Study On Adsorption Performance Of Several Quaternary-ammonium-Functionalized Metal-organic Frameworks(MOFs) For Palladium Cyanide

In recent years,metal-organic framework(MOFs)has been widely used to remove pollutants in water due to its high porosity,large surface area,thermal/chemical stability,hydrophilic/lipophilic matrix,low cost and easy modification.By means of the synthesis,special functional groups are introduced on the MOFs framework,and the adsorption characteristics of the MOFs material can be effectively changed by combining the ordered porous structure of the MOFs and the diversity of the graft groups.The modified MOFs material has high adsorption efficiency and molecular recognition performance.In this paper,Cu(BDC-NH2)(DMF),NH2-MIL-101(Fe),and NH2-MIL-101(Cr)were selected as modified matrix materials,and hydrophobic alkyl linkage branches were introduced into MOFs framework through the amine platform of MOFs.Three quaternary ammonium functionalized MOFs adsorbents were constructed to efficiently remove metal cyanide from aqueous solution by introducing active groups and increasing the stability of MOFs materials in water.In this paper,the following aspects are studied:preparation of quaternary ammonium functionalized MOFs adsorbents;The physicochemical properties of the adsorbents were studied by FTIR,TGA,SEM,XPS,XRD and nitrogen adsorption desorption experiments.Optimization of adsorption and elution conditions.Three kinds of quaternary ammonium functionalized MOFs adsorbents Et-N-Cu(BDC-NH2)(DMF),Et-N-NH2-MIL-101(Fe)and Et-N-NH2-MIL-101(Cr)were prepared by solvothermal method.A new system for adsorption of Pd(CN)42-by MOFs in three aqueous solutions was established.The key adsorption parameters,such as contact time,pH value,maximum adsorption capacity,elution condition,reusability,adsorption isotherm and kinetic model,were studied.The adsorption thermodynamic parameters and adsorption mechanism were also discussed in detail.The experimental results show that the three quaternary ammonium functionalized MOFs materials have high stability in neutral and weak alkaline aqueous solution.When the pH value is about 7.0,the adsorption of Pd(CN)42-is the best.Thermodynamic studies show three adsorption systems △G<0,△H<0,indicating that the adsorption process is a spontaneous exothermic reaction.The adsorption isotherms are in accordance with the langmuir model and the adsorption process conforms to the quasi-second order kinetic model.The adsorption rate is fast and the adsorption isotherms can reach equilibrium in 30min.Et-N-Cu(BDC-NH2)(DMF)can be used to remove metal cyanide efficiently,including Pd(CN)42-,Co(CN)63-,and Fe(CN)63-with maximum static adsorption capacity of 172.9(Pd(Ⅱ)),101.0(Co(Ⅲ))and 102.6(Fe(Ⅲ))mg g-1,Pd(CN)42-can be selectively recovered by a two-step elution process with a recovery of greater than 91.0%after five cycles.The experimental results show that the adsorption of Pd(CN)42-by the quaternary functionalized MOFs material is an ion-exchange reaction.The quaternary functionalized metal organic framework(MOFs)adsorbent has the advantages of simple preparation,high adsorption rate,easy elution and it has the potential application value in the fields of treating the metal cyanide and the recovery of the valuable metals.