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 (SM). 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 N-2, O-2, 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 (center dot 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 two- or three-component system on the reaction rates suggested that the reaction between H2O and excited POM* was reversible.