Photocatalyzed oxidation and mineralization of C1-C5 linear aliphatic acids in UV-irradiated aqueous titania dispersions-kinetics, identification of intermediates and quantum yields

The photocatalyzed degradation and mineralization of the linear carboxylic acid C1-C5 series (namely, formic acid, acetic acid, propanoic acid, butanoic acid, and valeric acid) were examined in UV-irradiated air-equilibrated aqueous TiO2 dispersions: (a) to assess the extent of (dark) adsorption and photoadsorption; (b) to determine their relationship with degradation; (c) to elucidate the dynamics of the degradative and mineralization processes (as loss of total organic carbon); (d) to identify intermediates by liquid chromatography (HPLC), gas chromatography (GC-FID) and gas chromatography coupled to a mass detector (GC-MS), and (e) to determine the quantum yields of degradation of this linear acid series based on formic acid (Φ = 0.12 ± 0.02) as a secondary actinometer proposed for aliphatic substrates. At low catalyst loading (2.0 g/L) and except for formic acid, adsorption of the other four C2-C5 acids increased with chain length, although the overall level was rather small, contrary to high catalyst loading (20 g/L) where all four C2-C5 acids adsorbed to nearly the same extent (15-19%) with adsorption of formic acid on TiO2 being two-fold greater (34%). The dynamics of degradation and mineralization decreased with increase in chain length, i.e. formic acid degraded and mineralized faster than its longer chain congeners. Several intermediates were identified in the degradation of the C3-C5 acids; in particular, 21 species were positively identified in the degradation of valeric acid by the techniques used. On the basis of partial charges and frontier electron densities of all atoms of the carboxylic acids, together with known species formed a route is proposed for valeric acid degradation. The quantum yields of degradation for the C2-C5 acid series increased with chain length from Φ = 0.010 for acetic acid to Φ = 0.067 for valeric acid.