【作者】 肖仕丽；

【导师】 史向阳； 王善元；

【作者基本信息】 东华大学 ， 纺织材料与纺织品设计， 2010， 博士

【摘要】 近年来,纳米技术在许多学科领域都引起了广泛的重视,而纳米材料的制备及其应用更是已经成为当前材料科学研究的热点。对环境修复材料而言,纳米结构材料的高表面积体积比能极大程度的提高修复材料与污染物之间的作用效率。零价纳米铁是近几年来广泛用于环境污染物修复的代表性纳米材料,良好的物理化学性能使其在难降解污染物如氯代有机污染物、重金属污染物以及染料废水等的净化处理方面表现出显著的效果。然而,大量研究发现,零价纳米铁在常规的制备和应用过程中极易团聚,使得纳米铁反应活性减小,从而降低了纳米铁对污染物的去除效率,而且分散在水体中的纳米铁颗粒容易产生二次污染。虽然已有的改进措施在一定程度上改善了纳米铁粒子在水中的分散性能,但寻求一种具有高比表面积的连续介质作为纳米铁粒子的载体材料,开发新型高效、无二次污染的水净化材料仍是我们努力的方向。作为一种能快速制备连续纳米材料的方法,静电纺丝技术被广泛用于各种纳米材料的制备。本文中,我们以静电纺丝技术为基础,以聚电解质高分子材料聚丙烯酸(Polyacrylic acid, PAA)为主体原料,以静电纺纳米纤维毡为模板,利用聚丙烯酸高分子的羧基官能团能络合金属离子的特性,通过原位化学还原的方法制备含有零价纳米铁颗粒的复合纳米纤维毡,开发功能性的废水净化材料。文中重点研究了含铁复合纳米纤维毡的制备过程,在对所制备的复合纳米纤维进行相应表征的基础上,优化制备过程中的工艺参数。同时,分别以印染废水中的代表性染料(酸性品红、甲基蓝和吖啶橙)、电镀废水中的重金属铜离子以及有机氯污染物三氯乙烯为模型污染物对所制备的含铁复合纳米纤维毡的去除水中污染物的性能进行了评价。本文采用自制的静电纺丝装置,以带负电荷的醋酸纤维素(Cellulose acetate,CA)为纺丝原料,制备CA纳米纤维,并以此为模板,结合层层自组装(LbLself-assembly)技术逐次将带正电的聚电解质聚二甲基二烯丙基氯化铵(Poly(diallyldimethylammonium chloride), PDADMAC)和表面带负电荷的聚丙烯酸层层组装在CA纳米纤维的表面,形成PAA/PDADMAC多层膜,以组装了PAA/PDADMAC多层膜的CA纳米纤维为反应器合成含纳米铁颗粒的复合纳米纤维毡。制备纳米反应器的过程中,首先研究了纺丝溶剂、CA的质量分数、流量、接收距离和施加电压对CA纳米纤维成丝性能的影响,优化纺丝工艺得到表面光滑、粗细均匀的CA纳米纤维,平均直径为295±145 nm。以CA纳米纤维为模板,借助于SEM表征,对组装在CA纳米纤维表面的聚电解质多层膜的层数进行优化得出,在CA纳米纤维的表面组装6个双层PAA/PDADMAC能较好的保持纳米纤维的形貌和纳米纤维毡的多孔结构。通过后续的化学还原方法,在组装了6个双层PAA/PDADMAC聚电解质多层膜的CA纳米纤维毡反应器上合成纳米铁颗粒。EDS、TEM以及FTIR测试表明,纳米铁颗粒成功固定在CA纳米纤维表面的聚电解质多层膜中,经过两次铁络合/还原后的纳米铁颗粒平均粒径为1.4 nm,均匀分散在多层膜中。以酸性品红为模型污染物,对含铁纳米纤维毡进行初步性能测试得到,所制备的复合纳米纤维毡能快速的脱除酸性品红的颜色,40min钟内对染料的去除率高达86.8%。对照试验证实,复合纳米纤维毡对污染物的高效降解能力仅与零价纳米铁的粒径大小有关。为简化工艺流程,我们在前期研究的基础上,直接以水溶性的聚丙烯酸高聚物为纺丝原料,制备聚电解质纳米纤维反应器。为制备具有良好表面形貌的PAA纳米纤维,我们系统研究了静电纺丝参数如纺丝液的浓度、电压、流量和接收距离对PAA纳米纤维形貌的影响。实验结果表明,溶液的浓度是影响PAA溶液可纺性能和纤维形貌的关键因素。同时,适当增加喷丝口到接收板的距离有利于溶剂的挥发和纳米纤维的固化。为制备具有良好水稳定性的PAA纳米纤维,我们将PAA高聚物与聚乙烯醇(Polyvinyl alcohol, PVA)的混合溶液进行纺丝后,145℃条件下热交联处理30min,形成不溶于水的PAA/PVA纳米纤维反应器。SEM测试表明,所制备的PAA/PVA纳米纤维表面光滑、粗细均匀,平均直径为170±27 nm。利用PAA上的羧基官能团络合铁盐溶液中的Fe(III)后,通过原位化学还原,将纳米铁颗粒直接固定在PAA/PVA纳米纤维中。SEM测试表明固定了纳米铁颗粒的复合纤维与PAA/PVA纳米纤维的形貌类似,纤维表面光滑,平均直径为205±27 nm,纤维毡仍保持良好的三维多孔结构。通过TEM观察含铁纳米纤维毡的截面发现,纳米铁颗粒均匀地分布在PAA/PVA纳米纤维的截面中,粒径为1.6±0.4 nm。验证含有纳米铁颗粒的PAA/PVA复合纳米纤维毡对模型污染物的脱色效率得到,所制备的纤维毡5 min内能使品红溶液的红色明显变浅,40 min内的脱色效率可达95.8%,而且该纳米纤维毡具有良好的再生性能。电感耦合等离子体测试表明,所制备的含铁纳米纤维毡对净化后的水体不会造成二次污染。以随机取向的方式沉积在接收板上的纳米纤维毡由于单根纤维内在的结合力弱,使得所制备的纤维毡机械强力低,为进一步提高静电纺纳米纤维毡的机械性能,我们将具有优良机械性能的碳纳米管掺杂到静电纺丝溶液中,以制备机械性能增强的含铁复合纳米纤维毡。实验结果得出,根据已经优化好的PAA/PVA纳米纤维的纺丝工艺,在浓度为10 wt%的PAA/PVA混合溶液中,掺入质量分数为1%的多壁碳纳米管(MWCNTs)可以显著地提高PAA/PVA纳米纤维上铁前后的拉伸机械强度和杨氏模量。固定纳米铁粒子之前,在PAA/PVA纳米纤维中添加1 wt%的MWCNTs,纤维毡的断裂拉伸应力从6.21 MPa上升到了10.1 MPa,提高了62.6%。杨氏模量则从88.3 MPa提高到了114 MPa。虽然固定纳米铁颗粒后,纤维毡的整体机械性能都明显下降,但含有MWCNTs的纤维毡的机械性能仍然略微高于不含MWCNTs的纤维毡。表明在静电纺丝溶液中添加碳纳米管是改善静电纺纳米纤维机械性能的有效途径。为全面考察含铁PAA/PVA/MWCNT复合纳米纤维毡对废水中污染物的去除能力,我们重点研究了含铁复合纳米纤维毡对印染废水中的常用染料吖啶橙、甲基蓝的脱色效果,对电镀废水中重金属铜离子的去除,以及对有机氯污染物三氯乙烯的降解效率。研究结果得出：(1)含铁PAA/PVA/MWCNT纳米纤维毡对吖啶橙和甲基蓝均具有快速脱色的效果。反应平衡时,对染料吖啶橙的脱色率为98%,甲基蓝的脱色率为96%,明显高于没有添加碳纳米管的含铁纳米纤维毡。(2)对模拟废水中Cu(Ⅱ)离子去除的系统试验得到,含纳米铁PAA/PVA/MWCNT纳米纤维毡和不含铁的PAA/PVA纳米纤维毡均能去除模拟废水中的Cu(Ⅱ)离子。分析两种纤维毡对Cu(Ⅱ)离子的去除机理发现,含铁纳米纤维毡对Cu(Ⅱ)离子的去除主要是纤维毡中的纳米铁与Cu(Ⅱ)离子置换反应形成铁-铜双金属粒子,而PAA/PVA纳米纤维则主要是PAA上的羧基官能团与Cu(Ⅱ)离子络合形成络合物,从而去除溶液中的Cu(Ⅱ)离子。该结果也进一步通过EDS谱图分析证实。对含铁PAA/PVA/MWCNT和不含铁的PAA/PVA/MWCNT纳米纤维毡的平衡等温模型研究得出,两种纤维毡与溶液中Cu(Ⅱ)离子作用的平衡等温模型均为langmuir模型。同时,其反应动力学也均符合准二级动力学方程。(3)含铁PAA/PVA/MWCNT纳米纤维毡对三氯乙烯模型氯化有机污染物的脱除实验证实,含铁纳米纤维毡也能高效的降解溶液中的三氯乙烯,3.5 h时,其脱除率高达93%。本文的研究为改善零价纳米铁在常规制备和应用过程中易于团聚、容易产生二次污染提供了一条可行的途径,研制了一种新型的废水净化纳米材料。同时,提出了以静电纺纳米纤维为纳米反应器的观点,为利用静电纺丝法制备三维多孔杂化纳米材料提供了借鉴。更多还原

【Abstract】 In recent years, nanotechnology has received much attention in many fields, and the synthesis and application of nanomaterials are the hot spot in the field of materials science in particular. For environmental remediation materials, the unique properties of nanostructured materials such as high surface area to volume ratio can improve the degradation effects of materials to the pollutants. Zero-valent iron nanopaticles (ZVI NPs), a representative nanomaterial used in environmental remediation, have exhibited excellent capacity because of their superior physical and chemical properties in the dechlorination of chlorinated organic contaminants, in the sequestration of toxic metal ions, and in the decoloration of dyes and so on. However, the formed particles are prone to agglomeration during the process of contaminant degradation and their transport process in the subsurface environment. This often leads to a reduced reactivity, which is a critical drawback in the environmental application of ZVI NPs. Although some promising new synthetic methods have been developed to produce more dispersible and stable ZVI NPs, ZVI NPs dispersed in water could cause second pollution. So, immobilizing ZVI NPs onto a continuous medium with a high surface area to volume ratio and good porosity is anticipated to meet the requirements for environmental remediation applications.Electrospinning technology has recently emerged as a straightforward method for synthesizing various polymeric nanofibers and nanostructured materials. In this study, polyacrylic acid (PAA)-containing nanofibers were first fabricated using the electrospinning method. Then, ZVI NPs were synthesized and immobilized in the electrospun polymer nanofibers through in situ reducing of ferric (or ferrous) ions complexed with nanofibrous mats, forming functional nanostructured materials for wastewater treatment. We focused on the study of synthesis, characterization and the processing parameter optimization of ZVI NP-containing composite nanofibrous mats. Meanwhile, we also evaluated the environmental remediation capability of ZVI NP-immobilized composite nanofibrous mats through decoloration of representative dyes in printing and dyeing wastewater (acid fuchsine, methyl blue, and acridine orange), removal of copper ions in the model electroplating wastewater, and degradation of chlorinated organic contaminant trichloroethylene (TCE).Electrospun cellulous acetate (CA) nanofibers were LbL-assembled with multilayers of poly(diallyldimethylammonium chloride) (PDADMAC) and PAA through electrostatic interaction. Then, the PAA/PDADMAC multilayers coated onto the CA nanofibers were used as a nanoreactor to complex Fe(II) ions through the binding with the free carboxyl groups of PAA for subsequent reductive formation and immobilization of ZVI NPs. We studied the effect of electrospinning parameters such as solvent, polymer concentration, flow rate, collection distance, and applied voltage on the formation of CA nanofibers. Smooth and uniform CA nanofibers were produced with a mean diameter of 295±145 nm under the optimized experimental conditions. To render the final formed PE multilayer-coated CA nanofibrous mats with porous structure, the number of PE multilayers deposited onto the CA nanofibrous mat was also optimized based on SEM characterization. Six bilayers of PAA/PDADMAC were chosen to coat onto the CA nanofibers and were used as a nanoreactor. Through the subsequent chemical reduction, ZVI NPs were successfully immobilized onto the (PAA/PDADMAC)6-CA nanofibrous mats. Combined EDS, TEM, and FTIR studies demonstrate that the synthesized ZVI NPs are uniformly distributed into the PE multilayers assembled onto the CA nanofibers with a mean size of 1.4 nm. The ZVI NP-immobilized nanofibrous mats were demonstrated to be able to decolorize an organic dye, acid fuchsin, a model contaminant in dyeing wastewater, and the decoloration percentage could be up to 86.8% within 40 min. The remarkable remediation capability of composite nanofibrous mats is solely attributed to the presence of ZVI NPs (1.4 nm).In order to further simplify the fabrication process, electrospun PAA nanofibers were directly used as a nanoreactor according to our previous study. For generating ultrafine, uniform, and stable PAA nanofibers, we systematically investigated the influence of processing parameters (namely solution concentration, applied voltage, flow rate, and collection distance) on the morphology of PAA nanofibers. We showed that the concentration of PAA solution was crucial to the fiber formation and nanofiber morphology. Meanwhile, longer collection distance was beneficial for the evaporation of solvent and fiber solidation. To retain the nanofibrous structure of PAA, we electrospun PAA and polyvinyl alcohol (PVA) mixture solution to form PAA/PVA nanofibers. Then thermal treatment was introduced to crosslink PAA/PVA nanofibers, resulting in water-stable PAA/PVA nanofibers. SEM results showed that PAA/PVA nanofibers exhibited smooth and uniform morphology with a mean diameter of 170±27 nm. ZVI NPs were directly immobilized into the PAA/PVA nanofibers after chemical reducing ferric ions complexed with PAA/PVA nanofibers. We show that the ZVI NP-immobilized composite nanofibers still retain uniform fibrous structure with a smooth surface similar to the electrospun PAA/PVA nanofibers without ZVI NPs. The mean diameter of composite nanofibers was 205±27 nm. TEM morphological studies show that the ZVI NPs are uniformly distributed in the cross-sections of PAA/PVA nanofibers with a mean size of 1.6 nm. Acid fuchsine decoloration experiment demonstrated that the red color of acid fuchsin solution was obviously decolorized after 5 min exposure of ZVI NP-immobilized composite nanofibrous mats. The decoloration percentage approached 95.8% within 40 min. The composite nanofibrous mats were easy reusable and recyclable. Inductively coupled plasma atomic emission spectroscopy studies showed that no iron was released into the water solution after 1 month exposure of the fibers, thereby avoiding second pollution of ZVI NPs.Nanofibrous mats deposited with randomly orientated nanofibers were always not as strong as desired due to the intrinsic weak mechanical properties of the nanofibers. In order to improve the mechanical properties of composite nanofibrous mats containing ZVI NPs, we doped MWCNTs into the PAA/PVA mixture solution before electrospining. The mechanical property testing suggested that the incorporation of MWCNTs could effectively improve the tensile strength and Young’s modulus before and after immobilizing ZVI NPs. With only 1 wt% MWCNTs incorporation, the tensile strength could be increased up to 10.1 MPa from 6.21 MPa, about 62.6% higher than that of pure PAA/PVA nanofirous mats. In addition, the Young’s modulus of PAA/PVA nanofibrous mats also increased up to 114 MPa from 88.3 MPa. After immobilization of ZVI NPs, the mechanical properties of nanofibrous mats exhibited a decrease compared to their counterparts, but still higher than those ZVI NP-immobilized PAA/PVA nanofibrous mats without MWCNTs, which demonstrated that it was an effective way to improve the mechanical properties of electrospun nanofibrous mats through electrospinning MWCNT-doped polymer solution.To systematically evaluate the environmental remediation capability of MWCNT-reinforced PAA/PVA composite nanofibrous mats containing ZVI NPs, we further investigated the efficiency of the fibers toward the decoloration of commonly used dyes such as acridine orange and methyl blue, the removal of copper ions, and the degradation of TCE. We showed that (1) MWCNT-reinforced PAA/PVA composite nanofibrous mats containing ZVI NPs could decolorize acridine orange and methyl blue rapidly. At the equilibrium state, the decoloration percentage of acridine orange and methyl blue could reach up to 98% and 96% respectively, obviously higher than that of ZVI NP-immobilized PAA/PVA nanofibrous mats without MWCNTs; (2) MWCNT-reinforced PAA/PVA nanofibrous mats with and without ZVI NPs could effectively remove the copper ions in the model electroplated wastewater. Mechanistic analysis showed that copper ions was removed via reduction by ZVI NPs to form Fe-Cu bimetal nanoparticles in the presence of ZVI NP-containing composite nanofibrous mats. While the removal of copper ions by PAA/PVA nanofibrous mats without ZVI NPs was mainly due to the complexation of free carboxyl group of PAA. Equilibrium isotherm investigation indicated that the Langmuir model was appropriate to describe the interaction between MWCNT-reinforced nanofibrous mats (with and without ZVI NPs) and copper ions. Meanwhile, the kinetics of copper ion removal was found to follow a pseudo second-order rate behavior for MWCNT-reinforced nanofibrous mats with and without ZVI NPs; (3) likewise, the same nanofibrous mats could effectively remove a chlorinated contaminant of TCE with a removal efficiency approaching 93% within 3.5h.In summary, we report a facile approach to synthesizing and immobilizing ZVI NPs onto or into nanofibrous mats, preventing the agglomeration and second pollution of the ZVI NPs. Meanwhile, the concept of using electrospun nanofibers as nanoreactors might open a new avenue in the fabrication of various three-dimensional porous nanostructured hybrid materials for various applications.更多还原