Research On The Removal Effect Of Epigallocatechin Gallate Modified ZIF-8 Nanoparticles On Heavy Metals In Water And The Mechanisms

Since the pollution of various heavy metals in water has been a global challenge,adsorption,one of the methods for treating heavy metals,has been a widely recognized method with great economic and time efficiency.Therefore,it is particularly important to choose an eco-friendly adsorption material to solve the problem of heavy metal pollution.Metal-organic frameworks(MOFs)is a new generation of nanoporous materials following functional porous materials such as activated carbon and zeolite,have undoubtedly attracted much attention in recent decades.In this research,epigallocatechin gallate(EGCG)with strong reducibility was used to modify ZIF-8(Zeolitic Imidazolate Framework-8)which is a popular adsorbent belonging to MOFs,and a new material ZIF-8-EGCG with enhanced reducibility was obtained for the first time.In order to study the properties of ZIF-8-EGCG,a series of characterization methods were combined to test its physicochemical properties and hydrothermal stability.Meantime,its ability to remove heavy metals from water was discussed.The main conclusions of this research are summarized as follows:(1)According to the adsorption and reduction effect on Cr(VI),the optimal synthesis conditions of ZIF-8-EGCG were determined as ZIF-8:EGCG(mass ratio)=5:5,and stirring 2 h.(2)The characterization results of SEM/TEM showed the ZIF-8 modified by EGCG changed from the original rhombic dodecahedron to the ellipsoid shape.XRD indicated that the modification of EGCG did not completely destroy the crystal structure of ZIF-8,and the crystal structure was maintained in ZIF-8-EGCG.FT-IR proved that the target functional group(phenolic hydroxyl group)was successfully introduced into ZIF-8,and the new modified material ZIF-8-EGCG was prepared.(3)When discussing the hydrothermal stability of ZIF-8-EGCG,the effects of temperatures and pH were studied.The changes in stability before and after modification was investigated by comparing with the original ZIF-8.Through detection and analysis of the zinc leaching from the material's metal node,it was found that the stability of ZIF-8-EGCG was slightly inferior to that of ZIF-8,but still maintained good performance in a wide range of pH(4?10)and temperature(25?45?).(4)The kinetics and adsorption isotherm analysis showed that the data of heavy metals removed by ZIF-8-EGCG fitted well with Langmuir model,indicating a uniform monolayer adsorption;and the adsorption kinetics of Cr(?),conforming to both pseudo-first-order and pseudo-second-order models,implied that there was a simultaneous physicochemical reaction in the process.Different from Cr(?),the adsorption of Cd(?),Cu(?)and Ni(?)by ZIF-8-EGCG was more in line with the pseudo-second-order kinetic model,indicating chemical adsorption dominated mechanism.The order of the maximum adsorption capacity(Qe)of ZIF-8-EGCG for Cd(?),Cu(?),Ni(?)and Cr(?)was Cu(232.97 mg·g-1)>>Cd(123.58 mg·g-1)>Ni(82.90 mg·g-1)>Cr(?)(12.82 mg·g-1).(5)The influence of environmental factors on the adsorption capacity of Cr(?)was studied.It was found that ZIF-8-EGCG had the best removal effect of Cr(?)at room temperature(25?)and pH=4(22.39%),and it was a spontaneous,exothermic and entropy reduction process.The removal effect of ZIF-8-EGCG on Cr(?)was positively correlated with ionic strength and dosage but negatively correlated with co-existing anions(i.e.,Cl-,NO3-,AsO2-and SO42-).Furthermore,the presence of fulvic acid(FA)in the solution had no significant effect on the removal effect of Cr(?).(6)Comparative analysis showed that the reduction ability of ZIF-8-EGCG to Cr(?)is one of the most prominent members of existing MOFs,and its reduction rate was as high as 97%,which was 2.6 times that of ZIF-8.Simultaneously,its adsorption capacity has also been improved by 1.6 times as compared to that of ZIF-8.(7)The removal efficiency of Cr(?)was greatly improved due to the phenolic hydroxyl functional group introduced by EGCG.The weak electrostatic attraction on the ZIF-8 surface and the electron-rich imidazole ligands(e.g.,-NH)facilitated in Cr(?)adsorption and partial reduction.As for ZIF-8-EGCG,the enhancement of Cr(?)adsorbing and reduction was controlled by the phenolic hydroxyl chelation and the oxidation of EGCG to quinone.(8)Further research found that the co-existing cations(i.e.K+,Na+,Ca2+,and Mg2+)had a slightly positive effect on the removal of Cd(?),Cu(?)and Ni(?)by ZIF-8-EGCG when the cation concentration was low;the impact of FA was negligible in the process of Cd(?),Cu(?)and Ni(?)adsorption.(9)In the heavy metal mixed solution,the morphology of ZIF-8-EGCG changed significantly(from ellipsoid to dead leaf).The ZIF-8-EGCG had a preference of adsorption following the order of Cu(?)>>Cr(?)>Cd(?)>Ni(?).Cationic heavy metals all had a positive effect on the adsorption of Cr(?),and Cu(?)had the most obvious promoting effect.