USE OF CATALASE AND SUPEROXIDE-DISMUTASE TO ASSESS THE ROLES OF HYDROGEN-PEROXIDE AND SUPEROXIDE IN THE TIO2 OR ZNO PHOTOCATALYTIC DESTRUCTION OF 1,2-DIMETHOXYBENZENE IN WATER

The effects of two antioxidant enzymes on the rate of disappearance, r, of the pollutant 1,2-dimethoxybenzene (1,2-DMB) in UV-irradiated (lambda > 340 nm) TiO2 or ZnO aqueous suspensions have been determined. Catalase, which catalyzes the overall reaction 2H2O2 --> 2H2O + O2, caused a relatively moderate decrease in r in the case of TiO2 and no effect in the case of ZnO, showing that hydrogen peroxide formed in situ is not essential for the pollutant removal. The effect of H2O2 added to ZnO was negative and in the case of TiO2 was either favorable or unfavorable depending on the initial ratio [H2O2]/[1,2-DMB] as a result of competition between these compounds for the adsorption sites and/or the photoproduced holes, the formation of additional OH. radicals and the detrimental modification of the TiO2 surface. Favorable and unfavorable effects of added H2O2 were suppressed by catalase. The detrimental effect on r of superoxide dismutase (SOD), which catalyzes the overall reaction 202.- + 2H+ --> O2 + H2O2, was very important for both TiO2 and ZnO. It is inferred that it stems from the catalytic action of SOD and not from competitive photocatalytic destruction of 1,2-DMB and SOD or from the formation of H2O2. Therefore, these results point to the essential role of the O2.- radical-anion as an active species in the photocatalytic degradation of the pollutant and this role is tentatively discussed, in particular with respect to the formation of the 1,2-DMB-radical-cation.