Differentiation Between Adsorbed and Precipitated Sulfate in Soils and at Micro-Sites of Soil Aggregates by Sulfur K-Edge XANES.

To investigate the potential of synchrotron-based X-ray Absorption Near-Edge fine Structure spectroscopy (XANES) at the sulfur (S) K-edge for a discrimination of adsorbed and precipitated sulfate in soils and soil particles, XANES spectra of ionic sulfate compounds and Al/Fe hydroxy sulfate minerals were compared with spectra of SO₄^2- adsorbed to ferrihydrite, goethite, hematite, gibbsite, or allophane. Ionic sulfate and hydroxy sulfate precipitates had broader white-lines (WL) at 2482.5 eV (full width at half maximum [FWHM]: 2.4 to 4.2 eV; Al hydroxy sulfates: 3.0 eV) than SO₄^2- adsorbed to Al/Fe oxyhydroxides or allophane (FWHM: 1.8 to 2.4 eV). The ratio of the white-line (WL) height to the height of the post-edge feature at 2499 eV (WL/PEF) of edge-normalized spectra was larger for SO₄^2- adsorbed to Al/Fe oxyhydroxides or allophane (8.1 to 11.9) than for Al/Fe hydroxy sulfates and ionic sulfates (3.9 to 5.7). The WL/PEF ratio of edge-normalized S K-edge XANES spectra can be used to distinguish adsorbed from precipitated SO₄^2- in soils and also at microsites of soil particles. The contribution of adsorbed and precipitated SO₄^2- to the total SO₄^2- pool can be quantified. The spectra of most soils could be fitted by Linear Combination Fitting (LCF), yielding a similar partitioning between adsorbed and precipitated SO₄^2- as an evaluation of the WL/PEF ratio. The SO₄^2- pool of German forest soils on silicate parent material in most cases was strongly dominated by adsorbed SO₄^2-; however, in three German forest soils subject to elevated atmospheric S deposition, a considerable portion of the SO₄^2- pool was precipitated SO₄^­2-, most likely Al hydroxy sulfate. The same is true for Nicaraguan Eutric and Vitric Andosols subject to high volcanogenic S input. Our spatially-resolved analysis proved that iIn the subsoil of the Vitric Andosol, adsorbed SO₄^2- and Al hydroxy sulfate coexist on a micron scale.