Photochemical reactions between superoxide ions and 2,4,6-trichlorophenol in atmospheric aqueous environments

The superoxide anion radical (O₂^•-) is an important reactive oxygen species (ROS), and participates in several chemical reactions and biological processes. In this report, O₂^•- was produced by irradiating riboflavin in an O₂-saturated solution by ultraviolet light with a maximum emission at 365 nm. And the contribution of O₂^•- to 2, 4, 6-trichlorophenol (2, 4, 6-TCP) was investigated by a combination of laser flash photolysis (LFP) and UV light steady irradiation technique. The results of steady-state experiments showed that the photochemical decomposition efficiency of 2, 4, 6-TCP decreased with the increase of the initial concentration of TCP, while the increase of pH and riboflavin concentration promoted the photochemical reaction. The second-order rate constant of the reaction of the superoxide anion radical with 2, 4, 6-TCP phenoxyl radical (TCP^•) was (9.9 ± 0.9) × 10⁹ L mol^-1 s^-1 determined by laser flash photolysis techniques. The dechlorination efficiency was 61.5% after illuminating the mixed solution with UV light for 2 h. The conversion of 2, 4, 6-trichlorophenol was accompanied by the reductive dechlorination process induced by superoxide ions. The main steady products of the photochemical reaction of 2, 4, 6-TCP with O₂^•- were 2, 6-dichlorophenol (DCP), 2, 6-dichloro-1, 4-benzoquinone (DCQ) and 2, 6-dichlorohydroquinone (DCHQ). The addition process was the preferred process in the total reaction of superoxide ions with 2, 4, 6-TCP phenoxyl radical. These results indicated that the reaction of 2, 4, 6-TCP with O₂^•- was a potential conversion pathway and contribute to atmospheric aqueous phase chemistry.