Doctor of Philosophy (PhD)


Engineering Science (Interdepartmental Program)

Document Type



Costal or shoreline waters are often characterized by high concentrations of suspended cohesive sediments (or clay) affected by varied organic matters (mostly extracellular polymeric substances or exopolymers), salt, and hydrodynamic disturbance. Resulting from the flocculation-disaggregation between the colloidal clay and exopolymer, the size and structure changes of the cohesive sediments are of key importance for understanding sediment transport processes (i.e., settling, breakage, and survivability) and the geotechnical and geophysical properties of the bottom bed. Because the coastal environment is extremely complicated, with many unpredictable and uncontrollable parameters, current knowledge is still insufficient to predict or fully explain the behavior of cohesive sediments. To obtain a comprehensive and in-depth understanding of cohesive sediment properties, especially with focus on the flocculation mechanism and microstructure models of clay-exopolymers, a series of sediment samples were laboratory-prepared by using four model clays, including kaolinite, illite, Ca-montmorillonite (Ca2+-Mt), and Na-montmorillonite (Na+-Mt), and three representative exopolymers (xanthan, guar, and chitosan) with different polarities in both fresh and salt waters. In order to determine the influence of each main factor in coastal environments, the suspended cohesive sediments are separated as four systems and studied systematically, which are pure clay, clay-exopolymer, clay-salt, and clay-salt-exopolymer systems. Particle size distribution (PSD), settling velocity, and microstructure of these systems were characterized by state-of-the-art techniques and developed routines. The primary PSDs of the four pure clays were first investigated via comparing different dispersion or disaggregation methods, which were used as the baseline for studying the PSD variations of other systems. Based on the synthesis of indirect PSD as well as settling velocity data and direct electron microscopy imaging, a conceptual microstructure model consisting of four hierarchical levels (i.e., primary particles, flocculi, microflocs, and macroflocs) was proposed for the clay-exopolymer flocs. The flocculation of clay-exopolymer was simplified and explained as a two-step process, including adsorption of exopolymer onto clay surfaces by intermolecular forces and packaging of clay-exopolymer groups into flocs via charge neutralization or polymer bridging. By synthesizing all the findings, a complete model was developed for clay-exopolymer floc ranging from submicron to micron scales.



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Committee Chair

Zhang, Guoping