Optimized Fabrication And Application Of Solution-Blown Micro/Nano Fibers In Heavy Metal Ions Adsorption And Removal

With the fast development of industries and economic society, heavy metal pollutions in wastewater have posed a serious threat to ecological systems and public health because of their high toxic, non-biodegradable and carcinogenic properties even at very low concentrations, and the pollution of wastewater from the contamination of heavy metals has become one of serious environmental problems in China and the whole world. To obtain clean water resources and non-threatening wastewater, heavy metal ions are commonly removed by chemical precipitation, ion exchange, solvent extraction, chemical oxidation-reduction and adsorption. Among these methods, adsorption is one of the most economical, effective and widely used techniques due to the great advantage of convenient operation, low-energy consumption, low generation of residues, high capacity and reusability. Efficiencies of conventional adsorbents such as resins, foams and conventional fibers are usually limited by the lower specific surface area or fewer active sites and the lack of selectivity and so on. Compared to the conventional adsorbents, polymer nanofiber materials offer significantly improved adsorption efficiency because of their extremely high specific surface area and associated sorption sites, short intraparticle diffusion distance, tunable pore size, and easy to physical and chemical modification.Nowadays, new advances in electrospinning and other nanofiber production methods show a clear trend of interdisciplinary technological convergence. Recent efforts to improve the quality and performance of the nanofibres, increase productivity, reduce the cost of electrospinning and other methods involve merging key concepts of conventional textile fiber manufacture processes with nanotechnology. Among the emerging techniques, solution blowing has attracted significant attention on account of its versatility as well as economic competitiveness. In solution blowing, the polymer solution jet is issued into coaxial high speed gas jet, and then attenuated by the aerodynamic force with bending instability as well as solvent evaporation. It combines the superiority of electrospinning fabricating fibers with diameters from a few micrometers down to hundreds of nanometers and the capability of melt blowing generating microfibers on a commercial production scale. Therefore, it has a strong practical significance to prepare and optimize the solution-blown micro/nano fibers and explore their application in adsorption and removal of heavy metal ions in aqueous solution.This paper first reviews the traditional fabrication methods and formation theory of micro/nano fibers, emphatically expounds the research status of preparation, application and theory of solution-blown micro/nano fibers at home and abroad, and then summarizes the research progress in adsorption and removal of heavy metal ions using micro/nano fibers. Through optimizing the preparation of solution-blown micro/nano fibers and exploring their application in adsorption and removal of heavy metal ions in aqueous solution, this topic aims to provide a more effective method and adsorbent material for heavy metal wastewater treatment. The research content and main conclusions of this work are listed as follows:(1) The response surface methodology, based on the four-factor, three-level Box-Behnken design, was utilized to facilitate a more systematic understanding of the solution and processing parameters of solution-blown polyethylene oxide (PEO) micro/nano fibers. The factors investigated include air pressure, solution concentration, nozzle diameter, and injection rate; and the average fiber diameter of solution-blown PEO was specified as the response value. A statistical analysis of variance (ANOVA) was implemented to evaluate the fitted full quadratic polynomial model, the backward model reduction method was utilized and insignificant effects (p>0.05) were removed from the model. The results indicate that solution-blown parameters like air pressure, solution concentration, nozzle diameter, injection rate and the interactive effect between pressure and injection rate have significant effects on the average fiber diameter. The ranges of process parameter for preparing smaller and uniform diameter solution-blown PEO fibers were determined by the3D response surface graphics. The adequacy and efficiency of the refined response surface model was verified successfully by other three independent experiments. Based on the model and the response surface analysis, the processing parameters of optimization and preparation of PEO fibers were determined; under the optimal conditions, the average diameter of fibers are basically identical with the predicted values.(2) The turbulent airflow field of a solution-blowing annular jet at the bottom of various spinning nozzle was numerically investigated using the computational fluid dynamic (CFD) approach; connected with practical experiments, the suitable and efficient configuration of the nozzles was selected. The contours of the velocity field below nozzles displays that the two parallel jets merge together and combine to form a single free jet at a certain position below the nozzle face. In addition, the velocity decreases down the spinning line and severely decays at large distances. The locally amplified velocity vector field shows that two recirculation zones adjacent to the nozzle plane emerge in a triangular space between the two converging jets. The numerical analysis results of different nozzle flow fields and the observation of practical solution-blowing processes indicate that the retracted and flush nozzles result in intermittent processes with polymer solution blocking the nozzle end, whereas the protruded nozzles especially nozzle A, has the lowest integral value of the turbulent intensity and is capable of producing fibers without such deficiencies.(3) The velocity and turbulent intensity of solution-blowing annular jet under various inlet absolute air pressures were investigated numerically, and the flapping motions of solution-blown polyacrylonitrile (PAN) solution jet under the different air pressures were captured with a high-speed camera. The simulation results show that the centerline velocity and turbulent intensity increase with the increase of air pressure. The flapping motion in the x-y plane shows similar trends at different air pressures, the frequency of flapping at a certain position increases with increasing air pressure, but the amplitude at a certain position first increases and then decreases; under the same air pressure, the amplitude of the flapping motion grows with increasing distance from the nozzle. And then fiber morphology was correlated with the physical quantities of the airflow field and the motion of polymer solution jet. The results demonstrate that the relative velocity between the air jet and polymer jet, length of the straight segment of the polymer solution jet, velocity fluctuations, flapping motion of polymer jet, and solvent evaporation all impact the final fiber morphology in solution blowing. The fiber morphology of solution-blown PAN varies obviously as the air pressure increases. At lower pressure, the fiber diameters become more uniform and the average diameter decreases with increasing air pressure, while further increasing the air pressure, the fiber diameters reduce slightly and become uneven with the advent of some fiber strands.(4) Based on investigating the adsorption performance of Cd(II), Cr(III), Cu(II),Ni(H), Pb(II) and Zn(II) onto ordinary polypropylene (PP) nonwovens, amidoxime modified PP nonwovens and solution-blown PAN micro/nano fiber membranes, the solution-blown PAN micro/nano fiber membranes were chemically modified with hydroxylamine hydrochloride under certain conditions to synthesize amidoxime-functionalized PAN (APAN) micro/nano fiber membranes. Compared with the former three kinds of materials, APAN micro/nano fiber membranes have advantages in adsorption types, adsorption capacities and adsorption rates of heavy metal ions. Moreover, the total adsorption capacities of the six metals in multi-metal system onto APAN micro/nano fiber membranes are higher than that of the single-metal system of Cd(II), Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) in aqueous solution. In multi-metal systems, when the target metal ion concentrations increase or the initial concentrations of all the heavy metal ions are kept at the same time, the adsorption capacities of APAN micro/nano fiber membranes for each kind of heavy metal ions in multi-metal systems are less than that in single-metal systems. With the increase of initial concentrations, the adsorption of Cd(II), Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) onto APAN micro/nano fiber membranes in multi-metal aqueous solution system shows obvious antagonistic competition effect. The calculation results of distribution coefficients and selectivity coefficients indicate that the distribution coefficients of Pb(Ⅱ) and Cr(Ⅲ) are higher than that of Cd(Ⅱ), Cu(Ⅱ), Ni(Ⅱ) and Zn(Ⅱ), which suggests that APAN micro/nano fiber membranes have higher affinity and selectivity towards Pb(Ⅱ) and Cr(Ⅲ). The competitive adsorption of heavy metal ion in multi-metal systems onto APAN micro/nano fiber membranes shows that the process is very complicated, which includes surface complexation, antagonism competition and "replacement" reaction. The diversity and selectivity adsorption of heavy metal ions are mainly related to the stability constant of heavy metal ions and the functional groups on the surface of the amidoxime-functionalized solution-blown PAN micro/nano fibers and its formation of micromechanism.(5) Further studies were implemented to investigated adsorption performance of APAN micro/nano fibers for the single-metal system of Cd(II), Cr(Ⅲ), Cu(Ⅱ), Ni(Ⅱ), Pb(Ⅱ) and Zn(Ⅱ) in aqueous solution. Batch experiments and quantitative analysis show that pH values, contact time, initial concentrations and temperatures all influence the adsorption processes. The optimal initial pH value was indentified as6.0, and the absorption equilibrium time was selected at8h; adsorption capacities of APAN fibers increase with the increase of initial concentrations and temperatures, and the higher temperatures favore the adsorption reaction. According to the Langmuir、Freundlich and Dubinin-Radushkevich models, the adsorption equilibrium data are better explained by the Langmuir model with the adsorption capacities being followed the descending order:Pb(Ⅱ)>Cr(Ⅲ)>Cd(Ⅱ)>Ni(Ⅱ)>Cu(Ⅱ)>Zn(Ⅱ). Based on pseudo-first-order kinetic and pseudo-second-order kinetic models, the kinetic data demonstrate that the adsorption processes fit the pseudo-second-order kinetic model, indicating that the chemical sorption is the rate-limiting step. The calculated thermodynamic parameters indicate that adsorption of metal ions onto APAN fibers is feasible, spontaneous and endothermic. Desorption and reusability of APAN micro/nano fibers were determined by five adsorption-desorption cycles, the obtained results exhibit that APAN micro/nano fiber adsorbent holds good reusability, and has a potential application for removal of heavy metals in wastewater. The adsorption of heavy metal ions onto APAN micro/nano fibers membranes is a complex and combined process, which predominantly consists of chemical chelation, also contains physical adsorption, hydrogen bonding and electrostatic attraction.This thesis optimized the fabrication of solution-blown micro/nano fibers via response surface analysis, numerical simulation with the combination of high-speed photography, which provides experimental and theoretical basis for the controllable preparation of solution-blown micro/nano fibers. And then the competitive adsorption in multi-metal system and adsorption performance in single-metal system of Cd(II), Cr(III), Cu(II), Ni(II), Pb(II) and Zn(II) in aqueous solution onto the amidoxime-functionalized solution-blown PAN micro/nano fibers were further studied, which offers a more effective method and adsorbent material for adsorption and removal of heavy metal ions in wastewater, has important scientific significance and application value for heavy metal wastewater treatment, and also broadens the application fields of solution-blown micro/nano fibers.