Physicochemical Modification And Deammonium Mechanism Of Iron-manganese Oxide-based Ammonium Oxidation Catalysts

In recent years,the shortage of water resources and the deterioration of water ecology restrict the development of our society.Ammonia nitrogen in water exceeds the standard,aggravating the shortage of water resources and seriously polluting the environment.With the in-depth research of treatment technology,it is found that anaerobic ammonia oxidation combined with catalytic oxidation method can effectively treat ammonia nitrogen in water.The key point of this technology is how to convert ammonia nitrogen into nitrous nitrogen with high selectivity and stability.In this subject,a precipitation method was used to prepare an iron-manganese oxide-based ammonium oxidation catalyst with catalytic oxidation（referred to as"ammonium catalyst"）for the treatment of ammonia nitrogen,and the following results were obtained:1.The self-prepared iron-manganese oxide-based ammonium oxidation catalyst was systematically evaluated for the catalytic oxidation performance of ammonia nitrogen.It was found that the product of the catalytic oxidation of ammonia nitrogen by the ammonium catalyst was nitrous,and its selectivity was close to 100%,and the selectivity was still high within 1000 h.At the same time,the conversion rate of ammonia nitrogen is more than 85%.In addition,the structure and composition of the ammonium catalyst were studied by SEM,XRD,XPS and other characterization methods.And the results showed that the iron-manganese oxide-based ammonium oxidation catalyst was mainly in the form of a block,and the crystal form was birnessite-type manganese oxide.Manganese mainly exists in the valence states of Mn（II）,Mn（III）and Mn（IV）.2.A series of modification experiments were carried out on the ammonium catalyst in view of the problem of product nitrate（reduced nitrite selectivity）in the long-term catalytic oxidation of ammonia nitrogen by iron-manganese-based ammonium oxidation catalysts.The results show that 75℃,90℃water bath heating treatment and NaClO treatment can restore the nitrite selectivity of the ammonium catalyst product to more than 95%,and can prolong the life of the ammonium catalyst by about 30days.3.The changes in the activation process of the ammonium catalyst were further revealed by microscopic characterization,and the activation time of the ammonium catalyst was optimized to 80 h and 100 h by soaking in NaClO and heating in a water bath at 80°C,respectively.In addition,the mechanism of the catalytic oxidation of ammonia nitrogen by ammonium catalyst was explored:the catalytic oxidation of ammonia nitrogen by ammonium catalyst was completed by the conversion of Mn between various valence states,and high-valent manganese（Mn（III）,Mn（IV））participated in the oxidation of NH₄⁺,the adsorbed NH₄⁺is dehydrogenated and oxygenated step by step on the catalyst surface to obtain the oxidation product NO₂^-,the high-valent manganese is reduced to low-valent manganese（Mn（II））,and the low-valent manganese is oxidized under the action of active oxygen species to restore high-valent manganese,high-valence manganese continues to catalyze new ammonia nitrogen,and then completes the entire catalytic oxidation process.4.The powder ammonium catalyst can be granulated and formed by using PAN or PVA as a binder.The experiment found that the ammonium catalyst catalyzed oxidation of ammonia nitrogen after forming is nitrous,and the selectivity is close to 100%,and the selectivity is still higher than90%after 800 h,and the conversion rate of ammonia nitrogen is higher than 85%,granulation and molding solve the problem of difficulty in recycling powder materials,the operation is simpler,and it is easy to apply in engineering.