Natural sunlight photodehalogenation of aromatic disinfection byproducts in water

Aromatic disinfection byproducts (DBPs) have been widely observed in treated wastewater effluents, and their toxicity has been well recognized. The presence of aromatic DBPs in treated effluents poses concerns regarding their impact on the safety of water reuse. Although natural sunlight photolysis is recognized as an important degradation pathway for aliphatic DBPs in water, its effectiveness for aromatic DBPs has not been comprehensively evaluated. Therefore, this study assessed the potential of natural sunlight photolysis to dehalogenate aromatic DBPs in water. Outdoor solar photolysis experiments were conducted for twenty halogenated aromatics, including halogenated phenols, nitrophenols, hydroquinones, hydroxybenzoic acids, hydroxybenzaldehydes, and benzoquinones. Total organic halogen was used as an analytical tool to quantify the dehalogenation kinetics of the selected DBPs. The photolytic dehalogenation of the selected DBPs followed first-order kinetics, with rate constants ranging from 0.074 to 0.677 h^-1 and half-lives between 1.02 and 9.38 h. Halohydroquinones and halobenzoquinones were the most photosensitive, followed by halonitrophenols, halohydroxybenzoic acids, and halohydroxybenzaldehydes, whereas halophenols were the most resistant to sunlight-induced dehalogenation. Photodehalogenation rates of aromatic DBPs were affected by the number of halogen atoms (tri- > di- > mono-) and halogen types (I > Br > Cl). The photodehalogenation rates of aromatic DBPs increased with increasing pH and decreasing organic matter concentrations. Adding TiO₂ and H₂O₂ enhanced dehalogenation for all DBPs at optimal doses of 100 mg/L and 15 mg/L, respectively. Overall, the results of this study demonstrate that natural solar photolysis can effectively dehalogenate a variety of aromatic DBPs in water.