Formation and Transformation of Iron Oxide–Kaolinite Associations in the Presence of Iron(II)

Abstract

Iron oxide–kaolinite associations are important components of tropical and subtropical soils and have significant influence on the physical and chemical properties of soils. In this study, the formation and transformation of Fe oxide–kaolinite associations as a function of pH, temperature, and time were investigated at different Fe(II)/Fe(III) molar ratios (R). Results show that the formation of crystalline Fe oxides was significantly inhibited due to the presence of kaolinite, while accelerated by Fe(II). The formation of lepidocrocite– and goethite–kaolinite associations were accelerated by Fe(II) at R = 0.04 to 0.06, an initial pH (pHi) of 5 to 8, and a temperature (T) of 50 to 70°C; the formation of hematite–kaolinite association was accelerated by Fe(II) at R = 0.06, pHi 7 to 8, and T = 60 to 80°C; magnetite–kaolinite association was obtained at R = 0.06, pHi 9, and T = 60°C or at R = 0.1 to 0.5, pHi 7, and T = 60°C. The pH as a function of time (pHt) decreased sharply when crystalline Fe oxides were formed in the presence of Fe(II), Fe(III), and kaolinite. The decrease in pHt was slow, however, in the system with Fe(III) and kaolinite but without Fe(II) and in the system with Fe(II) and kaolinite but without Fe(III). The morphologies of lepidocrocite, goethite, hematite, and magnetite in associations are strip shaped, nanorod like, pseudo‐cubic shaped, and nanosphere like, respectively. In a system with Fe(II), Fe(III), and kaolinite, Fe(II) weakened the inhibition of the formation of crystalline Fe oxides by kaolinite; the presence of kaolinite decreased the acceleration by Fe(II); and the morphologies of Fe oxides were influenced by both Fe(II) and kaolinite.