Study On Preparation Of NiFe₂O₄/ZnAl-LDH Intercalated With CO₃²⁺ Or EDTA^2- And Their Adsorption Of Cr（Ⅵ）

In this study,the magnetic composite adsorbents NiFe₂O₄/ZnAl-CO₃ LDH and NiFe₂O₄/ZnAl-EDTA LDH were synthesized through co-precipitation method.Isoelectric point determination,SEM,BET,TG-DAT,VSM,XRD,FT-IR and XPS were applied for characterization of the adsorbents.The results showed that both of the two composites were compounded by NiFe₂O₄ and ZnAl-LDH through physical processes,and were of high purity.The BET specific surface,pore volume and theaverage pore diameter of the NiFe₂O₄/ZnAl-CO₃ LDH were 6.769 m2/g,0.230 cm3/g and 29.799 nm,respectively,and those values of the NiFe₂O₄/ZnAl-EDTA LDH were 1.518 cm3/g,0.009 cm3/g and 4.074 nm,respectively.The SEM images of the NiFe₂O₄/ZnAl-CO₃ LDH displayed a more loose surface structure than that of NiFe₂O₄/ZnAl-EDTA LDH.Moreover,NiFe₂O₄/ZnAl-CO₃ LDH and NiFe₂O₄/ZnAl-EDTA LDH possessed good magnetic separation performance,and their saturation magnetization were 5.39 eum/g and 5.93 eum/g,respectively.Batch experiments were carried out to systematically investigate the adsorption properties of Cr（Ⅵ）by the two types of adsorbents under different experimental conditions.Adsorption kinetics,isotherms and thermodynamics were studied.The regeneration performance of the adsorbents and mechanism of Cr（Ⅵ）adsorption were also discussed.The results indicated that solution pH had a significant effect on the adsorption of Cr（Ⅵ）by NiFe₂O₄/ZnAl-CO₃ LDH,and the maximum adsorption amount of 12.00 mg/g was occurred at the strongly acidic condition of pH=2.However,solution pH had a relatively small effect on NiFe₂O₄/ZnAl-EDTA LDH for Cr（Ⅵ）removal,and within a wide pH range of 3-6,the removal efficiency of Cr（Ⅵ）were all higher than 88%.The adsorption processes of Cr（VI）by NiFe₂O₄/ZnAl-CO₃ LDH were very fast in the first 20 minutes at thron 90 minutesee different concentrations（50,100 and 200 mg/L）,then approached the equilibrium.The inhibiting effect of coexisting anions on NiFe₂O₄/ZnAl-CO₃ LDH followed the order:PO43->SO42->Cl->NO₃-.The Cr（Ⅵ）adsorption rate of NiFe₂O₄/ZnAl-EDTA LDH became flat from 120 minutes at these three concentrations.The anions with the same concentration of Cr（Ⅵ）such as SO42-、PO43-、CO₃2-、HCO₃-、Cl-、NO₃-had less effect on Cr（Ⅵ）adsorption on NiFe₂O₄/ZnAl-EDTA LDH.Kinetic and isotherms study suggested that the adsorption of Cr（Ⅵ）onto NiFe₂O₄/ZnAl-C03 LDH and NiFe₂O₄/ZnAl-EDTA LDH both fitted with pseudo-second-order and Langmuir models.Thermodynamic parameters revealed that the adsorption process of NiFe₂O₄/ZnAl-CO₃ LDH was spontaneous and exothermic,and the maximum adsorption capacity of Cr（VI）of 56.50 mg/g was at temperature of 298 K.The adsorption of Cr（VI）on NiFe₂O₄/ZnAl-EDTA LDH was spontaneous but endothermic,and the maximum adsorption capacity of Cr（VI）was 77.22 mg/g when the temperature was 318 K.Regeneration experiments and adsorption mechanism studies indicated that NiFe₂O₄/ZnAl-CO₃ LDH was of good reproducibility and reusability,and the removal efficiency of Cr（VI）was as high as 83.1%after four regeneration cycles for the adsorbent.The adsorption of Cr（VI）by NiFe₂O₄/ZnAl-CO₃ LDH mainly depended on the surface hydroxyl groups via electrostatic force.At solution pH below the isoelectric point 7.8,the hydrated surface of NiFe₂O₄/ZnAl-CO₃ LDH was protonated and therefore was positively charged（-OH2+）,which was favorable for attracting chromate anions and removing Cr（VI）from aqueous solution.On the other hand,the adsorption of Cr（VI）by NiFe₂O₄/ZnAl-EDTA LDH appeared to be mainly through ion-exchange interaction.Namely EDTA2+ anions existing in interlayer of the adsorbent were exchanged with Cr（VI）anions.In summary,compared with other adsorbents reported in the literature and were used to adsorb Cr（VI）,NiFe₂O₄/ZnAl-CO₃ LDH and NiFe₂O₄/ZnAl-EDTA LDH synthesized in this study could have potentially broad application in the field of water treatment because of their advantages including high adsorption capacity,quick separation property and simple preparation process.