Author(s): Hang Yin; Guoping Zhang; Allen H. Reed; Yoko Furukawa; Zhen Liu
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Keywords: Flocculation; Particle size distribution; Ca-montmorillonite and guar flocs; Floc breakdown; Multimodality
Abstract: Cohesive sediments are abundant in natural waters, especially in estuarine and coastal environments where organic matter (particularly extracellular polymeric substances (EPS) ) and high water turbulence can lead to the formation of clay-EPS flocs. This flocculation process affects the sediment transport and the geotechnical/geophysical properties of the bottom bed. In order to investigate how EPS concentration and hydrodynamic forcing affect sediment flocculation behavior (e. g., floc size distribution, floc breakdown), a pure clay mineral, Ca-montmorillonite, which is abundant in marine environment and a neutral EPS, guar gum, were used in laboratory to prepare flocs with varying EPS to clay weight ratio (E/C) and the hydrodynamic forcing in a laser diffraction-based particle size analyzer. Results indicate that the cohesive sediment flocs exhibit a multimodal particle size distribution (PSD) in all three flow conditions (i. e., laminar, transitional, and turbulent flows). Statistical deconvolution of the PSD curves suggest that the mixture suspensions consist of four discrete particle groups, consisting of primary particles, flocculi, microflocs, and macroflocs. Neutral guar in anionic Ca-montmorillonite suspensions causes the formation of macroflocs (>200 um) and significantly increases the median particle size by several orders of magnitude when compared with the pure clay. However, adding more guar into the clay does not provide a proportional increase in the macrofloc fraction. Further analysis showed that when the E/C is ~2% , which is proposed as a critical value, macroflocs have the largest fraction of the entire population, while microflocs make up the smallest one. This demonstrates that smaller microflocs are most likely to form macroflocs at an E/C of 2% through flocculation, assisted and influenced by van der Waals forces, H bond, and other mechanisms as well as physical and chemical properties of Camontmorillonite and guar (e. g., specific surface area, surface charges, swelling properties, number of surface active sites, interlayer cations, and EPS polarity). Further, considering the effect of hydrodynamic forcing, macroflocs are weak and can easily breakdown into smaller microflocs when the flow conditions change from laminar to transition or turbulence, due to the breakage of weaker bonds inside the macroflocs.
Year: 2013