| Ammonia nitrogen is the main nitrogen pollutant in the hydrosphere,but its removal often has disadvantages such as high cost or low efficiency,especially in water treatment with low concentration of ammonia nitrogen.Acidic birnessite manganese oxide is ubiquitous in soils and sediments.It has unique s urface charge,cation exchange and redox properties,which make it highly reactive to adsorption,but the use of acidic birnessite manganese oxide for the adsorption of ammonia nitrogen has not been reported.At the same time,the reaction mechanism betwee n manganese oxide and ammonia nitrogen is still unclear.Representative theories include oxygen free radical mechanism and active manganese oxidation mechanism.Therefore,this paper studied the removal process of ammonia nitrogen by acidic birnessite manganese oxide,and further explored the possible reaction process by adjusting the oxygen vacancy and the content of trivalent manganese respectively.First,the adsorption of ammonia nitrogen by acidic birnessite manganese oxide was studied.In order to verify its adsorption performance,the effects of contact time,p H,initial ammonia nitrogen concentration and temperature were investigated.The adsorption kinetics showed that the adsorption rate of ammonia nitrogen was fast,the removal rate could reach more than 90%within 5 minutes,and the adsorption behavior followed the pseudo-second-order kinetic model.The maximum monolayer adsorption capacity obtained from the Langmuir isotherm was22.46 mg/g at 283K.Thermodynamic analysis showed that the adsorption process is spontaneous and physical adsorption.Zeta potential,FTIR analysis and coexisting cation experiments showed that the higher ammonia nitrogen adsorption capacity could be mainly attributed to the water absorption process and electrostatic interaction.The results show that acidic birnessite can be used as an economical and efficient ammonia nitrogen adsorption material.Then,the acidic birnessite manganese oxide was modified by vacuum deoxidation and P doping,and the same method was used to study the adsorptio n of ammonia nitrogen.Compared with the pre-modification,the removal capacity of the vacuum deoxidized manganese oxide for ammonia nitrogen was reduced,and the maximum single-layer adsorption capacity obtained from the Langmuir isotherm was only 20.49 mg/g at 283K.The SEM and BET analysis showed that the doping of P increased the surface area of manganese oxide,and the low zeta potential showed enhanced electronegativity.Its maximum monolayer adsorption capacity could reach 26.24 mg/g at 313K,which was more effective for ammonia nitrogen.The removal effect is significantly enhanced.Finally,the reaction of manganese oxide with ammonia nitrogen was studied.The analysis of the water phase and the solid phase showed that there was a chemical conversion process with ammonia nitrogen.Elemental analysis showed that the conversion of ammonia nitrogen was dominated by N2 gas,and nitrate nitrogen accounted for a relatively small proportion.Dissolved oxygen experiments showed that O2 could promote the reaction.XPS analysis proved that trivalent manganese was involved in this process.The enhanced reactivity of manganese oxide with high oxygen vacancies to ammonia nitrogen and free radical capture experiments proved that hydroxyl radicals might be generated on the solid surface to promote the conversion of ammonia nitrogen,which is believed to be related to the activation of molecular oxygen by oxygen vacancies.This subject proved the high adsorption performance of acidic birnessite manganese oxide for ammonia nitrogen,and it can be used as a potential adsorbent.Doping P modification is an effective method to improve the removal capacity.At the same time,it is proposed that the molecular oxygen activated by oxygen vacancies on the surface of manganese oxide promotes the possible chemical conversion of ammonia nitrogen,which provides new ideas and evidence for the study of the mechanism. |
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