Speciation of Iron in Soil Including Structural Fe(II)/Fe(III) Ratios in Phyllosilicates by X-ray Absorption Fine Structure Spectroscopy

Soil is a complex system where iron (Fe) speciation plays a key role in redox reactions and biogeochemical processes. However, quantitative evaluation of the Fe(II)/Fe(III) ratio in phyllosilicates, which constitute a major Fe reservoir in soils, has remained challenging. In this study, we developed a method to quantify Fe speciation in soils using linear-combination fitting (LCF) of X-ray absorption fine structure (XAFS) spectra with four end members: ferrihydrite, Fe(II)-bearing biotite, Fe(III)-bearing montmorillonite, and adsorbed Fe2+ as hydrated Fe2+ (outer-sphere complex). Principal component analysis confirmed that these four components are sufficient to reproduce Fe K-edge X-ray absorption near-edge structure (XANES) spectra of various soil and sediment samples under both oxic and anoxic conditions. The LCF-XAFS results showed that phyllosilicates account for 40–70% of total Fe in the studied samples, and the ratio of Fe(II) and total Fe in the octahedral layer in phyllosilicates (= Fe(II)phy/Fetotal-phy ratio) varies systematically with redox conditions. Under reducing conditions, ferrihydrite was partially dissolved and replaced by adsorbed Fe2+, while the Fe(II)phy/Fetotal-phy ratio increased significantly. The speciation results were consistent with those obtained by Mössbauer spectroscopy and sequential extraction within ∼10% difference. Furthermore, μ-XANES analysis revealed spatial heterogeneity of Fe speciation at the micrometer scale, demonstrating that the Fe(II)phy/Fetotal-phy ratio in phyllosilicates reflects local redox conditions within soils. These results indicate that the Fe(II)phy/Fetotal-phy in phyllosilicates can serve as a quantitative proxy for redox conditions in soils and sediments.