Characterization of humic acid from leonardite coal: an integrated study of Py-GC-MS, XPS and XANES techniques

Abstract

We studied the humic-acid fraction isolated from a Spanish leonardite coal (Torrelapaja, Cretaceous basin belonging to the Utrillas facies) using a suite of chromatographic and spectroscopic techniques to characterize the structure of the carbon skeleton and the nature of the sulfur-containing compounds. In particular, analytical pyrolysis coupled to gas chromatography-mass spectrometry was used to gain detailed molecular information on the organic structures. Pyrolysis in the presence of a methylating agent (tetramethyl-ammonium hydroxide, TMAH) was used to characterize polar moieties. We employed non-destructive techniques, XANES and XPS, to investigate the composition of the sulfur functionalities (such as sulfide, polysulfide, thiophene, sulfoxide, sulfonate and sulfate). The combination of these different approaches allows a more complete understanding of the organic sulfur structures in the leonardite coal. In agreement with previous studies, our results show that oxidized sulfur functionalities, such as sulfonate and sulfate, represent the major forms of sulfur in leonardite coal.