TiO₂ Photocatalytic Oxidation Mechanism of As(III)

Abstract

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*/O2(-)*) 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 *OH) in UV/TiO2 or vacuum-UV lamp irradiation (lambda = 185 + 254 nm), As(III) 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 *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 *OH also plays a significant role along with VB hole in the TiO2 PCO of As(III). 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 *OH and VB hole are the main oxidants in the TiO2 PCO mechanism of As(III).

Pathway of Oxygen Incorporation from O2 in TiO2 Photocatalytic Hydroxylation of Aromatics: Oxygen Isotope Labeling Studies

“…The role of h vb + was first examined by adding formic acid (FA) to remove it selectively. [24] In the presence of only 2 mm of FA, which was 10 % of the BA concentration, the abundance of 16 O (from 16 O 2 ) in the hydroxylated product increased remarkably (Table 2, . In addition, most of the earlier studies seemed to indicate that the hydroxylation of aromatic compounds is directly related to h vb + or COH generated from it.…”

Perspectives and Advances in Photocatalysis

“…Based on transient absorption decay as a function of As +3 concentration these workers concluded that adsorbed hydroxyl radical/trapped holes play a central role in the photocatalytic oxidation. Most recently, Yoon et al (2009) have provided a conclusive evidence to support the opinion that hydroxyl radical and valence band hole are the main oxidants in the photocatalytic oxidation of As +3 over TiO 2 . These workers investigated the correlation between As +3 oxidation and superoxide by measuring both As +5 and H 2 O 2 simultaneously.…”

Disparate roles of doped metal ions in promoting surface oxidation of TiO 2 photocatalysis

“…Advanced oxidation processes (AOPs) are impressed as highly competitive technologies for elimination of a wide variety of recalcitrant organics resistant to conventional approaches [3][4][5][6][7][8][9][10][11]. Among all AOPs, TiO 2 photocatalysis is capable of converting photon energy into chemical energy, and proves to be a green, eco-friendly and efficient tool for wastewater treatment and water purification [8,9,12].…”