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 S2
2–) 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.