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Sol gel prepared tungsten (VI) doped TiO2 powders were evaluated as photocatalysts for crystal violet (CV) bleaching under UV irradiation, in the presence of oxygen. The incorporation of the hexavalent cation affects both surface and bulk properties. The optimum activity observed results from the competition between higher surface areas and lower specific activities per adsorption site. At pH 4.0, doped particles share a common photodegradation mechanism, where the relative contribution of the aromatic structure rupture over the loss of terminal methyl groups (N-demethylation) is more relevant than in the case of non-doped titania. This behavior can be minimized shifting the pH into basic condition.

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Please cite this article as: Donadelli JA, Carlos L, Arques-Sanz A, García Einschlag FS, Kinetic and mechanistic analysis of azo dyes decolorization by ZVI-assisted Fenton systems: pH-dependent shift in the contributions of reductive and oxidative transformation pathways,

Photolysis of nitroaromatic compounds in aqueous solution is a very slow and inefficient process. As already observed for a variety of organic pollutants, considerably faster degradation rates of nitrobenzene (NBE), 1-chloro-2,4-dinitrobenzene (CDNB), 2,4-dinitrophenol (DNP), and 4-nitrophenol (PNP) could be achieved, when the oxidative degradation of these compounds was initiated by hydroxyl radicals produced by UV-C photolysis of H2O2. Analysis of intermediate products formed during irradiation by HPLC and IC showed that cleavage of the aromatic ring should occur at an early stage of the oxidation process and that organic nitrogen was almost completely converted to nitrate. The optimal initial concentration of hydrogen peroxide ([H2O2]OPT) leading to the fastest oxidation rate, which depends on the initial substrate concentration ([S]0), could be evaluated using a simplified expression based on the main reactions involved in the first stages of the degradation process. Using only a minimum of kinetic and analytical information, this expression shows that the ratio R(OPT) (= [H2O2]OPT/[S]0) is related to the bimolecular rate constants for the reactions of hydroxyl radicals with substrate (kS) and H2O2 (kHP) and to the corresponding molar absorption coefficients (epsilonS, epsilonHP). Competition experiments between selected pairs of the substrates showed that their relative reactivity toward hydroxyl radicals could be correctly predicted using the same simplified approach. The results of our investigations as well as literature data support the general validity of the proposed procedure for optimizing oxidation rates of the UV/H2O2 process.

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