The fate of organic nitrogen under photocatalytic conditions: degradation of nitrophenols and aminophenols on irradiated TiO2

The photocatalytic transformation of 2-, 3- and 4-nitrophenol (NP) and 2-, 3- and 4-aminophenol (AP) over irradiated titanium dioxide was investigated in aerated aqueous solution. At pH < 6, extensive mineralization (more than 90%) of the substituted phenols occurs, whereas, at pH 11, organic carbon is persistently observed. Photocatalysis converts 80% of the nitro group in nitrophenols to NO3− through the intermediate formation of NO2− ions; the remaining 20% is transformed into NH4+ ions. This implies that, even in aerated solutions, reductive pathways are significant, as confirmed by the detection of aminophenol. By contrast, photocatalytic degradation of aminophenols converts the NH2 group into about 60%–70% NH4+ ions and only about 10% NO3−, when about 20% of the organic carbon remaining. A slow transformation of the ammonium ions into NO3− ions is observed at longer irradiation times.