Preliminary evaluation of the operation variables and degradation pathway in the alkali-catalyzed hydrothermal oxidation of triclosan

Abstract

Due to the lack of green treatment technologies for chlorinated organic contaminant detoxification, in this study, the effect of non-hazardous alkali-catalyzed hydrothermal oxidation (HTO) on triclosan (TCS) was investigated. The optimal parameters determined by single-factor experiments were: a H2O2/TCS (H/T) ratio of 2.5 mL/mg, a temperature of 180 °C, a residence time of 120 min. Using these parameters, the final dechlorination efficiency (DE) of 99.69% was obtained. Response surface methodology (RSM) was employed to further determine the optimal conditions under multi-factor interactions. The optimal parameters were an H/T ratio of 2.5 mL/mg, a temperature of 187 °C, and a residence time of 150 min. The determined DE was 99.47%. The results of three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy showed that the degradation of TCS generated humic acid-like organics with aromatic ring and phenolic hydroxyl functional groups. Furthermore, fluorescence spectroscopy analysis showed that the residence time increased from 30 to 120 min, and the fluorescence intensity difference increased from 100.5 to 218 au, which confirmed that •OH participated in the alkali-catalyzed HTO process. There were two different reaction pathways: (1) Cleavage of the C–O bond between the two benzene rings, followed by the opening and dechlorination of the benzene ring via oxidation; (2) The benzene rings were directly hydroxylated, opened, and dechlorinated through oxidation. The intermediate products were further oxidized to non-Cl-containing organics, achieving the harmless degradation of TCS. Further analysis showed that the total organic carbon (TOC) removal efficiency was about 80% at a residence time of 140 min, indicating that TCS was almost completely mineralized. This study provides a green and environmentally benign approach for the detoxification of chlorinated organic contaminants.