6533b857fe1ef96bd12b5151

RESEARCH PRODUCT

Photoassisted Mineralization of Aromatic and Aliphatic N Heterocycles in Aqueous Titanium Dioxide Suspensions and the Fate of the Nitrogen Heteroatoms

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Abstract The photoassisted degradation of aromatic heterocycles (pyrrole, imidazole, pyrazole, isoxazole, oxazole and thiazole) and N-containing alicycles (aliphatic heterocycles: pyrrolidine, 4-butanelactam and 5-pentanelactam) was examined in liquid–solid dispersions. Complete mineralization (TOC) of the aromatic heterocycles was attained within ca. 1 h of UV irradiation of the TiO 2 /heterocycle system in acidic (pH 3), near-neutral (pH 6.0–7.6) and alkaline (pH 11) media. Mineralization kinetics were, in general, not appreciably influenced by the presence of acid but tended to be somewhat slower in alkaline media. N-alicycles were photomineralized more slowly than were the aromatics. The former could be mineralized in acidic and near-neutral media in less than 2 h, but not in alkaline media in which pyrrolidine, 4-butanelactam and 5-pentanelactam (and for comparison 4-butanelactone) were not mineralized even after 3 h of UV irradiation. Final products were, in all cases, CO 2 , NH 4 + and NO 3 − ions and SO 4 2− ions in the case of thiazole. Nitrogen (N 2 ) gas was detected only during the photooxidation of pyrazole (contains two adjacent N heteroatoms) but not imidazole in which the N atom are separated by a C atom. The molar ratio NH 4 + /NO 3 − depended closely on the chemical structure of the substrates at the longer irradiation times. The site and mode of adsorption of these heterocycles onto the TiO 2 particle surface were inferred from point charge calculations and the point of zero charge of the TiO 2 (pH pzc ∼6.7); the position of elecrophilic attack by the photogenerated OH radicals was deduced from calculated frontier electron densities.