Abstract
The redox reactions of Iron (Fe2+ and Fe3+ ) are deeply related to the evolution of the earth's atmosphere and microbial metabolism, and are very important from a geochemical point of view. The redox state and chemical species of Fe have a significant influence on the behavior of various elements (e.g., arsenic and potassium; Takahashi et al., 2004; Khaled and Stucki, 1991) in the paddy soils discussed in this study. However, previous studies have focused only on the redox state of clay minerals (Stucki, 2011) and ferrihydrite in soils individually. In this study, we attempted to precisely analyze how Fe-containing clay minerals (mainly smectite, a 2:1 type clay mineral) in paddy soils are related to redox reactions in soils, where only precipitation-solution of ferrihydrite has often been emphasized. In particular, according to Gorski et al. (2012), the Fe2+/Fe3+ ratio in the smectite structure varies over a wide Eh range and thus may have different reducing (oxidizing) potentials depending on the Fe2+/Fe3+ ratio. Iron hydroxide in soil may be reduced and dissolved to Fe2+ and then oxidized by Fe3+ in the clay mineral structure to form ferrihydrite and lepidocrocite (Géhin et al., 2007), and we also focused on this process.
