Date of Award
Doctor of Philosophy (PhD)
Smectite clay minerals are present in diverse environments on Earth as a major product of silicate weathering. Although smectites are compositionally varied, they almost always incorporate iron. Iron-bearing smectites are an important component of the biogeochemical iron cycle, serving as a major iron fraction in many soils and altered rocks as well as participating in redox reactions within the surrounding environment. Iron incorporates into smectites in the ferric [Fe(III)] form at the modern, oxygenated surface of Earth but clays forming in anoxic settings instead incorporate ferrous iron [Fe(II)]. Owing to their ubiquity at the surface of the modern Earth, the redox behavior of Fe(III) smectites have been studied extensively. The redox behavior of Fe(II) smectites formed under anoxic conditions are less well examined due to the difficulty in sampling and preparing the minerals without oxidation. Fe(II) smectites however are major components of the marine subsurface and were likely widely abundant on the early Earth. Studying their redox behavior can shed light on a large and under-examined pool of Fe(II) contributing to abiotic redox reactions and potentially available to microbial metabolisms. Studying the products of Fe(II) oxidation in smectites may reveal markers of past redox events preserved in the rock record.
The main objective of this work is to utilize the laboratory synthesis of Fe(II)-bearing smectites to examine the characteristics and oxidation behavior of these minerals which are otherwise costly and difficult to study. The synthesis of smectites has been used to study variations in the chemical and physical properties of these clay minerals for decades, but only recently has the synthesis of pure Fe(II)-bearing smectites been achieved. Utilizing this advancement, this work consists of three research projects that were conducted using synthetic Fe(II)-bearing smectites. First, trioctahedral Fe(II)-smectites were synthesized and exposed to dissolved oxygen, nitrite, or hydrogen peroxide to examine the rates, extents, and products of oxidation. Second, mixed-valence iron smectites were synthesized and exposed to oxygen to examine differences in structure and oxidation behavior between these minerals and the ferrous smectites. Finally, Fe(II)-bearing smectites were synthesized with a minor component of Co(II), Mn(II), Ni(II), V(III), or Zn(II) incorporated. These trace metal-bearing smectites were oxidized by dissolved oxygen or hydrogen peroxide to observe the effect of oxidation on the fate of the incorporated metals.
Trioctahedral ferrous smectites oxidized only partially when exposed to dissolved oxygen for 20 to 30 days. Two compositons of smectite with varied iron content were tested at two oxygen concentrations representing a microoxic environment and the modern oxygenated atmosphere. Oxidation proceeded more rapidly and to a greater overall extent when the iron concentration in the smectite or the oxygen concentration of the solution was greater but was always incomplete. This behavior contrasts with reduced nontronites, which reoxidize completely following exposure to air. Oxidation by nitrite was minimal in the moderate-iron smectite and proceeded slowing in the high-iron smectite, achieving 17% oxidation after 54 days of exposure. Nitrite forms as a reactive intermediary in microbial denitrification and has been implicated as a possible abiotic cause for iron oxidation associated with denitrifying microorganisms. The rates observed here suggest that nitrite is not a likely cause for abiotic oxidation in trioctahedral ferrous smectites over laboratory timescales. Hydrogen peroxide caused rapid and nearly complete oxidation of iron in both smectites, and no rate was measured. Examining the products of each oxidation experiment, no secondary minerals were detected by either powder X-ray diffraction or Mössbauer spectroscopy, even in the case of near total oxidation. The ability of trioctahedral smectites to resist complete oxidation by oxygen shows that they may persist in the rock record in the modern oxygenated atmosphere as mixed valence smectites, although the long term stability of these minerals is unknown.
Mixed-valence smectites were synthesized in order to characterize such phases when they natively contain both Fe(II) and Fe(III) in their structure. The properties and oxidation behavior of these synthetic clays were then contrasted with both reduced ferric smectites and oxidized ferrous smectites. The mixed-valence smectites had an octahedral layer charge greater than partially oxidized ferrous smectites found in nature. Compared to oxidized synthetic ferrous smectites, mixed-valent phases exhibited a smaller range of octahedral sheet sizes, indicating that ferrous smectites may become distorted and unstable when oxidized fully. Structural Fe(II) in mixed-valent smectites was partially recalcitrant against oxidation upon exposure to dissolved oxygen. The fractional extent of oxidation was proportional to the amount of Fe(II) per formula unit. Non-Nernstian redox potentials controlled by neighboring cations may cause Fe(II) to be more resistant to oxidation when Fe(II) was less prevalent. Oxidation did not result in lattice contraction across all samples, as occurs for ferrous smectites. The relative non-reactivity of Fe(II) in these samples may place restrictions on the Fe(II) available as an electron donor for microorganisms or for the reduction and immobilization of contaminants in natural systems. Mixed-valence smectites may represent a potentially large and non-reactive pool of Fe(II) in the environment.
Smectites often incorporate trace metal components but how the oxidation of structural Fe(II) in such clay minerals influences the fate of incorporated trace metals is unknown. In addition, it is currently unclear whether properties of trace metals in smectites may serve as indicators of past redox changes in the rock record. To address these uncertainties, five Fe(II)-Mg(II)-Al(III) smectites were synthesized, one each containing Co, Mn, Ni, V, or Zn. These smectites were then oxidized via exposure to dissolved oxygen or hydrogen peroxide. Syntheses yielded smectite clay minerals with solid-phase trace metal concentrations corresponding to 0.89 to 1.27 mol.% of the octahedral cations. All trace metals except vanadium were incorporated into the octahedral sheet in their divalent form. Solid-phase vanadium occurred as a mixture of trivalent and tetravalent forms that substituted in both the octahedral and tetrahedral sheets. Oxidant exposure solubilized up to 44% of the solid-associate vanadium while >99.8% of all other trace metals remained associated with the smectite. Vanadium occurred as a mixture of V(III), V(IV), and V(V) after oxidation, with H2O2 creating greater amounts of the pentavalent form. Assuming all dissolved vanadium occurred as a vanadate species, this indicates that 79% of vanadium in the system oxidized to V(V). Approximately 3% of solid phase cobalt was oxidized to Co(III) when the clay was exposed to H2O2, but no oxidation was detectable after O2 exposure. Manganese showed greater oxidation, with 7% Mn(III) forming in the smectite after O2 exposure and 43% Mn(III) after H2O2 exposure; no Mn(IV) was detected. For the redox-inactive trace metals Ni(II) and Zn(II), oxidation resulted in no change in their local coordination environment other than greater structural disorder. No partitioning of these trace metals to edge or interlayer surface sites was observed. These findings demonstrate that redox-active trace metals differentially record oxidation of iron in the smectite structure. Minor cobalt oxidation is possibly an indicator of involvement of reactive oxygen species. The extents of manganese and vanadium oxidation reflect the redox potential of the oxidant but the impact of oxidant concentration, e.g., oxic versus microoxic conditions, is not yet established. Full retention of nickel and zinc in smectites during oxidation indicates that their isotopic compositions will not be perturbed and should record formation conditions. However, vanadium release during smectite oxidation may result in substantial changes in vanadium isotopic signatures. Together, trace metals in smectites having compositions reflective of altered mafic crust provide a series of potential proxies for formation conditions and post-depositional oxidation.
These results provide a new understanding for the behavior of Fe(II)-bearing smectites and their oxidized products. Synthesis techniques provide homogenous and single-phase Fe(II)-bearing smectites that allow for examination of the same smectite under varied conditions. They also enable creation of a series of smectites that vary in systematic ways and the examination of how specific compositional trends affect redox behavior. Using these tools, we observe that Fe(II) in smectites may be limited in its ability to participate in redox reactions in the environment and may persist in a mixed-valence state. Smectites natively containing both Fe(II) and Fe(III), thus without the structural disruptions caused by iron oxidation, also exhibited a recalcitrant ferrous iron pool. This suggests that recrystallization of the oxidation products of initially Fe(II)-bearing smectites may resist complete oxidation and persist in a mixed-valent state. Incorporated trace metal components display element-specific responses to oxidation that also differ from the behavior displayed by iron. The profile of trace metal chemical and structural properties in smectites may thus provide information about past redox conditions. Finally, this dissertation demonstrates that chemically- or microbially-reduced forms of ferric smectites, such as nontronite, that have been widely examined in prior work are poor predictors of the behavior of natively Fe(II)-bearing smectites. Such clay are abundant in the modern subsurface of Earth, were likely widespread at Earth’s surface prior to atmospheric oxygenation, and occur in diverse planetary settings. Accurate understanding of the role of clay minerals in abiotic and microbially-mediated redox processes and the associated signatures in the rock record require knowledge of the specific phases dominant in such systems.
Chair and Committee
Jeffrey G Catalano
Alex S Bradley
Kupper II, Robert Joe, "A Synthesis-Based Approach to the Study of Fe(II) Oxidation in Smectites" (2022). Arts & Sciences Electronic Theses and Dissertations. 2742.