Study On The Preparation Of Ammoniated Magnetic Adsorbent And Its Adsorption Performance For Cr(Ⅵ) In Water

Hexavalent chromium（Cr（Ⅵ））has been widely concerned by scholars for a long time due to its high toxicity to the ecological environment and organisms.Cr（Ⅵ）easily enters the human body through the food chain,causing a weakened immune system,visceral damage,cancer,genetic variation and even death.To reduce human exposure to chromium（Cr）,the concentration must be reduced to a specified threshold before discharge.Among several techniques for removing Cr（Ⅵ）in water,adsorption method is considered as one of the most economical and effective methods due to its low material cost,good practicability,simple operation and high efficiency.However,the adsorption method also has practical problems such as low adsorption capacity,difficult separation and recovery,and low reusability.In order to solve such problems,two ammoniated magnetic adsorbents were prepared in this study,which not only have the advantages of rapid magnetic separation,but also retain the high adsorption efficiency and good reusability of Cr（Ⅵ）,which is a very good solution.Attractive green adsorbent.The main research results of this paper are as follows:（1）Graphene oxide（GO）was prepared by the modified Hummers method.Magnetic graphene oxide（MGO）was prepared by attaching Fe₃O₄ to the surface of GO by co-precipitation,and then surface modification was carried out by polyethyleneimine（PEI）to obtain ammoniated magnetic graphene oxide（PMGO）.The morphology,crystal structure and chemical composition were analyzed by SEM,TEM,XRD,VSM,XPS,FTIR,BET and other methods,and the optimal amount of PEI added was discussed.The results show that the particle size of Fe₃O₄ attached to GO is 20-30 nm,the saturation magnetization of PMGO is 12.40 emu/g,and it takes 75 s to separate from water by magnetic separation;the maximum adsorption capacity was 148.98 mg/g when the amount of PEI was 2 g;XPS and FTIR The results indicated that part of the amino groups of PEI reacted with the carboxyl and epoxy groups indicated by GO to endow the material with a large number of amino groups.（2）In order to improve the adsorption capacity of the magnetic adsorbent and the reaction speed of magnetic separation,Fe₃O₄ was prepared by hydrothermal method,Fe₃O₄@SiO₂was synthesized by stober method,and then Fe₃O₄@SiO₂-NH₂ was obtained by grafting PEI through silane coupling reaction.Its morphology,crystal structure and chemical composition were analyzed by SEM,TEM,XRD,VSM,XPS,FTIR and other methods.The results show that the particle size of Fe₃O₄@SiO₂-NH₂ is 200-400 nm,the SiO₂ layer wrapped on its surface is about 20 nm,and it becomes thinner（17 nm）after grafting amino groups;the crystallinity of Fe₃O₄@SiO₂-NH₂ can reach 98.19%.In addition,the saturation magnetization of Fe₃O₄@SiO₂-NH₂ is 41.6 emu/g,and the experimental results show that it only takes 50 s for magnetic separation from water,which is significantly improved compared with PMGO;and the maximum adsorption capacity is increased to 251.83 mg/g.XPS and FTIR results indicated that some amino groups of PEI endowed the material with a large number of amino groups through silane coupling reaction.（3）The effects of initial concentration of Cr（Ⅵ）,adsorption time,p H value of solution and inorganic anions on the adsorption efficiency of Cr（Ⅵ）by two adsorbents PMGO and Fe₃O₄@SiO₂-NH₂were investigated in detail.The adsorption kinetics and adsorption isotherms of the two adsorbents are in line with the pseudo-second-order kinetic model and the Langmuir model,which proves that the rate-limiting step of adsorption is the intermolecular force between the adsorbates,and the adsorption has uniform binding sites and adsorption.There is no interaction between species and the characteristics of monolayer coverage.The adsorption thermodynamic results indicated that the adsorption of Cr（Ⅵ）by PMGO and Fe₃O₄@SiO₂-NH₂ was a spontaneous endothermic process.The adsorption process is obviously affected by p H value（the best adsorption effect is when p H=3）;SO₄^2-and Cr（Ⅵ）in the solution have obvious competition relationship,which can inhibit the adsorption effect,other anions（NO₃^-,Cl^-）It has little effect on the adsorption effect.（4）PMGO and Fe₃O₄@SiO₂-NH₂ exhibited excellent performance in removing Cr（Ⅵ）.From the XPS results,it is inferred that the adsorption mechanism of PMGO and Fe₃O₄@SiO₂-NH₂ for Cr（Ⅵ）may be that amino groups are ionized at low p H and Cr（Ⅵ）is adsorbed by electrostatic interaction.In addition,the electron donated by the amino group reduces Cr（Ⅵ）to Cr（III）,and finally Cr（III）is adsorbed on the amino group through chelation.After five adsorption-desorption cycles of PMGO and Fe₃O₄@SiO₂-NH₂,the adsorption capacity decreased by 14.4%and 21.8%,respectively.The results show that both PMGO and Fe₃O₄@SiO₂-NH₂ have good reusability and stability.