『Abstract
In dynamic natural systems such as soils and surface waters,
transient biogeochemical processes can induce strong chemical
non-steady-state conditions. In this paper, we investigate the
effects of non-steady-state conditions on ligand-controlled iron
oxide dissolution. The rates of goethite dissolution at pH 6 in
the presence of low molecular weight organic acids (oxalate, citrate
or malonate) were observed. Non-steady-state conditions were induced
by rapid additions of fungal, bacterial or plant siderophores.
In the presence of the low molecular weight organic acids, dissolved
iron concentrations are below detection limit as predicted by
equilibrium solubility calculations. The rapid addition of the
siderophores triggered reproducible, fast dissolution of kinetically
labile iron from the iron oxide surface. The same effect was observed
upon rapid additions of high citrate concentrations to goethite-oxalate
suspensions. The concentration of the labile iron pool at the
mineral surface was a function of the surface concentration of
the low molecular weight organic acids and of the reaction time
before addition of the siderophores. Isotopic exchange with 59Fe
independently confirmed the existence of the labile iron pool
before addition of the siderophore. A dissolution mechanism was
elucidated that is consistent with these observations and with
accepted models of ligand-controlled dissolution. We conclude
that the fast dissolution reaction observed here is an important
process in biological iron acquisition and that it is based on
a general geochemical mechanism.』
1. Introduction
2. Materials and methods
2.1. Materials
2.2. Adsorption experiments
2.3. Dissolution experiments
2.4. Isotopic exchange
3. Results and discussion
3.1. Adsorption studies
3.2. Dissolution of goethite at constant total ligand concentrations
3.3. Dissolution of goethite in the presence of DFO-B and oxalate
under non-steady-state conditions
3.4. Isotopic exchange
3.5. Influence of the oxalate concentration on the dissolution
under non-steady-state conditions
3.6. Formation of kinetically labile iron at iron oxide surfaces
in the presence of low molecular weight organic acids other than
oxalate
3.7. Effect of organic ligands other than DFO-B on the dissolution
of kinetically labile iron from iron oxide surfaces
3.8. Dissolution mechanism
4. Conclusions
Acknowledgments
Appendix A. Supplementary data
References