Study On The Adsorption Of U(Ⅵ),Eu(Ⅲ),Pb(Ⅱ)and Cs(Ⅰ)onto ZSM-5 Silica Alumina Molecular Sieve In Wastewater

With the rapid development and wild application of nuclear energy,more and more radioactive wastewater was produced.How to deal with the wastewater effectively is getting more attention.Based on the pervious literature research,this work systematically studied the adsorption behavior of four different valence elements（U（Ⅵ）,Eu（Ⅲ）,Pb（Ⅱ）and Cs（Ⅰ））onto ZSM-5 silica alumina molecular sieve,which is widely used in industrial catalysis area.The effect of adsorption kinetics and other factors such as solution p H,concentration of adsorbate,ionic strength,temperature,adsorbent concentration and coexisting ions on the adsorption of the above target elements onto the ZSM-5 silica alumina molecular sieve are investigated.Combined with spectroscopy and related characterization methods,the possible adsorption mechanism was deduced.In addition,in order to recover and reuse the radionuclides,the desorption of above-mentioned target element ions adsorbed on the ZSM-5 silica alumina molecular sieve was also studied,and on this basis,the decontamination method of the target radionuclide in wastewater was proposed.（1）Using a variety of characterization methods,the surface morphology,acid-base properties,element composition,and stability of the ZSM-5 silica alumina molecular sieve were studied comprehensively.The results of characterization show that the possible phase composition of the material is Al₂O₃·54Si O₂ and Na₂Al_1.9Si_94.1O₁₉₂,the specific surface area is 379.4 m²/g,the p H of zero-point charge（p Hpzc）is 2.40±0.05,and the material was proved to have good thermal stability.（2）The adsorption rates of U（Ⅵ）,Eu（Ⅲ）,Pb（Ⅱ）and Cs（Ⅰ）on the ZSM-5 silica alumina molecular sieve are relatively fast,and the equilibrium can be basically reached within 4 hours under the experimental conditions.The pseudo-second-order model can explain the experimental data well.Through the experimental study of the kinetic conditions of Eu（Ⅲ）,it was found that the rate constant changed with the variation of the system acidity,target element concentration and environmental temperature,indicating the rate constant is a conditional constant.It was found that the system temperature has a greater influence on the rate constant.The adsorption and the equilibrium adsorption capacity enhanced with the increase of temperature,indicating that the increase of temperature is beneficial to Eu（Ⅲ）adsorption.（3）The acidity has a greater influence on the adsorption of the target element ions onto the ZSM-5 silica alumina molecular sieve,and low acidity is more conducive to adsorption,which is mainly related to the morphological distribution of the target element in the solution and the acid-base properties of the molecular sieve surface.Under high acidity,all of the four target elements exist in the form of cations.Accordingly,the surface of the molecular sieve is positively charged,making it difficult for the metal cations to reach the surface of the molecular sieve and adsorb,so the adsorption performance is poor.（4）The coexisting cations in the system have different effects on the adsorption of the above four element ions.Among them,U（Ⅵ）and Eu（Ⅲ）still have a good adsorption performance onto ZSM-5 silica alumina molecular sieve even when the common cations Na⁺,K⁺,Ca²⁺and Mg²⁺are 1×10⁵ and 1×10⁶times more than their concentration,respectively.The change of ionic strength（0.001～0.5 mol/L Na Cl）has little effect on the adsorption of U（Ⅵ）and Eu（Ⅲ）,which indicates that an inner-sphere surface complexes would be formed between U（Ⅵ）or Eu（Ⅲ）and the active sites of the molecular sieve surface.Pb（Ⅱ）and Cs（Ⅰ）are more affected by coexisting ions.In comparison,bivalent ions have a greater impact on the adsorption of Pb（Ⅱ）and Cs（Ⅰ）than monovalent ions.The higher the concentration,the greater the effect on adsorption,which indicates that ion exchange would be a possible mechanism during adsorption.（5）U（Ⅵ）,Eu（Ⅲ）,Pb（Ⅱ）and Cs（Ⅰ）can be effectively enriched by ZSM-5 silica alumina molecular sieve in neutral solution（p H=6～8）at room temperature.U（Ⅵ）adsorbed on molecular sieve can be recovered quantitatively by 1 mol/L CH₃COOH,Eu（Ⅲ）can be recovered by 1 mol/L Na₂CO₃,Pb（Ⅱ）on the ZSM-5 silica alumina molecular sieve can be desorbed by 1 mol/L HCl,and Cs（Ⅰ）can be recovered by 1mol/L HNO₃,respectively.（6）Through the characterization of the materials after adsorption of U（Ⅵ）,Eu（Ⅲ）,Pb（Ⅱ）and Cs（Ⅰ）,it is found that the structure of ZSM-5 molecular sieve was not changed evidently after adsorbing the above four elements.The formation of U-O,Eu-O and Pb-O bonds indicates that the target element is connected to the active site of the molecular sieve in the form of a chemical bond.Combining the results of ionic strength and coexisting ions,it can be deduced that U（Ⅵ）and Eu（Ⅲ）may form inner-sphere surface complexes with the surface active sites of the molecular sieve,Pb（Ⅱ）is the combined effect of surface complexes and ion exchange,and Cs（Ⅰ）is mainly controlled by ion exchange mechanism on the ZSM-5 silica alumina molecular sieve.（7）As a low-cost and easy-to-obtain material,except good catalytic performance,ZSM-5 silica alumina molecular sieve shows a very efficient and rapid adsorption performance on U（Ⅵ）,Eu（Ⅲ）,Pb（Ⅱ）and Cs（Ⅰ）under neutral conditions.The efficient adsorption of the above-mentioned target element ions by the molecular sieve ensures the accurate removal of U（Ⅵ）,Eu（Ⅲ）,Pb（Ⅱ）and Cs（Ⅰ）from wastewater with complex components.Therefore,ZSM-5 silica alumina molecular sieve has potential application for the treatment of radioactive wastewater.