Reaction Mechanism on the Synergistic Photodegradation of Dye X3B and Photoreduction of Dichromate in the Presence of Polyoxometalate

Photodegradation of a textile dye X3B and photoreduction of dichromate (Cr(VI)) in an acidic aqueous solution were studied under 320 nm cut-off UV light irradiation in the presence of two polyoxometalates (POM), H3PW12O40 (PW), and H4SiW12O40 (SiW). The reactions in POM-X3B-Cr(VI) system were faster than those in POM-X3B, POM-Cr(VI), and X3B-Cr(VI) systems. For all reactions, PW was more photoactive than SiW. The reaction rates were proportional to the initial concentration of each component. The effects of N2, O2, and air were small but regular, indicating Cr(VI) photoreduction by a reduced POM. Quenching experiments with H2O2 and ethanol revealed that X3B photodegradation mainly occurred through hydroxyl radical (OH). It was proposed that the production of OH and a reduced POM− by the reaction between H2O and excited POM* was the rate determining step, with which all evidence could be well interpreted. Different effects of POM concentration in a two- or three-component system on the reaction rates suggested that the reaction between H2O and excited POM* was reversible.