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/O2− 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/O2− was typically 1.01 (±0.08) × 10−9 M at pH 5.80 in the absence of buffers. The reduction of O2 by photo-induced electrons (ecb−) was increased by the concentration of O2 (from 0 to 0.38 mM). However, in the presence of buffer anions the concentration of HO2/O2− was gradually decreased by increasing pH. This indicates that the suppression of HO2/O2− 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/O2− and OH (and/or hvb+). Furthermore, the concentration of HO2/O2− 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/O2− reached steady values. These results strongly suggest that HO2/O2− formed on TiO2 photocatalysis migrates into the water bulk, and this study can contribute significantly to the body of knowledge regarding HO2/O2− at very low levels.