Occurrence characteristics of free iron oxides in soil microstructure: evidence from XRD, SEM and EDS

Free iron oxides in soils usually have distinct electrochemical properties, relatively high surface energy, a strong ability to adsorb, and provide structural cementation in the soil. Thus, soil properties are influenced by the content, characteristics and occurrence of free iron oxides. This paper reports a study on Zhanjiang clay and Longmen laterite, both of which are rich in free iron oxides, to investigate the occurrence characteristics of free iron oxides in soils. Dithionite-citrate-bicarbonate solution was adopted to remove free iron oxides from the soils, and qualitative filter paper was used to extract particles with size d < 25 μm. X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry were used to analyze alterations in the physical and chemical properties, mineral composition, and microstructure caused by the removal of free iron oxides. The results obtained in this study show that free iron oxides act as a bridge in Zhanjiang clay and mostly in the form of cladding in Longmen laterite. Both forms enhance the cementation and structural strength of the soils and weaken the swelling-shrinkage capacity. The findings of this paper will contribute to a deeper understanding of the formation and failure of soils.