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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
李蕾
学位类型博士
导师杨传芳
2016-07
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工艺
关键词壳聚糖 静电纺丝 纳米纤维 六价铬 吸附
摘要

近年来,中国六价铬污染事件时有发生,造成周围水体污染,严重影响附近居民的饮水安全和身体健康,引起了全国范围内的恐慌,也使铬污染问题得到公众的普遍关注。六价铬离子是一种危害性非常大的重金属离子,被美国国家环境保护局(EPA)归类为A类致癌物,因此研制其有效去除的新技术与方法具有重要的意义。壳聚糖是仅次于纤维素的第二大天然高分子材料,分子中含有大量的氨基,对六价铬离子具有很好的吸附作用。同时,壳聚糖是一种优异的绿色环境友好型材料,本身无毒、抗菌、可降解,且不会造成二次污染。然而,壳聚糖原材料作为吸附剂,比表面积非常小,吸附能力有限,大大制约了其高效利用。将壳聚糖纳米纤维化,可有效提高其比表面积,增强吸附效果。本论文以此为出发点,选用静电纺丝技术来制备壳聚糖纳米纤维,并研究其对六价铬离子的吸附去除效果和吸附机理。同时,针对壳聚糖本身机械强度较低、电纺丝纳米纤维膜堆积过于紧密的问题,制备了壳聚糖纳米纤维/聚对苯二甲酸乙二醇酯(PET)复合材料,并研究了其对六价铬离子的动态吸附行为。进一步地,制备了壳聚糖纳米纤维复合膜的卷式膜组件,尝试了纳米纤维膜在含铬水处理中应用的可行性,并与纳滤膜分离进行了比较。针对该课题,主要取得了如下研究成果:(1)采用醋酸为溶剂,利用静电纺丝技术,成功制备了平均直径为75nm的壳聚糖纳米纤维。详细研究了溶液参数、过程参数和环境参数对纺丝过程和纤维形貌的影响,得到了制备壳聚糖纳米纤维的最佳纺丝条件,即采用90wt%的醋酸为溶剂,控制壳聚糖溶液浓度为5wt%,纺丝电压23kV,供液速度0.1mm/min,接收距离6.8cm。特别地,发现了湿度在壳聚糖静电纺丝过程中的重要作用。只有相对湿度低于30%时,才能得到直径均匀、表面光滑的壳聚糖纳米纤维。采用戊二醛蒸汽对制备的壳聚糖纳米纤维进行了交联,成功地提高了其在水中,尤其是在酸性溶液中的稳定性。(2)利用静电纺丝壳聚糖纳米纤维毡膜对六价铬离子进行了静态吸附,其吸附量较其原始壳聚糖粉末提高了2.5倍。详细研究了pH对吸附的影响,研究了吸附动力学、吸附等温线、共存离子对六价铬吸附选择性的影响以及吸附机理。研究结果表明,当pH为3时,吸附效果最好。壳聚糖纳米纤维对六价铬离子的吸附动力学符合拟二级反应,吸附等温线既符合Langmuir吸附模型又符合Freundlich吸附模型。吸附完成后,壳聚糖纳米纤维基本保持了初始的形貌。除了SO42-离子外,其他过量离子的存在,均不明显影响壳聚糖纳米纤维对六价铬离子的吸附。X射线光电子能谱分析(XPS)结果表明,在吸附过程中,壳聚糖上的氨基和羟基同时参与了吸附过程。推测了壳聚糖吸附六价铬离子的吸附机理:除质子化氨基对六价铬的静电吸引作用外,吸附过程还伴随着六价铬的还原反应,还原成的三价铬通过静电吸引作用吸附在壳聚糖纳米纤维表面。(3)电纺壳聚糖纳米纤维得到的是纤维毡膜,纤维的多层紧密叠加会造成部分比表面积的损失,并影响溶质向毡膜内孔的扩散传质。此外,壳聚糖纳米纤维毡膜的机械强度较低。针对以上问题,将壳聚糖纳米纤维电纺在PET无纺布基底上,制备了孔径可控的壳聚糖纳米纤维/PET复合膜。通过动态过滤吸附的方法来处理低浓度的含铬废水,提高了壳聚糖纳米纤维的利用效率。实验结果表明材料的动态吸附效果不受溶液初始浓度和复合材料层数的影响,而受溶液的pH、流速和膜的排列方式影响。复合膜的吸附能力和利用效率随壳聚糖纳米纤维的沉积密度的增加有较大幅度的提高。通过对比发现,复合膜的动态吸附效果优于静态吸附,该复合膜对六价铬是一种很好的化学过滤吸附材料。对不同初始浓度、流速和膜厚度下的数据进行了数学模型拟合,发现所有的数据都符合Adams-Bohart模型和Dose-Response模型。通过Bed depth service time(BDST)模型拟合发现,复合膜的层数仅为三层时就可以抵抗吸附床的快速穿透,表明了壳聚糖纳米纤维复合膜对六价铬离子具有高效的去除作用。 (4)进一步地,将壳聚糖纳米纤维/PET复合膜设计制备成卷式膜组件,用于低浓度含铬水溶液的净化处理,尝试了壳聚糖纳米纤维材料在水处理中的实际应用。详细研究了流速、初始浓度、壳聚糖纳米纤维沉积密度和其他重金属离子对卷式膜组件吸附效果的影响。研究表明流速对膜组件的吸附效果影响很大,低流速有利于卷式膜组件吸附量的提高。膜组件的动态吸附能力只与流过膜组件的六价铬离子的总量有关,而与初始浓度的大小无关。膜组件中壳聚糖纳米纤维的沉积密度越大、复合膜的孔径越小,越有利于吸附反应的发生。纳米纤维沉积密度为2g/m2的膜组件在10%穿透时的吸附量高达20.47mg/g。研究还发现,卷式膜组件对铬、铜、镉和铅离子都有较好的吸附效果,可有效去除水中存在的多种重金属离子。膜组件经过两次脱附再生之后,其穿透曲线与原始曲线几乎完全重合,表明具有很好的再生效果,重复利用性较好。同时,研究了纳滤膜对六价铬离子的截留效果,发现对镁离子有很高截留率的纳滤膜对六价铬离子的截留率低于15%,表明壳聚糖纳米纤维膜较纳滤膜在含低浓度铬的水处理方面具有更大的优势。

其他摘要

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.

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/22891
专题研究所(批量导入)
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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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