCZ). First, the key parameters influencing 3,6-CCZ degradation (PMS dosage, Fe-0 dosage, initial pH, temperature and co-existing ions) were determined. Under the determined optimum conditions, the removal rate of 3,6-CCZ reached 100% within 1.5 h. Sulfate radicals (SO4 center dot-), hydroxyl radicals (OH center dot), and singlet oxygen (O-1(2)) generated in the reaction were directly identified with 0.1 M 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) by in-situ electron paramagnetic resonance (EPR) and indirectly identified by radical quenching experiments. The main reactive oxygen species (ROS) were different from most reported hydroxyl radicals (OH center dot) and sulfate radicals (SO4 center dot-). In this study, it was found that OH center dot and O-1(2) play a major role. Then, fresh and reacted Fe-0 were characterized by XRD, SEM, and XPS. Iron corrosion products such as Fe2O3, Fe3O4, and FeOOH were generated. Finally, 3,6-CCZ degradation intermediates were identified by GC-MS and its degradation pathway was speculated. The intermediate pathway confirmed the combined action of (OH center dot) and (O-1(2)) in 3,6CCZ removal. This study provides new insight into the activation mechanism of Fe-0-activated PMS and the removal mechanism of 3,6-CCZ.