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

J Synchrotron Radiat. 2010 Mar;17(2):166-72. doi: 10.1107/S0909049509049917. Epub 2010 Feb 2.

Abstract

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 approximately 1 mm(3)) was analyzed by micro-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 microm x 300 microm 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 micro-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 micro-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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Iron / chemistry*
  • Soil / analysis*
  • Sulfur / chemistry*
  • X-Ray Absorption Spectroscopy / methods*

Substances

  • Soil
  • Sulfur
  • Iron