Photocatalytic Degradation of Herbicide Quinmerac in Various Types of Natural Water

The efficiency of the photocatalytic degradation of the herbicide quinmerac in aqueous TiO2 suspensions was examined as a function of the type of light source, TiO2 loading, pH, temperature, electron acceptors, and hydroxyl radical ((OH)-O-aEuro cent) scavenger. The optimum loading of catalyst was found to be 0.25 mg mL(-1) under UV light at pH 7.2, with the apparent activation energy of the reaction being 13.7 kJ mol(-1). In the first stage of the reaction, the photocatalytic degradation of quinmerac (50 mu M) followed approximately a pseudo-first order kinetics. The most efficient electron acceptor appeared to be H2O2 along with molecular oxygen. By studying the effect of ethanol as an (OH)-O-aEuro cent scavenger, it was shown that the heterogeneous catalysis takes place mainly via (OH)-O-aEuro cent. The results also showed that the disappearance of quinmerac led to the formation of a number of organic intermediates and ionic byproducts, whereas its complete mineralization occurred in about 120 min. The reaction intermediates (7-chloro-3-methylquinoline-5,8-dione, three isomeric phenols hydroxy-7-chloro-3-methylquinoline-8-carboxylic acids, and 7-chloro-3-(hydroxymethyl)quinoline-8-carboxylic acid) were identified and the kinetics of their appearance/disappearance was followed by LC-ESI-MS/MS. Tentative photodegradation pathways were proposed and discussed. The study also encompassed the effect of quality of natural water on the rate of removal of quinmerac.