Dissolved silicate is ubiquitous in natural waters with near millimolar concentrations and may sorb strongly onto natural Fe oxyhydroxides under a wide range of conditions, altering their sorption potential for contaminants, and affecting Fe cycling and availability. Our initial surface diffraction study indicated that silicate bound in a monodentate topology to the reactive Fe-OH sites on the hematite R-plane, and could also form bridging structures by linkup of these sorbed silicate tetrahedra. We have now extended this study to examine sorption on hematite, lepidocrocite and goethite nanoparticles by examining ATR-FTIR spectra of surface hydroxyls and their attenuation and frequency shifts when exposed to monomeric silicate solutions. Results verify the expected ratio of exposed hydroxyl types as a function of nanoparticle habit, and also confirm that the most reactive hydroxyl is that coordinated to a single surface Fe. With increased silicate sorption other hydroxyls are affected suggesting a range of surface interactions depending on silicate concentration and surface plane termination.<br/>Such spectroscopy can potentially reveal sorbed silicate-silicate linkages through vibrational and group theoretical analysis in ideal cases, e.g. by observing the frequency shifts, band splitting and intensity changes attending silicate-group vibration symmetry breaking. However we have attempted to test both the sorption geometry and sorption/desorption reactions by performing MD simulations using ReaxFF potentials on the R-plane hematite surface for different protonation schemes and silicate topologies. Additionally we have used umbrella-sampling approaches to investigate the effect of forcing and restraining potentials on the binding of silicate on the R-plane surface. Overall results suggest that monodentate and perhaps other types of silicate binding is favorable, but that there is a significant kinetic barrier to sorption. Continuing work will more fully investigate the energy landscape of these sorption reactions using improved reactive force fields, with the experimental FTIR results used as a guide for relative hydroxyl site reactivity.

Silicate sorption on Fe oxyhydroxides: Identification of reaction sites from molecular spectroscopy, and reactive force field modeling of sorption/desorption processes

249th National Meeting (2015)

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