Overlooked Role of Iodate in Micropollutant Degradation by UV/Periodate: Kinetic Modeling and Mechanism

The periodate (PI, IO₄^-) is known as an emerging oxidant and disinfectant in water treatment with iodate (IO₃^-) as the benign end product. However, new results herein strongly suggest that IO₃^- could contribute to pollutant degradation and trigger disinfection byproduct (DBP) formation in the UV/IO₄^- process. The degradation of micropollutants, e.g., 17α-ethinylestradiol (EE2), followed two-stage pseudo-first-order kinetics along with the conversion of IO₄^- (stage I) to IO₃^- (stage II) in the UV/IO₄^- process. The radical scavenging experiments and electron spin resonance technique confirmed both reactive oxygen species (e.g., ^•OH and O₃) and reactive iodine species (RIS) (e.g., IO₃^•), contributing to contaminant degradation in the UV/IO₄^- system. A kinetic model based on first-principles was further developed to simulate reaction kinetics, revealing that ^•OH was the primary reactive species responsible for EE2 degradation in stage I, while RIS, especially IO₃^•, played major contributions in stage II. The photolysis of IO₃^- in stage II could increase the risk of iodinated DBP (I-DBP) formation, especially under acidic conditions. The new findings of this work broaden the mechanistic knowledge on the UV/IO₄^- process and highlight the overlooked role of IO₃^- in the worrisome I-DPB formation in the wastewater treatment.