Preparation Of Metal Oxide Catalysts And Their Use In Catalyzing The Oxidation Of Ammonia Nitrogen In Water By Ozone

As one of the most important water pollutants,the emissions of ammonium-nitrogen?NH₄⁺-N?has attracted people's attention.The NH₄⁺-N was included in "Energy Conservation & Emission Reduction in 12 th Five-Year Planning" by State Council,which claimed that NH₄⁺-N emissions should be reduced 10%.Therefore,the research on the treatment of NH₄⁺-N from waste water with high efficiency becomes an important goal for environmental protection workers.The intermediate?·OH radical?is produced by catalytic ozonation,which is widely used in the treatment of organic wastewater due to its strong oxidation ability,fast reaction rate,convenient to use,small secondary pollution,and so on.It is effective to treat low concentration NH₄⁺-N wastewater by catalytic ozonation under a proper catalyst.Not only can NH₄⁺-N be degraded,but also avoid nitrate and nitrite in the oxidation process,and at last the converse of NH₄⁺-N is in the form of nitrogen gas?N₂?.In this paper,five metal oxides such as MgO,Fe3O4,Co3O4,NiO,CuO were prepared as catalysts by homogeneous precipitation method,in which their structure and surface properties were characterized.Through the study of the catalytic ability of oxidizing of NH₄⁺-N,the catalyst with high catalytic activity on NH₄⁺-N degradation and high selectivity on N₂ was screened.The experimental results showed that MgO had the highest catalytic activity,which the NH₄⁺-N removal rate reached 90.2%,but most of the products of NH₄⁺-N degradation was nitrate.Co3O4 had high selectivity on N₂ while its catalytic activity was low,the converse rate of N₂ was 17.2%.The SEM and BET analysis indicated that MgO existed in the form of floc with a fluffy and coarse surface,which meant the largest specific surface area,more active center of surface and high catalytic activity.Co3O4 formed into small floc and had uniform particles.In order to prepare catalyst with highly active and selective,Mg/Co composite metal oxide catalyst was prepared and the best preparation conditions by the homogeneous co-precipitation was optimized.The prepared samples were characterized by XRD,SEM,BET,pore analysis,TG-DTA and FT-IR.The results revealed that the catalyst prepared at the condition of calcination temperature 500?,the calcination time 3 h,Mg/Co molar ratio 8:2 had the highest activity for the degradation of NH₄⁺-N,and the highest selectivity for the generation of N₂.The SEM and BET analysis indicated that the optimized catalyst had flocculent structure,a fluffy and coarse surface,uniform and smaller particles,larger specific surface area with mesoporous structure,which can provide active sites in the reaction.On this basis,the influencing factors such as the initial pH of solution,reaction time,ozone flow,catalyst dosage,reaction temperature,co-existing ions on the degrade of NH₄⁺-N and the mechanism were studied.The results showed that when the initial NH₄⁺-N concentration was 50 mg/L,pH of solution 9,flow of ozone 12 mg/min,catalyst dosage 1 g/L,reaction temperature 50?,reaction time 2 h,the removal rate of NH₄⁺-N and the converse rate of N₂ were 81.6% and 44.1%,respectively.The experimental showed that co-existing SO42-and HCO3-could inhibit the catalytic activity while CO32-could promote it.The co-existing Br-promoted greatly the catalytic reaction because of its involving in the catalytic cycle and more nitrogen gas was produced.But Br-would generate a carcinogen BrO3-in the presence of ozone,which should be a serious threat to human health.Tert butyl alcohol experiments indicated that hydroxyl radical was the main active substance for Mg/Co catalytic ozonation of NH₄⁺-N.FT-IR of catalyst before and after reaction showed that the Mg/Co catalyst was rich in hydroxyl groups,which could improve the catalytic activity and selectivity.