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The photocatalytic degradation of 2-, 3- and 4-nitrophenol has been investigated in oxygenated aqueous suspensions containing TiO₂. The organic reaction intermediates have been determined by high performance liquid chromatography. The results indicate the complete mineralisation of the substrates and the formation of both nitrate and ammonium ions. The degradation pathways involve a rapid opening of the aromatic ring followed by a slower oxidation of the aliphatic compounds. The hydroxyl radicals are responsible for the primary attack of the nitrophenols with formation of dihydroxynitrobenzenes. The ring hydroxylation occurs in the positions activated by the contemporaneous presence of phenolic and nitro groups. The presence of ammonium ions confirms that reduction pathways are also operating.