Iron oxides are among the most abundant minerals in soils and are also a significant component of atmospheric mineral dust that can be transported through the atmosphere to oceans, making iron-containing minerals crucial players in oceanic and terrestrial biogeochemical and atmospheric processes. Organic compounds are also abundant in soil and atmospheric systems, originating from a variety of plant, humic, and bacterial sources, and are often closely associated with mineral surfaces. Hematite (alpha-Fe2O3) is one of the most common iron oxides in soils, and is the focus of these studies. Its solubility is dictated by solution pH, ionic strength, and the presence of organic ligands. This work presents scanning and high-resolution broadband sum frequency generation (SFG) and second harmonic generation (SHG) surface spectroscopic measurements of hydroxamate siderophore ligands at the hematite/water interface. Siderophores are biological organic ligands that are secreted by aquatic and terrestrial microbes as a means of solubilizing iron in oxic (aerobic) environments, where it exists primarily as insoluble Fe(III)-containing minerals at neutral to alkaline pH. The presented studies will examine the molecular structure and interactions of thin film hematite/water interfaces with and without the presence of desferrioxamine-B (DFO-B), a siderophore commonly found in soils, and acetohydroxamic acid (aHa), a simple analog for DFO-B. Experiments focus on monitoring the SFG and SHG signals as the hematite/water interfaces are exposed to the organic ligands at varying bulk pH and ligand concentration, providing information on surface coverage, structure and kinetics of complexation and dissolution. Preliminary work on molecular dynamics and quantum mechanical simulations for the aqueous aHa/hematite interface will also be presented.

Hydroxamate siderophore interactions with thin-film hematite/water interfaces studied using ultrafast laser spectroscopy techniques and molecular simulation methods

ACS Spring 2022

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