Why inorganic salts decrease theTiO2photocatalytic efficiency

Abstract

Methylene Blue (MB) has been chosen as a model molecule to evaluate the impact of inorganic salts, present in textile waste waters, on the adsorption properties and on the photocatalytic efficiency ofTiO2. NoOH∘radical scavenging by anions such asNO3−,Cl−,SO42−,PO43−, andCO32−was observed at neutral and basic pH, while this phenomenon can be suggested at acidic pH for some anions except carbonate anions which are totally neutralized and/or eliminated asCO2in these conditions. The decrease in the rate MB photocatalytic degradation in the presence of inorganic salts was shown to be due to the formation of an inorganic salt layer at the surface ofTiO2, which inhibits the approach of MB molecules. The correlation between the amount of MB adsorbed and the rate of its photocatalytic degradation, whatever the nature of the salt, its concentration and the pH of the solution, indicates (i) that photocatalysis occurs at the surface and not in the solution and (ii) thatOH−ions added at basic pH do not participate to the increase in the photocatalytic efficiency by inducing an increase inOH∘formation. They increase the surface density in adsorption sitesTiO−. The effect of various salts is similar on various titania samples of industrial origin (MillenniumTiO2PC 500, PC 50, and Degussa P 25). It is however more important on Millennium PC 10 probably because of its smaller surface area.