Preparation Of Binary Metal Modified Chitosan Composite Beads And Their Adsorption Performances Towards Tetracycline From Water

With the use of antibiotics,a large number of wastewater containing antibiotic pollutants enter the environment,which poses a serious threat to the ecosystem and human health.Therefore,how to efficiently remove antibiotics from wastewater has been widely concerned.Adsorption method has been gradually favored by scientists and technologists because of its advantages of high removal efficiency,simple operation,low cost and no by-products.Chitosan（CS）,a kind of natural alkaline polysaccharide,is widely available,cheap and easy to obtain,and has good adsorption performance for pollutants in water.However,powdered CS is not easy to separate and unstable in acidic solution,which limits its application in wastewater treatment.In order to improve the stability and adsorption performance of CS adsorption materials,this paper prepared small spherical CS adsorbent by sol-gel pelletization method,and proposed to use different binary metal systems to modify it,to prepare high adsorption performance and easy regeneration of binary metal modified chitosan composite ball adsorbent materials.The application of tetracycline（TC）antibiotics in wastewater adsorption was investigated.The main research work and results are as follows:（1）A Cu-Zn-CS composite beads with high adsorption capacity and easy regeneration was prepared by using chitosan as the main raw material and the binary metal hydroxides（copper and zinc）were injected into it by one-step in-situ precipitation method.The morphology,functional group structure,specific surface area,crystal structure and elemental chemical states were characterized and analyzed by SEM,FTIR,BET,XRD and XPS.The effects of different experimental conditions（including dosage,initial p H,coexisting ions,etc.）on Cu-Zn-CS adsorption and TC removal performance were investigated,adsorption kinetics model and adsorption isotherm model were studied,and its regeneration performance was analyzed.Finally,the mechanism of Cu-Zn-CS adsorption and TC removal was explored based on the above analysis results.The results show that Cu-Zn-CS has a small spherical shape with a direct diameter of about 2 mm,rough surface and uneven porous structure inside.Cu-Zn dihydroxides formed in situ are amorphous.The specific surface area of Cu-Zn-Cs is 14.13 m²/g.In addition,Cu-Zn-CS has better adsorption capacity and removal rate when the adsorbent dosage is 0.2 g·L^-1.Cu-Zn-CS has a large adsorption capacity at p H 5,and has a good adsorption effect on TC under weak acidic conditions.The effect of SO₄^2-,Mg²⁺and Ca²⁺on the adsorption of tetracycline was more obvious than that of K⁺and NO₃^-.The adsorption kinetics of Cu-Zn-CS was more consistent with the quasi-second-order kinetic model,which indicated that the adsorption was dominated by chemical adsorption.The Langmuir model can better fit the isothermal adsorption data,indicating that the adsorption of TC on Cu-Zn-CS is monomolecular adsorption,and the theoretical maximum adsorption capacity is865.98 mg·g^-1.After 5 cycles of regeneration,the regeneration capacity of Cu-Zn-CS is about 70%of the initial adsorption capacity.The adsorption mechanism of Cu-Zn-CS is electrostatic attraction,hydrogen bonding and endosphere complexation.（2）In consideration of the advantages of metal-organic framework（MOF）materials,such as high specific surface area,rich pore size,and functional metal adsorption sites that can coordinate various unsaturated coordination,the in-situ loaded cobalt（Co）-MOF（ZIF-67）-chitosan composite spheres were firstly prepared by using chitosan as a carrier.Then,Fe-ZIF-67-CS modified chitosan composite beads were obtained by means of iron doping,which were used to further improve the performance of adsorption materials in removing TC from wastewater.The morphology,functional group structure,specific surface area,crystal structure and elemental chemical states of Fe-ZIF-67-CS were characterized and analyzed by SEM,FTIR,BET,XRD and XPS.The influence of adsorption factors such as dosage,initial p H and coexisting ions on TC removal performance of Fe-ZIF-67-CS was investigated,and the adsorption kinetics and isothermal adsorption process of Fe-ZIF-67-CS were analyzed.Based on the above analysis,the adsorption mechanism was revealed,and the regeneration characteristics of Fe-ZIF-67-CS were investigated.The results show that ZIF-67 is successfully loaded in Fe-ZIF-67-CS composite spheres.Fe-ZIF-67-CS has abundant pore structure and BET specific surface area is183.16 m²/g.In addition,Fe-ZIF-67-CS has better adsorption capacity and removal rate when the adsorbent dosage is 0.2 g·L-1.The adsorption capacity of Fe-ZIF-67-CS gel spheres is large at p H 8,and it has good adsorption effect on TC under weak acidic and alkaline conditions.In addition to SO₄^2-,co-existing cation K⁺,Ca²⁺,Mg²⁺and co-existing anion NO₃^-all had inhibition effect on TC adsorption.The adsorption process of Fe-ZIF-67-CS was more consistent with the quasi-second-order kinetic model,which indicated that the adsorption was dominated by chemical adsorption.Freundlich model can better fit the isothermal adsorption experiment data,indicating that the adsorption of TC on Fe-ZIF-67-CS is heterogeneous surface adsorption,and the theoretical maximum adsorption capacity of TC on Fe-ZIF-67-CS is 1384.54 mg·g^-1.After 5 cycles of regeneration,the regeneration capacity of Fe-ZIF-67-CS gel spheres remained at about 75%of the initial adsorption capacity.Based on the above analysis results,it is speculated that the adsorption mechanism of Fe-ZIF-67-CS on TC in water is electrostatic attraction,hydrogen bonding and endosphere complexation.