The Atomic and Electronic Structure of Contaminants at the Mineral-Fluid Interface

Abstract

The mobility of environmental contaminants is largely controlled by chemical reactions at the mineral-fluid interface. How a contaminant interacts with a mineral surface is fundamentally dependent on the molecular-level structure and properties of both the contaminant and the mineral surface. This dissertation explores different types of adsorption/incorporation processes and contaminant species (U, Np, Cr, and an organometallic compound as a proxy for oil) in order to understand specific mechanisms that may lead to contaminant (im)mobilization at the mineral-fluid interface. New applications of high-resolution experimental approaches and molecular modeling are developed in order to achieve a detailed understanding of mineral-fluid interface reactions. Contaminants that are structurally incorporated into a host mineral remain immobilized until mineral dissolution, which has significant implications for the long-term sequestration of mobile contaminants. Chapter 2 describes a new method to model the incorporation aqueous uranium U(VI) and neptunium Np(V) into carbonate and sulfate minerals. Here, we use quantum mechanics to calculate the equilibrium energetics and thermodynamics of incorporation reactions and to consider the structural and electronic changes in the host mineral. U and Np incorporation is more likely to occur at crystallographic defect sites, such as vacancies or impurities. It was also found that the ionic radius of the replaced cation is not necessarily an indicator (as it is often used) of improved incorporation energies due to the influence of hydration. Dissolved contaminants may also be immobilized through redox reactions catalyzed by mineral surfaces. The interaction between chromium (Cr) and the surface of the Fe(II)-bearing mineral magnetite, Fe(II)Fe(III)2O4, as an example catalyst, are investigated using electrochemical atomic force microscopy (Chapter 3). With this method, the reductive precipitation of Cr on the magnetite surface is imaged over time as a function of redox potential and pH of the solution. The redox transitions between Cr(III) and Cr(VI) at pH 7 are in agreement with the Eh conditions predicted from thermodynamic calculations, yet these predictions break down at more extreme pH conditions (pH 3 and 11), demonstrating the need for more robust thermodynamic databases of contaminant speciation. Mineral surface wettability, which describes whether a surface is hydrophilic (water-wet) or hydrophobic (so-called oil-wet), determines the distribution and mobility of oil in contaminated soils and groundwater and in hydrocarbon reservoirs. The adsorption of the polar components of oil or other surface-active agents, such as hexamethyldisilazane ((CH3)3SiNHSi(CH3)3, HMDS), can result in a change in wettability and how easily oil can be extracted from a particular formation. Using calcite as a substrate, changes in the mineral surface structure and surface potential are observed as the calcite surface undergoes wettability alteration from water-wet to oil-wet. For the first time, surface potential measurements were used to interpret how mineral surfaces react to wettability alteration. Atomic force microscopy and computational modeling reveal that the surface structure, primarily edges and corner sites, determine where HMDS can be adsorbed on the surface. Therefore, calcite substrates with higher densities of edges and corner features, such as fine-grained chalk, may be more likely to become mixed-wet or oil-wet after hydrocarbon migration. Molecular simulations reveal that the HMDS molecule is amphoteric and capable of interacting with both H+ and OH- surface groups, meaning that this molecule may function as an efficient wettability modifier at a range of pH conditions.