Photocatalytic Oxidation of Arsenite over TiO₂: Is Superoxide the Main Oxidant in Normal Air-Saturated Aqueous Solutions?

Abstract

TiO(2) photocatalytic oxidation (PCO) of As(III) in the normal air-saturated aqueous solutions has been widely studied. Yet no consensus has been achieved on the mechanism whether superoxide is the main oxidant, although many approaches have been taken. (Photo)electrochemical method can minimize changes to TiO(2) surface and could therefore not alter the normal mechanism. In this Article, both this approach and As(III) oxidation kinetic measurements were performed to clarify the disputed mechanism. Under a sufficient cathodic bias potential, the dark oxidation of As(III) by superoxide could occur, but both the reaction rate and the columbic efficiency were rather low, suggesting that it is a weak oxidant. However, under UV light, both the reaction rate and the columbic efficiency were greatly enhanced even at potentials negative enough to eliminate photohole participation, indicating that more efficient oxidants than superoxide were produced. Under UV illumination and enough positive potential where superoxide was absent, the As(III) oxidation was the most highly efficient. The columbic efficiency of photoholes was much higher than that of superoxide. In the normal aerated aqueous solutions and at open circuit, although the total contribution of superoxide and its derivates to the PCO of As(III) was considerably high (up to 43%), it was not more than that of photohole (57%). In addition, the reported various approaches taken to elucidate the mechanism were discussed, and the resulting disputes can be clarified by these findings. It was demonstrated that (photo)electrochemical method could provide direct and undisputed evidence to reveal the truth mechanics issues.

Removal of arsenic from drinking water by photo‐catalytic oxidation on MoOx/TiO2 and adsorption on γ‐Al2O3

“…According to a recent study, photo‐oxidation of As(III) is caused by both h + and • OH ad ; that is, As(III) is initially oxidized to As(IV) by h + and • OH ad , and then As(IV) is immediately converted to As(V) by oxygen, h + , or • OH ad . However, in a more recent paper the contribution of superoxide and its derivates in the photocatalytic oxidation of As(III) was found to be significant, although less relevant than that of photoholes . In our study, the efficiency of photocatalytic oxidation of As(III) to As(V) is promoted by polymolybdate on the titania surface which induces the formation of superoxide as well as polyoxometalate .…”

“…25 However, in a more recent paper the contribution of superoxide and its derivates in the photocatalytic oxidation of As(III) was found to be significant, although less relevant than that of photoholes. 27 In our study, the efficiency of photocatalytic oxidation of As(III) to As(V) is promoted by polymolybdate on the titania surface which induces the formation of superoxide as well as polyoxometalate. 26 It is very important to underline that, even at the lowest initial concentrations of As(III) (1 mg L −1 ), the complete oxidation of As(III) to As(V) occurs within 90 min irradiation time.…”

New Advances in Heterogeneous Photocatalysis for Treatment of Toxic Metals and Arsenic

“…•− [32,33,38,39], HO • , or h VB + [30,31,35,[40][41][42][43]. However, whatever the oxidant, it is not possible to deny the efficiency of the HP process to transform As(III) into As(V).…”

Water‐soluble polymer and photocatalysis for arsenic removal

“…In the case of As(III)‐contaminated water, such as underground or anoxic waters, a preoxidation step is commonly applied to improve the removal yield. In other oxidation processes, including oxidants, such as hydrogen peroxide, oxygen and ozone, chlorine, manganese oxide, electrochemical oxidants, and Fe(III), TiO 2 has also been successfully used . The oxidation reaction occurs on the surface of TiO 2 , where photo‐excited electrons react with adsorbed molecular oxygen, producing the superoxide radical ( ), which can act as an oxidant.…”

“…Conversely, the hydroxyl radical adsorbed on the surface (•OH ads ), which is a powerful oxidant in aqueous media, is produced in holes (VB + ) by reacting with adsorbed water or hydroxide anions. Fei et al noted that in TiO 2 photocatalysis, the superoxide seems to be the dominant oxidant of As(III). More recently, Li and Leng conclude that the main reactive oxygen species responsible by the oxidation of As(III) is hydroxyl radical with a weak participation of superoxide anion.…”