Shear-induced flocculation of a suspension of kaolinite as function of pH and salt concentration.
The relation between the electrokinetic charge of kaolinite particles and their flocculation behavior has been investigated over a wide range of pH and added salt (for MgCl(2) and NaCl salts). All flocculation experiments have been done with a mixing jar (sediment volume concentration phi=3.84x10(-5)). The electrokinetic charge of particles in different suspensions has been assessed by electrophoresis while laser diffraction has been used to measure the floc size distribution. Mixing jar experiments can be successfully used to investigate the flocculation behavior of kaolinite at shear rates higher than or equal to G=35 s(-1), which is the shear rate used in the experiments. At lower shear rates, the floc size distribution is affected by particle settling. The electrophoretic mobility of kaolinite decreases in absolute value when the pH of the suspension decreases. This is reflected in an increase of both floc size and flocculation rate: the floc size at pH 4 is three times larger than at pH 7 and the flocculation time is one order of magnitude smaller (from 1000 to 100 min). When the ionic strength of the suspension is increased, the electrophoretic mobility and the mean floc size display the same variations. On addition of NaCl (pH 9) both the electrophoretic mobility and the floc size display an optimum around 1 mM of added salt, a feature that has been observed by other authors as well. The equilibrium floc size for a suspension (A) at 1 M of added NaCl and pH 9 is the same as for a suspension (B) at pH 2 with no added salt. However, the time needed to reach the equilibrium for suspension (A) is one order of magnitude larger than for suspension (B). This is due to edge-face Coulombic attraction in suspension (B). The equilibrium floc size obtained by addition of MgCl(2) or sea salt at pH 9 is similar to the size obtained by addition of NaCl. The flocculation rate for a suspension with added MgCl(2) is higher than for suspensions with other added salts.
Environmental Fluid Mechanics, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Box 5048, 2600 GA, Delft, The Netherlands. f.mietta@tudelft.nl
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