Advanced oxidation mechanism of UV photolysis of electrochemically generated free bromine

ABSTRACT

In recent times, some researchers have successfully demonstrated the efficacy of UV photolysis of electrochemically generate free chlorine (UV/electro-chlorine) as for an advanced oxidation process. Since bromine as well as chlorine is an element belonging to halogen, it is expected that UV photolysis of electrochemically generated free bromine (UV/electro-bromine) also shows an advanced oxidation effect. To elucidate the feasibility of UV/electro-bromine system, its advanced oxidation mechanism was investigated using radical probes of 1,4-dioxane and nitrobenzene. In contrast to the UV/electro-chlorine system, the advanced oxidation effect of UV/electro-bromine system was inhibited under acidic conditions due to the accumulation of photochemically inert Br₂. The most abundant radical in UV/electro-bromine system was dibromine radical anion (Br₂˙⁻) and the second-order reaction rate constant of Br₂˙⁻ with 1,4-dioxane was estimated to be 2.4 × 10⁵ M⁻¹ s⁻¹. As a result of the abundance and the reactivity of Br₂˙⁻, it was the main contributor to 1,4-dioxane degradation. On the other hand, nitrobenzene was mainly decomposed by direct UV photolysis because Br₂˙⁻ does not react with nitrobenzene. The contribution of hydroxyl radical (HO˙) to 1,4-dioxane degradation was much lower than that of Br₂˙⁻ because its concentration was 4–5 order of magnitude lower than that of Br₂˙⁻. However, the HO˙ concentration elevated with a decrease in the concentration of bromide ion (Br⁻). Consequently, the reactivity of Br₂˙⁻ with pollutants and the Br⁻ concentration have critical impacts on the advanced oxidation performance of UV/electro-bromine system.

GRAPHICAL ABSTRACT

KEYWORDS:

• Advanced oxidation process
• dibromine radical anion
• free bromine
• reactive bromine species
• UV photolysis

Acknowledgements

This work was supported by the Japan Society for the Promotion of Science (JSPS) under Grant KAKENHI 26340064. We would like to thank Editage (www.editage.jp) for English language editing.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Japan Society for the Promotion of Science (JSPS) [grant number KAKENHI 26340064].