Coupling of Dissolved Organic Matter Molecular Fractionation with Iron and Sulfur Transformations during Sulfidation–Reoxidation Cycling

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Abstract
Iron (oxyhydr)oxides and organic matter (OM) are intimately associated in natural environments, and their fate might be linked to sulfur during sulfidation-reoxidation cycling. However, the coupling of DOM molecular fractionation with Fe and S transformations following a full sulfidation-reoxidation cycle remains poorly understood. Here, we reacted Fh and Fh-OM associations with S(-II) anaerobically and then exposed the sulfidic systems to air. S(-II) preferentially reacted with Fh to form inorganic S (e.g., mackinawite, S0, and S22-) over being incorporated into OM as organic S and therefore indirectly affected OM fate by altering Fe speciation. Fh sulfidation was inhibited by associated OM, and the main secondary Fe species were mackinawite, Fe(II)-OM compounds, and lepidocrocite. Concomitantly, organic molecules high in unsaturation, aromaticity, and molecular weight were detached from solid-phase Fe species due to their lower affinities for secondary Fe species than for Fh. During the reoxidation stage, the previously formed Fe(II) species were reoxidized to Fh with a stronger aggregation, which recaptured formerly released OM with higher selectivity. Additionally, •OH was generated from Fe(II) oxygenation and degraded a portion of the DOM molecules. Overall, these results have significant implications for Fe, C, and S cycling in S-rich environments characterized by oscillating redox conditions.
Funding Information
  • Natural Science Foundation of Jiangsu Province (BK20200507, BK20200531)
  • Fundamental Research Funds for the Central Universities (B210201052)
  • National Natural Science Foundation of China (42007338, 42225701)
  • 2021 High-level Personnel Project Funding of Jiangsu Province (JSSCBS20210255)

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