铝同晶替代赤铁矿形貌演化机制及其对磷的吸附特征研究

发布时间：2019-02-12 08:57

【摘要】：铝(Al)作为地壳中第三丰富的元素,在土壤赤铁矿的形成过程中,它以同晶替代方式进入氧化铁晶格,占据Fe~(3+)位置。相比于纯赤铁矿,铝替代赤铁矿在晶胞参数、晶粒大小、结晶度、生长取向、形貌等发生明显改变。这些矿物学性质的改变又反过来影响了赤铁矿的表面电荷特性、吸附能力、热力学稳定性、磁性质以及溶解性等。因此,深刻地理解铝同晶替代赤铁矿纳米颗粒的生长过程及其对赤铁矿矿物学性质的影响具有一定的理论意义。本论文研究了铝同晶替代对赤铁矿纳米颗粒结构和形貌的影响;利用密度泛函理论(DFT)进一步计算出铝对与赤铁矿结构的影响;探讨了不同铝同晶替代赤铁矿对磷的吸附特征。主要结论如下;1.铝同晶替代诱导了赤铁矿纳米颗粒沿垂直c轴方向生长,随着铝替代量的增加,赤铁矿的晶胞体积v和晶胞参数a、c降低,结晶度减弱,结晶尺寸增大,比表面积增大,结构水增加;赤铁矿纳米颗粒的形貌从菱形,过渡到菱形和平板状混合形貌,最后到平板状。2.DFT计算显示Al以同晶替代方式进入赤铁矿体相结构后,使得体相结构内金属八面体发生不同程度的扭曲形变。这样的形变主要导致了六方紧密堆积的O原子网络在a轴方向产生较大的收缩,而在c轴方向上基本保持不变。从整体上看,六方紧密堆积的O原子网络在ab方向排列更为接近平面六边形。Al与a-Fe_2O_3表面作用影响了其表面的原子结构以及表面能。影响程度与Al的量及特定晶面有关。a-Fe_2O_3(001)和(100)与Al作用(吸附),两个晶面的表面能发生明显的翻转,而翻转后表面能较低的(001)也是平板状a-Fe_2O_3的主导晶面。无论是a-Fe_2O_3体相结构中的结构水还是表面吸附的水都会增强铝同晶替代a-Fe_2O_3形貌异质性的演化。对于体相结构水,它加剧了六方紧密堆积的O原子网络在a轴方向的收缩,对于表面吸附水,它钝化了(001)面,使其表面能进一步降低。3.铝同晶替代赤铁矿对磷的吸附过程表现为一个初始快速吸附过程和随后的慢速吸附过程。随着铝同晶替代量的增加,赤铁矿对磷的初始吸附速率是依次增大的,慢速吸附速率也呈现同样的趋势,相对于快速吸附过程,慢速吸附过程的吸附速率要小一个数量级。Langmuir和Freundlich等温吸附模型均可以描述磷在铝同晶替代赤铁矿上的吸附行为。相比于Langmuir模型,Freundlich模型能够更好的模拟磷在铝同晶替代赤铁矿上的吸附过程。
[Abstract]:Aluminum (Al) is the third richest element in the crust. During the formation of soil hematite, it enters the lattice of iron oxide in the form of homocrystalline and occupies the position of Fe~ (3). Compared with pure hematite, the crystal cell parameters, grain size, crystallinity, growth orientation and morphology of aluminum-substituted hematite changed obviously. These changes in mineralogical properties in turn affect the surface charge characteristics, adsorption capacity, thermodynamic stability, magnetic properties and solubility of hematite. Therefore, it is of certain theoretical significance to understand the growth process of Aluminum-isomorphism instead of hematite nanoparticles and its influence on the mineralogical properties of hematite. In this paper, the effect of aluminum isomorphic substitution on the structure and morphology of hematite nanoparticles was studied, and the effect of aluminum on the structure of hematite was further calculated by density functional theory (DFT). The adsorption characteristics of phosphorus by different aluminum-isomorphic hematite substituted hematite were discussed. The main conclusions are as follows: 1. Aluminum isomorphic substitution induces hematite nanoparticles to grow along the vertical c axis. With the increase of aluminum substitution amount, the unit cell volume v and unit cell parameter AOC of hematite decrease, the crystallinity weakens, the crystal size increases, and the specific surface area increases. Increase of structural water; The morphology of hematite nanoparticles changed from rhombic to rhombic and plate-like, and finally to flat. 2.DFT calculation showed that Al entered the structure of hematite by homocrystalline substitution. The metal octahedron in the bulk structure is distorted and deformed to varying degrees. Such deformation mainly results in the large contraction of the hexagonal compact stacked O atom network in the a-axis direction, while it remains basically unchanged in the c-axis direction. On the whole, the closely packed hexagonal O atom network is closer to the hexagonal plane in the ab direction. The interaction between Al and a-Fe_2O_3 surface affects the atomic structure and surface energy of the surface. The degree of influence is related to the amount of Al and the specific crystal planes. A-Fe_2O_3 (001) and (100) interact with Al (adsorption). The lower surface energy (001) is also the dominant crystal plane of flat a-Fe_2O_3. Both the structure water in the bulk structure of a-Fe_2O_3 and the water adsorbed on the surface can enhance the evolution of the morphology heterogeneity of aluminum isomorphism instead of a-Fe_2O_3. For bulk structure water, it intensifies the contraction of the hexagonal tightly packed O atom network in the a-axis direction, and passivates the (001) surface for the adsorbed water on the surface, which further decreases the surface energy. The adsorption of phosphorus on Aluminum-isomorphic hematite is characterized by an initial rapid adsorption process and a later slow adsorption process. With the increase of the amount of aluminum isomorphic substitution, the initial adsorption rate of phosphorus increased in turn, and the slow adsorption rate showed the same trend compared with the rapid adsorption process. The adsorption rate of slow adsorption process is smaller by an order of magnitude. Both Langmuir and Freundlich isothermal adsorption models can describe the adsorption behavior of phosphorus on aluminum-isomorphic hematite instead of hematite. Compared with Langmuir model, Freundlich model can better simulate the adsorption process of phosphorus on aluminum-isomorphic hematite instead of hematite.
【学位授予单位】：华中农业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P579;X505

【参考文献】