Characterization of the hydroperoxyl/superoxide anion radical (HO2•/O2•-) formed from the photolysis of immobilized TiO2 in a continuous flow

In this study, the steady-state concentrations of HO2 center dot/O-2(center dot-) formed from the photocatalysis of immobilized TiO2 were investigated quantitatively by using the kinetic method with a continuous flow injection. In air-equilibrated water, the concentration of HO2 center dot/O-2(center dot-) was typically 1.01 (+/- 0.08) x 10(-9) M at pH 5.80 in the absence of buffers. The reduction of O-2 by photo-induced electrons (e(cb)(-)) was increased by the concentration of O-2 (from 0 to 0.38 mM). However, in the presence of buffer anions the concentration of HO2 center dot/O-2(center dot-) was gradually decreased by increasing pH. This indicates that the suppression of HO2 center dot/O-2(center dot-) in the presence of buffer ions is attributed to the blocking of active sites of the immobilized TiO2 photocatalyst and to certain processes, that is, the reaction between HO2 center dot/O-2(center dot-) and (OH)-O-center dot (and/or h(vb)(+)). Furthermore, the concentration of HO2 center dot/O-2(center dot-) increased with the increasing concentrations of H2O2 and oxalate, and then on the further addition of H2O2 (>10 mM) and oxalate (>2 mM), the concentration of HO2 center dot/O-2(center dot-) reached steady values. These results strongly suggest that HO2 center dot/O-2(center dot-) formed on TiO2 photocatalysis migrates into the water bulk, and this study can contribute significantly to the body of knowledge regarding HO2 center dot/O-2(center dot-) at very low levels. (C) 2008 Elsevier B.V. All rights reserved.