Preparation And Application Of Cross-linked Chitosan/Polyvinyl Alcohol Blending And Affinity Membranes

Compared with the traditional chemical separation process,membrane separation has the advantages of simple process,low pressure drop,low energy consumption and easy to control.It is considered to be one of the most promising new technologies in the 21st century,and its separation effect depends on the properties of the membrane material.Chitosan(CS)is a kind of polymer membrane material with great development potential,because it has a wide range of sources,natural,non-toxic,and has excellent biological activity,biocompatibility and biodegradability.However,the weaknesses of brittleness,poor mechanical properties and swelling in acidic media of chitosan membrane limit its application.Polyvinyl alcohol(PVA)is a separation membrane material with excellent properties,such as good film-forming,flexibility,solvent resistance and chemical stability.Based on the complementarity of chitosan and polyvinyl alcohol,two polymer solutions were mixed together to prepare the CS/PVA blending membrane.Using epichlorohydrin(EPI)as cross-linking agent,the cross-linked CS/PVA membrane was prepared.The macropore CS/PVA blending membrane was prepared by using silica gel as porogen,it was then cross-linked by epichlorohydrin to prepare the macroporous cross-linked CS/PVA membrane.Finally,after being coupled with iminodiacetic acid and chelated with Cu2+ and Ni2+ ions,the macroporous cross-linked CS/PVA membrane was used as an immobilized metal ion affinity membrane for the separation and purification of His-tagged model protein,recombinant serine hydroxymethyltransferase(SHMT).The dynamic separation effect of SHMT by the cross-linked CS/PVA IMAM cartridge was comparable to that of a commercial metal ion affinity chromatographic column.The main results were summarized as follows:1.Using acetic acid and water as solvent,the CS/PVA blending membranes with three mass ratios were prepared from chitosan and polyvinyl alcohol by phase inversion method.Then the CS/PVA blending membranes were cross-linked by EPI under alkaline condition.The prepared membranes were characterized by FTIR,DMA,TG,XRD and SEM,and the swelling and mechanical properties of the blending membranes before and after cross-linking were investigated.The experimental results showed that the two components of the CS/PVA blending membranes were bonded by hydrogen bonds with good compatibility.The addition of polyvinyl alcohol enhanced the swelling and mechanical properties of pure chitosan membranes.Being cross-linked with EPI significantly improved the swelling performance of the CS/PVA blending membranes.This overcame the CS/PVA blending membranes' weakness of alkali resistant but not acid resistant,so that they could be used under wider pH conditions.2.This paper explored the isothermal adsorption and kinetic adsorption behaviors of Cu2+ and Ni2+ ions on the cross-linked CS/PVA membrane at 25 ?.Langmuir,Freundlich and langmuir-freundlich adsorption models,as well as the quasi-first-order kinetic model and quasi-second-order kinetic model were used separately to simulate the isothermal adsorption characteristics and dynamic adsorption behavior of metal ions on the cross-linked CS/PVA membrane.The results showed that Langmuir-Freundlich adsorption model and quasi-second-order kinetic model could fit the isothermal adsorption and kinetic adsorption data of Cu2+ and Ni2+ ions on the cross-linked CS/PVA membrane very well.The suitable pH conditions for the adsorption of Cu2+ and Ni2+ ions were pH 5 and pH 7,respectively.The adsorption equilibrium time was 300 min at 25 ?.The adsorption amounts of Cu2+ and Ni2+ ions increased with the increase of temperature and decreased with the increase of membrane thickness.3.Using silica gel as porogen,the macropore CS/PVA blending membranes were prepared by porogen dissolution method.The porosity and average pore size of the macroporous cross-linked CS/PVA membranes were measured by dry and wet method and filtration method respectively.When the silica dosage in 40 mL casting solution was 3.0 g,the porosity and average pore diameter of the macroporous cross-linked CS/PVA membrane both reached its maximum value,66%and 2.54 ?m respectively.The isothermal adsorption and kinetic adsorption behaviors of Cu2+ and Ni2+ ions on the macroporous cross-linked CS/PVA membrane could also be simulated by Langmuir-Freundlich adsorption model and quasi-second-order kinetic model separately.Compared with the cross-linked CS/PVA membrane,the saturated adsorption capacities of Cu2+ and Ni2+ ions on the macroporous cross-linked CS/PVA membrane at 25? increased from 37.19 and 14.49 mg/g to 51.13 and 18.17 mg/g.The adsorption equilibrium time was shortened from 300 min to 220 min.4.A cross-linked CS/PVA IMAM was prepared from the macroporous cross-linked CS/PVA membrane after being coupled with IDA and chelated with Cu2+ and Ni2+ions in turn.The cross-linked CS/PVA IMAM had proper pore size and porosity.It could immobilize adequate amount of metal ions for affinity separation.The capacities of immobilized Cu2+and Ni2+ ions on the cross-linked CS/PVA IMAM were 7.93 and 7.00 ?mol/cm2 respectively,about 144.95 and 115.92 mg/g,both were higher than the data reported on other IDA grafted regenerated cellulose-based membranes in the literature.To the model protein,His-tagged recombinant SHMT,the adsorption capacity and separation efficiency of the cross-linked CS/PVA IMAM were excellent,its dynamic separation effect could be comparable to a commercial IMAC column packed with agarose-based Ni-6FF,which meant that SHMT crude enzyme extract could be purified to single component of protein by the cross-linked CS/PVA IMAM cartridge.5.Due to the relatively high adsorption capacities of Cu2+and Ni2+ ions,the prepared macroporous cross-linked CS/PVA membrane coupled with IDA can be used as an adsorbent for heavy metal ions adsorption and will play an active role in the treatment of water pollution.This also means that the prepared cross-linked CS/PVA IMAM provides an affinity membrane material with excellent properties for the large-scale affinity purification process of His-tagged recombinant protein in the near future.