The Effects of Different Particle Size of Nano-ZnO and Alumina-based Catalysts on Removal of Atrazine from Water with Ozone

Abstract

Due to the widespread application of pesticides and herbicides in agricultural industries, these substances have been highlighted as emerging contamination of natural ground and surface water resources. Conventional water treatment processes are only effective in removing emerging contaminants in water. The mechanism of degradation of organic impurities present in water using ozone is known to either directly involve the ozone molecule or to occur by the indirect effect of free hydroxyl radicals (•OH). The latter are produced in the radical chain reaction of ozone decomposition. A series of experiments were carried out to investigate the effects of particle sizes of nano-ZnO catalysts on removal of atrazine (ATZ). Nano-ZnO catalysts increase the rate of ozone decomposition and atrazine removal by production of hydroxyl radicals as oxidative intermediates. However, different particle sizes have a minimal effect on the rate of ozone decomposition and atrazine removal. It is believed that molecular ozone is adsorbed on the surface of nano-ZnO followed by the oxidation of the ozone molecule. This leads to the production of OH radicals. Therefore, it is reasonable to assume that reaction is carried out in the bulk of the solution and the rate is independent of catalyst’s surface area. This is probably the reason for similar reaction rates of different particle sizes of nano-Zno catalysts. Additionally three different metal oxides (ZnO, Mn2O3 and Fe2O3) loaded on ƴ-alumina and ƴ-alumina (metal oxide-free) were used in catalytic ozonation of aquatic atrazine samples. The findings substantiate the strong influence of molecular ozone on degradation of ATZ and the partial involvement of hydroxyl radicals in the mechanism. Based on adsorption studies, atrazine has a low affinity towards adsorption on the surface of the catalysts. It is logical to assume that ozone reacts with the hydroxyl groups of the catalyst to form a highly reactive metal-ozone complex. This layer could react with a molecule of atrazine through an electron-transfer mechanism. The residual concentration of ATZ and total organic carbon (TOC) were determined by High Performance Liquid Chromatography (HPLC) and Total Organic Carbon (TOC) analyses.