Degradation Of Chlorinated Aromatics Over Three Different Crystal Forms Of Manganese Oxide Nanomaterials

Persistent organic pollutants threatened the human health and ecological balance due to their high toxicity, biological accumulation and biodegradability characteristics, At present, some of them has been forbidden to produce and use. However, its widely application in the whole world lead to the results of that could be seen in all kinds of environmental media,and become more and more important to degradate them. Because chlorinated aromatics have the similar characteristics with persistent organic pollution, it is urgent to develop a green economy and low cost method to achieve the reduction of such pollutants. Metal oxide nano catalyst material has the advantages of low cost, high activity, and can be used for many times, which provides a new way for the disposal of chlorinated aromatics. There are many studies on manganese oxides as catalysts carrier, whereas the effect of catalytic forms on degradation of chlorinated aromatics is less.In this paper, three kinds of MnO₂ metal oxide catalysts were prepared by hydrothermal method. Using 1,2,4-Tr CB as a model compound of chlorinated aromatics, the catalytic degradation of 1,2,4-Tr CB with three different types of MnO₂ was evaluated by self built pulse-reactor. This paper mainly had the following results:1. With potassium permanganate and manganese sulfate as raw materials, ?-MnO₂, ?-MnO₂, ?-MnO₂ were prepared using hydrothermal method by controlling the concentration of K+ and OH-. The results of XRD showed that the ?-MnO₂ has the advantages of high crystallinity and small grain size in the three different crystal types of MnO₂. The OH- and K+ played an important role in the process of crystal growth of material. ?-MnO₂ showed a clavate nanostructure. ?-MnO₂ had a filiform nanostructure after added K+ to the reaction system. After added OH- to the system which environment is suitable for the growth of ?-MnO₂ with a flower-like hierarchical structure. Three samples are classified as mesoporous structure. The order of the surface area is ?-MnO₂<?-MnO₂<?-MnO₂, and order of the average pore size is ?-MnO₂<?-MnO₂<?-MnO₂. ?-MnO₂ showed the highest capacity of adsorption in three samples.2. Three different crystalline MnO₂ materials all showed the high degradation activity to 1,2,4-Tr CB, and ?-MnO₂ showed the highest degradation efficiency, and the degradation efficiency of MnO₂ increased with the increase of temperature and reaction time.3. The degradation pathway of 1,2,4-Tr CB over ?-MnO₂ was hydrodechlorination, isomerization, anti-hydrodechlorination, oxidation.The hydrodechlorination degradation products was DCB and MCB, and the isomerization degradation products were 1,2,3-Tr CB and 1,3,5-Tr CB. CO2, CO were also generated in the reaction process. The main products were MCB and 1,2,3-Tr CB, DCB and 1,3,5-Tr CB was less. The amount of production is related to the crystal forms of the material. The ?-MnO₂ was the most favorable to hydrodechlorination degradation of 1,2,4-Tr CB, and most unfavorable for the isomerization degradation of 1,2,4-Tr CB. The difference amount of three DCB products was related to the difference of the dissociation energy and the stability of the C-Cl bond, and the 1,4-DCB was the most. The oxidation degradation products was benzoic acid, and anti-hydrodechlorination products was tetrachlorobenzene The main degradation pathway is 1,2,4-Tr CB?1,4-DCB?MCB?…?CO2+CO.