Fundamental Controls on the Reactivity of Aluminum Oxide and Hydroxide Surfaces: Contributions of Surface Site Coordination States and Interfacial Water Structure

Chemical reactions at mineral-water interfaces are of great importance in many geological and environmental processes. Essential to many of these is adsorption because it directly controls contaminant fate and nutrient availability, promotes the nucleation and growth of minerals, initiates surface redox reactions, and plays a crucial role in carbon cycling and sequestration. These reactions occur at mineral surface sites having multiple possible coordination states that interact with both adsorbates and water. While general ion adsorption mechanisms and surface charging behaviors are well established, the roles of individual surface functional group types and water in affecting the structure and reactivity of the interfacial region have not been systematically investigated. Previous studies suggest that surface functional groups in different coordination environments differ in their charge states, proton affinities, and kinetics of oxygen exchange with water. This indicates that these groups may also have different reactivities toward adsorbates and may induce distinct structural arrangements of water near mineral surfaces. This latter role for charged surface groups further suggests that adsorbates, which act as new charged surface sites, potentially may alter the structure of water near surfaces. Such restructuring of interfacial water may contribute to the energetics of many important reactions at environmental interfaces, but this remains unclear currently. Thus, a direct relationship between surface reactions (involving various surface functional groups and adsorbates) and interfacial water properties needs to be established. Aluminum (hydr)oxides, as important reactive and widespread minerals in nature and in engineered systems, play significant roles in many geological and environmental processes. Surfaces of these minerals are especially important due to their ability to control the degradation and transformation of contaminants in soils and sediments and affect the composition of natural waters and geochemical element cycling. Gibbsite, the most common form of aluminum hydroxide found in nature, and its rarer polymorph bayerite, display substantially different morphologies dominated by distinct crystallographic planes. Corundum, as the only thermodynamically stable form of aluminum oxide, has been widely investigated as a proxy for aluminum hydroxide mineral surfaces and surfaces of other phases, such as the edges of Al-rich smectites and the aluminol surface of kaolinite. This study is focused on these minerals because they can serve as model analogues for understanding the surface reactivity of other naturally abundant Al-bearing minerals in soils and sediments due to the similarity in functional groups exposed on their surfaces and for studying fundamental geochemical reactions at interfaces. The main objective of this dissertation is to determine how surface site coordination states on aluminum (hydr)oxide mineral surfaces affect ion adsorption mechanisms, interfacial water structure, and the feedback between these. Arsenate is employed as the probe adsorbate because of its environmental relevance and its uptake over a wide pH range. This research specifically seeks to (1) identify the effects of surface site coordination on macroscopic arsenate adsorption and its binding mechanisms on synthetic gibbsite and bayerite particles; (2) determine how ionic strength affects arsenate adsorption on gibbsite and bayerite; (3) characterize the response of interfacial water structure to pH variations and arsenate adsorption on corundum (001) surfaces; and (4) compare the response of interfacial water structure to pH variations and arsenate adsorption on corundum (012) and (001) surfaces. Synthetic gibbsite and bayerite have distinct particle morphologies, exposing different types of functional groups. Gibbsite platelets expose large (001) basal surfaces terminated predominantly by \u3eAl2O groups, whereas bayerite microrods display mainly edge surfaces dominated by \u3eAlO groups. Macroscopic adsorption isotherms at a single ionic strength show that gibbsite adsorbs less arsenate per unit surface area than bayerite at pH 4 and 7 and suggest that two surface complexes form on each mineral. Arsenate adsorption decreases with increasing ionic strength on both minerals, with a larger effect at pH 4 than pH 7. The observed pH-dependence corresponds with a substantial decrease in surface charge, as indicated by _-potential measurements. At a single ionic strength, similar electrokinetic behavior is observed at the same relative coverages of arsenate, suggesting that similar reactive surface groups (\u3eAlO) control surface charging on both minerals. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows no variation in arsenate surface speciation on a given mineral with different surface coverage, pH, and ionic strength. While bidentate binuclear inner-sphere complexes are the dominant surface species present, EXAFS results find that the number of second shell Al neighbors around arsenate is lower than required for this adsorbate to occur solely as an inner-sphere complex, suggesting that outer-sphere species also occur on both minerals, in greater abundance on gibbsite. Together, these observations reveal that arsenate adsorption mechanisms and capacities vary with mineral morphologies because of the distribution of distinct surface functional groups. These also demonstrate that arsenate displays macroscopic and spectroscopic behavior consistent with the coexistence of inner- and outer-sphere surface complexes. This dissertation also investigated interfacial water structure near single crystal corundum surfaces. Surface X-ray scattering measurements show that corundum (001) surfaces induce weak spatial ordering of interfacial water that varies little between pH 5 and 9 but is substantially altered by the adsorption of arsenate. In the absence of arsenate, interfacial water ordering near the (012) surfaces is also largely unaffected by pH. This general invariance observed on both surfaces suggest that over the pH range of most natural waters, surface site protonation-deprotonation appears inadequate to induce extensive restructuring of interfacial water. The adsorption of arsenate weakly perturbs interfacial water structure near the (012) surface, in contrast to the substantial restructuring of interfacial water seen near the (001) surface. Arsenate is observed to form coexisting inner- and outer-sphere surface complexes on both surfaces, suggesting that adsorption mechanisms may not control the resulting restructuring of interfacial water. Instead, the different surface functional groups present on the (001) and (012) surfaces, with their distinct charging behaviors, likely drive the response of interfacial water to arsenate adsorption. This study improves our understanding of the fundamental controls of chemical reactions at environmental interfaces through systematic studies of arsenate adsorption on aluminum hydroxide particles and aluminum oxide single crystals. Arsenate adsorption on aluminum hydroxide surfaces is complicated, with different types of surface complexes forming through reactions at multiple types of functional groups of different reactivities. The complex interactions between arsenate and aluminum hydroxides can be extended to systems with other naturally abundant Al/Fe-bearing minerals and this must be considered when predicting arsenate fate at environmental interfaces. The dynamic response of interfacial water structure to adsorbates observed on different corundum surfaces here suggests a relationship among surface functional group coordination states, ion adsorption mechanisms, and interfacial water structure. Such adsorbate-induced restructuring of interfacial water indicates that water structure plays an important role in the energetics of interfacial reactions. This study provides new insight into the roles of surface functional group coordination states and interfacial water restructuring in chemical reactions at mineral-water interfaces