Heterogeneous photocatalytic degradation of citric acid over TiO2 II. Mechanism of citric acid degradation

The degradation pathways for citric acid (Cit. 5 mM, pH 2.8) degradation by TiO2-heterogeneous photocatalysis were studied through the analysis of its degradation intermediate products. The most important product formed was 3-oxoglutaric acid, but several other compounds, like acetoacetic. lactic, pyruvic, malic, glyoxylic, acetic and formic acids plus acetone were found. While citric acid is present, the intermediates cannot be photocatalytically degraded. The importance of the geometry of the photoreactor and the photon flux on the formation of 3-oxoglutaric acid from citric acid was studied in a different irradiation setup (at Cit] = 2 mM, pH 2.0), where reaction rates were one order of magnitude higher. With these results and those obtained in part I of this work, mechanisms for TiO2-photocatalytic degradation of citric acid were proposed, and related with the OGA photocatalytic and thermal decays already studied. (C) 2010 Elsevier B.V. All rights reserved.