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 center dot) 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 center dot 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).