Geochemical Behavior of Low Valence Manganese Oxides: Effects of Structural Impurity and Secondary Mineral
|As the third most abundant transition metal in the Earth’s crust, manganese (Mn) is often found in various forms of (oxyhydr)oxides in the environment (hereafter, Mn-oxides). Mn-oxides possess a high sorption capacity and are a powerful oxidizer, and thus, play critical roles in regulating nutrient cycles and the speciation and distribution of trace metals and metalloids in the environment. The structure and geochemical behaviors of Mn-oxides have been extensively studied using birnessite minerals that contain mainly Mn(IV) and variable concentrations of Mn(II) and Mn(III) in layered structures. However, relatively little research has been done on the more common lower valent Mn(II/III) oxides, such as hausmannite and manganite, presenting a lack of knowledge on the reactivity and transformation process in those Mn minerals. Thus, this dissertation focuses on accurately reflecting natural environments to measure the adsorption and oxidation ability of lower valent Mn-oxides towards a toxic metalloid of concern, namely, arsenic (As). That is, conditions, such as the presence of metal impurities in the mineral’s structure and the co-existence of the secondary mineral phases, are simulated to examine how and to what extent these variations affect the geochemical reactivity and transformation processes of hausmannite and manganite. Furthermore, examination of the fate of metal impurities when the minerals undergo dissolution reactions is of critical importance to better understand the environmental behavior and cycling of trace metals that are associated with the minerals. Chapter 1 and Chapter 2 examine the impact of various quantities of the structural impurities nickel (Ni) and cobalt (Co), the most common metal impurities in lower valent Mn-oxides on the stability and reactivity of hausmannite and/or of manganite toward As. Chapter 3 investigates the surficial interaction modes of hausmannite and manganite and their synergistic reactivity toward As oxidation and removal processes when they co-present. To probe the changes in the structural and surficial properties, as well as in the size and morphology of Mn-oxides induced by the metal substitution, As(III) oxidation to As(V), and the formation of other Mn oxide phases, these research activities utilize an array of state-of-the-art instrumentations, including X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, attenuated total reflectance-Fourier transform infrared spectroscopy, and scanning and transmission electron microscopy. Metal substitutions at higher loadings induce greater structural changes in hausmannite and significantly enhances the As(III) oxidizing ability of hausmannite, but is at a lesser extent in manganite. As(III) oxidation by both oxides leads to the formation of multiple surface complexes, but binuclear bidentate As(V) surface complexes are dominant on oxides. Both Ni(II) and Co(III) occupy the octahedral Mn(III) sites in hausmannite, but only redox-active Co(III) is involved with the As(III) oxidation, while redox-inactive Ni(II) is not. In contrast, when Co(II) occupies the octahedral Mn(III) sites in manganite, the oxidizing ability of manganite is not affected. Therefore, both Co coordination chemistry and structural quantity are critical to determining the extent of structural Co effects in Mn(II/III)-oxides. In both oxides, a greater mineral dissolution is observed with metal substitution in the presence of As. Even when the oxides undergo accelerated mineral dissolution, releasing significant levels of structural Mn, the majority of structurally-incorporated Ni and Co remain in the mineral/solid phases, showing the limited solubility and mobility of these metal substituents. Furthermore, mixtures of manganite and hausmannite show a higher As(V) production and As(III) removal rate when comparing that of the individual mineral phase. Such synergistic effects arise from aggregation structures of these oxides, where manganite limits the aggregation of hausmannite, resulting in more highly reactive hausmannite being available/exposed for the surface-mediated As(III) oxidation reactions. Thus, it is important to consider the presence, property, and relative quantity of structural impurities and presence of secondary Mn oxides to better predict the oxidative and adsorptive capacity of Mn oxides toward As in the environment. The present work helps recognize such important, but overlooked, effects on the mineral’s properties and reactivity and hence, improve our understanding of the geochemical behaviors of natural Mn(II/III) oxides and their interaction with associated metals and metalloids in the environment.
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|Geochemical Behavior of Low Valence Manganese Oxides: Effects of Structural Impurity and Secondary Mineral
|Grandstaff, David E.
|Chemtob, Steven M
|Strongin, Daniel R.
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