Preparation Of Silk Fibroin Composite Nanofibrous Membranes And Their Adsorptions For Copper Ions In The Containated Water

Removal of trace heavy metal ions existed in water is always an issue because they cause a serious biological danger. The objective of this work was to study the feasibility of preparing silk fibroin （SF） composite nanofibrous membranes by electrospinning and layer-by-layer self-assembly technique （LBL） and their potential applications in the remediation of heavy metals contaminated water as affinity membrane. SF composite nanofibrous membranes were composed of three parts: SF/cellulose acetate （CA） blend nanofibrous membranes, SF-PEI multilayer-assembled nanofibrous membranes and zero-valent iron nanoparticles （Fe0） immobilized SF-PEI multilayer-assembled composite membranes. This investigation provides the relatively comprehensive data for the SF composite nanofibrous membranes applications to the removal of heavy metal ions in wastewater and has the important theory significance and practical application value.The as-spun CA nanofibers using trifluoroacetic acid （TFA） as solvent were continuous and smooth, indicating an excellent electro-spinnability of CA solutions in TFA solvent. The obtained CA nanofibrous membranes were mainly amorphous, but characterized for good mechanical properties. The morphologies of the blend nanofibers were uniform in macro and the fiber diameter increased when adding 5^-10% CA into SF. However, the phase separation was observed between SF and CA when the content of CA was more than 20%, moreover the diameter distributions of the blend nanofiber were decentralized and polarized. Adding a small quantity of CA （no more than 10%） could induce the conformation transition of SF molecules from random coil toβ-sheet, and the mass fraction of CA was the determining factor. Meanwhile, the content ofβ-sheet conformation in SF molecules increased with the content of CA increasing from 5% to 10%. Accordingly, the thermal and mechanical properties were improved. Especially, the mechanical properties of the blend membranes were greatly improved. Metal adsorption capacities of SF/CA blend nanofibrous membranes with different content of CA were examined in an aqueous solution after ethanol treatment. The anti-felting properties of the SF nanofibrous membranes were markedly improved after treatment with 100% ethanol when SF was blended with CA. Furthermore, the dimensions of SF/CA blend nanofibrous membranes were measured and the specific surface area and porosity were calculated, which was approximately 1-2×107m^-1 and over 80%, respectively. Since the electrospun nanofibrous membrane had very large surface area and highly porous structure with predominant penetrability, these characteristic properties were very effective for the adsorption of ions, particles and the permeation of liquid, and became an advantage for affinity membrane which separated specific molecules through adsorption based on physical or chemical affinity. Metal ion adsorption test was performed with Cu²⁺ as a model heavy metal ions in a stock solution. The blend nanofiber membranes showed higher affinity for metal ions in an aqueous solution than pure SF and pure CA nanofibrous membranes. Especially, the SF/CA blend nanofibrous membranes with 20% content of CA had an exceptional performance for the adsorption of Cu²⁺, and the maximum milligrams per gram of Cu²⁺ adsorbed reached 22.8 mg·g^-1. This indicated that SF and CA had synergetic effect. Furthermore, the parameters affecting the metal ions adsorption, such as running time and initial concentration of Cu²⁺, had been investigated. The results showed that the adsorption of the Cu²⁺ sharply increased during the first 60min（>90%） and the amount of metal ions adsorbed increased rapidly as the initial concentration increased and then slope of the increase decreased as the further concentration increased. The maximum milligrams per gram of Cu²⁺ adsorbed reached 65.91 mg·g^-1.The fiber shape and porous structured nanofibrous membranes were well maintained when the number of the assembled SF-PEI bilayers was below five. The SF-PEI multilayer-assembled nanofibers were fine and uniform with the fiber diameter mainly ranging from 300-500nm, demonstrating that the SF-PEI multialyer-assembled nanofibrous membranes could be a promising material for removing heavy metal ions in water due to its large surface area and high porosity. However, when the deposition number of bilayers was increased up to six, obvious adhesive substances were observed on the surface of the fibers, and the morphology deteriorated seriously. In addition, the pores began to be blocked. PEI molecules were successfully deposited onto the surface of SF nanofibers, and the SF-PEI multilayer composite membranes formed which depended mainly on a simple electrostatic interaction between the layers of SF and PEI. SF-PEI multilayer-assembled nanofibrous membranes were found to exhibit superior capability to remove heavy metal ions in a stock solution. The filtration performances of SF-PEI multilayer-assembled nanofibrous membranes changed alternately with the alternative self-assembly of SF and PEI. The filtering effect of Cu²⁺ ions reached 59.7 mg·g^-1 when the number of the assembled SF-PEI bilayers was five. However, no obvious increases of the filtration performance were observed as the further increasing of the number of bilayers. In order to determine the potential rate-controlling steps involved in the adsorption process and estimate a limiting value at the concentrations studied, kinetic models and adsorption isotherms were established. The experimental data was analyzed using two adsorption kinetic models, pseudo-first-order and pseudo-second-order with the latter system providing the best fit, which correlation coefficient R2 was 0.9958 and the theoretical value qeq estimated from the second-order kinetic model was closer to the experimental data than the calculated qeq values of the pseudo-first-order model. From the adsorption isotherm modeling, it was revealed that the adsorption of copper ions followed both the Langmuir and the Freundlich type isotherms. However, a comparsion of the correlation coefficients indicated that the experimental data were fitted somewhat better by Langmuir isotherm model than the Freundilich isotherm model. And the curve generated by the Langmuir isotherm data were very close to experimental data. The maximum copper adsorption capacity （qm） on SF composite nanofibrous membranes was determined (83.33mg·g^-1) using the Langmuir linearized equation.The fiber shape and porous structured nanofibrous membranes were well maintained when the assembled SF-PEI composite membranes were immobilized with Fe⁰ nanoparticles, which was very suitable for removing heavy metal ions in water. The high resolution TEM images exhibited that the diameter of Fe⁰ particles synthesized and immobilized onto SF-PEI composite membranes was mainly within 10nm, which was more than 10 times shorter than that of Fe⁰ particles dispersed in ethanol. This demonstrated that synthesizing and immobilizing Fe⁰ nanoparticles onto SF-PEI multilayer-assembled electrospun polymer nanofibers could prevent the Fe⁰ nanoparticles from agglomeration. For the case of the （SF-PEI）5–assembled SF nanofibrous membranes, the Fe⁰ nanoparticles loading percentage sharply increased with the cycles of binding and reduction processes. The Fe⁰ nanoparticles loading percentage was 1.24% at two binding-reduction processes. A sharp increase was observed at four cycles of binding and reduction, which increased to 6.39%. For the case of two cycles of binding and reduction processes, the Fe⁰ nanoparticles loading percentage increased with the numbers of SF-PEI bilayers deposited onto the electrospun SF nanofibers. But its effect was not as obvious as the cycles of binding-reduction processes on the Fe⁰ nanoparticles loading percentage. The filtration performances of Cu²⁺ were markedly improved after immobilizing Fe⁰ nanoparticles onto SF-PEI nanofibrous membranes. Cu²⁺ capacity of the SF nanofibrous membranes with five numbers of SF-PEI bilayers prapared by two and four cycles of binding-reduction reached 89.71mg·g-1,108.89mg·g-1, respectively. For the case of two cycles of binding-reduction processes, the filtration performance of SF nanofibrous membranes with six numbers of SF-PEI bilayers was not greatly improved in comparison to that at five bilayers. The mechanism of action was mainly reduction and adsorption.