Knowledge Management System Of Institute of process engineering,CAS
|关键词||壳聚糖 静电纺丝 纳米纤维 六价铬 吸附|
Recent years have frequently witnessed the severe contamination of hexavalent chromium in China, which leads to the pollution of water around, and poses serious impact on drinking water safety and body health of the surrounding residents. Chromium (VI) pollution has brought about a nationwide panic, and thus gained widespread concerns from the public. Hexavalent chromium is a toxic contaminant classified as a Group ‘A’ human carcinogen by the U.S. Environment Protection Agency (EPA) because of its mutagenicity, carcinogenicity and teratogenicity to human body. Therefore, developing new and effective technologies and methods for hexavalent chromium ion removal is of great significance. Chitosan is the second most plentiful biopolymer after cellulose on earth, containing large numbers of amino groups and having demonstrated outstanding removal capabilities for Cr (VI) ions. At the same time, chitosan is an excellent green and environmentally friendly material. It is non-toxic, anti-bacteria and biodegradable in nature, and has no tendency of generating secondary pollution. However, chitosan has low specific surface area in the form of powders or flakes, which limits its use as an adsorbent. Changing the form of chitosan from powders and flakes to nanosized fibers is expected to greatly increased chitosan’s specific surface area and hence improve its adsorption capacity. Therefore, in this work, we applied electrospinning technology to prepare chitosan nanofibers, and studied their adsorptive effectiveness and adsorption mechanism toward the removal of Cr (VI) ions. Meanwhile, in order to cope with the problems of low mechanical strength of chitosan itself and the unintended adsorption capacity loss due to the tightly packed nanofiber that hinder solute infiltration, chitosan nanofiber/polyethylene terephthalate (PET) composite membranes were prepared and carefully investigated for their dynamic adsorption behavior for Cr (VI) removal. Further, a spiral wound membrane module of chitosan nanofiber composite membranes was constructed, and the feasibility of using this module for treating Cr (VI) contaminated water was tested, and compared with nanofiltration membranes.The research achievements in this dissertation are summarized as follows:(1) Homogenous chitosan nanofibers with an average diameter of 75 nm were successfully produced by electrospinning using acetic acid as the spinning solvent. Parameters including solution variables, processing variables and environment variables were carefully inverestaged to reveal their effects on the spinning process and the morphology of the formed nanofibers. An optimized operation condition was obtained, in which the solvent was determined to be 90 wt% acetic acid, the chitosan concentration was fixed at 5 wt%, the spinning voltage was 23 kV, the feed rate was 0.1 mm/min, and the collection distance 6.8cm, respectively. In particular, humidity was found to be of critical importance in the electrospinning process. Homogenous chitosan nanofibers could only be obtained when the environment humidity was less than 30%. The as-prepared chitosan nanofibers were cross-linked by glutaraldehyde vapor to successfully enhance their stability in water, especially under acidic conditions.(2) The electrospun chitosan nanofibers were used to adsorb Cr (VI) from water directly. The maximum adsorption capacity of Cr (VI) from water using the nanofibers was more than doubled compared with the chitosan powders used as the raw material for electrospinning. The effect of contact time, pH, and co-ions on the adsorption was studied, the adsorption kinetics and isotherms were also determined. The highest removal occurred at pH 3. The pseudo-second order kinetic model provided the best correlation of the experimental data. Adsorption on the electrospun chitosan nanofibers followed both Langmuir and Freundlich isotherm models. After adsorption, the nanofibers kept their original morphology. The presence of excess sulfate ions influenced Cr (VI) adsorption more obviously while other co-ions tested did not. The result of X-ray photoelectron spectroscopy (XPS) indicated that both amino and hydroxyl groups were involved in the adsorption of Cr (VI) ions on the nanofibers. The main mechanism of Cr (VI) adsorption on electrospun chitosan nanofibers is electrostatic attraction and oxidation-reduction reaction. (3) It was discovered that the obtained nanofibers formed nanofibrous mat/membrane with low mechanical strength and tightly overlapped nanofibers that caused unavoidable fiber surface area loss and difficulties of solute infiltration for diffusion mass transfer. In order to cope with such problems, composite chitosan nanofiber membranes with different fiber deposition density, therefore controlled pore size, on polyester scrim were fabricated by electrospinning. These nanofiber membranes were used as a packed bed to adsorb small concentration Cr (VI) dynamically by filtration and showed increased utilization efficiency of the nanofibers. While the performance of the nanofiber membranes including adsorption capacity and bed utilization efficiency was pH, flow rate and membrane packing pattern dependent, it was less sensitive to feed Cr (VI) concentration and bed length change. However, the bed adsorption capacity and bed utilization efficiency can be greatly improved by increasing nanofiber deposition density of the composite membrane. The maximum adsorption capacity achieved by the composite nanofiber membranes exceeded the saturation capacity of static adsorption using chitosan nanofiber mats. The result indicated that the nanofiber membranes were effective for adsorptive filtration of Cr (VI) in a single-pass flow as adsorptive membranes, a promising outcome for applying such membranes for water decontamination. The bed breakthrough data obtained at different feed Cr (VI) concentration, flow rate and bed length were well fitted by Adams-Bohart model for the initial stage of sorption and Dose-Response model for large time sorption in general. By applying bed depth service time (BDST) model, it was found that a short bed length (less than 3 layers) was sufficient to avoid breakthrough, indicating the high efficiency of the chitosan nanofiber bed for adsorptive filtration of Cr (VI) ions. (4) A spiral wound membrane module of chitosan nanofiber composite membranes was fabricated, and the feasibility of this module for treating Cr (VI) contaminated water was tested. The effect of flow rate, initial Cr (VI) concentration, chitosan nanofiber depositon density, and other heavy metal ionson the adsorption was investigated in detail. It was found that the loading capacity of the module was dependent on flow rate and nanofiber deposition density, but independent on initial Cr (VI) concentration. Lower flow rate lead to higher adsorption capacity. The maximum adsorption capacity obtained using the module containing 2 g/m2 nanofiber membranes at 10% breakthrough was 20.47 mg/g. The module also exhibited high adsorption capacity for Cr (VI), Cu (II), Cd (II) and Pb (II) ions. The module was easy to regenerate for repeated use and the breakthroughcurves almost overlapped for the two cycles studied. Furthermore, separation of Cr (VI) ions by nanofiltration membranes were tested for the purpose of comparision with the nanofiber membranes. It was shown that the nanofiltration membranes having high magnesium ion rejection only intercepted Cr (VI) ions by less than 15%. Therefore, it was concluded that the spiral wound chitosan nanofiber membranes were advantageous over such commercial nanofiltration membranes in treating water contaminated by small concentration Cr (VI) ions.
|李蕾. 静电纺丝壳聚糖纳米纤维膜的制备及对六价铬离子吸附的研究[D]. 北京. 中国科学院研究生院,2016.|
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