Particle Size and Structural Arrangement of Suspended Cohesive Sediments

Coastal environments are often characterized by high concentrations of cohesive sediments influenced by the loaded organic matter (particularly extracellular polymeric substances (EPS)), salt, and hydrodynamic disturbance. The size and structural variation of suspended cohesive sediments due to flocculation and/or disaggregation is of key importance for understanding a variety of sediment transport processes (e.g., settling, breakage, survivability) in littoral environments and the geotechnical/geophysical properties of the bottom bed. To obtain a comprehensive understanding of sediment floc behavior and correlate the clay-EPS-ion interaction mechanisms with their structures, a series of sediment samples were synthesized in laboratory using four pure clays (i.e., kaolinite, illite, Ca-montmorillonite, and Na-montmorillonite), three EPS (cationic, neutral, and anionic) at different concentrations, and saltwater of different salinity under different hydrodynamic conditions. Particle size analysis of the pure clays, clay-EPS, and clay-salt flocs under three hydrodynamic conditions demonstrated for the first time in the laboratory that pure clays and clay-EPS mixtures exhibit lognormal, multimodal (i.e., 2-4 levels consisting of primary particle, flocculi, microfloc, and macrofloc) particle size distributions (PSDs) within the size range of ~0.1 to ~500 μm. The presence of EPS causes the formation of macroflocs (>200 μm) and can significantly increase the mean particle size by several orders of magnitude through flocculation, assisted by electrostatic forces, ion-dipole, van der Waals forces, and other mechanisms. The change in size of the pure clay flocs in saltwater showed different trends: Due to the clays' different properties and interaction mechanisms with EPS, their PSDs and size changes are also different in different flow conditions: the hydrodynamic turbulence may promote the flocculation of Ca-montmorillonite, but break kaolinite and Na-montmorillonite flocs; it rarely influences illite flocs. The microstructure of selected clay flocs and clay-EPS flocs was studied via free settling test and electron microscopy (e.g., SEM, TEM) to estimate their physical properties, including settling velocity, density, fractal dimension, porosity, and solids volume fraction. The free settling velocity ranges from 0.05-1 mm/s and their fractal dimension is from ~1.3-2, which indicates their high porosity. One interesting finding is that clay-EPS flocs usually settle slower than pure clay flocs, apparently due to the lower density of EPS molecules. As such, a larger clay-EPS floc may settle slower than a smaller pure clay floc. This contradicts with previous understanding that a larger floc always settle faster than a smaller one. SEM and TEM were conducted on carefully selected individual flocs to characterize the microstructure. Surface features of different types of flocs (e.g., the presence of EPS, particle compactness, porosity, and clay particle orientation) were obtained from SEM. TEM imaging on chemically fixed, stained and sliced floc samples illustrates more detailed features of flocs' interior structure. In addition, a complete series of thin section TEM images were used to reconstruct the 3D image of individual flocs.