Iron minerals in soils and sediments play important roles
in many
biogeochemical processes and therefore influence the cycling of major
and trace elements and the fate of pollutants in the environment.
However, the kinetics and pathways of Fe mineral recrystallization
and transformation processes under environmentally relevant conditions
are still elusive. Here, we present a novel approach enabling us to
follow the transformations of Fe minerals added to soils or sediments
in close spatial association with complex solid matrices including
other minerals, organic matter, and microorganisms. Minerals enriched
with the stable isotope57Fe are mixed with soil or sediment,
and changes in Fe speciation are subsequently studied by57Fe Mössbauer spectroscopy, which exclusively detects57Fe. In this study,57Fe-labeled ferrihydrite was
synthesized, mixed with four soils differing in chemical and physical
properties, and incubated for 12+ weeks under anoxic conditions. Our
results reveal that the formation of crystalline Fe(III)(oxyhydr)oxides
such as lepidocrocite and goethite was strongly suppressed, and instead
formation of a green rust-like phase was observed in all soils. These
results contrast those from Fe(II)-catalyzed ferrihydrite transformation
experiments, where formation of lepidocrocite, goethite, and/or magnetite
often occurs. The presented approach allows control over the composition
and crystallinity of the initial Fe mineral, and it can be easily
adapted to other experimental setups or Fe minerals. It thus offers
great potential for future investigations of Fe mineral transformationsin situunder environmentally relevant conditions, in both
the laboratory and the field.