Enhanced flocculation of colloidal dispersions by polymer mixtures

Bridging flocculation and electrolyte coagulation of negatively charged col loidal dispersions in the presence and absence, respectively. of uncharged polymers and polymer mixtures were studied. The relative coagulation and fl occulation rates of particles in the presence of electrolyte and small poly mer amounts were measured and the stability ratios have been calculated at various ionic strengths. Also, the structure of polymer layers formed in in dividual adsorption of polymers and in simultaneous competitive adsorption from binary polymer mixtures at particle/solution interfaces was investigat ed. The electrophoretic mobility and the diffusion coefficient of particles with and without adsorbed polymer were measured by laser Doppler-electroph oresis and photon correlation spectroscopy, respectively, and the electroph oretic and the hydrodynamic thickness of adsorbed polymer layers have been calculated. It was found that the adsorbed polymers may enhance or diminish the rate of successsful encounters between particles, even at low surface coverages, depending on the magnitude of the interparticle electrostatic re pulsion. In addition, competitive adsorption of chemically different polyme rs for particle surfaces may result in considerable alteration in the confo rmation of macromolecules in the mixed adsorption layer. Close correlation was found between the effectiveness of polymers as flocculants and the thic kness of adsorbed polymer layers formed at optimum polymer dosages on the p article surfaces. Binary mixtures of suitable polymers proved to be very ef ficient flocculants for the dispersions. The enhanced flocculating effect o f some mixtures can be ascribed to extended polymer layers formed in compet itive adsorption of chemically different macromolecules at particle/solutio n interfaces. These findings have relevance in many environmental technolog ies and offer a way of improving the effectiveness of solid-liquid separati on processes. (C) 2000 Elsevier Science B.V. All rights reserved.