Iron speciation in soils and soil aggregates by synchrotron-based X-ray microspectroscopy (XANES, μ-XANES)

Iron speciation in soils is still poorly understood. We have investigated inorganic and organic standard substances, diluted mixtures of common Fe minerals in soils (pyrite, ferrihydrite, goethite), soils in a forested watershed which constitute a toposequence with a hydrological gradient (Dystric Cambisol, Dystric Planosol, Rheic Histosol), and microsites of a dissected soil aggregate by X-ray Absorption Near Edge Spectroscopy (XANES) at the iron K-edge (7112 eV) to identify different Fe(II) and Fe(III) components. We calculated the pre-edge peak centroid energy of all spectra and quantified the contribution of different organic and inorganic Fe-bearing compounds by Linear Combination Fitting (LCF) conducted on the entire spectrum (E = 7085-7240 eV) and on the pre-edge peak. Fe-XANES conducted on organic and inorganic standards and on synthetic mixtures of pyrite, ferrihydrite and goethite showed that by calculating the pre-edge peak centroid energy, the Fe(II)/Fe(III) ratio of different Fe-bearing minerals (Fe sulphides, Fe oxyhydroxides) in mineral mixtures and soils can be quantified with reasonable accuracy. A more accurate quantification of the Fe(II)/Fe(III) ratio was possible with LCF conducted on the entire XANES spectrum. For the soil toposequence, an increased groundwater influence from the Cambisol to the Histosol was reflected in a larger contribution of Fe(II) compounds (Fe(II) silicate, Fe monosulphide, pyrite) and a smaller contribution of Fe(III) oxyhydroxides (ferrihydrite, goethite) to total iron both in the topsoil and the subsoil. In the organic topsoils, organically bonded Fe (33-45% of total Fe) was 100% Fe(III). For different microsites in the dissected aggregate, spatial resolution of mu-XANES revealed different proportions of Fe(II) and Fe(III) compounds. Fe K-edge XANES and mu-XANES allows an approximate quantification of Fe(II) and Fe(III) and different Fe compounds in soils and (sub)micron regions of soil sections, such as mottles, concretions, and rhizosphere regions, thus opening new perspectives in soil research.