Ozone is a powerful oxidant widely used in water treatment for the degradation of organic pollutants and removal of colour, odor, and pathogens. In aqueous solution, ozone decomposes to generate hydroxyl radicals through chain reactions that are accelerated by the addition of hydrogen peroxide in the peroxone process. Yet the precise initiation mechanisms of these chains and the efficiency of hydroxyl radical production have remained controversial, with prior models proposing adduct formation as the rate-limiting step and yielding only approximately 50% hydroxyl radicals in peroxone process. Here we show that the hydroxyl radical yield in the peroxone reaction is approximately 67%, substantially higher than previously reported. Through complete-capture scavenger assays, competition experiments, and high-precision quantum-chemical calculations informed by Marcus electron-transfer theory, we establish that ozone reacts with hydroxide exclusively by oxygen-atom transfer, while its reaction with the hydroperoxide anion proceeds through parallel electron transfer (approximately 50%) and oxygen-atom transfer (approximately 50%) pathways. Spin-orbit coupling enables spin-forbidden release of triplet oxygen in the atom-transfer channel. We also determine the pK_a of the hydroxyl radical precursor hydrotrioxide as approximately 6.15 and quantify the long-disputed hydroxyl radical–ozone reaction rate constant as 1.1 × 10⁸ M⁻¹ s⁻¹. These results revise classical ozonation and peroxone mechanisms and provide a mechanistic foundation for optimizing ozone-based advanced oxidation technologies for water purification.

Designing photocatalysts with high light utilization and efficient photogenerated carrier separation for pollutant degradation is one of the important topics for sustainable development. In this study, hierarchical core–shell material α-Fe₂O₃@ZnIn₂S₄ with a step-scheme (S-scheme) heterojunction is synthesized by in situ growth technique, and MXene Ti₃C₂ quantum dots (QDs) are introduced to construct a double-heterojunction tandem mechanism. The photodegradation efficiency of α-Fe₂O₃@ZnIn₂S₄/Ti₃C₂ QDs to bisphenol A is 96.1% and its reaction rate constant attained 0.02595 min⁻¹, which is 12.3 times that of pure α-Fe₂O₃. Meanwhile, a series of characterizations analyze the reasons for the enhanced photocatalytic activity, and the charge transport path of the S-scheme heterojunction/Schottky junction tandem is investigated. The construction of the S-scheme heterojunction enables the photo-generated electrons of α-Fe₂O₃ and the holes of ZnIn₂S₄ to transfer and combine under the action of the reverse built-in electric field. Due to the metallic conductivity of Ti₃C₂ QDs, the photogenerated electrons of ZnIn₂S₄ are further transferred to Ti₃C₂ QDs to form a Schottky junction, which in turn forms a double-heterojunction tandem mechanism, showing a remarkable charge separation efficiency. This work provides a new opinion for the construction of tandem double heterojunctions to degrade harmful pollutants.