Photocatalytic treatment of metribuzin herbicide over TiO2 aqueous suspensions: Removal efficiency, identification of transformation products, reaction pathways and ecotoxicity evaluation

The present study describes the detailed transformation and mineralization of metribuzin, a widely used herbicide, by TiO2 mediated photocatalysis under simulated solar irradiation. Transformation of MET by heterogeneous photocatalysis with TiO2 ([metribuzin] = 10 mg L-1, [TiO2] = 100 mg L-1 and I = 750 Wm(-2)) was completed within 40 min irradiation, whereas 80% of mineralization was achieved within 300 min irradiation. The degradation for both metribuzin and total organic carbon (TOC) followed pseudo-first order kinetics. Rate constants of k(app (MET)) = 10.85 x 10(-1) min(-1) and k(app (TOC)) = 5.2 x 10(-3) min(-1) and half-lives (t(1/2)) of 6.4 min and 133.3 min were recorded for metribuzin and TOC respectively. High resolution accurate mass spectrometry coupled with liquid chromatography (HR-LC-MS) and gas chromatography-mass spectrometry (GC-MS) techniques were used to identify the degradation products. The main metribuzin phototransformation pathways were observed to be the cleavage of -NH2 and -SCH3 groups, hydroxylation and dealkylation of tert-butyl chain. The investigation of the role of the reactive species suggests that HO center dot, h(+) and O-2(center dot-) play the principal role in the degradation mechanism of metribuzin. Substrate decomposition is accompanied by the formation of intermediates compounds more toxic than the parent molecule, as evidenced by the measurement of acute toxicity using the Vibrio fischeri bacteria test. (C) 2014 Elsevier B.V. All rights reserved.