Synergistic effects of redox processes and ligand controlled dissolution of iron(hydr)oxide

Dissolution of iron oxides is a key process in the natural iron cycle and in biological iron acquisition. We draw on newly developed concepts of electron mobility in iron oxide phases and on our own observations to elucidate and quantify synergistic effects between the two most important mechanisms in iron acquisition: reduction and ligand promoted dissolution. In the proposed work, we will test and investigate the key processes on the molecular level based on the following five hypotheses (H1-H5): H1) Redox processes transferring electrons to the iron oxide influence the bonding environment and the reactivity of surface sites. H2) Iron oxide dissolution is catalyzed by dissipation of electron density over the mineral structure and surface sites, whereby the reactivities of surface sites are influenced by adsorbed ligands. H3) The synergistic effect of reduction and ligands does not depend on the mechanism of iron reduction. Electrons can originate from adsorption of reducing metal ions or from ligands by thermal or light-induced ligand-to-metal charge transfer. H4) Reduced iron acts as a catalyst and only small steady state concentrations are necessary to have a significant effect. Kinetically relatively inert reduced states have sufficient lifetimes to support the synergistic effect even in oxic systems. H5) The structure of the iron(hydr)oxide minerals influences the effect of iron reduction on ligand controlled dissolution. We will test these hypotheses with experimental and theoretical studies with: - Adsorption and dissolution batch experiments in the dark and with UV-A and visible light - In-situ infrared measurements of bulk and surface structures and of reactions - Ab initio density functional theory (DFT) molecular orbital calculations. The research will be conducted by a Ph.D. student at the University of Vienna supervised by Prof. Stephan Kraemer (kinetic batch experiments), by a Ph.D. student at Eawag supervised by Prof. Janet Hering and Dr. Stephan Hug (infrared experiments) and by a postdoc at Eawag supervised by Prof. James Kubicki (density functional theory calculations) and Dr. S. Hug, drawing on the complementary expertise of the participants. The combined studies will lead to an improved molecular understanding of the processes proposed in (H1-H5) and to a consistent theory of the synergistic effects. The theory will quantify the impact of the degree and the persistence of reduction in iron(hydr)oxides on the kinetics of proton- and ligand-promoted dissolution. This is of key importance for the bioavailability of iron from different iron(hydr)oxides in various natural environments and for the behavior and properties of iron-phases in natural and technical systems.

Iron is an essential nutrient for most known organisms. Therefore, biological iron acquisition is of great importance for survival. However, the availability of iron is very limited in many natural systems by the low solubility of iron bearing phases such as iron(oxohydr)oxides. This includes earth-surface environments that are oxygen rich and characterized by neutral to slightly alkaline pH, including ocean surface waters or calcareous soils. In these systems, microorganisms and plants need to rely on efficient mechanisms of dissolution and mobilization of iron from iron oxides. In this project we investigated if and how a known dissolution mechanism, the so-called ligand-controlled dissolution, is catalysed by reductants including divalent iron. We studied the acceleration of the mechanism by reductants and investigated if biogenic or non-biogenic ligands and reductants could synergistically promote the dissolution process. Indeed, we found such synergistic effects that lead to a clear acceleration of the iron dissolution process. In summary, we report the following key findings: With Fe(II) as reductant for iron oxides, we observe a catalytic acceleration of ligand- controlled dissolution of Fe(III). We observe this catalytic effect even at very low Fe(II) concentrations (< 1 M). Other reductants capable of reducing iron also have this accelerating effect on iron oxide dissolution. Reductants can have this accelerating effect even in oxic systems, depending on the re-oxidation rate of Fe(II). We investigated the re-oxidation kinetics of Fe(II) in iron oxides and found a clear correlation between the re-oxidation rates and the time-course of the catalytic effect. The catalytic effect on ligand-controlled dissolution is observed in the presence of structurally diverse ligands (and therefore is not a property of a particular ligand structure). Diverse reductants promote the catalytic effect. The dissolution of all tested iron oxides was impacted by the catalytic effect. Therefore, we suggest that the observed mechanism plays an important role in the biological iron acquisition.