| As a new type of adsorption and separation material,electrospun nanofibers have been widely concerned by academia and industrial wastewater treatment in recent years due to their advantages of high specific surface area,high porosity,wide variety and controllable process.However,the surface active groups of unmodified electrospun nanofibers are less and the adsorption capacity of the nanofibers is weak,therefore it is not easy for them to be directly used in the treatment of wastewater with complex components and low concentration of heavy metal ions.In this paper,the electrospun nanofibers were modified by natural adsorption materials(chitosan,clay and tannin)with excellent adsorption performance in order to prepare nanofibrous separation materials with affinity for both organic compounds and heavy metal ions in wastewater,and their adsorption properties and mechanism were studied.It includes the following three aspects:(1)Preparation and adsorption properties of cellulose acetate@chitosan modified montmorillonite(CA@CS-MMT)composite nanofibers.Firstly,Chitosan modified montmorillonite(CS-MMT)was prepared by solution intercalation of chitosan(CS)into montmorillonite(MMT).Then,CA@CS-MMT nanofibers were fabricated by blending CA and CS-MMT through electrospinning process.Lastly,the adsorption properties of CA@CS-MMT nanofibers for trace Cr(Ⅲ)in wastewater were studied.The surface morphologies and chemical properties of the nanofibers were analyzed by transmission electron microscopy(TEM),scanning electron microscopy(SEM),and fourier transform infrared spectroscopy(FTIR),respectively.The adsorption capacity and removal rate of the composite nanofibers for trace Cr(Ⅲ)in water were measured.The adsorption behaviors of Cr(Ⅲ)on the CA@CS-MMT composite nanofibers were studied by adsorption kinetics,isothermal adsorption model and thermodynamic study,and the adsorption mechanism was explored.The results showed that the CA@CS-MMT composite nanofibers with regular morphology,large specific surface area and high porosity can be prepared by controlling the electrospinning parameters.The adsorption capacity and removal rate are the highest when the CS-MMT content is 2 wt.%and the adsorption pH is 5.5±0.1.The adsorption process is better in accordance with the Langmuir isothermal adsorption model and pseudo-second-order kinetics,and the calculated maximum adsorption capacity can reach 144.93 mg/g at 25℃.Importantly,the batch adsorption process could make the Cr(Ⅲ)concentration notably decrease from 20 or 10 mg/L to below 1.5 mg/L by the CA@CS-MMT nanofibers adsorbent,which showed great potential application in disposal of trace Cr(Ⅲ)in wastewater.The adsorption mechanism study shows that the adsorption of low concentration Cr(Ⅲ)of simulated wastewater by composite nanofibers is mainly through ion exchange and intercalation complexation.(2)Preparation and adsorption properties of hydrolyzed polyacrylonitrile/polyethylenimine/tannic acid(HPAN/PEI/TA)composite nanofibers.Firstly,polyacrylonitrile(PAN)electrospun nanofibers were hydrolyzed by alkali.Then,with polyethyleneimine(PEI)and tannic acid(TA)as self-assembly units,HPAN/PEI/TA nanofibers were fabricated by a layer-by-layer self-assembly method.Finally,HPAN/PEI/TA nanofibers were used to adsorb the trace Cr(Ⅲ)from water.The effects of PEI concentration,TA concentration and the number of assembly layers on the morphologies,structures and surface chemical properties of the composite nanofibers were investigated.The adsorption capacity and adsorption rate of heavy metal ions on nanofibers were measured.The adsorption behaviors of trace Cr(Ⅲ)on nanofibers were studied by adsorption kinetics,isothermal adsorption model and thermodynamic study,with the adsorption mechanism explored.The results showed that when the adsorption pH was 5.5 ± 0.1,the adsorption capacity and removal rate of(HPAN/PEI/TA)n nanofibers reached the highest.The adsorption process fitted better with the Langmuir isothermal adsorption model and pseudo-second-order kinetics.The calculated maximum adsorption capacity can reach 221.41 mg/g at 25℃,and the adsorption of Cr(Ⅲ)onto(HPAN/PEI/TA)n is mainly a chemical adsorption process.Importantly,the adsorption of trace Cr(Ⅲ)by HPAN/PEI/TA nanofibers showed that initial concentration of 10 or 20 mg/L could be reduced to less than 1.5 mg/L when the dosage of HPAN/PEI/TA nanofibers adsorbent was 1.0 or 2.0 g/L,showing its effective removal of heavy metal ions with low concentration.The adsorption mechanism study shows that Cr(Ⅲ)ions mainly are adsorbed by chelation of phenolic oxygen anion and the coordination of the amino groups belong to the(HPAN/PEI/TA)n nanofibers.(3)Preparation of chemical crosslinked polyacrylonitrile/tannic acid(CPAN-TA)composite nanofibers adsorbent and its adsorption properties for trace Cr(Ⅲ)in complex in water.Firstly,PAN-TA composite nanofibers were prepared by blending partially hydrolyzed PAN with TA.Then,CPAN-TA composite nanofibers were prepared by chemical crosslinking of PAN-TA with glutaraldehyde.Finally,CPAN-TA composite nanofibers adsorbent was applied to the adsorption of Cr(Ⅲ)-gelatin complex and its adsorption performance was studied.The effect of TA content on the morphology,structure and chemical properties of CPAN-TA composite nanofibers were investigated.The adsorption capacity and adsorption rate of Cr(Ⅲ)-gelatin on nanofibers were measured.The adsorption behaviors of Cr(Ⅲ)-gelatin on nanofibers adsorbent were studied by adsorption kinetics,isothermal adsorption model and thermodynamic study,and the adsorption mechanism was explored.The results showed that when the mass ratio of PAN to TA was 9:3,the adsorption capacity of CPAN-TA for Cr(Ⅲ)-gelatin was 79.48 mg/g at pH 7.0±L0.1,and the TOC value decreased nearly 80%.The adsorption of trace Cr(Ⅲ)in complex by CPAN-TA compsosite nanofibers showed that initial concentration of 10 or 20 mg/L could be reduced to less than 1.5 mg/L when the dosage of CPAN-TA was 1.0 or 2.0 g/L,showing its effective removal of heavy metal ions with low concentration and stable complexation of organic compounds.The adsorption process toward Cr(Ⅲ)-gelatin by CPAN-TA nanofibers fitted well with the pseudo-second-order model and Freundlich isotherm model.The adsorption mechanism study shows that the adsorption of Cr(Ⅲ)-gelatin in chromium tanning wastewater by CPAN-TA composite nanofibers is mainly through the chelation of phenolic oxygen anion on its surface and the synergistic adsorption of multi-point hydrogen bonds and hydrophobic bonds of tannin to hydrolyzed collagen.In summary,the modified polymer nanofibers with high adsorption performance were prepared on the basis of electrospinning polymer nanofibers and the low-cost natural adsorbents(montmorillonite,chitosan,tannin)with good adsorption performance.The adsorption conditions,behaviors and mechanisms of three kinds of adsorption materials for low concentration Cr(Ⅲ)and Cr(Ⅲ)-gelatin complexes were systematically studied,which has certain theoretical research significance and practical application value. |
