Differentiation between adsorbed and precipitated sulphate in soils and at micro‐sites of soil aggregates by sulphur K‐edge XANES

Abstract

SummaryTo investigate the potential of synchrotron‐based X‐ray Absorption Near‐Edge Structure spectroscopy (XANES) at the sulphur (S) K‐edge for a discrimination of adsorbed and precipitated sulphate in soils and soil particles, XANES spectra of ionic sulphate compounds and Al/Fe hydroxy sulphate minerals were compared with spectra of SO42− adsorbed to ferrihydrite, goethite, haematite, gibbsite or allophane. Ionic sulphate and hydroxy sulphate precipitates had broader white‐lines (WL) at 2482.5 eV (full width at half maximum (FWHM) of edge‐normalized spectra, 2.4–4.2 eV; Al hydroxy sulphates, 3.0 eV) than SO42− adsorbed to Al/Fe oxyhydroxides or allophane (FWHM, 1.8–2.4 eV). The ratio of the white‐line (WL) height to the height of the post‐edge feature at 2499 eV (WL/PEF) was larger for SO42− adsorbed to Al/Fe oxyhydroxides or allophane (8.1–11.9) than for Al/Fe hydroxy sulphates and ionic sulphates (3.9–5.7). The WL/PEF ratio of edge‐normalized S K‐edge XANES spectra can be used to distinguish adsorbed from precipitated SO42− in soils and also at microsites of soil particles. The contribution of adsorbed and precipitated SO42− to the total SO42− pool can be roughly quantified. Adsorbed ester sulphate may result in overestimation of precipitated SO42−. The spectra of most soils could be fitted by linear combination fitting (LCF), yielding a similar partitioning between adsorbed and precipitated SO42− as an evaluation of the WL/PEF ratio. The SO42− pool of German forest soils on silicate parent material in most cases was strongly dominated by adsorbed SO42−; however, in three German forest soils subject to elevated atmospheric S deposition, a considerable portion of the SO42− pool was precipitated SO42−, most likely Al hydroxy sulphate. The same is true for Nicaraguan Eutric and Vitric Andosols subject to high volcanogenic S input. In the subsoil of the Vitric Andosol, adsorbed SO42− and Al hydroxy sulphate coexist on a micron scale.