Methyl chloride produced during UV 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 (CHCl) from guaiacol and chloride was observed during UV irradiation. The results indicated that direct photolysis of guaiacol produced an arenium ion, and the reactive methoxy group was further transformed to CHCl in the presence of chloride. O-quinone was detected as the primary product of the degradation of guaiacol resulting from UV 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 UV irradiation. Scavenging experiments and removal of oxygen demonstrated that neither oxygen nor chlorine radicals were involved in CHCl formation. In seawater samples, CHCl was also detected in the presence or absence of added organic matter. These results demonstrate that CHCl can be formed during UV irradiation in saline water and that attention should be paid to this compound and structurally-related compounds in the application of UV processes.