Assessment of sulfur and iron speciation in a soil aggregate by combined S and Fe micro-XANES: Microspatial patterns and relationships

To test whether synchrotron-based spectromicroscopy can be used to identify spatial patterns of sulfur (S) and iron (Fe) speciation as well as relationships between the speciation of S and Fe in soil colloids or aggregates at the micrometre and sub-micrometre level, an anoxically prepared dissected soil aggregate (size ∼1 mm³) was analyzed by μ-XANES at the K-edges of S (2472 eV) and Fe (7112 eV). The experiment included (i) elemental mapping at the S K-edge (S, Si, Al) and the Fe K-edge (Fe, Si), (ii) acquisition of 300 μm × 300 μm images of the region of interest with X-ray energies of 2474 eV (addressing reduced organic and inorganic S), 2483 eV (total S), 7121 eV (divalent Fe) and 7200 eV (total Fe), as well as (iii) acquisition of S and Fe μ-XANES spectra at two different positions, where image analysis suggested the dominance of reduced and oxidized S and Fe, respectively. Image analysis revealed a heterogeneous distribution of total Si, S and Fe as well as of different S and Fe species in the aggregate. Microregions which were either enriched in reduced or in oxidized S and Fe could be identified. A microregion with a large contribution of oxidized S (sulfate, sulfonate) to total S contained exclusively Fe(III) oxyhydroxides (probably ferrihydrite) as S-bearing phase, whereas another microregion with a large contribution of reduced organic S (thiol, organic disulfide) to total S contained a small amount of Fe(II)-bearing silicate in addition to the dominating Fe(III) oxyhydroxides. Our results show that combined S and Fe μ-XANES is a powerful tool for studying microscale spatial patterns of S and Fe speciation as well as microscale relationships between the speciation of S and Fe in soil aggregates.