Characterization of particle size and settling velocity of cohesive sediments affected by a neutral exopolymer

Abstract

In natural waters, exopolymers or extracellular polymeric substances (EPS) exuded by microorganisms interact with clay particles, resulting in the flocculation of clays and hence alteration to the properties of suspended cohesive sediments. To investigate and further understand how neutral EPS affect cohesive sediment transport and the final sediment yield, an experimental study was conducted on laboratory-prepared clay and guar gum (used as an analog for neutral EPS) suspensions to characterize EPS-induced flocculation and the settling velocity of resultant flocs. Four different clays consisting of kaolinite, illite, Ca-montmorillonite, and Na-montmorillonite were studied to examine the influence of different layer charges on clay flocculation induced by neutral EPS. Floc size was determined by a laser particle size analyzer, and settling velocity estimated by analyzing the time-series floc settling images captured by an optical microscope. Results indicate that neutral EPS promote clay-EPS flocculation for all four clays with the particle/floc size significantly increased from ∼0.1-60 μm to as large as ∼600 μm. Clays' layer charge has a profound influence on the clay-EPS flocculation. With the same floc size, the settling velocity of clay-EPS flocs is typically smaller than that of pure clay flocs, which is attributed to the reduced density of flocs caused by the EPS. However, for flocs of the same composition (e.g. pure clay or hybrid clay-EPS mixture), the settling velocity increases with size. The fractal dimension of these clay-EPS flocs estimated from settling velocity ranges from 1.39 to 1.47, which are smaller than that of pure clay flocs, indicating that these flocs are less compacted than the pure clay flocs.