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

The catalytic effect of aqueous Fe(II) (Fe²⁺_aq) on the transformation of Fe(oxyhydr)oxides has been extensively studied in the laboratory. It involves the transfer of electrons between Fe²⁺_aq and Fe-(oxyhydr)oxides, rapid atomic exchange of Fe between the two states, and recrystallization of the Fe-oxides into more stable Fe-(oxyhydr)oxides. The potential occurrence of these reactions in natural soils and sediments can have an important impact on biogeochemical cycling of iron, carbon, and phosphorus. We investigated the possible isotopic exchange between Fe²⁺_aq and sedimentary Fe(III) in Fe-Si-C-rich lake sediments. ⁵⁷Fe Mössbauer spectroscopy was used to evaluate Fe mineral speciation in unaltered lake sediments. Unaltered and oxidized sediment laboratory incubations were coupled with a classical kinetic approach that allows a quantitative description of the reactivity of assemblages of Fe-(oxyhydr)oxides found in sediments. Specifically, unaltered and oxidized sediment samples were separately incubated with an ⁵⁵Fe²⁺_aq-enriched solution and exchange was observed between ⁵⁵Fe²⁺_aq and sedimentary Fe(III), highest in the top of the sediment and decreasing with depth with the ⁵⁵Fe²⁺_aq tracer distributed within the bulk of the sedimentary Fe(III) phase. Our results indicate that atomic exchange between Fe²⁺_aq and sedimentary Fe(III) occurs in natural sediments with electrons transferred from the Fe(III)-particle to Fe(III)-particle via Fe²⁺_aq intermediates.