Photocatalyzed oxidation and mineralization of branched and oxidized C4 and C5 aliphatic acids in UV-irradiated aqueous titania dispersions: Adsorption and degradation dynamics

The photocatalyzed degradation and mineralization of branched C4 and C5 acids {2-methylpropanoic acid (2-MPA), 2-methylbutanoic acid (2-MBA), 3-methylbutanoic acid (3-MBA), and trimethylacetic acid (TMA)} and oxidized C4 and C5 carboxylic acids {e.g., 2-hydroxybutanoic acid (2-HBA), 2-hydroxy-2-methylbutanoic acid (2-OH-2-MBA), 2-hydroxyvaleric acid (2-HVA), and 2-ketovaleric acid (2-KVA)} were examined in UV-irradiated air-equilibrated aqueous TiO2 dispersions to assess: (a) the extent of (dark) adsorption; (b) the relationship between the extent of adsorption and degradation dynamics; and (c) the dynamics of mineralization (loss of total organic carbon (TOC)). Branching had little, if any impact on the adsorption process by comparison with analogous linear acids reported earlier. Complete degradation of the branched acids occurred in relatively short time (≤2 h), whereas mineralization required longer irradiation times (3–4.5 h). The trends correlate with CH bond strengths. Branched acids degrade at rates slightly greater than unbranched acids with degradation dynamics decreasing with increase in chain length. The 2-HVA and 2-KVA acids degraded at nearly identical rates. Methylation of 2-HBA had no effect on the dynamics of degradation. However, oxidized carboxylic acids (2-HVA and 2-KVA) degraded five-fold faster than the corresponding linear valeric acid; similarly for 2-OH-2-MBA, which degraded four-fold faster than 2-MBA. These observations explain the difficulty in detecting primary hydroxylated intermediates in OH-radical oxidized linear carboxylic acids.