Effects of exopolymers on particle size distributions of suspended cohesive sediments

Abstract

[1] The effects of exopolymers on the particle size distributions (PSDs) of suspended cohesive sediments were investigated in laboratory using four abundant clay minerals, kaolinite, illite, Na-montmorillonite, and Ca-montmorillonite, and two exopolymers, xanthan and guar, at six different exopolymer to clay ratios (E/C; i.e., 0, 1, 2, 5, 10, and 20 wt %) to represent the compositional variability of cohesive sediments in natural waters. Results show that the clay-exopolymer suspensions possess multimodal PSD. Statistical deconvolution of the PSD curves indicates that the suspensions consist of four discrete particle groups, primary particles, flocculi, microflocs, and macroflocs, all of which exhibit a unimodal lognormal distribution. Furthermore, such deconvolution quantifies the mean size and fraction of each particle group, leading to a more quantitative understanding of PSD kinetics of these sediments. Both clay surface charges and exopolymer polarity as well as the E/C affect the PSD kinetics. While neutral guar causes flocculation for all four clay minerals, anionic xanthan only induces flocculation for kaolinite with very low surface charges, but not for the other three clay minerals with relatively high charges. The fraction of each particle group also varies with the E/C, and such complex changes depend upon the interfacial interactions between clay particles and exopolymer molecules. For each exopolymer, critical E/C exists that can lead to a maximum or minimum fraction of microflocs or macroflocs. The role of exopolymer bridging, Coulomb force, and hydrogen bond in affecting the PSD kinetics of cohesive sediments is also discussed.

An Approach to Modeling Biofilm Growth During the Flocculation of Suspended Cohesive Sediments

“…For example, Shen and Maa (2016b) have produced bimodal FSDs of kaolinite suspensions with different dosages of guar gum in one of their mixing chamber tests. Multipeak FSDs are also reported in other studies focusing on the effects of organic substances on montmorillonite flocculation with cationic (such as chitosan), neutral (such as guar gum), or anionic (such as xanthan) EPS (Furukawa & Watkins, 2012;Zhang et al, 2013;Tan et al, 2012Tan et al, , 2014Furukawa et al, 2014). The occurrence of multimodality may be because of heterogeneous particle composition, irregular shaped flocs, limitation of floc size measuring technologies (that refer to spherical particles), and so on.…”

Shear Wave Velocity and Its Anisotropy of Polymer Modified High-Volume Class-F Fly Ash–Kaolinite Mixtures

“…Positively charged chitosan will first interact with negatively charged kaolinite through Coulombic attraction (charge neutralization). Charge neutralization occurred locally on the surface of an individual kaolinite particle only, not contributing to the binding between kaolinite particles (Huang and Chen 1996;Parazak et al 1988;Zhang et al 2013). Therefore, chitosan does not induce large compacted aggregates like PEO does (Fig.…”

“…4), which could be attributed to the following two mechanisms. First, the interparticle forces are dominated by strong electrostatic repulsive forces between negatively charged xanthan gum and negatively charged kaolinite (Benchabane and Bekkour 2006;Zhang et al 2013). This is equivalent to a decrement of effective stress, which, in turn, leads to a decrease of V s .…”

“…This is equivalent to a decrement of effective stress, which, in turn, leads to a decrease of V s . Second, xanthan gum chains, which attract positively charged edges of kaolinite and repel negatively charged faces of kaolinite via electrostatic attraction (Benchabane and Bekkour 2006;Tan et al 2014;Zhang et al 2013), induced loose [high e 0 of 1.34 (Table 1)] and layered kaolinite fabric (Labille et al 2005) [Figs. 2(c and f)].…”

“…The main soil improvement mechanism by surfactants was that adsorbed organic surfactants reduced the net surface charge (indicated by zeta potential) of the soil particles, reduced the interparticle repulsive forces, condensed the soil, and thereby increased its stiffness and strength . The role of polymers in soil improvement mechanisms is more complicated, consisting of polymer-induced cohesion (at a macrograin scale) (Cole et al 2012), polymer bridging (Nugent et al 2010;Zhang et al 2013), Coulomb forces, and the interaction between the individual function groups on polymer chains and clay mineral surfaces (Nugent et al 2010;Podsiadlo et al 2007;Zhang et al 2013). The addition of organics could possibly increase the bonding between negatively charged Class F fly ash (zeta potential is around −8mV) and soil particles through polymer bridging and electrostatic forces.…”