Ozone mechanism, kinetics, and toxicity studies of halophenols: Theoretical calculation combined with toxicity experiment

Aromatic disinfection by-products (DBPs), which are generally more toxic than aliphatic DBPs, have attracted increasing attention. The toxicity of 13 typical halophenols on Scenedesmus obliquus was experimentally investigated, and the ozonation mechanism and kinetics of representative halophenols were further studied by quantum chemical calculations. The results showed that the EC50 values of halophenols ranged from 2.74 to 60.23 mg/L, and their toxicity ranked as follows: di-halogenated phenols > mono-halogenated phenols, mixed halogen-substituted phenols > single halogen-substituted phenols, and iodophenols > bromophenols > chlorophenols. The toxicity of halophenols was well described by the electronegativity index (ω) as lg(EC50)-1 = 6.228ω - 3.869, indicating halophenols capturing electrons as their potential toxicity mechanism. The reactions of O3 with halophenolate anions were dominated by three mechanisms: 1,3-dipolar cycloaddition, oxygen addition, and single electron transfer. The kinetic calculation indicated that O3 oxidized aqueous halophenols by reacting with halophenolate anions with the reaction rate constants as high as (0.91-3.47) × 1010 M-1 s-1. The number of halogen substituents affected the kO3, cal values of halophenolate anions, which are in the order of 2,4-dihalophenolate anions >4-halophenolate anions > 2,4,6-trihalophenolate anions. During the ozonation of 2,4,6-tribromophenol (246TBP), the toxic products (dimers and brominated benzoquinones) could be synergistically degraded by O3 and HO•. Thus, ozonation is feasible as a strategy to degrade aromatic DBPs.