Effects of exopolymers on particle size distributions of suspended cohesive sediments

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.