How I^– alters UV and UV/VUV processes' redox capacities: Evidences from iodine species evolution, hydrogen peroxide formation, and oxyhalides degradation?

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.