Iron oxyhydroxide (FeOOH) mineral plays an important role in a variety of atmospheric, terrestrial and technological settings. Molecular resolution of reactions involving these minerals is thereby required to develop a fundamental understanding of their contributions in processes taking place in the atmosphere, Earth’s upper crust as well as the hydrosphere. This study resolves interactions involving four different types of synthetic FeOOH particles with distinct and well-defined surfaces, namely lath- and rod-shaped lepidocrocite (γ), goethite (α) and akaganéite (β). The surface and bulk reactivities of these particles are controlled by their distinct structures. When exposed to ambient atmospheric or aqueous conditions their surfaces are populated with different types of (hydr)oxo functional groups acting as reaction centers. These sites consist of hydroxyl groups that can be singly- (≡FeOH, -OH), doubly- (≡Fe2OH, μ-OH), or triply-coordinated (≡Fe3OH, μ3-OH) with underlying Fe atoms. Moreover, these sites exhibit different types, densities, distributions, as well as hydrogen bonding patterns on different crystal planes for each mineral. Knowledge of the types and distributions of hydroxyl groups on minerals with different surface structures is fundamental for building a molecular-scale understanding of processes taking place at FeOOH particle surfaces. In this thesis, Fourier transform infrared (FTIR) spectroscopy was used to resolve the interactions between (hydr)oxo groups of FeOOH particles with (in)organic acids, salts, water vapor as well as carbon dioxide. The focus on such compounds was justified by their importance in natural environments. This thesis is based on 9 articles and manuscripts that can be found in the appendices. FTIR spectroscopic signatures of hydroxyl groups in the bulk of well crystallized FeOOH minerals were characterized for structural differences and thermal stabilities. Those of akaganéite were particularly resolved for the variable bond strength of bulk hydroxyls induced by the incorporation of HCl through nanostructured channels at the terminations of the particles. FTIR bands of hydroxyl groups at all particle surfaces were monitored for responses to thermal gradients and proton loadings, providing experimental validation to previous theoretical accounts on surface site reactivity. This site reactivity was resolved further in the fluoride (F-) and phosphate (PO43-) ions adsorption study to follow the site selectivity for ligand-exchange reactions. These efforts showed that singly-coordinated groups are the primary adsorption centers for ligands, doubly-coordinated groups can only be exchanged at substantially higher ligand loadings, while triply coordinated groups are largely resilient to any ligand-exchange reaction. These findings helped consolidate fundamental knowledge that can be used in investigating sorption processes involving atmospherically and geochemically important gases. The latter parts of this thesis were therefore focused on water vapor and carbon dioxide interactions with these FeOOH particles. These efforts showed how surface structure and speciation affect sorption loadings and configurations, as well as how water diffused into and through the akaganéite bulk. Hydrogen bonding is one of the most important forms of interactions between gas phase and minerals. It plays a crucial role in the formation of thin water films and in stabilizing surface (bi)carbonate species. The affinity of surface hydroxyl groups for water and carbon dioxide is strongly dependent on their abilities to form hydrogen bonds. These are controlled by coordination number and site accessibility/steric constraints. In agreement with the aforementioned ligand-exchange studies, surfaces dominated by singly coordinated groups have stronger ability to accumulate water layers than the ones terminated by groups of larger coordination number. Collectively, these efforts consolidate further the concept for structure-controlled reactivities in iron oxyhydroxides, and pave the way for new studies along such lines.

No description available.

This dissertation includes theoretical and applied components that address the effect of coatings on rates of mineral reactions that occur in acid mine drainage (AMD) environments. The two major projects investigated how diffusion-limited transport of reactants through pore spaces in coatings on mineral grains affects the reaction rate of the underlying mineral. The first project considered the growth of gypsum coatings on the surface of dissolving limestone in anoxic limestone drains (ALD), which reduces the neutralization rate of the dissolving limestone and the subsequent effectiveness of this treatment. The second project investigated the conditions where iron oxyhydroxide coatings form on oxidizing pyrite and the potential strategies to prevent "runaway" AMD by reducing the rate of acid production to the point that the acid can be neutralized by the surrounding rocks. In both studies, experiments were conducted to measure reaction rates for the underlying minerals, as coatings grew thicker. These experimental data were fit to a diffusion model to estimate diffusion coefficients of reactants through pore spaces in coatings. These models are extrapolated to longer times to predict the behavior of the coated grains under field conditions. The experimental results indicate that management practices can be improved for ALDs and mine waste piles. For example, supersaturation with respect to gypsum, leading to coating formation, can be avoided by diluting the ALD feed solution or by replacing limestone with dolomite. AMD can be prevented if the rate of alkalinity addition to mine waste piles is faster than acid is produced by pyrite oxidation. The diffusion model developed in this study predicts when iron oxyhydroxide coatings will become thick enough that alkalinity from the surroundings is sufficient to neutralize acid produced by coated pyrite oxidation and additional alkalinity is no longer required. / Ph. D.