Design And Synthesis Of Functionalized Metal-organic Frameworks Based On Calix[4] Arene And Adsorption Of Pb²⁺ Form Water

Wastewater containing heavy metal ions that is discharged into nature without being treated to the required standard can lead to water pollution,especially wastewater containing lead(Pb²⁺).Lead ions are widespread in industrial production and untreated industrial wastewater containing lead can cause significant damage to the human heart,brain and blood vessels when it enters the human body through the food chain.Adsorption is currently one of the most promising methods for the removal of lead ions.Therefore,the development of high-efficiency and cost-effective adsorbents for heavy metal removal is imminent.Metal organic frameworks（MOFs）are a new type of material that is favoured by researchers for their excellent physicochemical properties.MOFs have the advantages of very large specific surface area,developed pore structure and adjustable structure and functional groups,which makes MOFs have unique advantages in adsorption and removal of pollutants.In this paper,aiming at the pollution problem of wastewater containing Pb²⁺,the third generation supramolecular compound calixarene with a special cavity structure was selected and functionally modified by introducing pyridine groups or carboxylic acid functional groups at its lower edge.The designed functionalized calix[4]arene were selected as ligands to prepare two functionalized MOFs materials,and their structures and stability were characterized,and their adsorption properties on Pb²⁺in water were investigated in detail.The main research results and contents are as follows:（1）An O^-functionalized MOF,{[Zn（ADB）（L1）_0.5]·1.5DMF}_n（1,DMF=N,N-Dimethylformamide）,was prepared by the reaction of Tetrapyridylcalix[4]arenes（denoted as L1）,4,4’-azoxydibenzoic acid（denoted as H₂ADB）and Zn（NO₃）₂·6H₂O under solvothermal conditions.The activated MOF material,[Zn（ADB）（L1）_0.5]_n（1a）was obtained by vacuum drying.In the solution without p H optimization,the MOF showed a strong affinity for Pb²⁺in water（K_d=8.88×10~6 m L/g）and higher adsorption capacity（463.52 mg/g）.It can also efficiently reduce the concentration of Pb²⁺in water（1～10 ppm）to ppb level.In addition,in the presence of other background ions,the selective and efficient capture of Pb²⁺can be realized,and the removal rate is as high as 99.05%.After collecting the adsorption data and fitting,it is found that the adsorption process of MOF 1a to Pb²⁺conforms to the pseudo-second-order kinetic model and the Langmuir adsorption model.The samples before and after adsorption are characterized by X-ray photoelectron spectroscopy and Fourier infrared spectroscopy,and the adsorption mechanism is explored.It shows that a large number of O^-groups in mof1a channel have strong electrostatic attraction and coordination effect with Pb²⁺,which can realize the efficient adsorption and removal of Pb²⁺.（2）A Cu-MOF,{[Cu_1.5（HL2）（H₂O）₃]·H₂O}n（2）,was prepared by a self-assembly reaction using a tetracarboxylic acid cup[4]aromatic（H₄L2）and Cu Cl₂·2H₂O under solvothermal conditions.By X-ray single crystal diffraction,it is shown that MOF 2 has a two-dimensional layered structure and uncoordinated carboxylic acid functional groups are loaded in the calix[4]arene-built cavity.Based on this,in a mixed solvent of methanol and isopropanol,the MOF 2 can be exfoliated into ultra-thin 2D MOF nanosheets by ultrasonication.The two-dimensional MOF nanosheets were used as adsorbent and its adsorption behaviour on Pb²⁺in water was systematically investigated.The effects of p H,adsorbent dose and other factors on the adsorption and removal of Pb²⁺were investigated.The results showed that the adsorption effect was the best when the adsorbent dosage was 100 mg/L and p H=7,and the adsorption capacity reached 738.65 mg/g.Even in the presence of other background ions,MOF nanosheets still showed good selectivity and anti-interference ability for Pb²⁺removal.The adsorption data were fitted to the quasi-secondary kinetic model and the Langmuir adsorption isotherm model.The adsorption mechanism was studied by X-ray photoelectron spectroscopy and Fourier infrared spectroscopy,which showed that there was a strong interaction between Pb²⁺and uncoordinated carboxylic acid oxygen atoms.