Flocculation with Polyelectrolytes in Latex and Microbial Systems
Abstract: Adsorption of cationic polyelectrolytes onto polystyrene latices was studied as function of polymer chemical composition and molecular weight, polymer and surface charge density and ionic strength. The ratio between polymer charges and surface charges was close to one when distances between polymer and surface charges were equal. Overcompensation of the surface charge was obtained at higher polymer charge densities due to surplus of positive charges at the adsorption site and at lower polymer charge densities due to formation of loops and tails. Different lengths of side chains carrying the cationic groups had effects on the way the polymers neutralised the surface. Flocculation experiments under the same conditions as for the adsorption gave minima in floc size and strength when the distance between polymer and surface charges was the same. With reference to surface force measurements it was concluded that the polymers, depending on charge density, played several and different roles in capturing, binding and adhesion trough traditional bridging, electrostatic double layer bridging and electrostatic attraction. With low charge density polymers dominance of traditional bridging led to flexible and shear resistant flocs with dense structure on small length scales and a less dense fractal structure on larger length scales. With higher charge density increasing adhesion between the primary particles resulted in fractal structure on all length scales and decreasing shear resistance. This was compensated however with a god reflocculation ability. Different behaviour of flocs under and after high shear was related to shear history and to the degrees of traditional bridging and adhesion respectively. Separation properties of activated sludge bioflocs were related to the degree of crosslinking and shrinkage of the biopolymer matrix binding the bacterial cells together. This depended on the availability of calcium ions and on the energy input to the system. Flocculation of monocultured bacteria with polycations proceeded in two steps. First the anionic extracellular polymers from the bacteria were precipitated to polyelectrolyte complexes which on recharging flocculated the cells. Depending on the distribution of extracellular polymers between surface and liquid phase differently broad effective dosage ranges were obtained. For conditioning of sludges with large contents of biopolymers it was necessary with high charge density polycations and dosage need was mainly spent on polymer complexation as with the pure bacteria. For sludges with more particular character lower charge density polymers were advantaeous.
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