This study investigates possible geochemical interactions between microbial reduced U(IV) species (Bio-U(IV)O2(s)) and abiotic and biotic factors (Chemolithoautotrophic bacteria) affecting the stability of U(IV)-oxide minerals in anaerobic aquifers. Microbially mediated precipitation of Bio-U(IV)O2(s) produced a mixture of a U(IV) solid, mineral uraninite (UO2(s)) and trace amounts of U(VI). Flow-through column experiments were performed to quantify biotic or abiotic oxidation of Bio-U(IV)O2(s), along with spectroscopic and microscopic techniques to characterize reactants and products. Results from flow through column experiments showed that release of U under anaerobic conditions is higher in presence of oxidants (NO3-, NO2-, Fe(III)) than with no oxidants. The presence of Thiobacillus denitrificans enhanced U release in the presence of 1 mM NO3-. However, in the system in which 5 mM soluble Fe2+ and 5 mM NO3- were present the release was significantly suppressed. One possible explanation is that Thiobacillus denitrificans favors oxidation of soluble Fe2+ over Bio-U(IV)O2(s) . Soluble Fe2+ might form insoluble ferric oxide solids that sorb oxidized products of Bio-U(IV)O2(s). Spectroscopic data provided evidence of U(VI) in reacted columns in which Fe(II) was passed. These findings suggest that the presence or addition of another electron donor such as soluble Fe2+ in the field might suppress re-oxidation of uranium into soluble products that are released in aquifers.

Addition of Mn(IV)-oxide phases pyrolusite or birnessite was investigated as remedial amendment for Hg-contaminated sediments. Because inorganic Hg methylation is a byproduct of bacterial iron and sulfate reduction, reaction of Mn(IV) oxides with pore water should poise sediment oxidation potential at a level higher than favorable for Hg methylation. Changes in Mn(IV)-oxide mineralogy and oxidation state over time were investigated in sediment tank mesocosm experiments in which Mn(IV)-oxide amendment was either mixed into Hg-contaminated sediment or applied as thin-layer sand cap on top of sediment. Mesocosms were sampled between 4 and 15 months of operation and solid phases were characterized by X-ray absorption spectroscopy (XAS). For pyrolusite-amended sediments, Mn(IV)-oxide was altered to a mixture of Mn(III)-oxyhydroxide and Mn,Fe(II,III)-oxide phases, with a progressive increase in the Mn(II)-carbonate fraction over time as mesocosm sediments became more reduced. For birnessite-amended sediments, both Mn(III) oxyhydroxide and Mn(II) carbonate were identified at 4 months, indicating a faster rate of Mn reduction compared to pyrolusite. After 15 months of reaction, birnessite was converted completely to Mn(II) carbonate, whereas residual Mn,Fe(II,III)-oxide phases were still present in addition to Mn(II) carbonate in the pyrolusite mesocosm. Mn(IV)-oxides in the thin layer sand cap showed no changes in XAS spectra after 10 months of reaction. Equilibrium phase relationships support the interpretation of mineral redox buffering by mixed-valent (Mn,Fe)(II,III) oxide phases. Results suggest that longevity of the amendment treatment for redox buffering can be controlled by adjustment of the mass and type of Mn(IV)-oxide applied, mineral crystallinity, surface area, and particle size.

The chemistry of copper (Cu) in ambient aerosols is complex, constrained partially by corrosive weathering nuanced by local aerosol chemistry and overtly controlled by source factors which influence the chemical speciation of Cu in emissions. Without a natural source to the atmosphere the bulk of Cu in atmospheric systems is localized near emission sources which are most commonly due to abrasive wear of moving parts in a mechanical system. A prominent system for Cu is traffic emissions, of which dust from brake wear comprises the majority. Fine and ultra-fine particulate matter (PM2.5 and PM0.25), often enriched in Cu, is considered the greatest environmental hazard to human health, especially in urban locations where health outcomes are notably worse. Inadequate and often simplified characterization of Cu in ambient PM and automotive brake wear prevents full realization of this hazard and handicaps modelling and remediation efforts. This dissertation focusses on the chemical speciation of Cu in ambient PM in urban environments and seeks to elucidate the chemical formation pathways through which Cu is emitted and environmentally weathered. The overall goal was to determine a clear connection from source to sink through chemical analyses of Cu phases. Three main experiments were conducted with the following aims: 1.) Characterize the Cu phases present in weathered and fresh ambient size-fractionated PM; 2.) Investigate whether mechanical wear affects the phase of Cu emitted from car brake pads; 3.) Determine whether the Cu from brake dust forms species similar to those found in ambient PM. Ambient PM2.5 and PM0.25 were collected from several locations in California including at two separate times (2016 and 2020) in Los Angeles. Samples from 2016 were allowed to weather under ambient conditions in a laboratory setting while samples from 2020 were frozen immediately upon collection and protected from air to inhibit weathering/aging. Weathered ambient PM2.5 samples from three locations in the San Joaquin Valley were also studied for comparison. Speciation of Cu phases in bulk samples was performed via X-ray absorption spectroscopy to investigate composition of Cu in PM upon emission and its chemical fate. To investigate the effect of mechanical wear on Cu phases emitted from automotive brake pads a pin-on-disc tribometry experiment was employed. Utilizing commercially available brake pads, the effect of nominal contact pressure on Cu speciation in size fractionated (PM2.5 and PM0.25) samples was examined. Brake pad PM samples were collected during wear using a cascade impactor which allowed for separation of particles based on aerodynamic diameter. Finally, a closed chamber experiment utilizing tribometer-generated brake wear samples probed the effects of various weathering treatments on Cu speciation. Environmentally relevant conditions including high relative humidity, gaseous sulfur dioxide exposure, ultraviolet light, and free radical exposure (∙OH and ∙NOx) were progressively utilized to explore the production and development of Cu species from the parent brake material. Overall, the coupled nature of these experiments illustrates the causal links between automotive brake wear and urban ambient PM and elucidates the major Cu species relevant for environmental and human health considerations.

This dissertation focuses on understanding and quantifying how environmental conditions affect the fate of salinity and mercury in freshwater. Salinity and mercury concentrations are increasing due to anthropogenic activity and threaten ecosystem services of water bodies. Management is important to prevent further degradation. To reconcile management of multiple water quality parameters with ecosystem services, an improved basic knowledge and advanced tools are important. It is still difficult to identify main drivers for elevated salinity at a specific site and there are gaps in our basic understanding of mercury cycling. Therefore, this research focused on development and application of numerical models to improve our understanding of salinity and mercury cycling and to provide tools to plan management measures. The aim of the first chapter was to continue development of a module for seasonally managed wetlands as part of a real-time forecasting tool for salinity in the San Joaquin River watershed. Revising water sources, inflow time series, and model variables that determine the timing of outflow improved model performance by better representing the extensive reuse and recirculation within wetlands. Adequate simulation of conservative water quality parameters such as salinity is a pre-requisite to simulate non-conservative parameters such as mercury. The second chapter focused on critically reviewing published kinetic rate constants for mercury methylation and demethylation including application to a reaction-transport model. Mercury exists in multiple chemical forms and monomethylmercury (MeHg) is one of the most toxic forms. Two important variables that determine MeHg concentrations are the rate of MeHg production and degradation. The critical review informs selection of rate constants from literature and provides a tool to assess rate constants. Experimental conditions and mathematical assumptions were found to cause uncertainty and limit comparability. The aim of the third chapter was to apply a kinetic-thermodynamic model to field data to evaluate how environmental conditions affect MeHg production. The chapter shows how field data can be used to constrain model parameters. A novel rate formulation was developed to simulate precipitation of Hg minerals depending on concentrations of sulfur and organic matter. The addition of the rate improved simulated MeHg under a range of sulfide concentrations. Overall, numerical models proved suitable to identify knowledge gaps and improve basic understanding for both salinity and mercury in freshwater.

Determination of energetically favorable surface complexes that form at the mineral-solution interface is important for understanding the surface reactivity of common minerals such as kaolinite and gibbsite and the behavior of toxic pollutants such as Cd²⁺. In this study, experimental Cd L_III X-ray Absorption Near-Edge Structure (XANES) was combined with periodic density functional theory (DFT), theoretical XANES calculations of the Cd L_III edge, and surface complexation modeling to characterize Cd²⁺ complexes sorbed to surfaces. Cd L_III spectra were collected for Cd²⁺ reference compounds, aqueous solutions, and sorption samples. Linear combination fitting of sorption sample XANES spectra was performed with known compounds and a sorption sample spectrum at low Cd surface coverage (IS-Kaol) that was interpreted to represent a mononuclear inner-sphere complex. With increasing surface coverage, spectra showed a systematic decrease in the fraction of IS-Kaol reference, indicating that as surface coverage increases, the proportion of bidentate complexes decreases. In linear combination fits, a component interpreted as a dimeric component (CdAlO₄(s) or Cd(OH)₂) increased with surface coverage, suggesting that dimerization or surface reformation may be occurring when surface coverage is high. A postulated set of Cd²⁺ surface reactions on the (100) face of kaolinite and gibbsite were compiled as constrained by DFT calculations and Cd L_III XANES. The Charge Distribution MUlti-SIte Complexation (CD-MUSIC) model was used in PhreePlot with this constrained set of reactions and used to derive complexation constants. Sorption on gibbsite was fit with a mononuclear bidentate reaction at low surface coverage (1 Cd/300 nm²), and with bidentate and dimer reactions at high coverage (1 Cd/3 nm²). Sorption on kaolinite was fit with a bidentate reaction at low surface coverage (1 Cd/150 nm²), and with bidentate, monodentate, and outer-sphere reactions at high surface coverage (1 Cd/1.5 nm²). The results indicate that mononuclear inner-sphere complexes form first on both kaolinite and gibbsite. With increased surface coverage, dimer complexes form on gibbsite, and outer-sphere complexes form on kaolinite, which is consistent with results from linear combination fits.

The mechanisms controlling arsenic mobility, including competition with oxyanion ortho-phosphate, in the subsurface and in laboratory controlled experiments were investigated. Additionally, the solubility and meta-stability of GR as a sorbing surface for arsenic in sub-oxic environments was examined. A systems approach was employed to: 1) investigate a model system constrained by thermodynamic constants and a well defined field site, 2) examine a synthetic system to elucidated the effects of co-precipitating oxyanions of arsenic and phosphate with GR, a scenario expected as an environment transitions from oxic to suboxic conditions, and 3) to study the abiotic conditions where GR formation is favorable in suboxic conditions. A systems approach, defined here as the method of investigating a problem by looking at individual but inter-dependent components in the broader context of a larger interdisciplinary environment, was employed to investigate GR in natural and laboratory controlled settings. This research has investigated the mineralogical response, and subsequent surface complexation reactions, to a redox change from oxic to sub oxic conditions. A redox gradient is commonly encountered in nature, observed in wetlands and from flooding sediments. These findings can be applied to a contaminant management plan, and may be important when considering the anticipated precipitation pattern changes due to global climate change. The overall goal of this study was to characterize mineralogical structural changes and solubilities of the solid phase iron sorbate host and the dissolved constituents (incl. nutrients and contaminants) in suboxic environments. Generally, suboxic environments are found in wetlands and submerged soils. Because wetlands accumulate dissolved ions from a vast watershed, a geochemical response to a hydrologic perturbation will likely affect contaminant and nutrient cycling in the region, and should be included in any management response. Iron is one of the most abundant elements in soils and the most abundant redox active element in natural systems, is generally dissolved as a reduced species (FeII) and insoluble as an oxidized species (FeIII). Therefore, iron transitions are principally controlled by the availability of oxygen. The result is a variety of iron oxide and hydroxide mineral phases that are stable and meta-stable under different soil PO2 conditions. Due to the transient variability between the aqueous Fe(II) and solid phase Fe(III) redox states, iron plays a major role in controlling the nutrient and contaminant cycling in subsurface environment. This research has investigated green rusts that may form at the redox boundary between fully oxidized and sub oxic sediments (Root et al. 2007, 2009). A primary goal of this research was to increase the scientific knowledge base concerning arsenic clean up and remediation, removal from the drinking water supply, and sequestration in the solid phase thereby isolating arsenic from uptake into the biota.

Mercury (Hg) contamination in aquatic soils poses significant environmental and public health risks due to its ability to undergo microbial transformation into methylmercury (MeHg), a neurotoxin that bioaccumulates in food webs. Production of MeHg in soil is largely driven by microbial pathways associated with sulfate reduction or iron-reduction, which are active under anaerobic conditions and low redox potentials. Although MeHg is typically found with trace concentrations in soils, bioaccumulation and biomagnification of MeHg in the food web can lead to high, and often hazardous, concentrations in higher trophic levels. The complexity of the mercury cycle, involving interactions between redox conditions, microbial activity, bioaccumulation, chemical speciation, and many other factors necessitates a variety of treatment approaches to address the dynamic behavior of Hg in sediment-water systems. Among these, in-situ remediation strategies offer an advantage by targeting the source of Hg and Hg-methylation, limiting Hg and MeHg diffusing from soils into overlying waters and entering the food web. This dissertation explores the development and evaluation of manganese oxide-modified activated carbon (MOMAC) as an in-situ remediation strategy for redox-sensitive contaminants, with a focus on Hg and MeHg. MOMAC presents a novel approach, combining the redox buffering capacity of manganese oxides with the high sorption capacity of activated carbon to disfavor production of MeHg, while sequestering Hg and MeHg species in soil.The first chapter focuses on the synthesis and characterization of MOMAC compared to homogenously precipitated manganese oxide (MnOx) and unaltered activated carbon (AC). Factors influencing sorption capacity (surface area) and redox-buffering capacity (average Mn redox state and Mn speciation) were investigated and compared for all materials using a variety of techniques including X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, electron microscopy, and surface area analysis. This chapter also included incubation experiment to show how characterized properties translated into redox buffer and sorption capacity in a system with artificial creek water or Hg-contaminated soils. Results showed a lower average oxidation state in MOMAC compared to homogenous MnOx and a lower surface area compared to unaltered activated carbon. However, while MnOx-treated sediments exhibited a large release of Hg, MOMAC was able to buffer redox potential compared to untreated or AC-treated sediments while maintaining Hg concentrations similar to AC. The second chapter utilized flow-through column experiments to compare MOMAC treatments against untreated or AC-treated sediments. Addition of organic carbon was incorporated into these experiments to promote microbial activity by adding dissolved organic carbon into the influent solution as acetate and pyruvate or by homogenizing solid OC into sediment as powdered spirulina, lyophilized cyanobacteria. A stopped-flow state was implemented in the experiments to assess performance under saturated, stagnant conditions, reflecting flooding from precipitation events that can induce changes in redox potentials and reaction kinetics. Results showed that MOMAC can lower production of MeHg in soils compared to untreated and AC-treated soils. However, both abiotic and biotic reductive dissolution, particularly during the stopped-flow state, can rapidly exhaust redox-buffering capacity. The third chapter was a small-scale in-situ field trial to assess whether MOMAC could lower MeHg in soils compared to AC or untreated soils. Soil cores were collected from two floodplain sites and two bank sites and amended with or without treatment. These soils, or soil admixtures, were aliquoted into three fine mesh bags, packed into a plastic mesh tube, and returned to point of extraction. Fine mesh bags were retrieved over the course of 11 weeks. The main Hg-species in soils across sites was analyzed using high energy resolution X-ray absorption spectroscopy. Results did not show that Hg or MeHg were significantly different across treatments but showed variability across sites, suggesting the experiment did not perturb the system sufficiently to observe changes. In contrast, the Hg species were largely similar across sites with some differences in MOMAC-treated soils, suggesting oxidative reaction with Hg-bearing analytes, such as organic associated Hg complexes.

Formation of faujasite- and sodalite/cancrinite-type phases associated with caustic waste reactions in the environment may structurally incorporate contaminant species such as radioactive Sr2+ and Cs+, and thus provide a mechanism of attenuation. In order to investigate mineral evolution and structural incorporation of cations in simplified experiments, aluminosilicate solids were precipitated homogeneously at room temperature from batch solutions containing a 1:1 molal ratio of Si to Al and 10-3 molal Sr and/or Cs, and aged for 30 or 548 d. Syntheses were done with solutions in equilibrium with atmospheric CO2 and with gaspurged solutions. Experimental products were characterized by bulk chemical analyses, chemical extractions, XRD, SEM/TEM, TGA, solid-state 27Al NMR, and Sr EXAFS. Chemical analysis showed that solids had a 1:1 Al:Si molar ratio, and that Sr was sequestered at higher amounts than Cs. After 30 d of aging in purged solutions, XRD showed that zeolite X (faujasitetype) was the only crystalline product. After aging 30 and 548 d in solutions equilibrated with atmospheric CO2, a mixture of sodalite, cancrinite, and minor zeolite X were produced. Surface areas of solids at 30 d were much lower than published values for zeolite phases synthesized at high temperature, although particle aging produced more crystalline and less aggregated phases with higher bulk surface areas. Characterization of products by 27Al NMR indicated only tetrahedrally coordinated Al. Measured isotropic shifts of primary resonances did not change substantially with precipitate aging although the primary mineral phase changed from zeolite X to sodalite/cancrinite, indicating local ordering of Al-Si tetrahedra. Analysis of reaction products by Sr EXAFS suggested Sr bonding in hexagonal prisms and six-membered rings of the supercages of zeolite X that may be more site-specific than those of monovalent cations. For samples aged for 548 d, interatomic distances from Sr-EXAFS are consistent with partial Sr dehydration and bonding to framework oxygen atoms in sodalite cages or in large channels in cancrinite. Incorporation of Sr into both faujasite and sodalite/cancrinite phases is favored over Cs during room temperature synthesis, possibly because of increased cation site competition between Cs+ and Na+. Results of this study help to constrain cation incorporation into sodalite/cancrinite mineral assemblages that form at caustic waste-impacted field sites and may aid in the predictive modeling of contaminant release.