Technical Description. This project will explore the dissolution of iron and manganese oxides by siderophores using real-time, in situ X-ray diffraction to determine mechanisms and rate laws for these important soil reactions. Our preliminary results reveal that the siderophore desferrioxamine-B (DFOB) at concentrations ranging from 0.1 to 10 mM will induce complete dissolution of the layered Mn oxide birnessite within hours. Moreover, Rietveld analysis of our time-resolved synchrotron XRD data have revealed that Mn(III) is selectively removed from the birnessite structure relative to Mn(IV). Removal of 20 mol% Mn(III) induces a critical instability in triclinic birnessite, and the structure collapses when vacancy concentrations increase beyond this value.
These observations lead us to hypothesize that the mechanism by which siderophores dissolve minerals depends on the heterogeneity of metal valence state. In mixed-valence metal (hydr)oxides, siderophore-mediated dissolution occurs by a structural collapse after a critical 3-dimensional vacancy concentration is achieved. In contrast, homovalent metal (hydr)oxides dissolve by the more conventional mode of 2-dimensional surface depletion. We will test these ideas by applying TR-XRD techniques to DFOB-assisted dissolution of a variety of heterovalent oxides (e.g., magnetite [Fe3+ (Fe2+,3+)2O4], hausmannite [Mn3+(Mn2+,3+)2O4], riebeckite [Na2Fe2+3Fe3+2(Si8O22)(OH)2] and homovalent oxides (e.g., hematite [Fe3+2O3], goethite [Fe3+O(OH)]), in the presence and absence of light.
Broader Impacts. Mineral dissolution mechanisms are foundational to a range of societally important issues, including soil fertility and the cycling of metals in the critical zone, contaminant transport, the sequestration of CO2, and the chemistry of Earth?s surface waters. Most prior studies of the rates by which minerals dissolve in aqueous solutions have monitored reaction progress through changes in fluid chemistry. Here we propose a novel and complementary strategy that correlates the chemical evolution of the fluid with structural changes in the reacting solid. By this approach, we can rigorously couple the chemical kinetics of the fluid with structural mineral transitions, greatly expanding our understanding of the underlying reaction mechanisms.
The work described in this proposal will help reveal the factors that control the mechanisms and rates by which siderophores can extract insoluble metals to sustain healthy metabolic activity in soil and marine environments. Siderophores are low-weight, biogenic compounds that occur ubiquitously in micromolar concentrations in soil and marine waters, and they are produced by a wide variety of microbes, fungi, and grasses to gain a selective advantage in severely Fe-limited conditions. Siderophores can extract Fe(III) and Mn(III) from nearly insoluble Fe(III) and Mn(III,IV) oxides with extremely high specificity, transporting the cations to parent organisms to satisfy nutritive or redox metabolic needs. Insights gained from this work may lead to a better understanding of methods to increase metal bioavailability in iron-deficient agricultural regions, and they will improve our understanding of mineral weathering, a major means of atmospheric CO2 drawdown.
