Characterization of iron(III) in organic soils using extended X-ray absorption fine structure spectroscopy

The distribution of different iron (Fe) species in soils, sediments, and surface waters has a large influence on the mobility and availability of Fe, other nutrients, and potentially toxic trace elements. However, the knowledge about the specific forms of Fe that occurs in these systems is limited, especially regarding associations of Fe with natural organic matter (NOM). In this study, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize Fe(III) in organic soils (pH 4.6-6.0) with varying natural Fe content. The EXAFS data were subjected to wavelet transform analysis, to facilitate the identification of the nature of backscattering atoms, and to conventional EXAFS data fitting. The collective results showed the existence of two pools of iron: mononuclear Fe(III)-NOM complexes and precipitated Fe(III) (hydr)oxides. In the soil with lowest pH (4.6) and Fe content mononuclear organic complexes were the completely dominating fraction whereas in soils with higher pH and Fe content increasing amounts of Fe (hydr)oxides were detected. These results are of environmental importance, as the different iron pools most likely have markedly different reactivities.