Sulfur Speciation In Humic Acids and Sediments Along Salinity Gradients In Coastal Wetlands.

Coastal wetlands are known as one of the largest reservoirs of sulfur-containing compounds, which play a vital role in the formation and function of humic acids in coastal wetlands. In this study, the chemical transformation of sulfur in coastal wetland sediments along with salinity gradients was investigated. Sediment samples were taken from three different wetland ecosystems, namely forest swamp (FS), freshwater marsh (FM), and salt marsh (SM), from the Barataria basin, Southern Louisiana. Humic acid fractions from the collected sediment samples were extracted, purified, freeze-dried, and finely ground. Sulfur speciation of both bulk sediment samples and corresponding humic acids was carried using X-ray absorption near-edge structure (XANES) spectroscopy. Results showed that major forms of sulfur in these coastal soils/sediments were organic sulfides, di-sulfides, and organic sulfates. Along salinity gradient from swamp to salt marsh, there was a slight increase in highly oxidized organic sulfates in humic acid fractions. Bulk sediments exhibited similar total reduced sulfur as humic acids for freshwater marsh, but differed for forest swamp and salt marsh. Higher organic sulfates were observed in forest swamp sediments whereas significant amounts of pyrite were found in salt marsh.  Overall the results showed significant salinity influence in sulfur speciation in costal wetland sediments.