Methyl chloride produced during UV₂₅₄ irradiation of saline water

Ultraviolet (UV) irradiation is widely used for water treatment due to its effectiveness against a wide range of waterborne pathogens with minimal production of regulated disinfection byproducts. However, in this study, the formation of methyl chloride (CH₃Cl) from guaiacol and chloride was observed during UV₂₅₄ irradiation. The results indicated that direct photolysis of guaiacol produced an arenium ion, and the reactive methoxy group was further transformed to CH₃Cl in the presence of chloride. O-quinone was detected as the primary product of the degradation of guaiacol resulting from UV₂₅₄ irradiation. Other organic compounds containing methoxy, ethoxy, or methylamino groups with structures that are similar to guaiacol were also demonstrated to generate halocarbons in aqueous chloride or bromide solution under UV₂₅₄ irradiation. Scavenging experiments and removal of oxygen demonstrated that neither oxygen nor chlorine radicals were involved in CH₃Cl formation. In seawater samples, CH₃Cl was also detected in the presence or absence of added organic matter. These results demonstrate that CH₃Cl can be formed during UV₂₅₄ irradiation in saline water and that attention should be paid to this compound and structurally-related compounds in the application of UV₂₅₄ processes.