A comparison of photodegradation kinetics, mechanisms, and products between chlorinated and brominated/iodinated haloacetic acids in water

In order to better understand the haloacetic acid (HAA) photodegradation process, this study investigated the kinetics, mechanisms, and products of seven chlorinated (Cl-), brominated (Br-), and iodinated (I-) HAAs irradiated with 254 nm ultraviolet light (UV₂₅₄). The photodegradation rates of HAAs by UV₂₅₄ alone were found to increase with increasing halogenation degrees (i.e., tri > di > mono) and molecular weights (i.e., I > Br > Cl), and the kinetic trends from both literature and this study were well captured by a quantitative structure activity relationship model, which reveals that molar absorptivity of compound is the most important property of HAAs in defining its photo-susceptibility. Interestingly, although direct photolysis was dominant for all HAAs removal, indirect photolysis via generation of hydroxyl radicals due to photodecomposition of degradation intermediates such as formic acid also contributes partially (26–47%) to the losses of Cl-HAAs. Degradation products and pathways varied significantly among HAAs too. While Cl-HAAs mainly underwent mineralization with carbon dioxide being the major product, Br- and I-HAAs experienced stepwise dehalogenation and decarboxylation processes, which resulted in substantial halogen-free organic compounds. The common products of HAAs photolysis are halide(s), formic acid, formaldehyde, and halomethanes, symbolizing three concurrent pathways. Overall, the findings clearly demonstrated the dramatic differences between Cl- and Br-/I-HAAs, and indirectly evidenced the presence of hydroxyl radical during HAA irradiation even without addition of catalyst.