Degradation of iodinated disinfection byproducts by VUV/UV process based on a mini-fluidic VUV/UV photoreaction system

Abstract

This study investigated the VUV/UV photodegradation of three iodinated disinfection byproducts (I-DBPs), namely, triiodomethane (TIM), diiodoacetamide (DIAcAm) and triiodoacetic acid (TIAA), based on a mini-fluidic VUV/UV photoreaction system (MVPS). The pseudo-first-order rate constants (k) of TIM, DIAcAm and TIAA under VUV/UV irradiation (1769, 1301 and 1174 m2 einstein−1, respectively) were higher than those under UV irradiation (1003, 832 and 766 m2 einstein−1, respectively). The enhancement of photodegradation could be attributed to the indirect HO• oxidation rather than the direct VUV photolysis. As a whole, compared with the indirect HO• oxidation, the direct UV photolysis had a relatively greater contribution to the degradation of the three I-DBPs, especially DIAcAm and TIAA (ca. 57% and 60%, respectively). The two electron-withdrawing groups in DIAcAm and TIAA (i.e., acylamino and carboxyl) decreased the electron density of the C−I bonds, thus weakening the electrophilic attack of HO•. The iodine in the three I-DBPs was released to form I− and a small fraction (< 6%) of I− was oxidized to IO3−, indicating that HO• preferred to break the C−I bonds rather than oxidize I−. The direct UV photolysis proceeded via H2O-catalyzed deiodination reactions, while the indirect HO• oxidation proceeded via deiodination reactions along with HO• addition. The VUV/UV photodegradation of the three I-DBPs was more favored at an acidic pH but inhibited by the water matrix components (i.e., NOM, Cl− and alkalinity) to different extents.