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Enhancement of Photcatalytic Mineralization of Phenol in the Presence of Ferric Ions: Kinetic Modelling and Parameter Estimation

Aaron Ortiz-Gomez1, Hugo De Lasa1, Benito Serrano-Rosales2, and Miguel Salaices3. (1) Chemical and Biochemical Engineering, The University of Western Ontario, 1151 Richmond Street, Suite 2, London, ON N6A5B8, Canada, (2) Chemical Engineering Department, Universidad Autonoma de Zacatecas, Zacatecas, Zacatecas, Mexico, (3) Instituto de Investigaciones Electricas, Cuernavaca, Morelos, Mexico

Metallic components have proven to modify the rate of reaction of organic pollutants in photocatalytic process depending on their concentrations. Most of those metallic components tested so far are equally harmful than the targeted organic pollutants. The search for means to improve the rate of mineralization of hazardous components has led us to look for metallic complexes that can enhance the rate of mineralization yet be environmentally friendly. This study shows that the presence of low ferric ion concentrations significantly accelerates the rate of oxidation and mineralization of phenol. Ferric ions not only considerably affect the disappearance and total mineralization rates of phenol, but also the concentration-time profiles of its major oxidation intermediates. At reaction conditions, three major aromatic intermediates were identified para-dihydroxybenzene (p-DHB), ortho-dihydroxybenzene (o-DHB) and 1,4-benzoquinone (1,4-BQ) as well as carboxylic acids such as oxalic acid, formic acid, maleic acid and fumaric acid. For the phenol oxidation in the presence of ferric ions, the concentration profiles of o-DHB and p-DHB changed greatly with respect to free iron ion conditions. The formation of o-DHB increases significantly while that of p-DHB decreases in the same magnitude (thus promoting a major selectivity for o-DHB). Results reveal that from all major aromatic intermediates, o-DHB is the most affected by the presence of ferric ions, which explains why the rate of mineralization of phenol is increased by promoting a higher formation of o-DHB in the presence of iron. On this basis a new refined mechanism for the photocatalytic mineralization of phenol is proposed containing important modifications versus the parallel-series model proposed by Salaices et al (2002). It is concluded that this mechanism describes adequately photocatalysis with and without ferric ions with the various mechanistic steps involving detectable species. Our experimental data also reveals that while the photocatalytic oxidation of phenol share a close mechanism in the presence or absence of ferric ions, iron promotes a considerable modification of step-related kinetic constants.