Dissolution of iron oxides in the presence of ligands and reductants

The dissoution of iron oxides in natural systems is
controlled by reductive and ligand-controlled processes.
Their mechanisms have been elucidated in model systems and
their effects have been studied in complex natural matrices.
Microorganisms and plants are taking advantage of the
reactivity of ligands and/or reductants for the acquisition of
iron as a nutrient in oxic aqueous or terrestrial systems in the
neutral or alkaline pH ranges.
A key problem of biological iron mobilization using
reductants is the extremely fast Fe(II) re-oxidation rates
typically observed in the neutral to alkaline pH-range, which
seem to preclude transport of the reduced iron toward
microbial or root surfaces unless the Fe(II) becomes
chemically stabilized or re-oxidized Fe(III) remains in a
soluble complex. However, previous and ongoing research of
our group has demonstrated, that low transient Fe(II)
concentrations have a catalytic effect on non-reductive
ligand-controlled dissolution.
In this presentation we will summarize key findings of
our research focusing on the interplay of ligand controlled
and reductive processes. In our studies we used a range of
synthetic and biogenic ligands (e.g., synthetic ligands used in
agricultural iron fertilization, microbial and plant
siderophores) and biogenic reductiants. In our most recent
work we studied a class of biogenic compounds exuded by
iron limited plants that can both reduce and ligate iron. We
studied their effect on the dissolution of various iron oxides
and on iron mobilization in complex soils. The advances and
limitations of our current conceptual understanding of
dissolution mechanisms in the presence of ligands and
reductants as well as their significance in natural systems are
discussed. Funding for this work is provided by the Austrian
FWF grant I 2865-N34