Preparation And Properties Of Modified Chitosan-based Ion Imprinted Composite Adsorbent

At present,ion imprinting technology has become a research focus for its advantages of high selectivity and identification to the target ion.The research and development of functional carriers of ion imprinting technology is the decisive factor in determining the efficacy of this technology.The materials currently used for preparing the functional carriers of ion imprinting technology are mainly synthetic polymers,which not only have numerous active sites but also can be functionalized easily.However,there are potential hazards in most of these carriers such as safety factors and secondary pollution.In order to solve this problem,this paper selected chitosan,a type of natural biopolymer,as the carrier material of ion imprinting.After taking the structure,properties of chitosan and water characteristics such as temperature,acidity and ion concentration as well as chitosan’s mechanism in water treatment and other factors into consideration,we chose the cheap and widely used glutaric acid,citric acid and thiourea as modifier,polyvinyl alcohol（PVA）with high hydrophilicity,good chemical resistance and mechanical properties and magnetic Fe₃O₄ as support material to increase the adsorption and stability of chitosan,thus broadening its scope of application.The magnetic separation method is used to achieve the rapid separation of the adsorbent and the treated liquid.In addition,various forms of ion imprinted chitosan-based composite adsorbent materials were prepared to improve the selectivity of the adsorbent to template ions,including magnetic ion imprinted microspheres,glutaric acid-modified CTS/PVA blended blotting membrane,magnetic CTS/PVA composite blotting membrane,glutaric acid modified magnetic CTS/PVA composite blotting membrane,glutaric acid modified freeze-dried imprinted membrane,thiourea and citric acid modified blended imprinted membrane,etc.,and their modification properties were investigated in this paper.Besides,the structure of chitosan functional composites was systematically studied by means of FTIR,XRD,SEM,EDS and other modern analytical methods and the adsorption mechanism of imprinted composite adsorbents was discussed.The main findings obtained are as follows:（1）Using chitosan as functional carrier,epichlorohydrin as cross-linking agent,Pb²⁺as template ion,after being composited with magnetic nano-Fe₃O₄,core-shell spheroidal Pb（Ⅱ）imprinted magnetic composite adsorbent（Pb（Ⅱ）-MICA）was prepared.The SEM characterization results show that the prepared imprinted composite is hollow spherical particle with particle size of 1.2μm.The 5s rapid separation of solid phase and aqueous solution is achieved by using magnet body and the absorption behavior of Pb（Ⅱ）-MICA on Pb（Ⅱ）in aqueous solution is analyzed by FASS method.The maximum adsorption capacity of the composite adsorbent is 32.48 mg/g,the optimal adsorption pH is 5-6,the optimal adsorption time is 2h.The experiment of selectivity shows that the relative selectivity coefficients of composites to Pb²⁺/Cu²⁺,Pb²⁺/Cd²⁺,Pb²⁺/Ni²⁺,Pb²⁺/Zn²⁺are respectively28.11,91.14,76.54,33.06 times those of non-imprinted adsorbents,showing good adsorption selectivity to template lead ions.The kinetic study shows that imprinted magnetic composite adsorbent is in line with quasi-secondary reaction kinetics model and the adsorption process is mainly under chemical action.The adsorption isotherm model was well consonant with the equilibrium adsorption data and the adsorption belongs to Langmuir type chemical adsorption.The thermodynamic parameters indicate that adsorption is a spontaneous,exothermic,and helium-driven process.The ion imprinted adsorbent could be regenerated using 0.2M EDTA,and the regeneration conditions are simple.The regenerated adsorbent retains most of its initial capacity after repeated use for more than 5 times.（2）In order to solve the problems of the easy agglomeration and embedding of microspheres granules,which would affect elution and adsorption efficiency,this paper adopts template polymerization method,selects CTS/PVA blend membrane as functional carrier,and uses the glutaraldehyde（GA）crosslinking with higher activity of aldehyde group and glutaric acid modification to synthesize a novel composite material of glutaric acid modified ion imprinted blend membrane,and the synthesized adsorbent is used to selectively adsorb and remove Pb（Ⅱ）from aqueous solution.Besides,Response Surface Analysis method is applied to optimize the main conditions affecting the preparation,and the effects of eluent,temperature and pH on the elution effect are studied in this paper.The adsorbent can be regenerated,whose adsorption capacity can still reach 95%of its initial adsorption capacity after repeated use for more than 5 times.The physical properties test results show that the tensile strength and elongation at break of glutaraldehyde cross-linking blend membrane modified by glutaric acid increase 1.37 and 1.61 times,respectively when comparing glutaraldehyde cross-linking CTS/PVA blend membrane.Glutaric acid modification improves the physical properties and adsorption performance of the adsorbent,thus providing theoretical basis for optimizing the R&D and application of modified chitosan imprinted ion adsorbent.（3）Based on the above research results,after combining the advantages of modified blend membrane,magnetic material and ion imprinting,glutaric acid modified chitosan-based magnetic imprinted composite membrane（Pb-GUMCCP）is prepared,and its adsorption performance to Pb²⁺is compared with those of lead-imprinted magnetic membrane（Pb-MCCP）,lead-imprinted unmodified membrane（Pb-NCCP）and glutaric acid modified lead imprinted membrane（Pb-GUCCP）,which belong to the same series.The results show that the glutaric acid modified chitosan magnetic imprinted membrane composite can maintain its adsorptivity in acidic solution（pH=3）and solution with large Pb²⁺concentration（2000mg/L）.The comparison of absorption capacity:Pb-GUMCCP>Pb-MCCP>Pb-NCCP>Pb-GUCCP,and the adsorption equilibrium time of Pb-GUMCCP is 60 min,which is half that of other adsorbents.As proven by kinetic study,the Pb-GUMCCP adsorption process is in line with the quasi second order kinetics.The maximum saturated adsorption capacity is 147.05 mg/g at initial ion concentration of2000 mg/L,pH of 3 and temperature of 65℃.The adsorption process is under chemical action with the theoretical saturated absorption capacity of 144.51mg/g.The thermodynamic parameters indicate that the adsorption process of Pb-GUMCCP is non-spontaneous endothermic process.With the increase of temperature,the adsorption process is under physical action,and transforms into chemical action.The selective adsorption capacities of Pb-GUMCCP in the presence of Zn²⁺,Cu²⁺,Co²⁺,Cd²⁺and Ni²⁺are11.37,10.66,6.27,7.99,2.39 times that of glutaraldehyde modified CTS/PVA,respectively.Pb-GUMCCP retains more than 90%of its initial adsorption capacity after repeated use for6 times,which proves its good regeneration performance and practicability for the adsorption and removal of lead ions from complex samples.（4）The effect of different modifiers（citric acid,thiourea,glutaric acid）,different drying methods（freeze-dried,air-dried method）and different template ions（Pb（Ⅱ）,Cu（Ⅱ））on the adsorption performance of glutaraldehyde cross-linking CTS/PVA based blend membranes was studied.Compared with the adsorption of glutaric acid modified copper imprinted air-dried membrane F-（Cu-GUCCP）,glutaric acid modified copper imprinted freeze-dried membrane D-（Cu-GUCCP）to Cu（Ⅱ）,thiourea modified adsorbent（Cu-THICCP）has strong adsorption performance to the target ion Cu（Ⅱ）).Compared with the adsorption of thiourea modification to Pb（Ⅱ）,citric acid modification has strong adsorption performance to the target ion Pb（Ⅱ）.By studying the competitive adsorption of ions,the maximum adsorption capacities of F-（Cu-GUCCP）,D-（Cu-GUCCP）,Cu-THICCP to copper are 68.95 mg/g,70.12 mg/g,121.77 mg/g,respectively without significantly reducing the adsorption capacity.The maximum adsorption capacities of Pb-CACCP,Pb-THICCP to lead are103.45mg/g,89.30mg/g,respectively.The thermodynamic parameters indicate that the adsorption of Cu-THICCP and Pb-CACCP modified imprinting membranes to Cu（Ⅱ）and Pb（Ⅱ）is spontaneous and exothermic.Cu-THICCP and Pb-CACCP is in accordance to Langmuir adsorption model.The adsorption process is subjected to monomolecular layer adsorption,which is dynamic equilibrium process.In view of the above-mentioned results,this work summarizes the technical advantages of membrane technology,magnetic nanotechnology,ion imprinting technology,instrumental analysis of biopolymer materials and other fields,providing new research ideas and theoretical research basis for the study of new types of ion imprinting functional carriers.