壳聚糖稳定纳米铁的制备与修复地表水中六价铬污染的研究

【作者】 耿兵；

【导师】 金朝晖；

【作者基本信息】 南开大学 ， 环境科学， 2009， 博士

【摘要】 地表水是人类的重要资源,在人们的生产和生活中具有不可替代的作用。但是,随着工业和经济的快速发展,一些工厂企业不合理排放含铬废水,导致大量铬尤其是Cr（Ⅵ）进入地表水中,引起严重的污染。Cr（Ⅵ）以其较大的毒害作用引发了一系列负面效应,影响了周围动物,植物以及人类的健康。因此,解决地表水中Cr（Ⅵ）的污染问题成为了环境工作者关注的焦点。近几年的研究表明,零价铁材料尤其是纳米级零价铁材料由于具有高的反应活性,可修复多种环境污染物等特点,成为目前环境污染修复技术中一个非常活跃的研究领域。但是在实际应用中纳米铁材料仍然存在一些问题需要解决,如纳米铁合成条件不易控制,极易团聚,空气稳定性差。这些因素都对纳米铁材料的制备和使用提出了严峻的挑战。针对以上问题,本论文在天津市应用基础及前沿技术研究计划（天津市自然科学基金）的资助下,对纳米铁制备的液相还原法进行改进,制备出壳聚糖稳定的纳米铁材料并用于地表水Cr（Ⅵ）污染的修复。论文研究了壳聚糖稳定纳米铁的稳定性,在静态条件和动态条件下考察了壳聚糖稳定纳米铁对地表水中Cr（Ⅵ）的去除能力,探讨了壳聚糖稳定纳米铁去除Cr（Ⅵ）的动力学和机理。论文的主要研究内容分为以下四部分:1.通过优化制备条件得到了壳聚糖稳定纳米铁制备的最佳方法为:将分子量为100000的壳聚糖溶解于0.05 mol/L HNO₃溶液中,得到质量百分含量为0.5%的壳聚糖硝酸溶液。使用前通过0.22μm的微孔滤膜过滤去除不溶部分,然后向装有3 ml上述壳聚糖溶液的三口烧瓶中加入10 ml含有0.2978 gFeSO₄·7H₂O的水溶液,加入去离子水使体系的总体积为15 mL,用真空线向溶液中通入高纯氮气除去体系中的氧（通气时间为30min）,并在整个合成的过程中持续向体系中通入氮气保持无氧环境,然后再搅拌10分钟使之充分混合均匀。同时将10 mL新配制的含0.2889 g KBH₄的水溶液从恒压漏斗中逐滴加入三口烧瓶中,滴速控制约为2滴/秒,在20℃下反应90分钟,生成黑色的纳米铁颗粒。反应结束后,用磁选法分离出纳米铁粒子,再用脱氧去离子水洗涤三次,每次用水150ml,得到壳聚糖稳定纳米铁复合粒子。2.壳聚糖稳定纳米铁的表征分析结果显示,壳聚糖稳定纳米铁粒径的分布范围为20-150 nm,平均粒径为82.4 nm,且具有一定的抗氧化性能。与普通纳米铁相比,壳聚糖稳定纳米铁呈现了很好的分散状态。3.采用批实验和柱实验研究了壳聚糖稳定纳米铁对地表水中Cr（Ⅵ）的去除能力。壳聚糖稳定纳米铁对水中Cr（Ⅵ）的去除能力高于200目铁粉和普通纳米铁,每克壳聚糖稳定纳米铁可以去除铬148.08 mg。地表水中的Ca²⁺、Mg²⁺、CO₃^2-、HCO₃^-、有机物和溶解氧等因素都对Cr（Ⅵ）的去除产生影响,但是壳聚糖稳定纳米铁仍然显示出纳米材料所特有的优势。4.壳聚糖稳定纳米铁对Cr（Ⅵ）的去除是基于吸附和还原的双重作用。去除Cr（Ⅵ）反应的表观一级反应动力学常数随Cr（Ⅵ）初始浓度的升高而降低。增加Cr（Ⅵ）的初始浓度促进了表面钝化层的形成降低零价铁的腐蚀速率,从而降低了Cr（Ⅵ）的还原速率。壳聚糖稳定纳米铁对Cr（Ⅵ）的吸附量与Cr（Ⅵ）的初始浓度成正比。反应的表观活化能为33 kJ mol^-1,证明壳聚糖稳定纳米铁去除Cr（Ⅵ）的反应是由化学反应所控制的,而非物理过程控制的。壳聚糖分子鳌合了产物中的Fe（Ⅲ）,使得样品表面Fe（Ⅲ）的含量增加,这也阻碍了纳米铁表面氢氧化物钝化层的形成,促进Cr（Ⅵ）的还原和去除。更多还原

【Abstract】 Surface water is the most important water source for human beings.However, with the development of industry and economy,Cr（Ⅵ） is widely detected in surface water.Cr（Ⅵ） anions,including chromate(CrO₄^2-) and dichromate(Cr₂O₇^2-),are highly soluble in aquatic systems and are severe contaminants to environment.Much concern has been paid on the technology for Cr（Ⅵ） contamination remediation.Recently,due to large specific surface area and more active sites the use of zero valent iron（Fe⁰）,especially Fe⁰ nanoparticles,as reactive media for in situ subsurface environment remediation has been extensively investigated.Applications to in situ remediation require the Fe⁰ nanoparticles to be stable in water.However,due to van der Waals forces and magnetic interactions,these Fe⁰ nanoparticles tend to agglomerate and grow rapidly to micrometer or millimeter scale particles,thereby diminishing their mobility and chemical reactivity.On the other hand,Fe⁰ nanoparticles show high activity with large specific surface area,but are easily oxidized by air or ignite spontaneously when exposed to air.Therefore,intensive efforts have been made to coat and protect Fe⁰ nanoparticles from agglomeration and air oxidation.This research work was supported in part by Tianjin Natural Science Foundation of China under grant No.07JCZDJC01800.The objectives of this research are to:（1） to prepare chitosan-Fe⁰（chitosan-Fe⁰） by modification of solution method.（2） to evaluate the capacity of chitosan-Fe⁰ to remove Cr（Ⅵ） from surface water through batch experiment and column experiment.（3） to investigate the kinetics and mechanisms of Cr（Ⅵ） removal from water by chitosan-Fe⁰.There are four main parts in this research work:1.The condition for preparation of chitosan-Fe⁰ was optimized:chitosan was dissolved in 0.05 mol/L HNO₃ to make the final concentration of 0.5%by weight. Finally,chitosan solution was filtered through 0.22μm syringe filters to remove any suspensions.Chitosan-Fe⁰ was prepared in situ by reducing Fe（Ⅱ） with KBH₄ in the presence of chitosan as a stabilizer.To ensure all the Fe（Ⅱ） were reduced,excess of KBH₄ over the Fe（Ⅱ） was used.The detailed procedure was as follows:10 mL of solution containing 0.2978 g of FeSO₄·7H₂O was first mixed with 3 mL of 0.5% chitosan solution.The mixture was stirred for 30 min under nitrogen gas.Then,to the mixture,10 ml of freshly prepared aqueous solution containing 0.2889 g of KBH₄ was added dropwise.At this stage,gas was evolved vigorously and black precipitation was formed.Again,the mixture was stirred for another 90 min.The resulted black precipitate was collected and washed by deoxygenated water for three times to get rid of the excess chemicals.2.The results of characterization indicated that the distribution of particle size of chitosan-Fe⁰ is widespread ranging from 20 to 150 nm with a mean diameter of 82.4 nm and has good stability against oxidation.Compared with EW-Fe⁰,the chitosan-Fe⁰ can stay stable in water.3.The capacity of chitosan-Fe⁰ to remove Cr（Ⅵ） from surface water was examined through batch experiment and column experiment.The ability of chitosan-Fe⁰ to remove Cr（Ⅵ） was far greater than that of EW-Fe⁰ and 200 mesh iron powder with a Cr（Ⅵ） removal capacity at approximately 148.08 mg Cr（Ⅵ） per gram of Fe⁰ nanoparticles.Ca²⁺、Mg²⁺、CO₃^2-、HCO₃^-、organic matters and dissolved oxygen （DO） in the surface water have negative effect on the Cr（Ⅵ） removal capacity of chitosan-Fe⁰.However,chitosan-Fe⁰ still show the superiority over bulk material.4.Due to the adsorption and reduction,chitosan-Fe⁰ can remove Cr（Ⅵ） from water rapidly.The rate of reduction of Cr（Ⅵ） to Cr（Ⅲ） can be expressed by a pseudo-first-order reaction kinetics.The rate constants increase with the increase in temperature and iron loading but decrease with the increase in initial Cr（Ⅵ） concentration and pH.It is found that the amount of Cr（Ⅵ） been adsorbed is proportional to the aqueous Cr（Ⅵ） concentration.The apparent activation energy is found to be 33 kJ mol^-1,which is characteristic of a chemically controlled reaction. Chitosan can hinder the formation of Fe（Ⅲ）-Cr（Ⅲ） precipitate by virtue of its coordinating capability consequently enhances the reduction of Cr（Ⅵ） to Cr（Ⅲ）.更多还原