Adsorption And Photocatalytic Removal Of Water Pollutants By Zeolitic Imidazolate Frameworks

With the development of the industrialization process,large numbers of industrial pollutants are discharged into natural waters,resulting in harmful effects on the water ecological environment.Adsorption behavior and photocatalytic degradation of pollutants in water are the hot and frontier research area in the field of wastewater treatment while to find a suitable adsorbent and photocatalyst will be paramount importance.Zeolitic Imidazolate Frameworks(ZIFs)is widely used in the field of water treatment because of its large specific surface area,high porosity,stable chemical properties.For in-depth understanding of the efficiency and mechanism of ZIFs of adsorption on heavy metal ions and photocatalytic degradation on organic dyes,this paper took the ZIF-8 as representation which was synthesized through mixing method using Zn(NO3)2·6H2O and 2-Methylimidazole as raw materials and the synthesis conditions were optimized by the response surface methodology.The adsorption behavior and mechanism of ZIF-8 on heavy metal ions in water were investigated.The photocatalytic degradation of organic dyes in water by Ag/AgCl@ZIF-8 was studied and the photocatalytic mechanism was discussed.The objective of this paper is to provide theoretical basis and technical support for the practical application of ZIFs in the treatment of the actual polluted water.The conditions of the synthetic experiments were optimized using an optimal experimental design based on central composite design and response surface methodology.The results showed that the optimal parameters molar ratio of Zn2+:Hmim = 1:45.63(ZIF-8 W) with 200 mL of DI water as solvent(Zn(NO3)2.6H2O=1.0g)while the optimal parameters molar ratio of Zn2+:Hmim = 1:60(ZIF-8 M)with 200 mL of the anhydrous mechanol as solvent(Zn(NO3)2·6H2O=1.0 g),which the adsorption capacities of ZIF-8 on Pb2+were 668.00 and 801.501 mg/g,respectively.The prepared ZIF-8 showed pure hexagonal octahedral structure,BET surface area was more than 1100 m2/g.The Gibbs free energy of the adsorption process was negative,which indicated that the adsorption process was a spontaneous process.The adsorption kinetics of ZIF-8 on Pb2+followed the pseudo-second-order model and the equilibrium adsorption time of ZIF-8 was 4 h.The optimal adsorption pH was 6.0-7.0,the coexistence of Cu2+ and Ni2+(Cu2+particularly)would weaken the adsorption effect of ZIF-8 on Pb2+.The adsorption capacity of ZIF-8 on Pb2+ was basically maintained at a high level after 3 cycles of adsorption.After adsorption,the structure and morphology of ZIF-8 remained the same.FT-1R and XPS results indicated that the adsorption mechanism was probably the coordination bond between Pb2+ and Zn-OH in ZIF-8.Photocatalytic degradation on MB and MO in aqueous solution by Ag/AgCl@ZIF-8 has been investigated.The experimental results showed that the optical energy gap of Ag/AgCl@ZIF-8 was significantly decreased but the photocatalytic activity of the modified ZIF-8 on MB and MO was improved.The degradation kinetics followed the pseudo-first-order model.Higher or lower pH of the solution was beneficial to the degradation reaction.The photo-degradation rate of MB and MO were affected by catalyst dosage,dye concentration and calcium ion concentration.According to the results of Fluorescence excitation-emission matrix spectroscopy(EEM)and HD UHR-Q-TOF-MS analysis,it was speculated that a large amount of active substances produced in the photocatalytic process and·OH were the main mechanism of Ag/AgCl@ZIF-8 on photocatalytic degradation of dyes.In conclusion,ZIF-8 can be used as an excellent material for wastewater treatment which shows a great potential in terms of adsorption and photocatalytic degradation of pollutants in water.