Degradation of 1,3,6,8-tetrabromocarbazole by sulfidated zero-valent iron activated peroxydisulfate: Mechanistic insight and transformation pathways.

[Display omitted] • 1,3,6,8-BCZ could be efficiently degraded in S-ZVI/PDS system. • Both radical and nonradical pathways play roles in the degradation of 1,3,6,8-BCZ. • •OH and 1O 2 are the dominant reactive oxygen species (ROS). • 1,3,6,8-BCZ could be degraded by 1O 2 via 1,4-cycloaddition. • O 2 •− plays a synergistic role in the intermediate degradation and 1O 2 generation. Polyhalogenated carbazoles (PHCZs) are a kind of emerging contaminants which have widespread distribution, strong potential persistence, high bioaccumulation, and dioxin-like toxicity. In this study, we used sulfidated zero-valent iron (S-ZVI) synthesized by ball-milling to activate peroxydisulfate (PDS) for the degradation of 1,3,6,8-tetrabromocarbazole (1,3,6,8-BCZ). The initial 1 mg/L 1,3,6,8-BCZ was degraded by 92.9 % within 30 min in S-ZVI/PDS system. The pseudo-first-order rate constant was estimated as high as 0.182 min−1. Electron paramagnetic resonance (EPR) study and quenching experiments results revealed that •OH, 1O 2 , O 2 •−, and SO 4 •− played roles in the degradation of 1,3,6,8-BCZ, with their contribution percentages of 32.8 %, 32.5 %, 24.8 % and 9.9 %, respectively. By further density functional theory (DFT) calculations, radical addition by •OH and 1,4-cycloaddition by 1O 2 were suggested as the dominant channels for 1,3,6,8-BCZ degradation, while O 2 •− exerted an indirect synergistic influence. Based on the identification of intermediates and analysis of Fukui indices, 1,3,6,8-BCZ degraded through three degradation pathways, namely hydroxylation, debromination, and C–N bond cleavage. The S/Fe molar ratio, S-ZVI dosage, PDS dosage, reaction temperature, initial pH, and water matrices had influences on the degradation in 1,3,6,8-BCZ, result in a great toxicity reduction. Overall, this research developed an effective and feasible approach for the treatment of PHCZs, and which gave deep insights into the vital roles and synergistic effects of various reactive species based on the combination of experimental study and theoretical calculation. [ABSTRACT FROM AUTHOR]