Effects of aquatic dissolved organic matter redox state on adsorption to goethite

Direct electrochemical methods were used to alter the redox state of dissolved organic matter (DOM) rich in carbon–hydrogen–oxygen–sulfur (CHOS) and sulfur deficient (CHO) composition to explore how their reduced and oxidized forms sorbed to iron oxides. The CHOS-rich DOM was isolated from Prairie Pothole wetland surface waters, while Suwannee River natural organic matter (SRNOM) represented a sulfur poor DOM. CHOS-rich DOM showed decreased adsorption to goethite following electrochemical reduction compared to its oxidized or unaltered forms. In contrast, no difference was observed between the adsorption of unaltered or reduced SRNOM to goethite. Our results suggest that the organic sulfur moieties in Prairie Pothole DOM are responsible for the decline in adsorption following reduction. Changes in DOM molecular weight after adsorption were observed for all samples, however, reduced Prairie Pothole DOM resulted in increased fractionation relative to its unaltered or oxidized forms. This was not observed for SRNOM. Furthermore, increases in Fe(II) in solution were observed following interaction of reduced Prairie Pothole DOM with goethite, a phenomenon not seen in oxidized or reduced SRNOM. This observation suggests that reduced CHOS-rich DOM enhanced the reductive dissolution of goethite, which may play an important role in the cycling and bioavailability of iron in systems where sulfur is important. This work illustrates the potential importance of redox active, high sulfur DOM in aquatic biogeochemical cycles.