A clearer path to cleaner water

Ozone-based water treatment is widely used to remove pollutants, odors, color, and pathogens, yet one of its most important chemical engines has remained uncertain: how efficiently ozone reactions generate hydroxyl radicals (•OH), the highly reactive species that drive advanced oxidation. A new study revisits this long-standing question and finds that the peroxone process—ozone (O₃) combined with hydrogen peroxide (H₂O₂)—produces an approximately 67% •OH yield, higher than the commonly assumed value of about 50%. By clarifying how radical chains begin, the work offers a more accurate chemical foundation for designing ozone-based systems for cleaner and safer water. 

Ozone (O₃) oxidation has been used for decades in drinking water and wastewater treatment because O₃ can directly oxidize contaminants and also decompose into •OH through chain reactions. Adding H₂O₂ accelerates this decomposition, making the peroxone process one of the major advanced oxidation process (AOP) based on O₃. However, previous mechanism proposed that adduct formation was the rate-limiting step and suggested limited radical yield, leaving uncertainty over how O₃ reactions actually begin in water. Because pollutant removal depends not only on ozone decay but also on radical generation efficiency, a clearer mechanism is needed. Based on these challenges, in-depth research is needed into the initiation pathways and •OH yield of ozone-based oxidation reactions.

The study, conducted by Yishi Wang, Wei Qiu, Yongbo Yu, and Jun Ma from the State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, was accepted on May 16, 2026, and published (DOI: 10.1016/j.ese.2026.100704) in Environmental Science and Ecotechnology. The article combines radical-capture experiments, competition assays, and quantum-chemical calculations to revise the mechanism of how O₃ and H₂O₂ initiate •OH-forming chain reactions in water.

The researchers first measured how pH and H₂O₂ concentration affected O₃ decay and pollutant degradation, using compounds such as atrazine (ATZ) and p-chlorobenzoic acid (pCBA) to track •OH-driven oxidation. They found that increasing pH and adding H₂O₂ both enhanced •OH exposure, but H₂O₂ was the more practical route under near-neutral water conditions. Complete-capture assays using tert-butanol (t-BuOH) and dimethyl sulfoxide (DMSO) showed that the O₃/H₂O₂ system generated •OH at a stable yield of about 67%, while O₃-only reactions showed pH-dependent yields. Competition experiments with multiple probe compounds further supported this value and helped resolve the disputed reaction rate between •OH and O₃ as 1.1 × 10⁸ M⁻¹ s⁻¹. Theoretical calculations then revealed the chemical reason: O₃ reacts with hydroxide ion (OH⁻) through oxygen-atom transfer (OAT), while O₃ reacts with hydroperoxide ion (HO₂⁻) through two nearly equal routes—electron transfer (ET) and oxygen atom transfer (OAT). This dual-pathway mechanism explains why peroxone chemistry produces more •OH than earlier models predicted.

The authors said the findings show that peroxone chemistry is not simply an ozone-decomposition shortcut, but a finely balanced radical-generating process driven by competing molecular pathways. They said the approximately 67% •OH yield provides a clearer benchmark for evaluating O₃/H₂O₂ systems, while the identification of ET as a key initiation route helps explain why previous adduct-based models underestimated radical production. By connecting bench-scale radical measurements with Marcus electron-transfer theory, the study turns a debated reaction sequence into a more testable and design-ready mechanism.

These findings could improve how engineers design advanced oxidation processes (AOPs) for water purification. A more accurate •OH yield allows treatment systems to better estimate oxidant doses, reaction efficiency, and pollutant removal potential. The work also suggests that simply tracking O₃ decay may not be enough; operators need to understand how much decay is converted into useful radical chemistry. By resolving the roles of OH⁻, HO₂⁻, ET, and OAT, the study provides a mechanistic map for optimizing peroxone reactions under realistic water conditions. In the longer term, this knowledge may support more efficient degradation of persistent organic pollutants while reducing unnecessary chemical use in ozone-based treatment systems.

###

References

DOI

10.1016/j.ese.2026.100704

Original Source URL

https://doi.org/10.1016/j.ese.2026.100704

Funding information

The financial support by the National Natural Science Foundation of China (NFSC20240010) and State Key Laboratory of Urban-rural Water Resources and Environment, Harbin Institute of Technology (No. 2025DX23).

About Environmental Science and Ecotechnology

Environmental Science and Ecotechnology (ISSN 2666-4984) is an international, peer-reviewed, and open-access journal published by Elsevier. The journal publishes significant views and research across the full spectrum of ecology and environmental sciences, such as climate change, sustainability, biodiversity conservation, environment & health, green catalysis/processing for pollution control, and AI-driven environmental engineering. The latest impact factor of ESE is 14.3, according to the Journal Citation Reports^TM 2024.