Photocatalytic degradation and mineralization of the herbicide bentazon in aqueous solutions has been investigated. The pseudo-first order degradation kinetics was studied under different operational conditions, such as type of photocatalyst, catalyst loading, initial pH and addition of electron acceptors. The degradation rates proved to be strongly influenced by these parameters. Organic sulfur and nitrogen were only partially converted into inorganic in the presence of TiO 2 P25 or TiO 2

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... s vlp 7000 and UV-A. The presence of H 2 O 2 led to either enhancement or decrease of the initial degradation rates, depending on the type of catalyst employed. Intermediate products formed during photocatalytic treatment were identified by means of a high resolution LIT Orbitrap LC-ESI/MS system. 21 intermediates including dimers, hydroxy and/or keto bentazon derivatives, and further oxidized species were identified and detailed transformation pathways have been proposed. Acute toxicity profiles using marine bacteria Vibrio fischeri showed an increasing trend at the first stages of oxidation, implying the progressive formation of intermediates more toxic than the parent molecule, or synergistic effects among the transformation products. Thereafter, ecotoxicity was completely eliminated within 90 min of irradiation under the investigated conditions. Phytotoxicity tests, carried out on the monocotyl Sorghum saccharatum and the dicotyls Sinapis alba and Lepidium sativum, showed similar responses of the three plant species to the photocatalytically treated samples, resulting to complete removal of phytotoxicity under the employed conditions.