Adsorptive Removal Of Anionic Contaminants In Aqueous Solution Using Magnetic ZIF-67:Adsorption Performance And Mechanism

The wastewater discharge pollution containing anionic contaminant has become more and more serious.The human health and aquatic environment was threatened by the various types and complex components of pollutants.Thus,adsorption is a promising technology for anionic contaminants treatment,because it has the advantage of easy operation,high efficiency and no secondary pollution.Metal organic frameworks（MOFs）is a new class of crystalline inorganic-organic hybrid materials,and zeolite imidazolate frameworks（ZIFs）is an important subclass of MOFs with excellent properties.In addition to its high specific surface area and advanced pore structure,the surface active sites provided by metal precursor and stability given by organic ligands made it a potential adsorbent for water remediation.However,most of the ZIFs are powdered crystals,which have many constraints such as difficulty in recovery and potential loss during processing.To overcome the above shortcomings of ZIFs,this study designed and synthesized two types of magnetic ZIF-67composites materials with simple preparation routine,fast adsorption rate,large adsorption capacity,and high efficiency of separation and recovery.The measurements of XRD,FT-IR,SEM,and TEM and other techniques were used to characterize the composite materials.The effects of factors such as contact time,initial pollutant concentration,temperature,p H value,and ionic strength on the adsorption process were studied using static batch adsorption experiments.The adsorption-desorption experiments were performed to value the regeneration and reuse probability of magnetic ZIF-67composites.Various adsorption kinetic models and adsorption isotherm models were used to fit the adsorption process and equilibrium data,and the interaction on the interface of the adsorbent and adsorbate was discussed.The main results of this study are shown as follows.（1）Fe₃O₄ with a particle size of about 200 nm was selected as the magnetic cores,and anionic surfactant polystyrene sulfonate sodium was used for magnetic core modification.Then,cobalt nitrate hexahydrate and dimethylimidazole were used as metal precursors and organic ligands for the further growth of ZIF-67.At 60℃,the nucleation and growth of ZIF-67 on the surface of Fe₃O₄ were carried out in a step-by-step method to synthesize the core-shell magnetic composite material named Fe3O4-PSS@ZIF-67.（2）Two types of anionic dyes,direct blue 86（DB-86）and methyl orange（MO）,were selected as target pollutants,and Fe₃O₄-PSS@ZIF-67 was used to remove the two dyes.After 5 h,the equilibrium capacities of direct blue 86 and methyl orange were172.42 and 104.17 mg/g,respectively（C₀=50 mg/L）,and the adsorption kinetics is in accordance with the pseudo-second-order model.The adsorption of DB-86 is dominated by chemical adsorption and its isotherm fits the Langmuir model better,while the MO adsorption over Fe₃O₄-PSS@ZIF-67 is the combination of physical and chemical adsorption,thus follows to the Freundlich model.The optimum p H value of both dyes adsorption is neutral,but the adsorption performance of DB-86 is better than that of MO.Finally,the selective adsorption and separation of multi-component dyes with different concentration ratios were investigated to analyze the mechanisms of anionic dyes.Through the XPS analysis of the material surface before and after adsorption,the mechanisms of anionic dyes adsorption on Fe₃O₄-PSS@ZIF-67 are determined as hydrogen bonding and electrostatic interaction.（3）Fe₃O₄ with a particle size of about 100 nm was prepared by a hydrothermal method as a magnetic core.To avoid the problem of surfactant pollution,the magnetic core was modified using sodium acetate（Na AC）instead of sodium polystyrene sulfonate.ZIF-67 crystals formed and grew between the adjacent Fe₃O₄.By investigating the ratio of magnetic core and cobalt precursor,the maximum ZIF-67 loading capacity was determined.（4）Phosphorus is one of the common contaminants in natural water.Thus,phosphate was used to simulate phosphorus-containing wastewater,and the removal performance of phosphate by Fe₃O₄-COO^-@ZIF-67 was investigated.At 25℃,the adsorption reached saturation within 180 min,and the maximum adsorption capacity was116.59 mg P/g.Since the distribution of phosphate species varies with the p H value of the aqueous solution,the effects of p H and coexisting ions on the adsorption process were mainly investigated.Remarkably,Fe₃O₄-COO^-@ZIF-67 exhibited faster phosphate adsorption in the presence of Ca²⁺.Further mechanisms of the enhanced phosphate removal were investigated through monitoring the detailed XPS spectra,and the improved adsorptive performance was mainly owing to the synergistic effect of adsorption and precipitation.The results confirm that the exchange of hydroxyl groups on the surface of Fe₃O₄-COO^-@ZIF-67 has played a significant part for phosphate removal.In the study,anionic dyes and inorganic phosphates were selected as target anionic pollutants.We prepared core-shell and non-core-shell structures of magnetic ZIF-67 with different modification agents,and the materials ratio of composites was optimized to achieve the best adsorption performance.And the mechanisms of anionic contaminants removal using magnetic ZIF-67 composites were further investigated.In conclusion,this work is expected to provide a theoretical basis for the design and application of magnetic ZIFs/MOFs composites in the water remediation.