Adsorption and flocculation behaviors of polydiallyldimethylammonium (PDADMA) salts: Influence of counterion

Abstract

Two modified polymer flocculants of PDADMA salts, PDADMA nitrate (PDADMAN) and PDADMA sulfate (PDADMAS), were prepared from the most widely used commercial product of polydiallyldimethylammonium chloride (PDADMAC). Solution properties such as conductivity and viscosity were investigated, showing that the conductivity follows the order of PDADMAS > PDADMAC > PDADMAN, while the reduced viscosity increases in the order of PDADMAS < PDADMAC < PDADMAN. The results indicate that, compared with PDADMAC, PDADMAN has a more cationic density and a more extended polymer chain, whereas PDADMAS exhibits a smaller coiled polymer size because of the stronger charge affinity of sulfate ion to the polycation of PDADMA. Flocculation experiments were performed on kaolin suspensions. It was found that PDADMAN is the most efficient in turbidity removal, while PDADMAS has wider optimum dosages, larger floc size and more rapid settling rate. Atomic force microscopy (AFM) was employed for imaging the adsorption structures of the three polymers on the negatively charged mica surface. Remarkable differences in molecular conformation were observed. Compared with the pearl necklace-like aggregation of hemispheroids of PDADMAC polymer, PDADMAN takes a sponge-like thin layer structure and PDADMAS exhibits a uniform hemispherical structure. The above differences in both the solution property and the adsorption and flocculation behaviors are attributable to counterion effects. Different counterions may lead to different adsorption and flocculation mechanisms that PDADMAN adopts a flat and layer–layer adsorption without loops and tails extended from the surface, implying that a charge neutralization mechanism plays more roles, while adsorbed PDADMAS is in the form of coiled spheres with loops and tails so that a bridging mechanism is more involved. Namely, it is the adsorption structure and morphology of the polymer flocculants that is the main factor influencing the flocculation mechanism.