How I^- alters UV and UV/VUV processes' redox capacities: Evidences from iodine species evolution, hydrogen peroxide...

Although UV and/or VUV tandem I^- are often proposed as advanced reduction processes (ARPs) to eliminate micropollutants by generating e_aq^-, the fate of I^- and its byproducts formation remain to be explored. Therefore, this study investigated the iodine species evolution during UV/I^- and UV/VUV/I^- processes under different influencing factors. Results show that UV/VUV oxidized most of I^- to IO₃^- whereas UV only oxidized a portion of I^- to intermediate reactive iodine species (RISs, including I₂, HOI, and I₃^-); meanwhile, substantial H₂O₂ was generated only in UV/VUV/I^- process but not in UV/I^- process, proving that UV/VUV owns stronger oxidation ability than UV alone. Spiking I^- into water exerted triple-sided effects by consuming •OH, generating e_aq^-, and shielding light, thus complicating the systems. Holistically, increasing pH or decreasing dissolved oxygen converted oxidizing environment into reducing condition and caused less RISs formation, especially for UV/VUV/I^-. For oxyhalides, neither UV/I^- nor UV/VUV/I^- degraded ClO₄^-. While UV/I^- cannot remove ClO₃^-, UV/VUV/I^- reduced ClO₃^- to Cl^-. Expectedly, both UV/I^- and UV/VUV/I^- reduced BrO₃^- to Br^- more efficiently than UV and UV/VUV, confirming that I^- can enhance the reduction capacities of UV/VUV and UV technologies.