TiO2 Photocatalytic Oxidation Mechanism of As(III)

There has been a controversy over the TiO2 photocatalytic oxidation (PCO) mechanism of As(III) for the last several years. The key argument has been whether superoxide (HO2 center dot/O-2(-)center dot) is the main oxidant of As(III) in the UV/TiO2 system. Previously we and other groups have refuted the claim that superoxide plays the main role in the TiO2 PCO of As(III). Nevertheless, thereafter,the superoxide-mediated As(III) oxidation mechanism has been repeatedly claimed, making it difficult to draw a clear conclusion regarding this mechanism. The objective of this study is to draw a unanimous conclusion on the TiO2 PCO mechanism of As(III) and thus finish the controversy regarding this issue. To investigate the correlation between As(III) oxidation and superoxide, both As(V) and H2O2 were measured simultaneously. When excess formic acid (FA) was added as a scavenger of valence band (VB) hole (or center dot OH) in UV/TiO2 or vacuum-UV lamp irradiation (lambda = 185 + 254 nm), As(Ill) oxidation was greatly inhibited while H2O2 generation was promoted. Since H2O2 is photochemically produced through the disproportionation of superoxide, this result definitely shows that superoxide has little role in the oxidation of As(III) not only in UV/TiO2 but also in other advanced oxidation processes (AOPs). Interestingly, not only FA (a scavenger of VB hole) but also methanol (a scavenger of adsorbed center dot OH) showed an inhibitory effect on the TiO2 PCO of As(III). Excess methanol retarded the TiO2 PCO of As(III) moderately but not completely, which indicates that adsorbed center dot OH also plays a significant role along with VB hole in the TiO2 PCO of As(Ill). Although the conclusion is not based on the rate constant between As(III) and superoxide but derived from indirect inference from the experimental data, this study provides convincing evidence to support that adsorbed center dot OH and VB hole are the main oxidants in the TiO2 PCO mechanism of As(III).