Overlooked Role of Peroxides as Free Radical Precursors in Advanced Oxidation Processes

Research efforts on advanced oxidation processes (AOPs) have long been focused on the fundamental chemistry of activation processes and free radical reactions. Little attention has been paid to the chemistry of the precursor oxidants. Herein, we found that the precursor oxidants could lead to quite different outcomes. A counterintuitive result was observed in the photoreduction of bromate/iodate: the combination of H₂O₂ and UV enhanced the reduction of bromate/iodate, whereas the addition of persulfate to the UV system led to an inhibitory effect. Thermodynamic and kinetic evidence suggests that the reduction of bromate in UV/H₂O₂ was attributable to the direct reaction between HOBr and H₂O₂. Both experimental determination and kinetic simulation demonstrate that the reaction between HOBr and H₂O₂ dominated over the ^•OH-mediated reactions. These results suggest that H₂O₂ possesses some particular redox properties that distinguish it from other peroxides. The prototypical UV/H₂O₂ process is not always an AOP: it can also be an enhanced reduction process for chemicals with intermediates that are reducible by H₂O₂. Considering the increasing interest in persulfate-based AOPs, the results of this study identify some novel advantages of the classical H₂O₂-based AOPs.