Solar photolysis kinetics of disinfection byproducts

Disinfection byproducts (DBPs) discharged from wastewater treatment plants may impair aquatic ecosystems and downstream drinking-water quality. Sunlight photolysis, as one process by which DBPs may dissipate in the receiving surface water, was investigated. Outdoor natural sunlight experiments were conducted in water for a series of carbonaceous DBPs (trihalomethanes, haloacetic acids, halopropanones, and haloacetaldehydes) and nitrogenous DBPs (nitrosamines, halonitromethanes, and haloacetonitriles). Their pseudo-first-order rate constants for photolytic degradation were then used to calibrate quantitative structure-activity relationship (QSAR) parameters, which, in return, predicted the photolysis potentials of other DBPs or related compounds. Nitrogenous DBPs were found to be more susceptible to solar irradiation than carbonaceous DBPs, with general rankings for the functional groups as follows: N-nitroso (N-NO)>nitro (NO₂)>nitrile (CN)>carbonyl (CO)>carboxyl (COOH). Compounds containing a high degree of halogenation (e.g., three halogens) were usually less stable than less halogenated species (e.g., those with two halogens). Bromine- or iodine-substituted species were more photosensitive than chlorinated analogs. While most bromine- and chlorine-containing trihalomethanes and haloacetic acids persisted over the 6-h test, nearly complete removal (>99%) of nitrosamines occurred within 1 h of sunlight exposure. Indoor laboratory experiments using simulated sunlight demonstrated that the degradation of nitrosamines was ∼50% slower when organic matter was present, and ∼11% slower in non-filtered water than in filtered water.