Study On Adsorption Performance And Mechanisms Of Intercalated MgAl-layered Double Hydroxide For Heavy Metals From Water

With the rapid development of urban industrialization and economy,various heavy metal ions are discharged into water bodies and reducing the use of water environment.The more serious is that heavy metal ions（such as copper,lead,cadmium）may enter into drinking water,and endangering human health.Therefore,the treatment of heavy metal polluted wastewater is particularly important.Among the various treatment methods,the use of adsorbents is considered as an efficient technique for removing toxic metal ions from aqueous medium because of their numerous advantages,such as high removal efficiency,easy operation and low-cost.Layered double hydroxide（LDH）is an inorganic mineral material,and has aroused much interest in the field of removal of heavy metal ions due to their layer structure,simple synthesis,low-cost and greenness.In this dissertation,MgAl-LDH was used as the bulk material,and different intercalated LDH materials were synthesized to remove heavy metals（Pb（Ⅱ）,Cu（Ⅱ）and Cd（Ⅱ））from water.Adsorption experiments and various characterizations were performed to study the adsorption performance and mechanisms,which provide a theoretical basis for the application of LDHs to remove heavy metals in water.The results are as follows:（1）L-cysteine（Cys）intercalated MgAl-LDH（MgAl-Cys-LDH）was synthesized by the co-precipitation method and characterized using transmission electron microscope（TEM）,scanning electron microscope（SEM）,Zeta potential,Fourier transform infrared spectroscopy（FTIR）,Brunauer-Emmett-Teller（BET）,X-ray diffraction spectroscopy（XRD）,X-ray photoelectron spectroscopy（XPS）and elemental analysis.The results showed that Cys was successfully intercalated into the MgAl-LDH layers.After the intercalation,the crystal structure of MgAl-Cys-LDH was well,and the introduction of sulfhydryl（-SH）,amino（-NH₂）and carboxyl（-COOH）functional groups provided abundant sites for the adsorption of heavy metals.According to adsorption experiments,the adsorption performance of MgAl-Cys-LDH for Pb（Ⅱ）,Cu（Ⅱ）and Cd（Ⅱ）was significantly better than that of LDH.The adsorption process was not affected by the initial solution pH.The adsorption data fit to the pseudo-second-order kinetics equation and Langmuir isotherm model.The maximum adsorption capacities of MgAl-Cys-LDH for Cu（Ⅱ）,Pb（Ⅱ）and Cd（Ⅱ）were 58.07,186.2 and 93.11 mg/g,respectively.The adsorption mechanisms were analyzed through various characterizations and included precipitation,isomorphous substitution and complexation.The adsorption experiment of simulated wastewater containing heavy metals proved that MgAl-Cys-LDH had a good value for practical applications in heavy metal wastewater treatment.（2）The polyethylene glycols with different molecular weights（PEG 200 and PEG 4000）intercalated MgAl-LDHs were synthesized using the precipitation method and named as LDH-200 and LDH-4000.A series of characterization（XRD,FTIR,XPS,TEM,SEM and Zeta potential）wereu uesd to analyze the morphology,surface functional groups of the material.The introduction of PEG made LDH-200 and LDH-4000 had a large number of hydroxyl groups,which significantly improved the adsorption performance for Cu（Ⅱ）,Pb（Ⅱ）and Cd（Ⅱ）.The fitting results of adsorption data showed that the adsorption process fit to the pseudo-second-order kinetic equation and Langmuir model.The equilibrium adsorption capacities of LDH-200for Cu（Ⅱ）,Pb（Ⅱ）and Cd（Ⅱ）were increased to 359.5%,118.3%and 196.6%,respectively.The maximum adsorption capacities of LDH-200 for Cu（Ⅱ）,Pb（Ⅱ）and Cd（Ⅱ）were 142.8,125.9and 69.11 mg/g.The adsorption mechanisms were qualitatively analyzed through various characterizations.The contributions of each mechanism were further analyzed quantitatively through the chemical extraction method.The adsorption mechanisms included precipitation,complexation,isomorphous substitution and electrostatic attraction.The precipitation was the main interaction mechanism and the contributions to the total adsropiton capacity were 44.20%-76.25%.In conclusion,the above intercalated MgAl-LDH materials had the advantages of simple preparation,easy functional modification,environmental friendliness and low-cost,and can efficiently remove Cu（Ⅱ）,Pb（Ⅱ）and Cd（Ⅱ）in water.It provides a certain theoretical reference for the modified LDH to remove heavy metals from aqueous solutions.