Catalytic effect of low concentration carboxylated multi-walled carbon nanotubes on the oxidation of disinfectants with Cl-substituted structure by a Fenton-like system

Abstract

Carbon nanotubes (CNTs) have attracted increasing attentions due to their numerous potential applications. Their applications as catalysts or catalyst carriers have been the focus of many recent studies. In this paper, the role of carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) played in the degradation of triclosan (TCS), chlorofene (CF) and dichlorofene (DCF) by a Fenton-like system Fe3+/H2O2 was investigated. The results indicated that MWCNTs-COOH exhibited significant catalytic effect on TCS, CF and DCF removal. Meanwhile, the structure and chemical composition of MWCNTs-COOH did not show significant change after the oxidative reaction process, which has been characterized by TEM, Raman spectroscopy, XPS, and FT-IR. Taking TCS for example, increasing initial Fe3+ ion and H2O2 concentrations and reaction temperature favored the decomposition of TCS, while the optimal concentration of MWCNTs-COOH at 2.0ppm was observed. When [Fe3+]=0.04mmoldm−3, [H2O2]=0.6mmoldm−3, [MWCNTs-COOH]=2.0ppm, [TCS]=0.02mmoldm−3, at pH 4.0 and 25°C, the TCS removal efficiency was 97.0% after 30min and the total organic carbon (TOC) removal was 64.5% after 90min. Mechanism study showed that the reduction rate of Fe3+ ion to Fe2+ ion and the decomposition rate of H2O2 were both increased in the presence of MWCNTs-COOH, thus lead to the enhanced production of hydroxyl radicals (OH), which is the dominant reactive species responsible for TCS oxidation. Seven intermediates of TCS were identified by TOF-LC-MS, and their structures were further rationalized by frontier electron density (FED) calculations. Major oxidation pathways including hydroxylation, dechlorination and cleavage of the ether bond were tentatively proposed, and detailed underlying mechanisms were also discussed.