Characterization of sulfur in asphaltenes by sulfur K- and L-edge XANES spectroscopy

Abstract

Bitumens exposed to atmospheric conditions undergo various types of weathering including evaporation, biodegradation, abiotic oxidation, and water washing. Sulfur present in bitumens is a good marker of oxidation processes as it is very sensitive to oxidation and exhibits a range of oxidation states from -2 to +6. Sulfur K-and L-edge XANES spectroscopy were applied to the asphaltera extracts of a series of recent geological and archeological bitumens from the Middle East. In all the samples, dibenzothiophene is the major sulfur moiety. In the least altered asphaltene, minor species include disulfide, alkyl sulfide and sulfoxide. The proportion of oxidized sulfur (sulfoxide, sulfone, sulfonate and sulfate) increases with the alteration of the samples, whereas the disulfide and alkyl sulfide content decreases. This work illustrates the interest of XANES spectroscopy as a selective probe for determining sulfur speciation in natural samples, and the complementarity of S K-and L-edge for identifying the oxidized and reduced forms of sulfur respectively.