Ammonium Removal From Surface Water Using A Natural Zeolite Modified By Combined Modification Of Sodium Nitrate Solution Impregnation And Calcination

Ammonia nitrogen is an environmental factor that causes eutrophication and pollution of surface water bodies.The economic and effective control of ammonia nitrogen pollution in water bodies has always been an important task of environmental governance today.Since biological activity is severely inhibited in low-temperatureenvironments（<5℃）,thecommonlyused nitrification-denitrification denitrification process is not suitable for cold winter conditions.In this project,natural zeolite is used as an adsorbent to remove low-concentration ammonia nitrogen in surface water.Sodium nitrate solution impregnation combined with roasting modification process is proposed to improve the adsorption capacity of natural zeolite.The modification mechanism is revealed through the characterization of zeolite structure.In the study of mild adsorption thermodynamics,the effects of coexisting ions,p H,temperature and dosage on ammonia removal efficiency were analyzed through static adsorption experiments,and dynamic adsorption experiments were also conducted.Sodium nitrate solution impregnation combined roasting modification effect is better than single roasting and single sodium nitrate solution impregnation modification.When the concentration of sodium nitrate solution is 3.00mol/L and the calcination temperature is 673K,the modified zeolite has the best performance.When the concentration of ammonia nitrogen solution is 5mg/L and the dosage is0.60g/L,the removal rate of the zeolite to ammonia nitrogen is as high as 81.68%,which is far better than 41.80%of natural zeolite.The zeolite also has good regeneration capacity,and the adsorption rate of ammonia nitrogen is as high as79.34%after 6 cycles of regeneration liquid treatment.The modified zeolite introduces a large amount of Na⁺that can be used for ion exchange,an increase of302.44%compared to natural zeolite,and the mesopore ratio of the modified zeolite also increases;the surface of the modified zeolite is observed to be rougher and the surface heterogeneity is significantly increased by scanning electron microscopy.Residual nitrate helps to enhance the thermal stability of natural zeolite,thereby maintaining its structural integrity.Both natural zeolite and modified zeolite conform to the quasi-first-order kinetic model.The adsorption rate of modified zeolite is much greater than that of natural zeolite.The intra-particle diffusion model shows that the ammonia nitrogen diffusion rate in the modified zeolite is twice that of natural zeolite.For natural zeolites,the Langmuir isothermal model shows a better fitting relationship,while the Freundlich isothermal model is more suitable for the adsorption of modified zeolites.The main reason may be attributed to the rough surface of the modified zeolite.The saturated adsorption capacity of modified zeolite is 16.96mg/g,which is43.97%higher than that of unmodified zeolite.Adsorption free energy in modified thermodynamics of modified zeolite△G<0,which shows that the adsorption behavior of modified zeolite is spontaneous;the change of adsorption enthalpy△H is 29.32k J/mol,which shows the exchange of cation and ammonia nitrogen in modified zeolite The behavior is endothermic;the entropy change△S is 116.40 k J/（mol·K）,indicating that the adsorption of ammonia nitrogen by the modified zeolite is an entropy-increasing reaction,and the ion exchange process increases the degree of chaos in the system.Within a certain temperature range,increasing the temperature enhances the diffusion of ammonia nitrogen inside the zeolite,thereby improving the ammonia nitrogen removal efficiency.When treating ammonia nitrogen solution with a concentration of 5mg/L,the optimal dosage of modified zeolite is 0.60g/L;the zero point charge of the modified zeolite is increased from 6.58 to 8.70 of natural zeolite,which means the introduction of sodium;The most suitable p H range of modified zeolite to remove ammonia nitrogen is 5-8,too small or too large p H will affect the adsorption effect of zeolite;The ability of modified zeolite to remove ammonia nitrogen when anions and cations coexist with ammonia nitrogen was investigated.The results showed that as long as there were other ions,the adsorption rate of modified zeolite to ammonia nitrogen was disturbed,and the adsorption capacity was significantly weakened with the increase of coexisting ion concentration The effect sizes are K⁺>Na⁺>Mg²⁺and SO₄^2->Cl^->NO₃^-,respectively;the main mechanism of modified zeolite adsorption is ion exchange,so it has no adsorption effect on non-cationic pollutants such as urea and penicillinRegarding the factors affecting the adsorption of ammonia nitrogen by the modified zeolite in the dynamic experiment,the smaller the flow rate of the ammonia nitrogen solution,the lower the initial ammonia nitrogen concentration,and the more the modified zeolite filling amount,the longer the saturation breakthrough time of the adsorption column.Under the same experimental conditions,the working time of modified zeolite is much longer than that of natural zeolite.When the inlet water flow rate is 20m L/min,the initial ammonia nitrogen concentration is 5mg/L,and the bed depth is 8cm,the adsorption capacity of natural zeolite is 2.18mg/g,reaching saturation after running for 1500min,while the adsorption capacity of modified zeolite is 6.81mg/g,and the penetration time is3840min,which is far better than the adsorption capacity of natural zeolite.In this study,the natural zeolite is modified by sodium nitrate solution impregnation and roasting.The Na⁺ion exchange function and NO₃^-thermal decomposition are used to improve the pore structure.The combination of natural zeolite and ammonia nitrogen adsorption performance is improved to maximize the use of modifier.At the same time,it avoids the secondary pollution caused by the washing process after the modification of the traditional salt solution zeolite,which has a positive significance for the cost control of large-scale industrial applications.