Photocatalytic degradation of azo dye in aqueous TiO2 suspension: Reaction pathway and identification of intermediates products by LC/MS

Azo dyes, aromatic moieties linked together by azo (-NN-) chromophores, represent the largest class of dyes used in textile-processing and other industries. The release of these compounds into the environment is undesirable, because of color, and their toxic breakdown products can be mutagenic. The aim of the study is to degrade azo dyes in the presence of artificial UV light and photocatalyst, and to identify degradation products of C.I. acid orange 7 (AO7) by liquid chromatography-mass spectrometry (LC/MS) and ion chromatography (IC). Thereby, establishing the mechanism of photocatalytic degradation. The substrate was chosen as a simple model for the study of reactions involving the more complex commercial products used for the dyeing of textile fibers. For degradation of AO7, batch experiments were carried out by irradiating the aqueous solution of dye, in the presence of TiO2/UV light. The rate of decolorization was estimated from residual concentration spectrophotometrically. The effect of process parameters viz. pH (2-10), initial concentration of dye (5-100 ppm), and photocatalyst loading (0.5-2 g/l) on decolorization of dye has also been assessed. The chromatographic separations achieved by liquid chromatography were analyzed by UV-vis diode array and the separated products were further subjected to mass spectrometry and five new reaction intermediates were identified from mass spectral data. The aliphatic ions were identified by IC. On the basis of identified products the mechanism of photocatalytic degradation of azo dye was proposed.