Effects of Al substitution on local structure and morphology of lepidocrocite and its phosphate adsorption kinetics

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.