Now showing items 1-20 of 229

• #### Accelerating the discovery and optimization of thermoelectric materials

Widespread application of the Materials Genome Initiative (MGI) promises to revolutionize the discovery and realization of next-generation materials for a diverse set of applications. Fundamental to the success of the MGI is the synergistic effect of computational and experimental material science, wherein computation serves as a guide for experimentation, as opposed to the \textit{ex-post-facto} approach which has dominated the literature in the prior decades. The field of thermoelectrics, in particular, has historically been dominated by experimental work motivated largely by chemical intuition. Complex coupling between scattering phenomena, electronic transport, and thermal transport renders optimization in thermoelectric systems difficult, both experimentally and computationally. We have formulated a computationally inexpensive metric, deemed $\beta_{SE}$, which was applied in a high-throughput computational survey of over 40,000 compounds. This metric was validated and refined through the experimental work within this thesis. Our survey revealed many intriguing material classes, two of which were examined experimentally in detail: 1) the n-type Zintl phases, and 2) the quaternary diamond-like semiconductors. The n-type Zintl phases are a particularly interesting example of where chemical intuition and historical precedent can mislead the discovery of new materials. The p-type Zintl phases are a well-known and historically successful family of thermoelectric materials. The thermoelectric community has long-held that Zintl phases must be p-type due to the proclivity of the typical chemistries to form alkali or alkali-earth vacancies. However, our computational search indicated that the n-type Zintl phases should both outnumber and outperform their p-type counterparts. We proceeded to discover and dope two n-type Zintls, KAlSb$_4$ and KGaSb$_4$, finding them to be promising thermoelectric materials. The quaternary diamond-like (DLS) semiconductors are another class of materials identified through our search. The quaternary materials were predicted to exhibit both high electronic mobility and low lattice thermal conductivity, properties that are generally inversely related to each other. We experimentally investigated the 3x3 matrix of compositions Cu$_2$(Zn,Cd,Hg)(Si, Ge, Sn)Te$_4$, finding the Hg-containing DLS to have an unusually high electronic mobility and abnormally low lattice thermal conductivity- ideal for thermoelectrics. However, while computations predict high performance when doped n-type, all of the quaternary DLS present as degenerately doped p-type semiconductors. To overcome the degenerate p-type doping, applied the concept of phase boundary mapping'' to reduce the carrier concentration nearly 5 orders of magnitude through intrinsic defect manipulation alone. Our work within the quaternary DLS demonstrated that material discovery in thermoelectrics is also an optimization problem with many dimensions -- which is onerous to perform using classical synthesis techniques. The complex optimization problem presented by the DLS was the impetus for the last study presented in this work. We demonstrated that high-throughput experimental synthesis (particularly with bulk ceramics) has the potential to dramatically increase the rate of material optimization, potentially allowing better synergy with existing high-throughput computational efforts. Together, our work ultimately moves the field of thermoelectrics towards the vision described by the MGI. We have produced new metrics for understanding thermoelectric materials, identified potential materials for thermoelectric applications, and built-upon existing experimental techniques to accelerate material optimization. Together these efforts have begun to unravel the complex structure-property relations that dictate thermoelectric performance.
• #### Accumulated inelastic strain based damage modeling of pressure vessel steels in air

A great deal of research is currently being conducted on methods which can lead to a more robust clean energy infrastructure. One such method is using hydrogen gas as an energy carrier to store energy during low demand and convert it to electricity when demand increases. Many of the current vessels designed to transport and store pressurized hydrogen gas are designated DOT3AA cylinders. The materials used for the construction of DOT3AA cylinders designed for hydrogen service must be qualified by use of ISO-11114 Transportable Gas Cylinder- Compatibility of Cylinders and Valve Materials with Gas Contents standard: Part 4 - Test methods for selecting steels resistant to hydrogen embrittlement. The standard uses three disparate methods to certify materials for this use. This work creates a framework for modeling each of the three test methods as well as in-service conditions of a DOT3AA cylinder in the finite element platform ABAQUS. To determine the deformation response at areas of interest, a non-linear combined kinematic/isotropic hardening model has been implemented and calibrated to strain-life fatigue tests conducted in air on AISI4130 steel removed from an DOT3AA cylinder. The modeling results are used to compare each of the three test methods to in-service conditions. The resulting framework can be built upon to incorporate the effects of hydrogen and is a first step towards the ability to unify results of each test method as well as in-service conditions.
• #### Advanced control of converters with multitask functionalities in distribution grid systems based on conservative power theory

Distributed generation (DG) play a very important role in the modernization of electric power systems, it is estimated their increasing share of operation in the near future. In addition, there is growing concern on the environmental issues, lack of transmission capacity and limitation in constructing new lines, and increasing demand of energy, that would support a more flexible inverter control, capable of interacting users with the utility grid. The main objective of such a system is providing active power, which is the primary use to balance loads. However, power electronic systems can provide power quality improvement and DG systems would then be used as multi-functional compensators for improving power quality, instead of only balancing or selling active power to the grids. In this context, this dissertation first studies the well-known instantaneous current decomposition theories highlighting the important contributions based on the Instantaneous Power (PQ) theory and the Conservative Power Theory (CPT) and presents a comprehensive comparison from performance and computational complexity perspectives. Although these theories are quite distinct in their formulations, the central idea is to make a comparative study between the current portions and their respective portions of power, in order to show the similarities and divergences between them in terms of characterization of the physical phenomena and in terms of disturbing current compensation. The studied instantaneous current decomposition techniques are then used to provide selective functionalities in distribution systems. Therefore, it is possible to inject active power plus compensate selectively unwanted current terms (reactive, unbalance, and distortion), enabling full exploitation of the inverter capability and increasing its overall cost-benefit and efficiency. Afterwards, control structures with multitask functionality to the grid side converter of the renewables to carry out the power quality ancillary services in the distribution system are developed. The key diversity of the methodologies we proposed in this project with respect to others in the literature is that the developed control structures on the grid side converters are based on the CPT theory. This choice provides decoupled power and current references for the grid side inverter control, which offers very flexible, selective and powerful functionalities. These qualities make the system to be the benchmark for achieving 100% renewable and sustainable grid with multifunctional capabilities. This thesis then proposes the coordinated control of the aforementioned multifunctional interfacing DG systems to enhance the operation of microgrid systems. Based on our proposed method, a hybrid cooperative strategy is developed that overcomes limitations in communication-based and non-communication-based approaches for the coordinated operation of multifunctional distributed generators in islanded microgrid systems. Two important issues that are addressed are the power quality and undesirable current sharing, particularly in the low-voltage distribution network, where electronic devices are drawing distorted and unbalanced currents. The interactions of such current disturbances with high feeder/line impedances, in a low voltage system cause considerable voltage deterioration and possibly affect sensitive loads showing the requirement for power quality enhancement. Finally, this thesis explores the study and implementation of cascaded multilevel converters, in which the primary concepts relating to modulation, structure, and control schemes are detailed. These topologies are composed of series-connected H-bridge converters with isolated DC links. Therefore, it is possible to integrate renewable energy and storage resources to power grids. The experimental findings validate the applicability and performance of the proposed control strategies in distribution grid systems.
• #### Advanced organic characterization of hydraulic fracturing wastewaters

Advancements in technology have allowed for the utilization of previously unattainable natural gas resources. Hydraulic fracturing (HF) is a process used in the extraction of underground resources to increase oil, natural gas, and water production rates when these resources are located in rock formations with a naturally low permeability. Horizontal fracturing, often referred to as high volume fracturing, is the preferred method for removing natural gas from shale facies. After the fracturing event is complete, injection water returns to the surface as HF wastewater (HFWW). In the beginning of the flowback period, this wastewater is thought to be more representative of the injection water and is referred to as flowback water. As the flowback period continues, this water is more influenced by the shale facies and are referred to as produced water. The United States produces 870 billion gallons of produced water annually. Produced water is comprised of a geogenic portion, consisting of compounds native to the geologic formation, and additives, which contain chemicals used to stimulate the fracturing formation and aid in production. Recently there has been an increased push from industry, the scientific community, and the public, suggesting produced water from oil and gas (O&G) operations could potentially represent a new water source for areas with water scarcity problems, such as Colorado. Although alternative uses for this water could greatly benefit communities, careful consideration of the chemical composition must be given before reuse or treatment. The objective of this dissertation was to characterize HFWW throughout the fracturing process and their interaction in the environment in the event of a spill. Four research efforts were undertaken to evaluate this topic: identify new analytical methods needed for complete chemical characterization (Chapter 2), describe the temporal variability known chemical constituents of HFWW (Chapter 3), identify and describe the unknown chemical variation (Chapter 4), and simulate a HFWW surface spill in an agricultural soil under environmentally relevant conditions (Chapter 5). Chapter 2 focused exclusively on the organic fraction of this wastewater. It was found that many organic chemicals remain unidentified, and targeted approaches for organic chemical analysis alone will be insufficient for complete organic chemical characterization. This dissertation presents applications of under-utilized approaches that may serve as potential solutions to address the issues created by the complex matrices inherent to flowback and produced water. The temporal variation identified in Chapter 3 found the presence of numerous surfactant homologs, including biocides, with the highest levels at the beginning of the flowback period. It was also discovered that three different stages exist in the flowback period: the flowback stage, the transition stage, and the produced water stage. The results from Chapter 4 found that numerous homologous series were present. The increase in homologous series during the transition stage corresponded with variability described in the principal component analysis of nontargeted high resolution mass spectrometry data. Finally, Chapter 5 demonstrated that no surfactants or their transformation products were found in leachate samples. Thus, in this environment, under these time constraints, these compounds are unlikely to travel far from the initial spill site. However, the leaching of trace metals due to salts was observed and could pose a threat to ground and surface waters. The results of this dissertation motivate further efforts for complete characterization of HFWW; these efforts may lead to significant improvements in HFWW treatment, potentially leading to the beneficial reuse of these waters.

Fiber-reinforced polymer composites are an intriguing class of engineering materials that are increasingly exploited in the construction, aerospace, and energy sectors. Their high specific properties make them an ideal design choice where traditional engineering materials like metals are too heavy, or where unreinforced polymers are not stiff or strong enough. Furthermore, their anisotropic nature can be exploited for unique applications such as airfoils in aircraft wings or wind turbines. However, most structural composites use thermosetting polymers as their matrix, which presents several issues. Foremost is that thermosets cannot be easily recycled, so massive amounts of composite waste are landfilled at the end of a part’s service life. Secondly, thermoset subcomponents of a larger structure can only be joined using adhesives. Conversely, thermoplastic composites enable recycling after a part is retired from service and facilitate thermal joining of multi-part structures. Liquid infusible thermoplastic resins are beginning to emerge for use in vacuum-assisted resin transfer molding, which is the method of manufacture for wind turbine blades. While infusible thermosetting resins have been well characterized, basic characterization of rheological and kinetic behavior for thermoplastic resins is lacking. The present work provides important experimental development and data aimed at characterization of infusible thermoplastic resin systems. A novel thermoplastic biobased resin system is also developed, which has potential for commercial use.
• #### Analysis of military protective structures: a framework for quantifying cost-benefit of existing and new protective systems

The United States military is constantly evolving into an organization equipped by the latest technology and seeking the greatest protection per cost ratio for its members in harm’s way. While new protection methods are steadily produced by the Engineering Research and Development Command, most protective structure options fall into either very expensive or very labor-intensive structures with widely varying degrees of reusability and transportability. Furthermore, there is currently no widely accepted quantitative approach to help the decision-making process when choosing which system to use in a specific condition. This study will seek to create a framework which can be used to aid the decision-making process based on quantitative calculation of cost benefit of various protective systems. The framework will encompass resource metrics of man-hours, machine hours, and monetary cost. The calculations and assessments will also be affected by quantitative evaluations of military situations which can increase or decrease each value of resource metric. This study will also investigate the potential of using a mass timber product, namely Cross Laminated Timber (CLT) panels, as a protective structure that may be useful in certain military situations. While not designed to replace other systems, it is another option for military commanders and staffs to consider when choosing the most efficient and economical protection method for their soldiers.
• #### Analysis of the stratigraphy, sedimentology and reservoir quality of the Dean sandstone within Borden and Dawson counties, Midland Basin, West Texas, An

Located in West Texas and Southeastern New Mexico, the Greater Permian Basin has recently become the largest petroleum-producing basin in the United States and the second largest in the world, having produced over 30 billion barrels of oil as of January 2018 (Mercador, 2018). Although the Permian Basin has been conventionally drilled since the 1920s, horizontal drilling and hydraulic fracturing have recently generated a resurgence in activity in the once thought uneconomical, low permeability, basinal plays of the Lower Permian (Wolfcampian and Leonardian Series) stratigraphy. The current most popular unconventional targets within the Midland Basin, a sub-basin of the broader Permian Basin region, are the fine-grained, low permeability siliciclastic intervals of the Leonardian Series (Spraberry and Dean formations) and the organic-rich calcareous mudstones of the Wolfcampian interval. The Permian Basin has been subject to a great number of geologic studies to establish age, stratigraphy, regional setting, and depositional facies in support of a long history of conventional oil field reservoir development in the basin. A few recent studies have been conducted on the most popular unconventional play in the Midland Basin, sometimes referred to as the Wolfberry Play (Wolfcamp interval and Spraberry Formation), but fewer yet have studied the Dean Formation. In response to growing industry interest in the Permian Basin, this study focuses on the sedimentology, stratigraphy and reservoir quality of the Dean Formation within Borden and Dawson counties, West Texas. Based on observations, analyses, and interpretations, multiple conclusions were made regarding the Dean formation in this thesis study. The Dean Formation can be divided into two parts, the Upper and Lower Dean, which are separated by a cemented carbonate zone, mineralogically unique from one another, and consequently contain different reservoir properties due to these differences. Overall, the Upper Dean contains a higher percentage of authigenic and detrital carbonate minerals that are prone to occlude porosity and restrict permeability, while the Lower Dean has higher silica content, lower authigenic and detrital carbonate content, and higher overall porosity and permeability. Furthermore, the Lower Dean displays lower water saturation, higher TOC values, and a higher fracture count than the Upper Dean. Six facies were determined through three Dean Formation core descriptions and analyses. All facies can be subsequently broken up into three separate facies associations: Facies 1-3 are basinal facies associations (Facies 1 (Laminated argillaceous siltstone), Facies 2 (Bioturbated argillaceous siltstone), and Facies 3 (Massive/Microburrowed argillaceous siltstone)); Facies 4 and 5 are turbidite facies associations (Facies 4 (Clean siltstone (Bouma sequences Ta, Tb, and Tc)) and Facies 5 (Silty shale (Bouma seqeuences Td and Te)); Facies 6 is a transitional facies association (Facies 6 (Wavy-laminated/rippled stiltstone)). Dean Formation sediments are interpreted as turbidite deposits that have passed through submarine canyons/channels and fans, which were later deposited deep in the basin. Facies within the Dean Formation have both high vertical and lateral variability due to localized turbidite deposits making it difficult to correlate without well logs. Using a technique called core luminance it was found that the standard logging tool resolution used to sample the Dean Formation is inadequate due to the extreme variability of lithology and facies as it typically misses all of these changes. These differences between the core luminance curve and the standard gamma ray curve causes major differences in net sand in the core versus the net sand in well logs. Sediments found in three cores examined exhibited signs of high terrigenous input, moderate amounts of paleoproductivity, and low to moderate amounts of anoxia. XRF and XRD analyses generally showed that silica content within cores increased with depth while carbonate content decreased. Overall, the Dean Formation has relatively low to moderate average porosity values and low average permeability values, ranging from negligible up to ~12% porosity and negligible to 0.4 mD permeability. Facies 1 (average porosity ranging from 9.6 - 9.8% and 0.2 mD), Facies 2 (average porosity ranging from 8.2 - 8.6% and 0.4 mD), and Facies 4 (porosity ranging from 8.7 - 12.0% and 0.2 mD) represented the highest porosity and permeability values while Facies 3 (average porosity of 2.0% and 0.2 mD) and Facies 5 (average porosity ranging from 3.2 – 8.0% and 0.2 mD) had the lowest. Microfractures found within the facies greatly enhanced porosity when present.
• #### Application of a custom-built, 400 MHz NMR probe on Eagle Ford Shale core plug samples, Gonzales and La Salle counties, Texas

Nuclear magnetic resonance (NMR) has become an increasingly important tool for estimating porosity, permeability, and fluid characteristics in oil and gas reservoirs since its introduction in the 1950s. While NMR has become common practice in conventional reservoirs, its application is relatively new to unconventional reservoirs such as the Eagle Ford Shale. Porosity and permeability estimates prove difficult in these exceptionally tight rocks and are routinely below the detection limit and/or resolution of low frequency (2 MHz or less) NMR. High frequency (400 MHz) NMR has been applied to address these issues; however, previous studies have been limited to crushed rock samples or millimeter-sized core plugs. In response, a custom-built NMR probe has been constructed, capable of measuring 0.75-inch diameter, 0.45-inch length core plugs at 400 MHz, to determine if larger core plug sizes yield higher resolution T2 distributions in the Eagle Ford Shale. The tool is composed of two primary elements, the structural framework and the radio frequency circuit. Each element was designed and constructed iteratively to test various layouts while maintaining functionality. The probe's structural design was initially based on retired, commercial probes then modified to operate within a Bruker Ascend 400WB NMR spectrometer. Designs were drafted and 3D-printed multiple times to determine proper physical dimensions and clearances. Once designs were deemed satisfactory, structural components were manufactured and assembled to create the structural framework. A radio frequency circuit was then built to measure T2 distributions at the desired frequency and sample size. Multiple inductor designs and capacitor combinations were tested until a stable circuit, capable of matching impedance and tuning to the proper frequency, was achieved. The probe's stability and data quality were then confirmed by measuring the NMR spectra of deuterated water in a Teflon container. The NMR probe was validated by comparing high frequency (400 MHz) data acquired in-house to low frequency (2 MHz) data measured at a commercial laboratory. Twelve core plugs (0.75-inch diameter, 1-inch length) were cut from two Eagle Ford Shale subsurface cores located in Gonzales and La Salle counties, Texas. Low frequency T2 distributions were measured twice: first after drying core plug samples in a vacuum oven and again after spontaneous imbibition with various brine solutions (deionized water, 8 wt.% KCl, or 17.9 wt.% KCl) for one week. These contrasting saturation states were applied to highlight immovable water in the core plugs. For high frequency data measurements, samples were trimmed to 0.45-inch lengths to fit inside the newly-built NMR probe, leaving two sub-samples for each of the original core plugs. T2 distributions were first acquired "as-is" (e.g., without drying or imbibition). After as-is data acquisition, samples were dried in a vacuum oven then allowed to spontaneously imbibe the same brine solutions used in the low frequency study. T2 distributions were measured again after imbibition and compared to the low frequency data acquired by the commercial laboratory. Qualitatively, high frequency T2 distributions resemble low frequency data; however, the absolute T2 values are routinely higher by one order of magnitude. The difference may be caused by data acquisition, data processing, fluid-rock interactions, magnetic field inhomogeneities, or some combination thereof. In spite of not attaining the higher-resolution T2 distributions desired, the project still provides a proof-of-concept that T2 relaxation times can be measured in conventional-sized core plugs using 400 MHz NMR. Although limited in its outcomes, the study delivers promising results and elicits future research into utilizing high frequency NMR spectroscopy as a petrophysical tool for unconventional reservoirs.

• #### Assessment of performance and efficiency of membrane distillation for treatment of impaired water and brine with high scaling potential

Water recovery is limited in pressure-driven membrane processes such as reverse osmosis (RO) and nanofiltration (NF) due to increase in scaling risk and osmotic pressure of the feed water with concentration. Membrane distillation (MD) is an emerging thermally-driven membrane desalination processes that utilizes a difference in vapor pressure across a microporous, hydrophobic membrane as the driving force. Thus, it is not limited by differences in the osmotic pressure between the feed and permeate and is tolerant of much higher salinity than RO. Nevertheless, MD still suffers from problems associated with membrane fouling, which is one of the major challenges that hinder its commercialization. The overall objective of this dissertation is to elucidate scaling and fouling behavior in MD by various inorganic contaminants relevant to inland brackish desalination, which typically must achieve high water recovery to minimize brine disposal costs. Water flux, thermal efficiency, and rejection were experimentally measured in laboratory experiments using real and synthetic solutions supersaturated with respect to soluble salts, sparingly soluble salts, and silica. Various mitigation and cleaning strategies were tested, and the long-term effects of scaling on MD performance were evaluated by performing repeated experiments on previously-fouled membranes using new solutions. Impacts and control of silica scaling was emphasized because it is ubiquitous in natural water supplies and is of particular concern in brackish desalination. Cleaning of MD membranes scaled by silica was impractical, but several mitigation strategies were effective at preventing silica scale, including modification of feed pH and optimization of feed temperature. Silica scaling propensity in MD was increased by the presence of calcium and magnesium, but the effects were reduced with increased carbonate alkalinity. Desalination of hypersaline brines with high mineral scaling potential were also investigated using water obtained from the Great Salt Lake (GSL). NaCl scaling occurred rapidly at its saturation limit, resulting in immediate loss of performance, and gradual decline in performance was also observed due to both mineral scaling and organic fouling. However, sustainable operation was achieved by operation at low feed temperatures combined with periodically reversing the direction of water flux.

• #### Bastnaesite beneficiation by froth flotation and gravity separation

Rare earth elements are in high demand in the United States. Independence from the importation of rare earths is essential to alleviate dependence on China for these rare earth elements. Bastnaesite, a rare earth fluorocarbonate, is one of the most abundant sources of rare earths in the United States. It is a fluorocarbonate mineral containing primarily cerium and lanthanum. The largest rare earth mine in the United States is Mountain Pass. This research was done to find a way to combine flotation with novel collectors and gravity separation techniques to reach an enhanced grade and recovery of rare earth elements while rejecting the gangue minerals, calcite, barite and silicate minerals. The main economic driving force is the price of hydrochloric acid in downstream processes, as calcite is an acid consumer. Surface chemistry analysis was completed using adsorption density, zeta potential, and microflotation on both gravity concentrates and run of mine ore samples. Four collectors were examined. These were N,2-dihydroxybenzamide, N-hydroxycyclohexanecarboxamide, N,3- dihydroxy-2-naphthamide, and N-hydroxyoleamide. Through this analysis it was determined that, to obtain the desired results, that flotation would be the rougher stage and the gravity separation would be utilized as the cleaner stage. Bench scale flotation tests were conducted on the run of mine ore using conditions that were determined using a previously calculated Stat Ease model. The bench tests that produced the most desirable results were then scaled up to a 10 kilogram float test. A concentrate from this test showed a rare earth oxide grade of 44%, while rejecting 91% of the calcite. This concentrate was used for gravity separation. Through gravity separation it was found that another 40% of the calcite could be rejected with a final rare earth oxide grade of 47% in the concentrate that was produced. Through economic analysis and the results from this project the use of gravity separation is not economical as a cleaner stage and more research should be done on flotation using lock cycle testing.
• #### Beneficiation and hydrometallurgical treatment of Norra Kärr eudialyte mineral

Due to the demand for rare earth elements for everyday technology and applications, there has been much research initiated into the extraction and recovery of rare earth elements. An otherwise unknown mineral, eudialyte, is a zirconium silicate consisting of rare earth oxides, specifically the heavy rare earth oxide yttrium (III), with only trace amounts of thorium and uranium. The focus of this research project was to investigate and develop a beneficiation and leaching procedure for processing the Norra Kärr eudialyte ore. The development of the type of beneficiation and leaching experiments conducted was aided by a review of different physical separation methods and the treatment of iron and silica in other industries. After mineral characterization, a two-stage beneficiation process was developed, consisting of gravity and magnetic separation. The gravity separation portion comprised of preliminary heavy liquid separation tests done using both sodium polytungstate and methylene iodide at different size fractions. Different size fractions were studied for liberation purposes. This gravity separation step was implemented for the removal of the heavy iron-bearing mineral aegirine. This float product is then processed in a wet high-intensity magnetic separation (WHIMS) at 1 Tesla to separate the paramagnetic eudialyte from the non-magnetic gangue minerals. The implementation of this process resulted in limited success for a clear separation of eudialyte from its gangue. The overall results yielded no significant upgrade of eudialyte using the beneficiation process proposed. However, the proposed process did show that iron could be rejected through either gravity or magnetic separation, a definite benefit for further hydrometallurgical treatment. After the conclusion of the beneficiation tests, hydrometallurgical testing was done. The samples used in these leaching experiments were non-magnetic concentrates, where most of the iron was rejected via WHIMS. Two separate leaching processes were investigated to eliminate or minimize the formation of silica gel within the solution, while still recovering the total rare earth elements (TREEs). The first leaching process treated the concentrate in a 0.1 M solution of sulfuric acid at 25, 50 and 75°C at two and four-hour intervals. This leaching process resulted in gelation of the leach liquor as well as filtrate solution, but recovered the TREEs and Zr. The second leaching process limited the amount of water and acid available to the concentrate by only adding enough concentrated sulfuric acid to completely wet the sample. The acid-wet samples were then left for 30 minutes, one hour (then oven dried) or air dried before leached with DI water. While no gelation was observed during or after this leaching process, little to no rare earth elements and zirconium were recovered. It has become evident through these beneficiation and leaching experiments, that a generalized method, applicable in many other mineral processing industries for commonly known minerals, may not be the best method for processing eudialyte. In all, the mineralogy of eudialyte should be more heavily investigated so an appropriate mechanism can be applied. However, it is worth noting that due to the complex chemical composition of eudialyte, a specialization is required within the eudialyte mineral group.
• #### Biochemical and chemical controls on sedimentation, sequence stratigraphy, and diagenesis, in the Phosphoria rock complex (Permian), Rocky Mountain region, USA

Biochemical sedimentation, near-surface diagenesis, stable isotopes, and porosity vary systematically stratigraphically and regionally in the Phosphoria Rock Complex (PRC), Rocky Mountain region, USA, in response to dynamic paleo-environmental conditions spanning the Middle Permian. Environmental, biochemical, and isotopic trends mimic diagnostic trends of the End-Permian Mass Extinction (EPME), occurring through Kungurian-Wordian time (~274Ma to 265Ma), ~13MY before the EPME and ~5MY before the end-Guadalupian crisis. These indicate PRC trends are of a similar genesis to the EPME, and EPME dynamics were driven by locally modified global processes spanning the Middle to Late Permian. Biochemical, isotopic, and environmental trends are heterogeneous across the PRC, a second-order (~9MY) cycle, and the third-order (2-5 MY) Franson (latest Kungurian - Wordian) and Ervay (Wordian) cycles. During transgressions, influx of cool, acidic, low-oxygen, nutrient-rich waters warmed and interacted with hot, oxygenated marine and evaporitic waters. Flourishing sapropelic algal and anaerobic microbial communities resulted in phosphorites and sulfidic-OM-rich mudrocks seaward of calcitic biota and micritic carbonates, redbeds, evaporites and microbialites. Values of δ18O and δ13C in carbonates and silica are depleted in distal settings due to microbial decay of OM and increase landwards due to evaporative fractionation. Values of δ18O in carbonate fluorapatite are depleted in distal environments, increase landwards and towards maximum transgression, indicating warming. Porosity in transgressions is low due to near-surface cementation and recrystallization, as well as compaction and infill of pore space by secondary OM (bitumen) in OM-rich mudrocks. S-rich OM catalyzed early secondary-OM generation and inhibited OM-hosted porosity generation through burial. During highstands warm, oxygenated, alkaline marine waters dominated in and became increasingly hot, shelf-confined, and evaporitic. With limited nutrient influx, and input of eolian-sourced silica, resulted in widespread spiculites and calcitic-biota carbonates at maximum transgression and in distal highstands. Increasingly restricted and evaporitic conditions through highstands resulted in dolomitized bioturbated muds and sandstones, aragonitic molluscs, ooids, and peritidal microbialites. Values of δ18O and δ13C became increasingly enriched throughout highstands in marine carbonates and widespread authigenic silica. This and moganite-bearing chalcedony suggest evaporitic reflux drove silicification and dolomitization. Porosity is most abundant in dolomites deposited in restricted, evaporitic highstand conditions.
• #### Biogeochemical controls of uranium remediation and transport

In the U.S. almost 140 sites have been contaminated by uranium mining and milling operations or by the storage of radioactive materials. In-situ recovery (ISR) facilities still face challenges restoring water to pre-mining conditions and leave behind elevated uranium concentrations. Bioremediation and reactive transport modeling are potential tools to mitigate the impact of uranium contamination on human and environmental health, through their ability to immobilize uranium and assess the effectiveness of natural uranium attenuation. This project investigated biogeochemical aspects of both active and natural remediation of uranium contaminated subsurface for two field sites: The Smith Ranch Highland (SRH) site in WY, and the Rifle, Integrated Field Research Challenge (IFRC) site in CO. Our project objectives were to study the transformation of organic substrate during biostimulation and assess uranium retardation due to sorption with sediments taken from an ISR site. This thesis presents two research projects that address the remediation and risk assessment of uranium contaminated sites. The first project evaluated the impact of added organic carbon on the long-term biogeochemical attenuation of uranium in the subsurface of a former mill tailings site. Fluorescence and specific ultraviolet absorption (SUVA) analyses were used together with dissolved organic carbon (DOC) measurements to track organic carbon dynamics during and post-biostimulation of the 2011 Rifle IFRC experiment. An electron mass balance was performed on well CD01 to determine if any carbon sinks were unaccounted for. DOC values increased to 1.76 mM-C during biostimulation, and 3.18 mM-C post-biostimulation over background DOC values of 0.3-0.4 mM-C. Elevated DOC levels persisted 90 days after acetate injections ceased. The electron mass balance revealed that assumed electron acceptors would not account for the total amount of acetate consumed. Fluorescence spectra showed an increase in signals associated with soluble microbial products (SMP), during biostimulation, which disappeared post-biostimulation despite an increase in DOC. SUVA analyses, indicated that DOC present post-biostimulation is less aromatic in nature, compared to background DOC. Our results suggest that microbes convert injected acetate into a carbon sink that may be available to sustain iron reduction post-stimulation The second project consisted of two sets of column experiments and attempted to evaluate the impact of alkalinity and pH on the sorption of uranium in sediments from an ISR site. The ability of thermodynamic models to predict uranium behavior under conditions relevant to ISR restoration sites was also tested. Sediments at three different depths from a monitoring well at the SRH site were used in nine column studies and six batch experiments to study the sorption capacity of SRH sediments and estimate uncertainties associated with fitted parameters. Sediments were characterized by X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF) for dominant mineralogy and Brunauer-Emmett Teller (BET) measurements to determine sediment surface area. Uranium transport in the columns was modeled with PHREEQC using a generalized composite surface complexation model (GC SCM). A parameter estimation program (PEST) was coupled to PHREEQC to derive best parameter fits according to correlation coefficients and lowest sums of residuals squared. In the first set of sorption experiments a GC SCM utilizing one, two, and, three generic surfaces, was evaluated in 5 column studies to find the simplest model with the best fit. A 2-pK model with strong and very strong sorption sites was found to produce model results in best agreement with observed data. Uranium breakthrough was delayed by a factor of 1.68, 1.69 and 1.47 relative to the non-reactive tracer for three of the 5 experiments at an alkalinity of 540 mg/l. while a sediment containing smectite and kaolinite retained uranium by a factor of 2.80 despite a lower measured BET surface area. Decreasing alkalinity to 360 mg/l from 540 mg/l in the kaolinite containing sediments increased retardation by a factor of 4.26. Model fits correlated well to overall BET surface area in the three columns where clay content was less than 1%. For the sediment with clay, models consistently understated uranium retardation when reactive surface sites were restricted by BET results. Calcite saturation was shown to be a controlling factor for uranium desorption as the pH of the system changed to a lower value. A pH of 6 during a secondary background water flush remobilized previously sorbed uranium resulting in a secondary uranium peak at twice the influent concentrations. The first set of sorption experiments demonstrated the potential of GC SCM models to predict uranium transport in sediments with homogenous mineral composition, but highlighted the need for further research to understand the role of sediment clay composition and calcite saturation in uranium transport. The second set of experiments consisted of duplicate column studies on two sediment depths. Columns were flushed with synthesized restoration waters at two different alkalinities (160 mg/l CaCO3 and 360 mf/l CaCO3) to study the effect of alkalinity on uranium mobility. Low alkalinity (160 mg/l CaCO3) water at pH of 7.5 was introduced after 143 hours, to mimic background water entering the restoration zone. Uranium breakthrough occurred 25% - 30% earlier in columns with 360 mg/l CaCO3 over columns fed with 160 mg/l CaCO3 influent water. Fitted models produced R2 values of > 0.9 for all columns using a 2-surface site model with strong and very strong sorption sites. The results demonstrated that the GC SCM approach is capable of modeling the impact of carbonate on uranium in flow systems. Derived site densities for the two sediment depths were between 135 and 177 µmol-sites/kg-soil, showing similar sorption capacity despite heterogeneity in sediment mineralogy. Model sensitivity to alkalinity and pH was shown to be moderate compared to fitted site densities, when calcite saturation was allowed to equilibrate. Calcite kinetics emerged as a potential source of error when fitting parameters in flow conditions. Fitted results were compared to data from batch experiments conducted on SRH sediments prior, and column studies from the first set of experiments, to assess variability in derived parameters. Parameters from batch experiments were lower by a factor of 1.5 to 3.9 compared to column studies completed on the same sediments. The difference was attributed to errors in solid-solution ratios and the impact of calcite dissolution in batch experiments. Column studies conducted at two different laboratories showed almost an order of magnitude difference in fitted site densities suggesting that methodology may play a bigger role in column sorption behavior than actual sediment heterogeneity. Our results demonstrate the necessity for ISR sites to remove residual pCO2 and equilibrate restoration water with background geochemistry to reduce uranium mobility. In addition, the observed variability between fitted parameters on the same sediments highlights the need to provide standardized guidelines and methodology for regulators and industry when the GC SCM approach is used in subsequent risk assessments. This study demonstrates the impact of biogeochemical parameters on uranium remediation and transport at current and former mining and milling sites. Subsurface bioremediation projects need to incorporate microbial transformation of injected organic carbon into conceptual models and operational procedures. Furthermore, the potential of thermodynamic models to predict uranium behavior at ISR restoration sites was shown to depend highly on accurate representation uranium geochemistry and experimental methodology to derive sorption parameters. The work herein advises regulators and industry on the best practices for the management of uranium contaminated field sites to protect the public health and the environment.
• #### Biomechanical modeling of gait in children with cerebral palsy

Cerebral palsy, a severe motor disability among children, is a chronic neuromuscular disorder that affects an individual’s ability to control basic motor tasks, posture, and muscle coordination. Children diagnosed with cerebral palsy often have reduced walking ability compared to their typically developing peers, which limits their independence and overall quality of life. Despite early intervention to address anatomical and functional deficits for children with cerebral palsy, some children do not respond to treatment, in part, because the driving musculoskeletal sources of reduced mobility are challenging to identify separate from compensatory muscle action. For example, children with cerebral palsy often walk with a hip compensation strategy; however, how this strategy is related to the child’s self-selected walking speed remains unclear. In addition, tibial torsion, a common bone deformity seen in children with cerebral palsy, results in reduced capacity of lower limb muscles to support the body during gait. However, implications for compensatory gait strategies adopted by children with bone deformities have not yet been explored. Musculoskeletal modeling and simulation is a non-invasive tool used to evaluate muscle forces and functional roles during gait. These tools can be used to identify sources of altered walking patterns and evaluate current physical therapy and surgical procedures. However, the application of musculoskeletal models for children with cerebral palsy remains limited by model assumptions. The purpose of this work was to provide a quantitative analysis of walking in children with cerebral palsy. Both joint and muscle level analyses were completed to evaluate how children with cerebral palsy walk, including how children walk at faster self-selected walking speeds and how children use compensatory gait patterns with lower limb bone deformities. Current assumptions limiting the accuracy and use of musculoskeletal modeling and simulation for children with cerebral palsy were addressed using sensitivity analyses and subject-specific model development. The results from this work provide novel information regarding gait mechanics in children with cerebral palsy and methods that have potential to guide therapy interventions.