Isotopic Exchange between Aqueous Fe(II) and Solid Fe(III) in Lake Sediment─A Kinetic Assemblage Approach

Area of Science:

• Geochemistry
• Environmental Science
• Soil Science

Background:

• Laboratory studies show aqueous iron(II) catalyzes iron oxyhydroxide transformation.
• These reactions involve electron transfer, Fe atomic exchange, and recrystallization.
• Such processes in soils and sediments affect biogeochemical cycles of Fe, C, and P.

Purpose of the Study:

• Investigate isotopic exchange between aqueous iron(II) and sedimentary iron(III) in lake sediments.
• Quantify the reactivity of iron oxyhydroxide assemblages in natural sediments.

Main Methods:

• Used 57Fe Mössbauer spectroscopy for Fe mineral speciation in sediments.
• Incubated unaltered and oxidized sediment samples with 55Fe(II)-enriched solution.
• Applied a kinetic approach to describe iron oxyhydroxide reactivity.

Main Results:

• Observed isotopic exchange between 55Fe(II) and sedimentary Fe(III).
• Exchange was highest in surface sediments and decreased with depth.
• The 55Fe(II) tracer distributed within the bulk sedimentary Fe(III) phase.

Conclusions:

• Atomic exchange between aqueous iron(II) and sedimentary iron(III) occurs in natural sediments.
• Electrons are transferred from Fe(III) particles to other Fe(III) particles via aqueous Fe(II) intermediates.
• This confirms the role of Fe(II) in sedimentary iron transformations and biogeochemical cycling.