Molecular and Surface Interactions between Polymer Flocculant Chitosan-g-polyacrylamide and Kaolinite Particles: Impact of Salinity

Solution salinity plays an important role in the interactions between polymer flocculants and solid colloid particles which determine the flocculation performance. In this work, chitosan-graft-polyacrylamide (chi-g-PAM) was synthesized and characterized. The influence of solution salinity (viz., addition of NaCl and CaCl2) on the flocculation of chi-g-PAM on kaolinite suspension and the interactions between the polymers and solid surfaces was investigated via several complementary measurements and techniques, including settling tests, zeta potential analyzer, quartz crystal microbalance with dissipation (QCM-D), surface forces apparatus (SFA), and atomic force microscope (AFM). Our results show that the initial settling rate (ISR) of kaolinite suspension decreases with increasing the concentration of NaCl and CaCl2 (0.01 to 1 M). The high salinity condition leads to a relatively weak adsorption and more compressed conformations of chi-g-PAM chains on both silica and alumina surfaces (two facets of kaolinite), as well as weakened bridging interactions between two mica surfaces (with silicate structure similar to kaolinite), therefore resulting in relatively poor flocculation performance. The interaction mechanism between chi-g-PAM polymer chains and kaolinite particles during flocculation was discussed in terms of the intermolecular and surface forces involved.