Characterization And Self-assembly Synthesis Of Biomineralized Nano-FeOOH And Its Environmental Significance

Environmental nanotechnology emphasizes understanding of the distribution, constitution, origin and behavior of nanostructure in large environment of nature which has the multiplicity of physics/chemistry. The research of nanostructural biomineralization in environmental nanoscience becomes the focal point on intersectional subjects in recent years. At present, the research of biomineralization mechanism of iron (hydr)oxide under different environmental conditions needs to be carried out deeply, and the effects of those iron minerals in nature environment should be concerned. We chose iron hydroxide biomineralization mechanism, envirochemical effect of biomineralized nanophase goethite including interfacial catalysis and oxidation in natural light as the object of study. It not only promotes the development of enviromineralogy, chemistry, imitative material synthesis and biometallurgy, but also expands the research fields of environmental science and provides theoretic foundation in further research of iron biominerals and its application in environmental pollution remediation as well as its industrial application.On the basis of an extensive survey of literatures, the dissertation systematically summarizes research of FeOOH biomineralization, molecular self-assembly synthesis technology and its application in heavy metal and toxic organic materials management, especially photocatalysis degradation of organic contaminants. In addition, it proposes theoretical questions needed to be focused on, innovative research route and experimental methods.The composition, microscopic structure, crystal structure and surface chemistry of selected natural biomineralized iron hydroxides are systematically investigated. In normal temperature and neutral pH aqueous solution system, we lay the emphasis on organic metabolite of iron bacteria, as well as the inducing and modulating function of biological polysaccharide on FeOOH mineralization and nanostructural self-assembly progress through sphaerotilus biomineralization and polysaccharide template biomimetic experiments. The chemistry, biochemistry action and formation condition of the crystallization of iron hydroxide gel in the presence of organic matrix are discussed and a new possible mechanism of the crystallization and phase transformation of iron hydroxide gel during preliminary stage of mineralization are put forward.At the same time, a series of research of adsorbing heavy metal elements, dyes and photocatalysis with use of nano FeOOH minerals and synthetic materials have been developed at the point of theory and application of enviromineralogy and nano-enviromaterials. We investigate the surface chemical reaction and microstructure effect of the biomineralized FeOOH by several experiments of adsorption chromium ions in aqueous solution, and further proceed to do contrastive research of water treatment which contains heavy metal elements and azo dyes using the production of iron bacteria biomineralization, self-assembly synthesis by organic molecular inducing mineralization, and inorganic chemical synthesis iron oxyhydroxide respectively. In particular, it reveals a mechanism of the adsorption—photocatalysis oxidation co-effect on the surface of FeOOH through the interfacial adsorption and photocatalysis of nanophase iron hydroxide with or without solar light's irradiation. The progress of our study is shown as follows:1. The biomineralization product is proven to be an amorphous iron oxide or weakly crystalloid goethite formed by natural iron bacteria biomineralization, and it appears to have microtwist, microtubular and strawberry microstructure. The concentration of iron oxide is 56.60 percent. It also contains some organic matter such as polysaccharide. High resolution electron microscopy shows that the biomineralized tubular or twist iron hydroxide are formed via nanophase goethite regularly arranged. and are a kind of nanophase aggregate composed of goethite from several to tens nanometers in size, which may be grown through nucleating at active sites of amorphous Fe(OH)₃ firstly, then the nanosized nuclei adjust their growing direction to orientation arrangement. The mechanism of nucleation and crystallization of twist and tubular nanophase goethite is self-assembled growth.2. The biomimetic experiments using polysaccharid organic matrix as template to chemically synthesize nanoscale iron hydroxides showed that the dextran could modulate the mineralization behavior of Fe(OH)₃ gel. The dextran controlled over the mineralization behavior of Fe(OH)₃ gel formβ-FeOOH toα-Fe₂O₃ by the selecting mechanism of nucleation and transformation. 13-FeOOH would be the transition phase of Fe(OH)₃ gel to formα-Fe₂O₃. The different concentration of dextran in solution could control the rate of crystal growth and the behavior of crystallization, so the nanoparticles of iron (hydr)oxides could be attained.3. The mineralization and crystallization of Fe (OH)₃ gel occurred both inside and outside the sphaerotilus cells. The weak crystalline akaganeite (β-FeOOH) nanoparticles was the main product and some polysaccharide and protein metabolized by sphaerotilus were attached (bound) on their surfaces. Under the conditions of different substrates and the same concentration of Fe(â…¢) in solution, the morphology and structure of iron (hydr)oxides formed through the mineralization of Fe(OH)₃ gel varied with the ingredients and concentrations of organic matrices which were metabolized by sphaerotilus.4. The experiments of heavy metal adsorption from aqueous solution showed that there was strong adsorption of Cr(â…¢)and Cr(â…¥)on goethite and both Langmuir and Freundlich adsorption isothermal models were able to accurately describe the adsorption of these two ions. Under the conditions of this experiments, the goethite had a greater adsorption capacity for Cr(â…¥) than for Cr(â…¢). The adsorption of chromium on naturally biomineralized goethite was heterogeneous and affected by microstructures of goethite. Furthermore, XPS data showed that redox reaction of chromium on the surface of biomineralized goethite took place with the adsorption of both Cr(â…¢) and Cr(â…¥). Fe-O-Cr had a stronger action of interface chemistry. The Cr₂O₇^2-/CrO₄^2- adsorbed on the surface of goethite was much easier to transform into Cr(â…¢) than the oxidization of Cr(â…¢) on the surface of FeOOH. The proportion of lower chemical state N_1S of NO₃^- increased with the increase of adsorption capacity of Cr(NO₃)₃. So the oxidation reaction of Cr(â…¢) to form Cr(â…¥) was probably promoted by the reduction of NO₃^- on the surface of goethite. These results indicated that the adsorptive action on the naturally biomineralized goethite is more complicated.5. Comparative experiments were conducted about the adsorption of Cr(â…¥) in the wast water on the four different iron oxyhydroxides (FeOOH)--biomineralized byiron bacteria, self-assembled using respective chitosan and gelation respectively as organic substrates, and synthesized simply with chemical techniques. The results showed that the crystal structures of FeOOH (e.g.α-FeOOH orβ-FeOOH) exerted slight effect on the adsorptive capacity of chromium and their specific surface area were not also the main decisive factors. However the variety and microstructure of organic matrices attached on their surface led to serious effect on the adsorptive capacity of heavy metal. The naturally biomineralized goethite has a strong adsorptive capacity and fixed effect owing to its nanoscale polycrystalline structure and some polysaccharide organic matrices bound on its surface. The nanosizedβ-FeOOH by self-assembly synthesis using polysaccharide template has also better ability to adsorb and fix chromium ions in the aqueous solution.6. Comparative experiments were conducted to study the treatment of the dye-containing waste water by two kinds of iron oxyhydroxides—self-assembly syntheticβ-FeOOH using polysaccharide organic molecule as template and chemical syntheticα-FeOOH without organic template. The FTIR spectra showed that methyl orange and Acid Red G were apt to form complex on the surface of iron oxyhydroxides. Under the dark condition, the decoloration process of this dye solution occurred firstly when the azo bond of organic molecules adsorbed on the surface of iron oxyhydroxides ruptured. The decoloration took place mostly because the electron transfer occurred between the organic reductive complex adsorbed and the iron oxyhydroxides surface's Fe³⁺ centre, leading to the rupture of azo bond and the degradation and decoloration of the dye solution. This reactive mechanism was mainly the action of complex adsorption—oxidation.Under the irradiation of solar light and in the neutral aqueous solution, the oxidation of methyl orange was obviously accelerated by the FeOOH photocatalysis. The photocatalytic degrading rate was higher in the system ofβ-FeOOH/visible light, owing to the stronger co-effect of interface chemical adsorption- photocatalysis ofβ-FeOOH than that ofα-FeOOH. After 5 hours reaction, the benzene rings of a part of methyl orange molecues were opened and the decoloration ratio was up to 83% in this system.However, in the system of FeOOH /visible light/H₂O₂, for this two kinds of catalysts the benzene rings of most methyl orange molecules were opened to form a series of little molecules and the decoloration ratio was up to 90% within the same reactive time (5h). In particular, there was deeper degradation of benzene rings in the system ofβ-FeOOH/visible light/H₂O₂. This reactive mechanism was mainly the action of organic complex--photocatalytic oxidation on the surface of FeOOH.7. The biomineralized nanoscale goethite formed by the action of iron bacteria can be used as the functional materials to remove the contaminants in the soil and water environment due to its unique microstructure and surface activity. And the self-assembled synthetic nanometric 13-FeOOH through using polysaccharide molecules as the template has high surface activity and can be used to adsorb and fix the heavy metal ions (e.g. chromium) and to photocatalytically degrade the hazardous organic substances in the environment.