Ferrihydrite is one of the most common iron (oxyhydr)oxides in the environment.Owing to its large specific surface area and unique features of structure,ferrihydrite plays an important role in controlling the transport and fate of various pollutants.As a metastable mineral,ferrihydrite can be transformed into other iron (oxyhydr)oxides under certain conditions.Aluminum (Al) commonly exists in the environment during the formation and transformation of various iron (oxyhydr)oxides,and thus may probably play an important role in the formation and transformation of ferrihydrite,which is rarely explored.In the present study,modern spectroscopy techniques including powder X-ray diffraction and X-ray absorption spectroscopy were used to study the effect of Al3+ substitution on the transformation process of ferrihydrite into hematite.The results showed that selfassembly process of primary nanoparticles and Ostwald ripening process was involved in transforming ferrihydrite into hematite.On one hand,Al3+ substitution reduced the conversion rate of ferrihydrite to hematite.On the other hand,Al3+ substitution promotes the conversion of edge-sharing [FeO6] to face-sharing [FeO6],resulting in the transformation of ferrihydrite into hematite.