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

Bitumens exposed to atmospheric conditions undergo various types of weather ing including evaporation, biodegradation, abiotic oxidation, and water was hing. 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 Ledge XANES spectroscopy were applied to the asphaltene extracts of a series of recent geological and archeological bitu mens from the Middle East. In all the samples, dibenzothiophene is the majo r sulfur moiety. In the least altered asphaltene, minor species include dis ulfide, alkyl sulfide and sulfoxide. The proportion of oxidized sulfur (sul foxide, sulfone, sulfonate and sulfate) increases with the alteration of th e samples, whereas the disulfide and alkyl sulfide content decreases. This work illustrates the interest of XANES spectroscopy as a selective probe fo r determining sulfur speciation in natural samples, and the complementarity of S K- and L- edge for identifying the oxidized and reduced forms of sulf ur respectively.