The Study Of Electrospun Chitosan-based Nanofiber Membranes For Hexavalent Chromium Ions Adsorption

With the rapid development of modern industry,the environmental pollution problems have gained much concerns.Water pollution,especially for heavy metals pollutants in water,is one of the most serious pollution to humans and other organism.Heavy metal ions are highly soluble in water,cannot be naturally degraded and eventually are enriched in the organisms through the biologic chain,which will lead to a serious threat to human health.Hexavalent chromium(Cr(VI))is a toxic contaminant which is commonly produced in leather tanning,electroplating,metal polishing and chemical industries as well as drinking water sources.The hazardness of Cr(VI)includes destroying normal physiological metabolic activities of organisms,causing carcinogenesis,distortion and mutation.Therefore,developing new methods for removal of Cr(VI)from natural water and wastewater is significant.The traditional Cr(VI)treatment methods mainly include adsorption,chemical precipitation,ion exchange,biological treatment and membrane filtration.However,most of these methods have problems such as high cost,difficulty in achieving emission requirements or secondary pollution.Among them,the adsorption has received extensive attention for its high efficiency,low cost and easy operation.In the adsorption process,the porous material with large specific surface area is the best choice.Due to their advantages of small diameter,large specific surface area,fast adsorption speed,recyclability,easy desorption and continuous preparation,electrospun nanofibers have good application prospects in the fields of Cr(VI)adsorption and removal.In this study,chitosan-based(CS-based)core-shell nanofibers and multi-component nanofibers were prepared by coaxial electrospinning and mixed electrospinning,respectively.The performance of CS-based fiber membranes for Cr(VI)adsorption was studied in detail.The main research were described as follows:(1)Core-shell structured nanofibers(labeled as CA-PCL/CS)with cellulose acetate(CA)as core and polycaprolactone/chitosan(PCL/CS)as shell,were electrospun via coaxial electrospinning.The effect of core solution electrical conductivity on the morphology and structure of core-shell fibers was studied emphatically.The scanning electron microscope(SEM)and transmission electron microscope(TEM)results showed the increasing electrical conductivity of core solutions would result in smaller diameter ratio of core to shell,thinner overall diameter and more inhomogenous diameter distribution.Additionally,fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD)and differential scanning calorimetry(DSC)all demonstrated that the interaction between core and shell weakened with the increase of electrical conductivity of core solutions,this might lead to the different diameter ratios of core to shell in the fibers.The results provide a basis for further design and optimization of processing conditions to control the nanostructure of core-shell composite nanofibers.(2)The as-prepared CA-PCL/CS core-shell composite nanofiber membranes was used for the removal of Cr(VI)ions.The adsorption performance of the three core-shell nanofiber membranes(S1,S2 and S3)was significantly better than CS powder.The maximum adsorption capacity of core-shell nanofibers increased by 69.6% compared with CS powder.The electrospun CA-PCL/CS membrane showed excellent durability in acidic environment and there was almost no weight loss after soaking in water(pH = 3)for 48 h.The effect of pH,initial concentration and contact time on the adsorption behavior was studied.Besides,the adsorption kinetic and isotherms were analyzed.The results showed that the highest removal for Cr(VI)occurred at pH = 3.As initial concentration and contact time increased,the amount of adsorption gradually increased.The adsorption process of CS powder was in accordance with the pseudo second-order model and Langmuir model.Adsorption on core-shell nanofiber membranes simultaneously followed pseudo second-order,Langmuir and Freundlich isotherm models.(3)Chitosan/poly(ethylene oxide)/permutit(CS/PEO/PT)composite nanofibers were prepared via electrospinning and their potential application on removing Cr(VI)from aqueous solutions was studied systematically.1% permutit dispersed well in nanofibers.With adding permutit into precursor solutions,surface area and pore volume of nanofibers increased obviously.In addition,XRD and DSC results showed there was a strong interaction among CS,PEO and permutit.The effects of permutit content,pH value,contact time and initial Cr(VI)concentration on the adsorption behavior were studied.It was concluded that nanofibers with 1% permutit showed the highest removal of Cr(VI).Acidic(pH = 3)solution and low Cr(VI)concentrations contributed to the removal of Cr(VI).The maximum adsorption capacity of CS/PEO/PT could reach 229 mg/g.The adsorption kinetics and equilibrium data were well described by pseudo-second-order kinetic and Langmuir isotherm models,respectively.According to the results of FTIR and XPS,it was found that protonation,ion exchange,electrostatic interactions and redox reactions were involved during the adsorption process.