Oxidation of Refractory Benzothiazoles with PMS/CuFe2O4: Kinetics and Transformation Intermediates

Benzothiazole (BTH) and its derivatives 2-(methylthio)bezothiazole (MTBT), 2-benzothiazolsulfonate (BTSA), and 2-hydroxybenzothiazole (OHBT) are refractory pollutants ubiquitously existing in urban runoff at relatively high concentrations. Here, we report their oxidation by CuFe₂O₄-activated peroxomonosulfate (PMS/CuFe₂O₄), focusing on kinetics and transformation intermediates. These benzothiazoles can be efficiently degraded by this oxidation process, which is confirmed to generate mainly sulfate radicals (with negligible hydroxyl-radical formation) under slightly acidic to neutral pH conditions. The molar exposure ratio of sulfate radical to residual PMS (i.e., Rcₜ) for this process is a constant that is related to the reaction condition and can be easily determined. The reaction rate constants of these benzothiazoles toward sulfate radical are (3.3 ± 0.3) × 10⁹, (1.4 ± 0.3) × 10⁹, (1.5 ± 0.1) × 10⁹, and (4.7 ± 0.5) × 10⁹ M–¹ s–¹, respectively (pH 7 and 20 °C). On the basis of Rcₜ and these rate constants, their degradation in the presence of organic matter can be well-predicted. A number of transformation products were detected and tentatively identified using triple-quadruple/linear ion trap MS/MS and high-resolution MS. It appears that sulfate radicals attack BTH, MTBT, and BTSA on their benzo ring via electron transfer, generating multiple hydroxylated intermediates that are reactive toward common oxidants. For OHBT oxidation, the thiazole ring is preferentially broken down. Due to competitions of the transformation intermediates, a minimum PMS/pollutant molar ratio of 10–20 is required for effective degradation. The flexible PMS/CuFe₂O₄ could be a useful process to remove the benzothiazoles from low dissolved organic carbon waters like urban runoff or polluted groundwater.