Characterization of Sulfur-Containing Functional Groups in Sedimentary Humic Substances by X-ray Absorption Near-Edge Structure Spectroscopy

Humic substances, which are randomly cross-linked heterogeneous organic materials of high molecular weight, comprise a major fraction of the organic matter in soils and sediments. Because humic substances have a variety of functional groups, they play a major role in controlling the physical and chemical characteristics of natural waters, soils, and sediments. Sedimentary humic substances are considered to be the precursors of petroleum-forming kerogens. Sulfur is thought to play a major role in forming humic substances in anoxic marine sediments. Because of their structural heterogeneity, humic substances are not easily amenable to biochemical degradation and, thus, are generally considered to be biologically refractory. The complexity and heterogeneity of humic substances render the elucidation of their chemical structure a major analytical challenge. Degradative techniques, such as pyrolysis GC−MS, can change a sample's structure during preparation and analysis and, thus, may give inaccurate information. In this study, we investigated the sulfur-containing functional groups in humic acids from various marine sediments using XANES spectroscopy which has proved to be an important, nondestructive tool for analyzing sulfur forms. Humic acids were isolated from near-surface sediments from three locations: a salt marsh in Shelter Island, NY; the Peru Margin; and Florida Bay. We found that organic sulfides, di- and polysulfides, sulfonates, and organic sulfates are the major forms of sulfur in these sedimentary humics. The reduced-sulfur structures, organic sulfides, and di- and polysulfides, are essentially intramolecular as opposed to the highly oxidized forms of sulfur, sulfonates, and ester-bonded sulfates, which can only be present as end groups. The abundance of the reduced-sulfur structures reflects the extent to which sulfur is involved in forming intramolecular cross-links which are crucial for building up the macromolecular structures of humic substances. The significance of the presence of the different sulfur functionalities in humic substances is discussed in relation to early diagenesis and preservation of organic matter in marine sediments.