Effects of polymer molecular weight on adsorption and flocculation in aqueous kaolinite suspensions dosed with nonionic polyacrylamides

The effects of polyacrylamide (PAM) molecular weights (MWs) on the PAM adsorption capacities and PAM-mediated flocculation of kaolinite suspensions were investigated using a series of nonionic PAMs with different MWs. Adsorption tests using aqueous kaolinite suspensions dosed with a series of PAMs with MWs of 1.5 kg/mol, 10 kg/mol, 0.6-1 Mg/mol, 5-6 Mg/mol, and 18 Mg/mol (referred to as 1.5 K, 10 K, 0.6-1 M, 5-6 M, and 18 M PAMs) indicated that the adsorption capacity of the kaolinite for PAM increased with increasing MW. However, the capacity for 18 M PAM was 20 times smaller than those for the 0.6-1 M and 5-6 M PAMs, although it has the highest MW. In steady-shear viscosity tests, a 1 g/L stock solution of 18 PAM was found to cause polymeric chain entanglements, which reduced the adsorption capacity. The 0.6-1 M and 5-6 M PAMs were further used in flocculation tests, in order to investigate the effect of PAM MW on the flocculation capability. The 5-6 M PAM was found to have higher flocculation capabilities than 0.6-1 M PAM; 5-6 M PAM was more subject to nonequilibrium flocculation, resulting in the development of unstable, stretched polymeric structures on solid surfaces and increasing particle-particle bridging and flocculation. Higher-MW PAMs are more effective flocculation agents, because of their higher adsorption capacities and flocculation capabilities. However, an extremely high-MW PAM, such as 18 M PAM, decreases adsorption/flocculation, and the preparation and handling of working solutions become difficult, because of polymeric chain entanglements.