Size Kinetics and Mechanics of Clay-biopolymer Flocs

The suspended cohesive sediments, mostly clay minerals with negative surface charges, usually interact through an array of intermolecular/surface forces (e.g., Coulomb force, van der Waals attractions, and hydrogen bonding) with waterborne organic matter (e.g., exopolymers) and dissolved salts, resulting in fractal, tenuous, and hybrid organic-inorganic flocs or “marine snow”. A pilot and systematic laboratory and theoretical work have been conducted towards a synthetic nanoscale understanding of the flocculation kinetics and mechanics of the clay-exopolymer micro-sized flocs. This systematic study has achieved two impressive objectives: (1) the effect of exopolymer polarity and concentration on the particle size kinetics of clay-exopolymer mixtures has been investigated, based on extensive flocculation experiments involving four clay minerals (i.e., kaolinite, illite, Na-montmorillonite, and Ca-montmorillonite), three exopolymers (i.e., xanthan, guar, chitosan), six exopolymer to clay weight ratios (i.e., 0, 1, 2, 5, 10, and 20 wt.%), and three hydrodynamic conditions (i.e., laminar, transitional, and turbulent flows); (2) the floc mechanical properties affected by salinity and exopolymers have also been investigated by means of extensive floc nanocompression testing involving the four aforementioned clay minerals, four neutral exopolymer (guar) to clay weight ratios (i.e., 0, 2, 10, and 20 wt.%), and four salinities (i.e., 0, 2, 10, and 30 PSU). Results from flocculation experiments indicate that both the pure clay and clay-exopolymer mixture suspensions possess discrete particle groups consisting of primary particles, flocculi, microflocs, and macroflocs that exhibit subordinate unimodal lognormal distributions. There exists a critical exopolymer to clay ratio that can lead to a maximal or minimal fraction of microflocs or macroflocs. Meanwhile, the highly scattered floc mechanical properties led to the theoretical finding that both floc’s elasticity and yield shear strength satisfy Weibull’s Law. These findings are of great importance to geotechnical engineering