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

ABSTRACT

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 (Fe⁰) to remove 3,6-dichlorocarbazole (3,6-CCZ). First, the key parameters influencing 3,6-CCZ degradation (PMS dosage, Fe⁰ 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·^-), hydroxyl radicals (OH·), and singlet oxygen (¹O₂) 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·) and sulfate radicals (SO₄·^-). In this study, it was found that OH· and ¹O₂ play a major role. Then, fresh and reacted Fe⁰ 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·) and (¹O₂) in 3,6-CCZ removal. This study provides new insight into the activation mechanism of Fe⁰-activated PMS and the removal mechanism of 3,6-CCZ.

Highlights

• Fe⁰ is a long-lasting and efficient catalyst of PMS for the degradation of 3,6-CCZ.

• The key parameters influencing 3,6-CCZ degradation were determined.

• The degradation pathways of 3,6-CCZ were inferred.

• OH· and ¹O₂ were the main ROS in Fe⁰-activated PMS system.

GRAPHICAL ABSTRACT

KEYWORDS:

• 3,6-Dichlorocarbazole(3,6-CCZ)
• peroxymonosulfate (PMS)
• degradation pathways
• radical identification
• mechanism‌

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

Additional information

Funding

This work was supported by Research project of ecological environment protection and restoration of Yangtze River in Zhoushan: [Grant Number SZGXZS2020068]; the National Key Research and Development Project: [Grant Number 2019YFC0408604].