Abstract Aluminum (Al) substitution into the lepidocrocite structure commonly occurs in redoximorphic soils. However, little is known about its effects on the local structure, morphology, and surface properties of lepidocrocite due to its metastability in the environment. In this study, a series of Al-substituted lepidocrocite samples was synthesized and the local structure and surface properties were characterized by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, high resolution transmission electron microscopy (HRTEM), zeta potential and phosphate (P) adsorption kinetics. With the substitution of Al into the lepidocrocite structure, the lepidocrocite unit cell volume decreased due to the smaller ionic radius of Al3+ (r = 0.535 A) than Fe3+ (r = 0.645 A), and both the corner-sharing (along c-axis) and edge-sharing (along a-axis) Me-Me (Me = Al, Fe) distances decreased, with a more significant decrease in corner-sharing Me-Me distances. In addition, hydrogen bonding in the interlayer regions of the mineral along the b-axis was weakened with Al substitution. These changes of the local structure led to a gradual evolution of the lepidocrocite morphology: the aspect ratio (length to width) decreased with increasing Al substitution, and the Al-substituted lepidocrocite crystals became thinner, resulting in a decrease in the final ratios of coherent scattering domain (CSD) sizes of face (020) to those of terminating face (200) and (002). Theorfore, with increasing Al substitution, the relative proportion of face (020) with doubly coordinated oxygens per specific surface area decreased, leading to an increase of active hydroxyls and P adsorption density at the surface. These results shed new insights on the evolution mechanism of local structure and morphology of Al-substituted lepidocrocite and provided important information for evaluating the surface reactivity and environmental behaviors of lepidocrocite in soils.

中文翻译：

---

铝取代对纤铁矿局部结构和形貌及其磷酸盐吸附动力学的影响

摘要 铝 (Al) 取代到纤铁矿结构中通常发生在氧化还原土壤中。然而，由于其在环境中的亚稳定性，其对纤铁矿的局部结构、形态和表面性质的影响知之甚少。在这项研究中，合成了一系列铝取代的纤铁矿样品，并通过 X 射线衍射 (XRD)、扩展 X 射线吸收精细结构 (EXAFS) 光谱、高分辨率透射电子显微镜 ( HRTEM)、zeta 电位和磷酸盐 (P) 吸附动力学。随着 Al 取代纤铁矿结构，由于 Al3+ (r = 0.535 A) 的离子半径小于 Fe3+ (r = 0.645 A)，纤铁矿晶胞体积减小，角共享（沿 c 轴）和边缘共享（沿 a 轴）Me-Me (Me = Al, Fe) 距离均减少，角共享 Me-Me 距离下降更显着。此外，矿物层间区域中沿 b 轴的氢键被 Al 取代减弱。这些局部结构的变化导致纤铁矿形貌的逐渐演变：纵横比（长宽比）随着铝取代度的增加而减小，铝取代的纤铁矿晶体变薄，导致最终的相干比降低面 (020) 到终止面 (200) 和 (002) 的散射域 (CSD) 大小。因此，随着Al取代度的增加，单位比表面积中具有双配位氧的面（020）的相对比例降低，导致表面活性羟基和磷吸附密度的增加。这些结果为铝取代纤铁矿局部结构和形态演化机制提供了新见解，并为评估纤铁矿在土壤中的表面反应性和环境行为提供了重要信息。