M. Sandström,1 F. Jalilehvand,2 E. Damian1
1Department of Structural Chemistry, Stockholm University
2Department of Chemistry, University of Calgary
Synchrotron-based XANES spectroscopy can with high sensitivity in natural samples at atmospheric pressure identify sulfur species in a large number of valence states, from -II to +VI. This makes XANES spectroscopy a powerful tool for investigating the many important, but often incompletely known, environmental effects of sulfur compounds. By means of the dedicated SSRL sulfur beam line 6-2 the unexpected discovery was made that elemental sulfur has often accumulated in large quantities in marine-archaeological artifacts, evidently when hydrogen sulfide in anoxic aqueous solution penetrates the submerged wood. For the seventeenth century Swedish warship Vasa analyses of oak core samples show that the outermost 2 cm often contain several mass percent elemental sulfur, which stepwise is being oxidized into sulfuric acid, probably catalyzed by iron compounds. Fluorescence EXAFS shows if iron in the wood is present as sulfides or oxides. We have also found similar sulfur accumulation for several other marine-archaeological shipwrecks, with implications for the conservation procedures. The complex sulfur XANES spectra are curve-fitted using standards to find intermediates and possible oxidation pathways. Simulations of standard spectra by density functional computations reveal details of the electronic transitions and bonding in the sulfur species. Similar methods will allow studies of the active sulfur species in other environmental problems, e.g. exposure of growing plants to gaseous sulfur dioxide and hydrogen sulfide. The low sulfur concentrations then require dedicated beam lines with even higher intensity than that presently available.
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