Preparation And Characterization Of Magnetic Metal Organic Framework ZIF-8 Composite Adsorbent And Its Application In Water Environment

The rapid advancement of industrialization and urbanization has caused many serious water environment pollution problems.Once the highly toxic,carcinogenic and non-biodegradable pollutants（especially organic dyes and heavy metals）enter the aquatic system,they will pose a serious threat to the entire ecological environment and human health.Adsorption method has been widely used in the field of wastewater treatment because of its advantages such as high efficiency,low cost and low risk of secondary pollution.Metal-Organic Framework（MOF）is characterized by high specific surface area,high porosity and wide variety,among which Zeolitic Imidazolate Framework（ZIF）has become one of the most ideal MOF adsorbents which can be applied in water environment due to its excellent water stability.However,its application is limited due to its difficult recovery and poor acid resistance.In this paper,the magnetic metal-organic framework ZIF-8 composite adsorbent（Fe₃O₄@Carbon@ZIF-8）was prepared,and the preparation of adsorbent was optimized.The structure and physicochemical properties of adsorbent were characterized and analyzed by various characterization methods.Fe₃O₄@Carbon@ZIF-8 was applied to treat Congo red（CR）dye wastewater and Cu（II）heavy metal wastewater,and the influence factors of adsorption,selective adsorption performance,adsorption mechanism,recycling capacity and application of simulated real wastewater were systematically studied.The main conclusions obtained from this paper are as follows:（1）The magnetic nanoadsorbent Fe₃O₄@Carbon@ZIF-8,which is composed of a magnetic core,carbon-based inner shell and ZIF-8 outer shell,with specific core-double shell structure,has been successfully synthesized by the layered self-assembly method.Fe₃O₄@Carbon@ZIF-8 not only has a high specific surface area,excellent adsorption performance,but also can be quickly recovered from the aqueous solution by magnetic separation.（2）In the optimized preparation of Fe₃O₄@Carbon@ZIF-8,the influences of main factors were evaluated with iodine value,methylene blue value,CR and Cu（II）adsorption capacity as indexes.The results showed that the optimal synthesis conditions of adsorbent were as follows:zinc source（Zn（NO₃）₂）,Zn²⁺:2-Hmim molar ratio（1:8）,reaction solvent（methanol）,time（6 h）,temperature（348.15 K）.（3）Fe₃O₄@Carbon@ZIF-8 were characterized and analyzed by SEM,TEM,EDS,BET,FTIR,XPS,XRD,VSM,XPS,TGA and Zeta potential.The results showed that Fe₃O₄@Carbon@ZIF-8 had an obvious core-double shell structure,and the ZIF-8 shell layer accounted for 10.42%of the adsorbent;It is a typical microporous material with a specific surface area of 658.92 m²/g and an average pore size of 1.6409 nm;Its saturation magnetization is 52.08 emu/g,which can meet the magnetic separation conditions;It contains Fe₃O₄ and ZIF-8 with good crystallinity and has good thermal stability,and its equipotential point is 4.50;It has good water stability over a wide p H range（4～9）.（4）The adsorption properties of Fe₃O₄@Carbon@ZIF-8 for Congo red were studied.The optimum dosage of adsorbent and the optimum initial solution p H value of adsorption reaction were 0.3 g/L and 4.0,respectively;The increase of adsorption temperature is beneficial to the adsorption process;The adsorption equilibrium was reached after about 400 min;Interfering ions such as Cl^-,HSO₃^-,NO₃^-,HCO₃^-,SO₄^2-and C₂O₄^2-will produce competitive adsorption with CR and inhibit the adsorption.The Langmuir isotherm model and PSO kinetic model fit the adsorption process well,indicating that the mode of adsorption reaction is monolayer adsorption,mainly chemical adsorption.The adsorption reaction is a spontaneous,endothermic and orderly process.FT-IR and XPS were used to characterize Fe₃O₄@Carbon@ZIF-8 before and after the adsorption of CR,the results show that the adsorption mechanism includes electrostatic interaction,hydrogen bond andπ-πbond.Fe₃O₄@Carbon@ZIF-8 has excellent adsorption and regeneration ability for CR,and the desorption effect of 0.01mol/L Na OH is the best.After 5 cycles of adsorption-desorption,the adsorption effect still remains above 80%of the original value.（5）The adsorption properties of Fe₃O₄@Carbon@ZIF-8 for Cu（II）were studied.The optimum dosage of adsorbent and the optimum initial solution p H value of adsorption reaction were 0.3 g/L and 5.0,respectively;Although the increase of reaction temperature is beneficial to the adsorption process,the effect is not significant;The adsorption equilibrium was reached after about 900 min;Interfering ions such as Na⁺,K⁺,Mg²⁺,Ca²⁺and Al³⁺will produce competitive adsorption with Cu（II）and inhibit the adsorption.The Langmuir isotherm model and PSO kinetic model fit the adsorption process well,indicating that the mode of adsorption reaction is monolayer adsorption,mainly chemical adsorption.The adsorption reaction is a spontaneous,endothermic and orderly process.FT-IR and XPS were used to characterize Fe₃O₄@Carbon@ZIF-8before and after the adsorption of Cu（II）,the results show that the adsorption mechanism includes ion exchange,electrostatic interaction and microchemical precipitation.Fe₃O₄@Carbon@ZIF-8 has good adsorption and regeneration ability for Cu（II）,and the desorption effect of 0.01 mol/L Na OH is the best.Although the adsorption effect is only 24%of the original after five adsorption-desorption cycles,the adsorption capacity at this time is 70.63 mg/g,which is still at a high level.（6）The selective adsorption properties of Fe₃O₄@Carbon@ZIF-8 for CR and Cu（Ⅱ）in complex wastewater were investigated.The results show that it has an excellent selectivity for CR in the constructed anion-cation and anion-anion binary dye systems,i.e.,CR/MB,CR/Rh B and CR/SY and CR/Mo binary systems.Its selective adsorbability of Cu（Ⅱ）in containing Cu（Ⅱ）,Ni（Ⅱ）,Co（Ⅱ）,Cr（Ⅲ）,Zn（Ⅱ）of binary,ternary and multiple heavy metals system is good on the whole,preferential adsorption order is Cu（II）>Cr（Ⅲ）>Ni（Ⅱ）>Zn（Ⅱ）>Co（Ⅱ）.（7）The removal efficiency of Fe₃O₄@Carbon@ZIF-8 on CR and Cu（Ⅱ）in different simulated wastewater was investigated,and compared with commercial adsorbents.The results show that the removal efficiency of Fe₃O₄@Carbon@ZIF-8 on CR and Cu（Ⅱ）is much better than that of powdery activated carbon and zeolite.The equilibrium adsorption capacity q_max value was higher（CR,756.88 mg/g;Cu（Ⅱ）,230.63mg/g）,the adsorbent has great application potential.