Experimental and Quantum-Mechanical Approaches to Explore Oxide, Phosphate, and Sulfate Minerals as Redox Catalysts, Geochemical Probes, and the Stability of Their Solid Solutions

Abstract

In this dissertation, the role of mineral catalysis for redox reactions, and the thermodynamics, mechanism, and atomic and electronic structure of ionic substitution in minerals are investigated using experimental and computational approaches. There has been an increasing understanding in the literature and from our group’s previous work that semiconducting minerals catalyze otherwise-slow redox reactions in the environment, either by partially dehydrating the reactants or by shuttling electrons from the reductant to the oxidant. In this context, Chapter II investigates redox transformation of selenium (Se) catalyzed by magnetite (Fe3O4). A novel electrochemical setup using a powdered mineral as the working electrode and catalytic surface was applied to measure the redox thermodynamics and to identify the reaction mechanisms as a function of speciation, pH, and Eh. We find direct evidence of a multi-electron transfer process for Se reduction and nucleation mediated by magnetite. This study advances our understanding of Se oxidation state changes and catalytic effects of Fe-bearing minerals in subsurface environments. Chapter III investigates mineral catalysis further by adding photons as the main driving force of the electron transfer. The photocatalytic reactivity of anatase (TiO2) indicates interaction between uranyl (as an electron acceptor), organic ligands (electron acceptor and ligand), catalytic surfaces, and UV light at different wavelengths, all of which are necessary to promote the redox kinetics. Uranyl removal was almost independent of wavelength for acetate- and oxalate-containing solutions whereas greater removal was observed for EDTA (using UV-C) and hydroquinone (UV-A) solutions. Our results suggest that formation of uranium-ligand complexes plays a critical role in controlling the reactivity of uranyl species and the stability of reduced uranium species in the course of the photoreaction. In Chapters IV and V, quantum-mechanical modeling has been applied to simulate ionic substitution in minerals and establish the thermodynamic basis for using these incorporated phases as a geochemical probe and the structural stability of solid solutions in the environment. Chapter IV addresses the energetic stability and geometry of sulfur (S) in multiple oxidation states in apatite [Ca5(PO4)3(F,OH,Cl)]. These properties of S-incorporated apatite vary depending on (1) the major/minor ions in apatite and their site preferences, and (2) the molecular geometry and orientation of S oxyanions in the structure. These new computational results provide the thermodynamic framework required to investigate the potential role of S in apatite as a proxy to trace redox conditions in hydrothermal-magmatic systems. In Chapter V, thermodynamic mixing properties of sulfate-chromate (S-Cr) and sulfate-selenate (S-Se) solid solution and of sulfate-phosphate-arsenate (S-P-As) solid solutions in alunite supergroup minerals are investigated. This work includes the first geochemical application that combines first-principles calculations, statistical thermodynamic analysis, and the convex hull method to derive phase diagrams of binary and ternary solid solution. S-Cr and S-Se solid solutions in alunite [KAl3(SO4)2(OH)6] and jarosite [KFe3(SO4)2(OH)6] tend to be complete at room temperature and no ordering is acquired at or above ambient conditions. Our computed phase diagrams of S-P-As mixing suggest that binary solid solutions between pairs of sulfate, phosphate, and arsenate in alunite-like minerals scarcely occur below 100 °C, are limited to temperatures from 100 to 300 °C, and become extensive or complete above 300 °C. Our computational model demonstrates the potential role of alunite and jarosite as an indicator for the equilibrium temperature on magmatic-hydrothermal processes as well as in controlling toxic elements for long-term immobilization.