The significant role of montmorillonite on the formation of hematite nanoparticles from ferrihydrite under heat treatment

Abstract With high surface reactivity and weak crystallinity, ferrihydrite (Fh) tends to be transformed to other thermodynamically stable iron (hydr)oxides, and its transformation is strongly influenced by the coexisting substances. Clay minerals widely coexist with Fh in the near-surface areas, but there is limited information about their effects on the transformation of Fh. In this work, the effect of montmorillonite (Mt) on the formation of hematite (Hem) nanoparticles from Fh under heat treatment was studied. Fh, Mt, and their mixed samples (Fh-Mt) were heated at different temperatures (350, 450, 600, 700, and 800 °C) for various times (3, 5, and 10 h), and the samples before and after heating were well characterized. X-ray diffraction and 57Fe Mossbauer spectroscopy results revealed that Fh was transformed to Hem after heat treatment in both the presence and absence of Mt. Scanning electron microscopy and transmission electron microscopy images indicated that Mt could well disperse Fh and distinctly decrease the particle sizes of the formed Hem after heating. In particular, the particle size of Hem reached ~1000 nm in the pure Fh system, while it was only 15–75 nm in the systems containing Mt. Fourier transform infrared spectroscopy patterns displayed the strong interactions between Mt and Fh (through the formation of Si−O−Fe and Al−O−Fe bonds), which may also reduce the aggregation of Fh and inhibit the phase transformation of Fh particles into large Hem particles. This work verifies the important protective effect of clay minerals on coexisting iron oxide under high-temperature conditions, which would contribute to better understanding the formation and preservation of natural nano-sized minerals (particles) in nature.