Determination of sulfur environments in borosilicate waste glasses using X-ray absorption near-edge spectroscopy

Sulfur can be the waste-loading limiting constituent for vitrification of sulfur-bearing radioactive wastes due to low solubility in silicate melts. Methods to improve sulfur loading would benefit from improved understanding of the structural aspects of sulfur incorporation in borosilicate and other glasses. To this end, sulfur XANES spectra were collected for eight crystalline standards and twenty-four glasses, including borosilicate, phosphate, and borate compositions. Spectra for the standards show a systematic energy shift of the sulfur K-edge from 2469 to 2482 eV, as sulfur valence increases from 2− (in sulfides) to 6+ (in sulfates). Most crucible glasses investigated have simple edges near 2482 eV that indicate sulfur in the form of sulfate only. Other glasses, some synthesized under reducing conditions, have complicated edges, indicating sulfate, sulfite, and more reduced species that may include S, S–S doublets, or short polysulfide chains. Sulfide species (S 2−) were not dominant in any of the samples over the range of redox conditions investigated. These results indicate that sulfur incorporation is considerably more complex than would be suggested by the conventional interpretation of the redox-dependence of sulfur solubility, which considers only sulfate and sulfide species. Raman data indicate that several of the glasses investigated are not homogeneous with regard to all sulfur species.