Advanced oxidation mechanism of UV photolysis of electrochemically generated free bromine.

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.