Degradation of 3-chlorocarbazole in water by sulfidated zero-valent iron/peroxymonosulfate system: Kinetics, influential factors, degradation products and pathways

As an emerging class of organic contaminants, polyhalogenated carbazoles (PHCZs) have been increasingly detected all over the world since 1980s. Due to the environmental persistence, bioaccumulation, and dioxin-like toxicity, PHCZs have aroused widespread concerns in recent years. However, efficient approach for the degradation of PHCZs is quite limited so far. Therefore, in this study, an advanced oxidation process (AOP), sulfidated zero-valent iron/peroxymonosulfate (S-ZVI/PMS) system was used to degrade 3-chlorocarbazole (3-CCZ), which is one of the mostly detected PHCZs congeners. The degradation of 3-CCZ was systematically studied under different conditions by varying the molar ratio of S/Fe, the dosage of S-ZVI or PMS, pH and temperature. The results indicated that S-ZVI/PMS was an effective strategy for PHCZs treatment. The 20-min degradation efficiency of 3-CZZ was up to 96.6% with the pseudo-first-order rate constant of 0.168 min(-1) under the conditions of 5 mg/L 3-CZZ, 0.3 g/L S-ZVI (S/Fe = 0.2), 1.0 mM PMS, pH 5.8 and 25 degrees C. HCO3-, Cl- and humic acid (HA) showed inhibitory effects to different degrees. Results of the electron paramagnetic resonance (EPR) and scavenging experiments clarified the dominant role of center dot OH, followed by O-1(2) and SO4 center dot. The product analysis and DFT calculation revealed three degradation pathways of 3-CCZ, namely hydroxylation, dechlorination and C-N bond cleavage, which largely alleviated the toxicity of the parent compound. This study showed the effectiveness of SZVI/PMS system in PHCZs treatment and provided a comprehensive investigation on the degradation behaviors of PHCZs in AOPs.

Automated summary

Polyhalogenated carbazoles (PHCZs) are an emerging class of organic contaminants that have aroused widespread concerns due to their environmental persistence, bioaccumulation, and dioxin-like toxicity. However, efficient methods for the degradation of PHCZs are limited. In this study, an advanced oxidation process using sulfidated zero-valent iron/peroxymonosulfate (S-ZVI/PMS) was shown to effectively degrade 3-chlorocarbazole (3-CCZ), one of the most commonly detected PHCZs congeners. The results demonstrated the potential of the S-ZVI/PMS system for PHCZs treatment and provided insights into the degradation pathways of PHCZs in AOPs.