Photocatalytic oxidation mechanism of arsenite on tungsten trioxide under visible light

• The oxidation of As(III) to As(V) on WO3 primarily proceeds through two-hole transfer.
• OH radical scavengers had little effect on the oxidation rate of As(III) to As(V).
• As(III) enhanced the photocurrent by reacting with hole.
• As(III) on WO3 acts as an external charge recombination center.

The oxidation of arsenite (As(III)) to arsenate (As(V)) and the involved oxidants on tungsten trioxide (WO3) under visible light were mechanically investigated. Three oxidant species, valence-band hole (hvb+), hydrogen peroxide (H2O2), and hydroxyl radical (OH), can be generated on WO3 under visible light. However, As(III) was not oxidized to As(V) in the presence of H2O2. In addition, the oxidation rate of As(III) to As(V) was not reduced in the presence of OH scavenger. These results indicate that the oxidation of As(III) to As(V) is primarily initiated by hole, but both OH and H2O2 are little involved in As(III) oxidation to As(V). This hole-mediated oxidation of As(III) to As(V) on WO3 was further confirmed by X-ray photoelectron spectroscopy (XPS) analysis (i.e., the adsorption of As(III) on WO3) and photoelectrochemical measurement (i.e., the enhanced photocurrent by As(III) addition). Although OH is little involved in As(III) oxidation to As(V), the production of OH was reduced in the presence of As(III). This behavior implies that As(III) on WO3 acts as an external charge recombination center, where the oxidation of As(III) to intermediate As(IV) species by hole or OH is immediately followed by the reduction of As(IV) to As(III) by conduction-band electron (ecb–). Therefore, the direct oxidation of As(III) to As(V) by two-hole transfer can be proposed as the primary oxidation pathway of As(III) to As(V) on WO3 under visible light.

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