Abstract

Titanium dioxide, irradiated by ultraviolet/visible light was used to mediate the photocatalyzed degradation of several methylated creosote phenolics: ortho-, meta-, and para-cresol, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-xylenol as well as 2,3,5-trimethylphenol. Total mineralization of the phenols to CO$\sb2$ and H$\sb2$O ensues as evidenced for m-cresol, 3,4-xylenol, and 2,3,5-trimethylphenol. The photo-oxidation of methylhydroquinone and 4-methylcatechol, two intermediates detected in the photodegradation of cresols, also leads to total mineralization to CO$\sb2$. The effect of such parameters as pH, initial phenol concentration, TiO$\sb2$ concentration, oxygen concentration and radiant power levels on the degradation of methylated phenols was examined in detail for m-cresol and 3,4-xylenol. The rates of photodegradation were found to increase as a function of pH and temperature. Concentration dependence experiments show the photo-oxidation process follows saturation-type behaviour. Intermediates were identified in these photo-oxidation reactions: these were typically methylated derivatives of dihydroxybenzenes or p-benzoquinone. Photochemical efficiencies (lower limits of quantum yields) were determined for the cresols and the xylenols; these were: 0.0096 (o-cresol), 0.0076 (m-cresol), 0.0104 (p-cresol), 0.0067 (2,3-xylenol), 0.012 (2,4-xylenol), 0.0074 (2,5-xylenol), 0.015 (2,6-xylenol), 0.0060 (3,4-xylenol) and 0.0067 (3,5-xylenol). A practical case study was carried out using aqueous mixtures of coal tar creosote; irradiation of the solutions in the presence of TiO$\sb2$ resulted in the complete mineralization of the organic components (75 wt% C) to CO$\sb2$. The reactions of xylenols and pentahalogenated phenols with $\cdot$OH radicals were examined in homogeneous solution using pulse radiolysis in order to identify the primary organic radicals formed. Reaction of $\cdot$OH radicals with 2,3-, 2,4-, 2,5-, 2,6- and 3,5-xylenol yields exclusively the $\cdot$OH-adducts (dihydroxydimethylcyclohexadienyol radicals) as the initial products. 3,4-Xylenol reacts with $\cdot$OH to form both an $\cdot$OH-adduct and a phenoxyl radical. At pH $\le$ 8, the 3,4-phenoxyl radical is formed via H$\sb2$O elimination from the $\cdot$OH-adduct; by contrast, at pH $\ge$ 9, both radicals appear to form concurrently. The principal products from the reaction of OH$\cdot$ with pentabromophenoxide and pentachlorophenoxide are the corresponding pentahalophenoxyl radicals; other products formed are the $\cdot$OH-adducts (dihydroxypentahalocyclohexadienyl radical anions), and the semiquinone radicals. The reaction of OH$\cdot$ with pentafluorophenoxide anion yields exclusively the dihydroxypentafluorocyclohexadienyl radical. The sonochemical oxidation of phenol was also examined in aerated solutions. Major intermediates detected were hydroquinone, catechol and p-benzoquinone. In contrast with photo-oxidation, complete mineralization to CO$\sb2$ did not occur.