Advanced oxidation mechanism of UV photolysis of electrochemically generated free bromine

Abstract

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.