Photoinduced Decomposition of Nitrate in Drinking Water in the Presence of Titania and Humic Acids

The photoreduction of nitrate in aqueous medium was investigated at 292 K in a batch system open to the ambient. Titania was tested as a photocatalyst and humic acids were added as promoters. Conversions of 28% were reached after 80 hours when a 44 mg/l nitrate solution was irradiated with a high pressure Xe-lamp; the major product was nitrite. The addition of humic acids (20 mg/l) promoted reduction of nitrate to nitrite but the mechanism of promotion could not be unambiguously identified. Titania (0.1 g/l) itself did not catalyze the photoreduction of nitrate but rather seemed to act as a catalyst for the reoxidation of nitrite to nitrate. The most successful system was a combination of 44 mg/l nitrate, 20 mg/l humic acids and 0.1 g/l Kronos-1002 titania: the nitrate conversion reached 32% after 76 hours, with little nitrite formed. Photocatalytic nitrate degradation is accompanied by homogeneous reduction to the more toxic nitrite; requiring any effective catalyst system to also reduce nitrite concentration.