Transformation of bromine species in TiO2 photocatalytic system.
Bromine species have six oxidation states from Br(-) to BrO(3)(-), 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 (BrO(3)(-)) 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 BrO(3)(-) by OH. and reduction of BrO(3)(-) to Br(-) by e(-) were concurrent. However, due to a higher reaction rate for reduction of BrO(3)(-) under a pH range of 3-11, oxidation of Br(-) was totally offset and hence, only the reduction of BrO(3)(-) was observed with hydrobromous acid and hydrobromite formed as intermediates. Apart from e(-), H(2)O(2), to a certain extent, was involved in the photocatalytic reduction of BrO(3)(-). 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 BrO(3)(-). 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).
Zhang
Xiwang
X
School of Civil and Environmental Engineering, Nanyang Technological University, 639798 Singapore. XWZhang@ntu.edu.sg
Zhang
Tong
T
Ng
Jiawei
J
Pan
Jia Hong
JH
Sun
Darren Delai
DD
D000327:Adsorption; D001966:Bromine / Q000737:chemistry*; D002384:Catalysis; D010084:Oxidation-Reduction; D010777:Photochemistry; D014025:Titanium / Q000737:chemistry*
C009495:titanium dioxide; D014025:Titanium; D001966:Bromine