Preparation Of Modified Zeolite Adsorption Materials And Its Removal Performance Of Nitrate From Aqueous Solution

To meet the discharge standard of TN is a technical issue for the upgrading process of wastewater treatment plants,and nitrate is the main component of TN in biochemical effluent.Therefore,the removal of effluent NO₃^--N is great significant for ensuring the discharge of sewage to meet the standard.Adsorption is considered as an effective method in wastewater treatment due to its excellent selectivity,simple operation and less secondary pollution.Zeolite materials,as a new type of environmental adsorption material,had attracted much attention of scholars for its rich pore structure,low costs and high adsorption performance.However,the poor adsorption selectivity,difficult separation with aqueous phase and high regeneration costs limit its further application.Further modification is needed to expand the scope of application in the environment.In this study,SMZ-La composite adsorption material was prepared with NaY zeolite used as substrate.The external surface was loaded with double cationic surfactant hexadecyl trimethyl ammonium bromide?HDTMA?and hydroxyl lanthanum was loaded on the super cage inside the zeolite.Therefore,the space of the NaY zeolite's super cage could be fully utilized,and the nitrate adsorption performance in the wastewater was improved.The main results are concluded as follows:1.Preparation and characterization of SMZ-La composite adsorbents:NaY-La?loading condition:pH=9,NaY:La=5:1?was prepared by improved solution impregnation method could achieve effective adsorption of NO₃^--N,and the La loading capacity was 0.61 mmol/g.SMZ-La was furtherly prepared and loaded with HDTMA?0.35 mmol/g?and La?0.61mmol/g?.The materials were further characterized by Scanning Electron Microscopy?SEM?and Thermogravimetric Analysis?TGA?to analyze the morphology,chemical group structure and HDTMA/La loading of the SMZ-La composites.Results showed that La could be effectively loaded on the NaY carrier with the increase of loading pH,while HDTMA was mainly loaded on the surface of NaY with double layer molecular structure.2.Study of static adsorption properties:The optimum adsorption conditions,adsorption kinetics,adsorption isotherm,competitive adsorption effect and regeneration performance of SMZ-La composites were investigated by static adsorption experiments.The results showed that SMZ-La could maintain efficient adsorption of NO₃^--N?qmax=3.82 mg/g?in a wider pH range?pH=3-9?,compared with the SMZ material.The adsorption process was endothermic and was well fitted by Langmuir isothermal adsorption model,and the maximum adsorption capacity was 14.49 mg/g?45??.The adsorption rate V0 was 2.30 mg/?L· min?and the adsorption equilibrium was reached within 60 min,the reaction process could be fitted by the Pseudo 2nd dynamic model.The adsorption capacity of NO₃^-decreased slightly with the coexistence of Cl^-and SO₄^2-in water,while PO₄^3-would compete for the adsorption sites with NO₃^-on the surface of SMZ-La and reduced the adsorption of NO₃^-(When the concentration of PO₄^3-reached 10 mg/L,the adsorption capacity was reduced from 3.82mg/g to 2.89 mg/g).At the same time,SMZ-La exhibited excellent adsorption-desorption regeneration performance,and it could maintain more than 80%adsorption capacity after 5 recycles.3.Study on dynamic adsorption properties of fixed-bed reactor:A fixed bed dynamic adsorption reactor was designed to provide technical support for engineering application.The breakthrough curves of SMZ-La under different conditions were studied.Varied influent and adsorption-desorption cycles with the influence of pH and competitive ions were investigated.Results showed that SMZ-La could dispose higher water content?70 BV?under the condition of expected effluent water qualities?Ct/C0<0.6?compared with SMZ.It indicated that La loaded in NaY super cage played an important role in maintaining efficient adsorption for a long time.Compared with NaY-La?At=47 h?and SMZ?At=30 h?,the At of SMZ-Lawas only 22 h,which indicated a better adsorption performance of SMZ-La.When the concentration of NO₃^--N in water was low?15 mg/L?,the adsorption performance of SMZ-La was poor?Ct/C0=0.336?and the penetration point was reached when the influent amount is 40 BV.The selective adsorption performance of SMZ-La gradually increased with the increase of NO₃^--N concentration in water,and was less influenced by competitive ions.Using 4-6 BV desorbing agent could achieve high desorption efficiency at a flow rate of 0.8 BV/h.Then using 5 BV deionized water to regenerate the desorbed adsorbent.The SMZ-La could keep stable and recycle under the condition of experimental operation.4.Study on the adsorption mechanism of SMZ-La composites on NO₃^--N:The adsorption mechanism of SMZ-La composite adsorbents was further analyzed with the method of pHPZC,BET,SEM-EDS,XRD,FTIR and XPS.Results showed that the surface area of SMZ-La was up to 43.258 m2/g.The pHPZC of SMZ-La was significantly improved compared with the SMZ,and the material surface had a positive charge in a wider pH range?pH<8.1?.The results of SEM-EDS analysis showed that La was loaded inside the NaY and adsorbed NO₃^--N efficiently.XRD?FTIR and XPS analyses showed that La was mainly loaded on NaY zeolite with the form of La?OH?3 and La2O3,and was chemically adsorbed with NO₃^--N in water.All the results above showed that the adsorption mechanism of SMZ-La adsorbed on NO₃^--N mainly included electrostatic attraction,anion exchange,ligand exchange and complexation.