Adsorption Of Direct Dyes In Water By ZIF-8 And Its Modified Materials

As a natural or synthetic colorant,dyes are widely used in various industrial production.However,the harm caused by dyes in water environment and human beings can not be underestimated.Therefore,it is very important to develop efficient and environmentally friendly technologies to remove dyes from wastewater.Adsorption method just meets this demand and has become a popular technology to treat dye wastewater.Metal-organic frameworks（MOFs）have been widely used as adsorbents in recent years because of their large specific surface area,stable structure and simple modification.Focusing on the nano-material ZIF-8 and its modified materials ZIF-8@PDA and ZIF-8@PDA@Fe₃O₄,this paper dicusses the adsorption of direct pink（DR 12B）and direct dark brown（DDB M）in water by these three materials,and analyzes their adsorption mechanisms.The following research results were obtained:（1）ZIF-8 was prepared by stirring at room temperature,and was characterized by SEM,XRD,BET,FTIR and XPS,and it was confirmed that ZIF-8 material was successfully synthesized.The influence of single factor on the adsorption of DR 12B and DDB M by ZIF-8 was discussed.Under the optimum conditions（25℃,pH=8,5 mg adsorbent,50 mg/L initial dye concentration,120 min reaction time）,the adsorption capacity of ZIF-8 for DR 12B and DDB M was 144.95 mg/g and 127.54 mg/g respectively.The bivalent anions(CO₃^2-and C₂O₄^2-)have great influence on the adsorption of DR 12B and DDB M by ZIF-8.The adsorption of DR 12B and DDB M by ZIF-8 was more consistent with the pseudo-second-order kinetic model,indicating that chemical adsorption was the main method.The Freundlich isotherm model is more consistent with the adsorption reaction,which shows that it is multimolecular layer adsorption.Thermodynamic study shows that with the increase of temperature,the adsorption capacity of ZIF-8 on DR 12B and DDB M decreased,which is a spontaneous exothermic reaction.The adsorption mechanism of ZIF-8 to DR 12B and DDB M is mainly hydrogen bond,electrostatic attraction andπ-πbond.（2）ZIF-8 was modified with dopamine hydrochloride to obtain ZIF-8@PDA composite material,which was characterized by SEM,XRD,BET,FTIR and XPS.Meanwhile,the adsorption properties of ZIF-8@PDA on DR 12B and DDB M were discussed.Under the optimal conditions（25℃,pH=8,5mg adsorbent,50 mg/L initial dye concentration,120 min reaction time）,the adsorption capacity of ZIF-8@PDA for DR 12B was 168 mg/g,and that of DDB M was 139 mg/g.Different anions have different effects on the adsorption of DR 12B and DDB M by ZIF-8@PDA,Cl^-and NO₃^-have little effect on the adsorption of DR 12B and DDB M by ZIF-8@PDA,but CO₃^2-and C₂O₄^2-have great effects on the removal of DR 12B and DDB M by ZIF-8@PDA.ZIF-8@PDA adsorption DR 12B and DDB M were more consistent with pseudo-second-order kinetic model,indicating that chemisorption was the main method.Freundlich isothermal model can better describe the adsorption of ZIF-8@PDA on DR 12B and DDB M,indicating that the reaction is multi-molecular layer adsorption.Thermodynamic studies show that with the increase of temperature,the adsorption of DR 12B and DDB M by ZIF-8@PDA decreases,which is a spontaneous exothermic reaction.The adsorption mechanism of ZIF-8@PDA on DR 12B and DDB M is mainly the hydrogen bond,electrostatic attraction andπ-πbond.（3）The magnetic metal-organic skeleton material ZIF-8@PDA@Fe₃O₄was obtained by solvothermal loading Fe₃O₄on the basis of ZIF-8@PDA,through SEM,XRD,BET,FTIR,VSM and XPS characterization,it was found that Fe₃O₄was successfully loaded and the material had superparamagnetism.The adsorption properties of ZIF-8@PDA@Fe₃O₄on DR12B and DDB M were investigated.Under the optimal conditions（25℃,pH=8,5 mg adsorbent,50 mg/L initial dye concentration,120 min reaction time）,the adsorption capacity of ZIF-8@PDA@Fe₃O₄for DR 12B and DDB M was 147.7 mg/g and 127.5 mg/g respectively.The effect of anions on the adsorption of DR 12B and DDB M by ZIF-8@PDA@Fe₃O₄is ranked as Cl^-<NO₃^-<C₂O₄^2-<CO₃^2-.The adsorption of DR 12B and DDB M by ZIF-8@PDA@Fe₃O₄is more consistent with pseudo-second-order kinetic model,indicating that chemisorption is the main method.Freundlich isothermal model can better describe the adsorption of ZIF-8@PDA@Fe₃O₄on DR 12B and DDB M,indicating that the reaction is multi-molecular layer adsorption.Thermodynamic studies show that with the increase of temperature,the adsorption capacity of ZIF-8@PDA@Fe₃O₄on DR 12B and DDB M decreases,which is a spontaneous exothermic reaction.The adsorption mechanism of ZIF-8@PDA@Fe₃O₄on DR 12B and DDB M is mainly the hydrogen bond,electrostatic attraction andπ-πbond.In summary,ZIF-8,ZIF-8@PDA and ZIF-8@PDA@Fe₃O₄prepared in this study have good adsorption effects on DR 12B and DDB M,which provides a feasible point of view for ZIF-8 and its modified materials to adsorb direct dyes.