Investigation Of Catalyzed Ozonation Of Dichloroacetic Acid In Drinking Water By Nano-Zinc Oxide

In recent years, along with the speeding up industrialization of our country advancement,a large amount of sewage and industrial wastewater have been discharging riotously which caused serious pollution of potable water sources, the conventional water treatment technology can hardly meet the requirement of human health. More and more drinking water treatment plants add chlorination process as a strengthening conventional water treatment before the traditional craft, which will inevitably cause large of chlorinated by-products (CBPs) appear in water and make a serious threat to human health and survival. So it has become a top priority to study and develop an effective removal technology of chlorinated by-products in drinking water. Haloacetic acids (HAAs), a principal of the CBPs in drinking water, although the concentration may be lower than trihalomethanes (THMs), the HAAs accounted for 91.9% of the total cancer risk. Previous studies mainly focused on HAAs formation mechanism and measures of precursor removal, while relatively little work was done on the removal of already formed HAAs in drinking water. In present work, nano-ZnO catalyst was prepared by homogeneous precipitation to degrade dichloroacetic acid (DCAA), which was chosen as a typical representative of haloacetic acids. The efficiency and impact factors of the ozone/nano-ZnO catalytic degradation of trace DCAA were studied using static methods, the mechanism of nano–ZnO catalytic ozonation of DCAA and the reactive species formation on the catalyst surface were investigated in depth.As to single ozone process, the oxidation efficiency of trace DCAA in water is lower and was greatly impacted by the initial DCAA concentration, ozone dosage and solution pH values. The presence of radical inhibitor, tert-butanol, significantly inhibited the ozone degradation of DCAA, showing that DCAA ozone process follows the hydroxyl radical mechanism. The removal efficiency of DCAA in filtered water and Songhua River raw water decreased by 7.06% and 19.58%, which showed that the organic matter and bicarbonate inhibited the ozone degradation on DCAA.The catalyst activities of 11 common metal oxides were evalutated and Nano- ZnO was finally chosen as the catalyst for ozonation because of its higher catalyst activity. The prepared conditions of ZnO were optimized and several material detecting technologies, including XRD, BET, SEM, etc. were applied to characterize the synthesized nano-ZnO. The synthesized ZnO is a type mesopore material which has narrow distribution of pore size. The ZnO catalyst has hexagonal wurtzite structure, and the catalyst is in the form of flcos which consituted by like-spherical particles. There are two types of biding state oxygen, i.e. adsorbed oxygen and lattice oxygen. The surface of ZnO catalyst was rich in hydroxyl groups and the pHzpc of nano-ZnO is 9.39.The DCAA degradation and concerning influencing factors for ozone/nano-ZnO process were also studied. The addition of ZnO improved the removal of DCAA during ozonation, while DCAA adsorption by ZnO played a little role. The removal ratios of DCAA can also be improved by increasing the initial concentration of DCAA, mixing speed, ozone dosage, catalyst dosage and solution temperature. The removal ratio of TOC was much less than that of DCAA. Increasing the concentration of HCO₃^- can decrease the removal of DCAA. With the existence of low concentration humic acid, the removal ratios during ozone/nano-ZnO were enhanced, while restriction effect was observed in the presence of high concentration humic acid. Few Zn²⁺can be detected after ozone/nano-ZnO process, and there was no significant decrease on the catalyst activity of ZnO. Therefore, the synthesized nano-ZnO can be used in practical water treatment process. The reaction rate constant of ozonatizing DCAA by ozone molecule, i.e. 0.21L/ (mol·s), was determined at strong acidic pH. Using pCBA as the competition probe compound, the reaction rate constant of oxidizing DCAA by hydroxyl radicals, i.e. 1.33×10⁹L/(mol·s), was determined at strong alkaline pH.The mechanisms of heterogeneous catalytic ozonation of DCAA were studied. The results showed that the solution pH could influence the degradation of DCAA by affecting the surface charge status and the existing form of the target. In the catalytic system, the removal rate of DCAA was obviously inhibited by free radical scavenger tert-butyl alcohol, which indicated that the degradation process followed hydroxyl free radical mechanism. The hydroxyl radicals were also identified in the processes of ozonation alone and ZnO catalytic ozonation by spin-trapping/ESR technology, which indicated that hydroxyl radicals were generated in the two processes mentioned above. And the two reaction paths of hydroxyl radical degradation of DCAA in aqueous solution were speculated in this work.The surface properties of the catalyst were also determined by FTIR analysis. The results showed that the surface hydroxyl groups which generated by the Lattice oxygen in aqueous solution and Lewis acid sites of the catalyst were the active site. and for the first time use the Density Functional Theory (DFT) to study the adsorption and reaction between ozone molecule and water molecule on the surface of ZnO and discuss the interaction between the ozone and catalyst surface. The generation of hydroxyl groups on the surface of catalyst was studied. The electronic properties of the adsorbate on the surface of the catalyst and the production were in-depth analyzed. The experimental results showed that the adsorption of ozone molecule on the surface of non-polar ZnO was chemisorptions. The ozone molecule on the surface of the catalyst was not decomposition. The adsorbed ozonide was interacted with water molecule to generate surface groups of OH and HO₃. The adsorption of ozone molecule on the surface of polar ZnO was symmetric bridge site adsorption. The ozone molecule could react with H₂O to generate adsorption groups OH and free radical groups, and release an oxygen molecule.