Metribuzin and metamitron degradation using catalytic ozonation over tourmaline: Kinetics, degradation pathway, and toxicity

The reaction kinetics and degradation mechanisms of the herbicides metribuzin (MTZ) and metamitron (MTT) were evaluated during conventional and tourmaline-catalyzed ozonation processes. The tourmaline catalyst was characterized by X-ray diffraction, field emission-scanning electron microscopy, Brunauer-Emmett-Teller surface analysis and Fourier transform infrared spectroscopy. The reactivities of these two herbicides with ozone (O₃) are low due to their refractory characteristics, so catalytic ozonation is an ideal way to effectively degrade them. The effects of catalyst dosage, pH, temperature and ozone concentration on catalytic degradation of MTZ and MTT were investigated. Based on the results from quenching experiments and calculation of second-order rate constants (k_{O3, MTZ} (4.82 ± 0.42 M⁻¹ s⁻¹), k_{O3, MTT} (13.35 ± 0.62 M⁻¹ s⁻¹) and k_{•OH, MTT} ((6.90 ± 0.51) × 10⁹ M⁻¹ s⁻¹)), it was found that oxidation of the pollutants was mediated by decomposition of O₃ to produce ^•OH. Twelve MTZ degradation products and five MTT degradation products were identified by gas chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry, and the reaction mechanisms were analyzed. In addition, the potential toxicities of the derivatives formed during catalytic ozonation were examined. The degradation products of the two pollutants exhibited different toxicity profiles. The environmental-friendliness and efficient catalytic properties of tourmaline suggest that it has great potential as a heterogenous and green catalyst.