【作者】 李勇超；

【导师】 金朝晖；

【作者基本信息】 南开大学 ， 环境科学， 2012， 博士

【摘要】 铬及其化合物是金属加工、冶金工业、制革、电镀、油漆、印染、制药、照相制版等行业必不可少的原料。由于不能对这些行业产生的含铬废水进行有效的控制和不合理排放，地下水常存在着严重的铬污染现象。Cr（Ⅲ）和Cr（Ⅵ）是铬的两种主要形态。其中，Cr（Ⅵ）的溶解度和迁移性较大，且具有致癌、致畸和致突变作用。相对来说，Cr（Ⅲ）的毒性较小并且是人体内的一种必需元素，其易发生沉淀、迁移性较弱。因此，将Cr（Ⅵ）还原为Cr（III）是一种重要的Cr（VI）污染控制技术。纳米零价铁由于粒径小、比表面积大、还原性强，成为一种新型污染控制技术。但是纳米铁颗粒极易团聚，迁移能力差，不易到达污染地区，从而降低了反应活性即去除污染物的能力。此外，纳米铁颗粒在空气中容易氧化，也为它的使用和运输带来不便。最常见的增强纳米铁稳定性的方法是对纳米铁表面进行包覆或者将其负载在载体上，这是因为包覆层和载体可以提高纳米铁颗粒之间的空间位阻和静电排斥力，从而防止纳米铁颗粒团聚。但是常规的纳米铁载体价格较高，不适应大规模生产，而包覆层材料的选择会影响到纳米铁的活性、稳定性、成本及二次污染。针对上述问题，本文选用合适的载体和包覆剂制备了功能化核壳型纳米铁复合材料—硅微粉负载型纳米铁（SF-Fe）和二氧化硅包覆型纳米铁（Fe@SiO₂），既保证纳米铁的活性、稳定性，又能在地下含水层具有较强的迁移能力。论文的主要研究内容有以下四部分：（1）用经济、无害的固体废弃物硅微粉作载体，通过KBH₄与FeCl3反应的简单液相还原法制备SF-Fe。采用XRD、TEM、SEM、FTIR等对SF-Fe的形貌与结构进行表征，并将其作还原剂用于地下水中Cr（VI）的去除。考察了硅微粉投加量对SF-Fe去除Cr（VI）效果的影响。结果表明：通过TEM分析，SF-Fe为不完全核壳型复合材料，载体硅微粉为复合材料的内核，球形纳米铁为外壳，均匀的分布在硅微粉的表面。纳米铁粒径分布为20¹10nm，整个微球粒径为0.15⁰.45μm。硅微粉表面的羟基对稳定纳米铁起到了关键作用。当硅微粉投加量为84wt%时，所制备的SF-Fe去除Cr（VI）效果最佳。与未负载纳米铁相比，SF-Fe对Cr（VI）的去除率升高了22.55%。（2）利用液相还原与改进的St ber法相结合，通过向原硅酸乙酯（TEOS）和氯化铁混合溶液直接添加硼氢化钾，一步合成了Fe@SiO₂复合材料。本制备方法无需表面改性剂和刺激性氨水，无需改变反应体系，简便实用。通过XRD、EDAX、TEM、UV-Vis、FTIR、XPS等对所得样品的形貌、结构和组成进行表征。将制备的Fe@SiO₂用于水体中Cr（Ⅵ）还原去除。XRD和EDAX分析证实了，KBH₄促进了TEOS发生水解缩合反应，生成了无定形的SiO₂。TEM分析表明：KBH₄滴加速度和TEOS的投加量对复合材料的结构形态具有一定的影响。TEOS投加量为0.1mL，KBH₄滴加速度为5mL min-1时，所制备的Fe@SiO₂是一种完全核壳型复合材料，它具有清晰的核壳结构，多孔的SiO₂层包裹1-2个球形纳米铁粒子，纳米铁平均粒径为25nm，SiO₂层的厚度为9nm。随着TEOS投加量的增加，SiO₂层变厚，纳米铁核具有更好的分散性。KBH₄滴加速度太快会导致单体二氧化硅生成，太慢又会导致纳米铁团聚。通过FTIR和XPS分析证明，TEOS水解缩合生成的SiO₂具有表面活性羟基，通过化学键合作用均匀的包覆在纳米铁颗粒表面。与未包覆型纳米铁相比，Fe@SiO₂对Cr（Ⅵ）的去除能力显著提高。TEOS投加量为0.1mL，KBH₄的滴加速度为5mL min-1，所制备的Fe@SiO₂对Cr（Ⅵ）去除能力（以Fe的质量计算）达到最大，为466.67mg g-1，而未包覆型纳米铁仅为76.35mg g-1。（3）通过批实验，考察了各种干扰物质对SF-Fe和Fe@SiO₂去除Cr（Ⅵ）的影响。研究了SF-Fe与Fe@SiO₂去除水中Cr（Ⅵ）的动力学。通过XPS测定对反应产物进行分析，探讨载体硅微粉、二氧化硅外壳的作用。（4）对SF-Fe与Fe@SiO₂填充的石英砂柱来说，随着时间的推移，出水的Cr（Ⅵ）的浓度基本呈上升趋势，但零价纳米铁并没有被完全消耗掉。求得SF-Fe与Fe@SiO₂去除铬的能力分别为每克纳米铁可以去除12与65mg Cr（Ⅵ），远低于批实验能力。这主要归因于干燥使纳米铁团聚、未研磨，导致所制备的核壳型复合材料的表面积大大降低。通过研究纳米铁在具有一定孔隙度和水力条件下的石英砂柱中的透过率来评价其在模拟土壤中的迁移性能。考察了离子强度和腐殖酸对SF-Fe、Fe@SiO₂复合材料在模拟土壤中的迁移特性的影响。结果表明：与未负载纳米铁相比，SF-Fe及Fe@SiO₂在模拟土壤迁移能力增强；在去离子水条件下，未负载纳米铁很少通过石英砂柱，而51.50%的SF-Fe能高效的通过竖直石英砂柱，88.03%的Fe@SiO₂能通过竖直石英砂柱。此外，在水平方向，两种纳米铁硅复合材料的迁移性仍然很强。离子强度对SF-Fe的具有一定负面影响，但腐殖酸能促进SF-Fe迁移。总之，本论文首次选用被称为固体废弃物的硅微粉作为载体，开发出一种分散性强并且具有较高活性的核壳型纳米铁复合材料；发明了一步法制备二氧化硅包覆型纳米铁的“绿色”合成方法，发现试剂KBH₄在体系中既可以作为合成纳米铁的还原剂又可作为生成SiO₂的催化剂。本论文对SF-Fe和Fe@SiO₂在石英砂柱竖直和水平方向的迁移能力的研究，是一次有实际意义的尝试，除了为同行在研究方法上提供参考外，其结果也可为进行实际地下水纳米铁修复技术提供理论指导。更多还原

【Abstract】 Chromium, with its great economic importance for industrial use, is widely usedin a variety of industrial processes. It is one of the important pollutants found in soiland groundwater. Chromium exists in the environment primarily in two valence states:Cr（Ⅵ） and Cr（III）. Cr（Ⅵ） is a toxic, carcinogenic substance to human and animals.Furthermore, Cr（Ⅵ） has high solubility in aqueous phase over almost the entire pHrange and it is only weakly sorbed onto inorganic surfaces, thus it is quite mobile inthe natural environment. On the contrary, Cr（III） is nontoxic and an essential humannutrient, which does not readily migrate in groundwater since it usually precipitatesas Cr（OH）3or as mixed Fe（III）–Cr（III）（oxy）hydroxides.In situ chemical reduction of Cr（Ⅵ） by zero-valent iron nanoparticles （nZⅥ）represents a potentially more effective, lower cost alternative to other remediationtechniques. However, there are still serious technical challenges associated with itsuse. For example, nZⅥ prepared using traditional methods tend to either agglomeraterapidly or react quickly with the surrounding media （e.g., dissolved oxygen or water）,resulting in rapid loss in reactivity. Also, they are essentially not transportable ordeliverable in soils, and thus, cannot be used for in situ applications. Several methodshave been proposed to solve this problem. One approach is attaching nZⅥ to supportmaterial such as resin and carbon. Another approach is to directly modify the surfacesof individual nZⅥ by adding poly acrylic acid, starch and carboxymethylcellulose. Astabilizer can enhance dispersion of nanoparticles through electrostatic repulsion andstatic hindrance.However, the cost of general used support materials is high. And the choice ofmodifier can affect the activity, stability and cost of nanoiron. To solve theseproblems, a suitable support and modifier were chosen in this study. The core-shellsilica fume-supported Fe0（SF-Fe） nanoparticles and SiO₂-coated Fe0（Fe@SiO₂）nanoparticles were prepared. These nanoparticles had high activity and mobility. Themain contents are as follows:（1）Fe0nanoparticles supported on commercial and friendly silica fume weresynthesized by borohydride reduction of ferric chloride in the presence of a support material. Transmission electron microscopy, X-ray diffraction and Fourier transforminfrared were used to characterize the SF-Fe. The feasibility of using this SF-Fe forreductive immobilization of hexavalent chromium Cr（Ⅵ） in soil and groundwaterwas studied. The effect of silica fume percentage on Cr（Ⅵ） removal by SF-Fe wasinvestigated, and was compared with those of unsupported Fe0nanoparticlescontaining the same amount of iron as SF-Fe. By analysis of TEM micrographs, itcan be seen that, the nanomaterial had core-shell structure and approximatelyspherical Fe was dispersed well on the surface of silica fume. The composite particlesize range was on the order of0.15-0.45μm, and the Fe size ranged from20nm to110nm. FTIR study indicated that hydroxyl group was the anchor for silica fume oniron which was accountable for the stability of Fe0nanoparticles. Compared withunsupported Fe0, SF-Fe was significantly more active in Cr （Ⅵ） removal especiallyin84wt%silica fume loading. The experiment results showed that the removal ratioof Cr（Ⅵ） by SF-Fe was22.55%higher than that of unsupported Fe0.（2）Without using aqueous ammonia and a surface modifier, a facile one-stepmethod was developed to fabricate Fe0nanoparticles coated with a SiO₂shell（Fe@SiO₂） by a modified St ber method combined with an aqueous reductionmethod. The Fe@SiO₂was prepared by directly adding KBH₄to a mixed solution oftetraethylorthosilicate （TEOS） and anhydrous ferric chloride. The structure andmorphology of the as-synthesized powders were investigated by X-ray powderdiffraction, energy dispersion analysis of X-ray, transmission electron microscopy,ultraviolet-visible absorption spectroscopy, Fourier-transform infrared spectrometryand X-ray photoelectron spectroscopy. The XRD and EDAX analysis had showedthat the amorphous SiO₂was prepared by the KBH₄. The TEOS amount and KBH₄injection speed had influence on Fe@SiO₂structure. For preparation of0.015g bettercoated Fe0nanoparticles, the perfect TEOS dose was0.1mL and the optimal KBH₄injection speed was5mL min-1. The TEM images showed that under this optimumcondition the prepared Fe@SiO₂had a distinct core-shell structure. One or two Fe0nanoparticles （25nm in diameter） were homogeneously coated by a porous SiO₂shell（9nm）. With an increase in the amount of added TEOS the Fe0nanoparticles hadbetter dispersion and the thickness of the SiO₂coating increased gradually. Lower KBH₄injection speed was preferable to assemble Fe0nanoparticles. The feasibility ofusing the prepared Fe@SiO₂for the reductive immobilization of Cr（Ⅵ） in water wasstudied. Compared with uncoated Fe0nanoparticles, Cr（Ⅵ） removal by Fe@SiO₂increased greatly. The removal ability of the prepared Fe@SiO₂was the highest atTEOS dosage of0.1mL and KBH₄injection speed of5mL min-1. The highestremoval ability of Fe@SiO₂was466.67mg·g-1and it was only76.35mg·g-1foruncoated Fe0nanoparticles.（3） Batch experiments were conducted to evaluate the influences ofinterference factors on Cr（Ⅵ） reduction by the SF-Fe and Fe@SiO₂. The rate ofreduction of Cr（Ⅵ） to Cr（III） can be expressed by a pseudo-first-order reactionkinetics. In order to study the role of silica fume and SiO₂shell, the reaction productwas analyzed by XPS.（4）The column experiments had showed that at certain hydraulic conditions,the Cr（Ⅵ） removal rate gradually slowed down as time went on. However, thenanoiron was not used up. The removal ability of Fe@SiO₂and SF-Fe were only65,12mg·g-1respectively, which were much lower than batch experiment. This may bethat the nanomaterials were heavily aggregated by drying and was not grinded.The mobility of SF-Fe and Fe@SiO₂through sediments was tested usingvertical and horizontal column transport experiments. The experiments resultsconfirmed that the SF-Fe and Fe@SiO₂moved more effectively through model soilsthan unsupported Fe0nanoparticles. It was indicated that51.50%and38.29%ofSF-Fe were eluted from the vertical and horizontal columns under the specifiedconditions, respectively. And88.03%and61%of Fe@SiO₂were eluted from thevertical and horizontal columns, respectively. Increasing the solution ionic strengthwould decrease the mobility of SF-Fe and Fe@SiO₂due to the increased attachmentto sand grains. However, the mobility of Fe0nanoparticles was enhanced by thepresence of humic acid.In summation, this study was the first time to use silica fume as a support for Fe0nanoparticles, and the prepared core-shell nanomaterials had high dispersion andactivity. A facile and friendly one-step method to fabricate monodispersed core-shellFe@SiO₂nanocomposites was discussed. Borohydride was acted not only as a reductant for iron salt but also a catalyst for hydrolysis and polycondensation reactionof TEOS. The study of mobility of SF-Fe and Fe@SiO₂in vertical and horizontalcolumn filled with quartz sand was a meaningful effort. It not only served as areference for the research methods, but also provided theory guide for the practicalapplication of nanoiron technology.更多还原