Transformation of Bromine Species in TiO2 Photocatalytic System

Bromine species have six oxidation states from Br− to BrO3 −, and their transformation between each oxidation state is complex and has been a common debate topic in photocatalytic systems where oxidants (h+ and OH·) and reducers (e−) coexist. In this study, the lowest oxidation state (Br−) and highest oxidation state (BrO3 −) were used as the starting compounds in a photocatalytic reaction to investigate the transformation of bromine species. The experimental results showed that oxidation of Br− to BrO3 − by OH· and reduction of BrO3 − to Br− by e− were concurrent. However, due to a higher reaction rate for reduction of BrO3 − under a pH range of 3−11, oxidation of Br− was totally offset and hence, only the reduction of BrO3 − was observed with hydrobromous acid and hydrobromite formed as intermediates. Apart from e−, H2O2, to a certain extent, was involved in the photocatalytic reduction of BrO3 −. A low concentration of organic matter (3 mg/L) reacted with hydroxyl radicals to inhibit combination of holes and electrons, hence promoting photocatalytic reduction of BrO3 −. It is important to note that pH had an influence on the transformation of bromine species, because it affects adsorption of reactants on the photocatalyst and controls the amount of aqueous H+ and OH− ions present. Photocatalytic oxidation of Br− became dominant under a strong acidic condition (pH 1.5) while both photocatalytic oxidation and reduction were inhibited under a strong basic condition (pH 13.5).