Roles of H2O2 and OH radical in bactericidal action of immobilized TiO2 thin-film reactor: An ESR study

To evaluate the bactericidal roles of hydrogen peroxide (H2O2) and hydroxyl radical (OH) in immobilized titanium dioxide (TiO2) thin-film reactor, the mechanism of H2O2 and OH formation in photocatalytic reaction was investigated by applying electron spin resonance (ESR) technique. The results showed that the formation of OH largely depended on H2O2 formed in photocatalytic reaction. OH were mainly originated from the H2O2 via the reduction reaction although a relatively small portion came from the direct photolysis of H2O2. Due to the convertible relationship between H2O2 and OH, the approaches that can increase H2O2 concentration such as bubbling air or adding H2O2 could greatly result in enhancement of OH. The results of bactericidal experiments showed that killing effect was completely inhibited by catalase, in contrast, the inactivation efficiency was proportionally increased with elevated H2O2 levels. Considering the fact that H2O2 alone was not enough to complete the disinfection action, it is concluded that OH, not H2O2 was the primary bactericidal radicals in the immobilized TiO2 thin-film reactor; however, due to its conversion to OH and oxidative toxicity, H2O2 was also indispensable for photocatalytic bactericidal effect.