The effect of adsorption on photocatalytic degradation of phenyl urea herbicides on zinc oxide

Abstract

The effect of adsorption on photocatalytic degradation was studied by using zinc oxide with different morphologies and exposed surfaces as the photocatalyst. Polar zinc-terminated surface and polar oxygen-terminated surface are dominating planes on the top and bottom of a crystal of ZnO conventional particles, while non-polar surface is the main plane on the side of ZnO nanorods. These surfaces affect the adsorption behavior of phenylurea herbicides, i.e. diuron, linuron and 3,4-dichloroaniline. Consequently, they cause the difference in photocatalytic degradation rate and intermediate products formed during the degradation. The adsorption capacity of ZnO conventional particles is about twice as high as that of ZnO nanorods although the surfaces area of the ZnO conventional particles is one order of magnitude less than that of ZnO nanorods because of the greater amount of adsorption sites and affinity of the polar surfaces. However, the extents of the photocatalytic degradation achieved by two forms of ZnO are insignificantly different. In addition, the adsorption configuration of the herbicides on the surface of ZnO also affects the performance in degradation. The intermediates detected even at the shortest residence time investigated depend on the adsorption behavior of the herbicides. The attack of hydroxyl radicals occur on adsorbed position. Therefore, the different alignment on the surface results in variation in intermediates. The toxicity of intermediates were assessed and were found that intermediates produced on ZnO nanorods is less toxic than that on ZnO powder.