土壤中铝同晶替代对赤铁矿形貌影响机制的研究

Aluminum substitution in iron oxides is very common in soils. It results in important changes in the structure, morphology and physicochemical properties of iron oxides. Though hematite with a variety of morphologies can be synthesized in laboratory, it is commonest as platy crystal in soils, and the thickness of the crystal gradually decreases as the content of Al substitution increases. This phenomenon was also observed in synthesized Al-substituted hematite. Different ionic radius of Al3+ from that of Fe3+ might be the key factor, however the mechanisms governing the decrease in the thickness of the crystals during crystallization is still obscure. In this proposal the crystal growth mechanism of hematite in the presence of Al was to be investigated using several traditional and modern analytical techniques, from the respects of microstructures with comparison to those of hematite crystals doped with several other cations (Cr3+、Cu2+、Ni2+) with different radii. It aims to discover the relationship between crystal morphology and crystal habits, which was determined by the ionic radius, and finally to reveal the basic mechanism of Al influencing the morphology of hematite. The results can help us decipher the interactions of metals with soil hematite and the geochemical behaviors of hematite during soil genesis, and thus enrich the theories of iron oxides in soils. Further it can provide important information on the roles of iron oxides in the migration and transformation of nutrients and contaminants in soils and the genesis and properties of soil aggregates.

氧化铁矿物中的Al同晶替代是土壤中普遍存在的现象。由于Al3+对晶格中Fe3+的替代，使赤铁矿的结构、形貌和表面性质发生改化。尽管赤铁矿可有多种形貌，但在土壤中常为板柱状晶体，且随着Al同晶替代量的增加, 晶体变薄, 这一现象亦存在于合成的Al替代赤铁矿中。离子半径不同可能是其关键因素，但离子半径如何使其晶体变薄的结晶学机制并不清楚。本项目以几种不同半径的金属离子(Cr3+、Cu2+、Ni2+等)同晶替代赤铁矿为对照，应用常规实验方法和现代分析技术，从微观结构研究Al替代赤铁矿晶体的生长规律，明确由离子半径变化所决定的结晶学特性与形貌的关系，揭示Al同晶替代对赤铁矿形貌影响的结晶学机制。研究成果将有助于揭示成土过程中金属离子与赤铁矿的相互作用及赤铁矿的晶体化学行为，丰富土壤氧化铁的理论体系；为阐明氧化铁对土壤养分、环境污染物的迁移转化、以及土壤结构体的形成与性质的影响等提供理论依据。

土壤氧化铁矿物中常常存在铝同晶替代现象。赤铁矿中Al同晶替代会使其形貌由菱块状变为薄片状，但是其机制尚不清楚。本项目针对这一科学问题，应用同步辐射X射线吸收光谱、多晶X射线衍射及Rietveld结构精修和高分辨电镜等，研究不同量Al替代赤铁矿形貌变化与其微结构之间的关系，并与一系列不同量过渡金属离子掺杂赤铁矿进行对比，取得了以下主要结果：1. 实验室合成赤铁矿样品中, Al的引入使赤铁矿形貌由菱块状变为薄片状，且薄片状晶体直径随着Al含量增加而增大。微结构分析表明，赤铁矿结构中共面Fe-Fe和沿c轴方向上O-O键长增加比a-b方向上共边Fe-Fe、O-O键长增加更多，使得赤铁矿沿c轴方向原子面网密度减小，而a-b方向上面网密度增加，赤铁矿晶体最终沿平行于a-b面的晶面保留下来。这可能是Al替代使赤铁矿形貌由菱块状转变为薄片状的机制。2. 穆斯堡尔谱研究表明，由于Al对赤铁矿晶格中Fe的替代，使得Fe局域配位数减小，四级分裂值ΔEq随着Al含量增加而逐渐减小。Al替代使赤铁矿结构OH含量和结构缺陷如Fe空位增多，导致磁超精细分裂值Bhf减小。3. Al的替代使赤铁矿对磷酸根和砷酸根单位面积吸附量降低;且对砷酸根的吸附量更大。4. 不同量过渡金属离子(Cr3+、Cu2+、Ni2+、Mn2+、Zn2+和Pb2+)替代对赤铁矿晶胞参数、形貌等的影响各不相同。低含量的Cr并未显著改变赤铁矿的形貌，而Cr10样品为短棒状；Cu、Ni、Mn掺杂赤铁矿均为颗粒状；Pb、Zn掺杂使得赤铁矿的形貌向扁平状转变。Fe K边EXAFS分析表明，离子半径较大的金属离子进入赤铁矿中使得共面Fe-Fe键长的减小程度小于共边Fe-Fe键长的减小程度，而小离子半径的金属离子进入赤铁矿中使得共面Fe-Fe键长的增大程度大于共边Fe-Fe键长的增大程度，即与Fe3+离子半径不同的金属离子同晶替代进入赤铁矿的结构中都会使得赤铁矿的共面Fe-Fe键长与共边Fe-Fe键长比值发生改变。5. 过渡金属离子掺杂(Co3+、Cu2+、Ni2+、Mn2+、V3+、Cr3+、Si4+、Sn4+、Ti4+)使得赤铁矿芬顿反应去除亚甲基蓝的效率降低。