Browsing by Subject "colloids"
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Item Confocal microscopy study of colloidal sedimentation and crystallization(2009-05-15) Beckham, Richard EdwardColloidal crystallization in sedimenting systems is an incompletely understood process, where the influence of interparticle forces on the three-dimensional (3-D) microstructure remains to be fully elucidated. This dissertation outlines work that is intended to improve our knowledge of this subject by studying sedimentation equilibrium and phase behavior for electrostatically repulsive systems, as well as the interfacial crystallization of attractive depletion systems. Towards this end, several analytical and experimental tools have been developed to explore the thermodynamic behavior of these systems. For example, the experimental challenges necessitated the development and implementation of the following in this work: (1) core/shell silica particles incorporating molecular fluorophores or semiconductor nanocrystals; (2) modification of silica particle surfaces; (3) the design of specialized sedimentation cells; and (4) the development of a novel fluorescent intensity-based approach to quantifying colloidal sediments. Analysis of the experimental data required the use of the following tools: (1) location of particle centers from images; (2) deconvolution of intensity profiles using a novel Monte Carlo-type algorithm; and (3) prediction of colloidal phase diagrams using perturbation theory. On the basis of this work?s experimental and simulation data, it is concluded that competing orientations of crystal grains may suppress crystallization at grain boundaries, resulting in a non-uniform depth of the fluid/solid transition. Also, it was demonstrated that the grain size in depletion crystals formed from quantum dot-coated silica particles can be increased by localized annealing with the confocal microscope?s laser. Additional findings include the ability of the intensity-based approach to measure interparticle forces in colloidal sediments, as well as the inability to use perturbation theory to predict two-dimensional colloidal fluid/solid transitions. While significant progress has been achieved, work on 3-D imaging of colloidal depletion crystals in a refractive index-match medium is ongoing. This work improves our understanding of 3-D colloidal crystallization at interfaces, as well as provides new tools for future research. Also, this work demonstrates a potential route for zone refining of colloidal crystals, a technique that may be important in the search for low-defect 3-D arrays that can be used as templates for photonic bandgap materials.Item Molecular Level Characterization and Mobility of Radionuclide-Carrying Natural Organic Matter in Aquatic Environments(2012-10-19) Xu, ChenRadionuclides, 129I and 239,240Pu, are major products or by-products of nuclear fission and among the top risk drivers for waste disposal at the Savannah River Sites (SRS) and Rocky Flats Environmental Technology Sites (RFETS), respectively, due to their perceived mobility in the environment, excessive inventory, toxicity, and long half-life. The objective of this study is to investigate the role of natural organic matter in retarding or facilitating the migration of 129I and 239,240Pu in the Department of Energy (DOE) sites. Measurements of 127I and 129I in humic acids (HAs) and fulvic acids (FAs) obtained by five successive alkaline, two glycerol and one citric acid-alkaline extractions, demonstrated that these extractable humic substances (HS) together account for 54-56 percent and 46 percent of the total 127I and 129I in the soil, respectively. The variations among 127I and 129I concentrations, isotopic ratios (129I/127I), chemical properties of all these humic substances indicated iodine was bound to a small-size aromatic subunit (~10 kDa), while the large-size subunit (~90 kDa), which likely linked the small-size unit through some weak chemical forces, determined the relative mobility of iodine bound to organic matter. Soil resuspension experiments simulating surface runoff or stormflow and erosion events were conducted with soils collected from SRS. Results showed that 72-77 percent of the newly-introduced I- or IO3- were irreversibly sequestered into the organic-rich soil, while the rest was transformed into colloidal and dissolved organo-iodine by the soil. The resulting iodine remobilization contradicts the conventional view that considers only I- or IO3- as the mobile forms. Quantitative structure analysis by 13C DPMAS NMR and solution state 1H NMR on these humic substances indicate that iodine is closely related to the aromatic regions containing esterified products of phenolic and fomic acid or other aliphatic carboxylic acids, amide functionalities, quinone-like structure activated by electron-donating groups (e.g., NH2) or hemicelluloses-lignin-like complex with phenyl-glycosidic linkage. The micro-molecular environment, such as the hydrophobic aliphatic periphery hindering the active aromatic cores and the hydrophilic polysaccharides favoring its accessibility towards hydrophilic iodine species, play another key role in the interactions between iodine and SOM. NMR spectra of the colloidal organic Pu carrier which can potentially be released from the soil during the surface runoff or stormflow showed Pu was transported, at sub-pM concentrations, by a cutin-derived soil degradation products containing siderophore-like moieties and virtually all mobile Pu.Item The role of colloidal particles on the migration of air bubbles in porous media(2009-05-15) Han, Ji-seokThe contamination of groundwater and soils has been a big issue of great interest and importance to human health. When organic compounds from leaking underground storage tanks or accidental spills on the surface infiltrate into the subsurface environment, they migrate downward through the unsaturated zone. These contaminants are dissolved into groundwater and move with groundwater flow. Thus, there is a need for remediation technologies. Air sparging is relatively cost-effective, as well as an efficient and safe technique for recovering organic contaminants in the subsurface. This technique introduces air into the subsurface system to enhance the volatilization and bioremediation of the contaminant in the groundwater system. In this operating system, the movement of air phase can take place either as a continuous air phase or as discrete air bubbles. However, the present research focused on continuous air phase assumption and mass balance equations of individual phases rather than taking into account the movement of air bubbles and colloidal particle capture on discrete air-water interface. Generally colloidal particles are treated as suspended particles in the water, so the hypothesis is that the rising air bubble can collect the particles and transport them up to the water table where the pump extracts the dirty bubbles from the groundwater system to the processing unit on the ground surface. This dissertation developed a pore-scale study to model the migration of discrete air phase in the presence of colloidal particles captured on the air-water interface. The model was based on the pore-scale balance equation for forces acting on a bubble rising in a porous medium in the presence of colloids. A dimensional analysis of the phenomenon was also conducted to provide a more generalized methodology to evaluate the effect of individual forces acting on an air bubble. The results indicate that the proposed model can predict the terminal velocity of a rising bubble without or with colloidal particles and provide the effect of numbers of colloidal particles, properties of colloidal particles, and solid grain size. The results showed that the terminal velocity of a discrete bubble was affected by the attachment of particles on a bubble, and then the volatile organic compound (VOC) removal rate was changed by the various radii of a bubble and the number of colloidal particles on a bubble.