Author(s): Xiaoling Tan; Liming Hu; Guoping Zhang; Allen H. Reed; Yoko Furukawa
Linked Author(s):
Keywords: Cohesive sediments; Exopolymer; Fractality; Salt water; Settling velocity
Abstract: As the main constituents of cohesive sediments in natural waters, clay particles always interact with the waterborne extracellular polymeric substances (EPS) exuded by microorganisms, or exopolymers, resulting in fractal clay-EPS flocs and hence change in sediment transport. During the transition of cohesive sediments from fresh water to salt water in the estuarine areas, the clay-to-clay and clay-to-EPS interactions will be influenced by the salinity change and thus the alternation to sediment transport. This study investigated the influence of a neutral exopolymer, guar gum, on the size, settling velocity, and fractality of clay flocs in both fresh and sea waters. To account for the variety of different clay types in natural cohesive sediments, four clays, namely kaolinite, illite, Ca- and Na-montmorillonite, were used as the inorganic constituents of flocs. To simulate the water chemistry in coastal areas, fresh water and salt water with a salinity of 35 ppt were used. Experimental results showed that the neutral EPS increased the floc size by promoting flocculation. In fresh water, the floc size increased from ~0. 1–60 um to as large as ~100 um. The settling velocity of clay-EPS flocs was smaller than pure clay particles of the same size in both fresh and salt water, which may be caused by the reduced density. Due to flocculation, more clay particles are attached to flocs and hence become settable. The fractal dimension of flocs, which was estimated based on a simple floc size-settling velocity model, ranges from 1. 763-1. 861 for pure clay flocculi, 1. 387-1. 473 for the clay-EPS flocs, 1. 861-1. 954 for clay flocculi in salt water, and 1. 436-1. 905 for clay-EPS flocs in salt water. Both the floc size and settling velocity in salt water are greater than that in fresh water. The increased ion concentration also promotes the flocculation of clay particles and clay-EPS flocs by reducing the double layer thickness and cation bridging, which lead to an increase in floc density and fractal dimension.
Year: 2013