Effect of cationic polyacrylamide on precipitated calcium carbonate flocculation: Kinetics, charge density and ionic strength

We quantified the adsorption kinetics of cationic polyacrylamide (CPAM) of three charge densities but constant molecular weight and their effects on the dynamics of a polydisperse calcium carbonate (CC) flocculation at various ionic strengths. The adsorption kinetics of CPAM onto PCC can be explained by the balance of the electrostatic and van der Waals interactions, hydrogen bonding and steric hindrance between the adsorbed and dissolved CPAM molecules and CC. While the amount of adsorbed CPAM onto CC increased as a function of the CPAM charge density in the absence of salt, this relationship reversed at increased ionic strength (I = 0.1). Decreasing charge on CPAM molecules allows more amide groups available for hydrogen bonding although reducing electrostatic attraction of CC. So the lower-charge CPAM benefits from the additional hydrogen bonding at higher ionic strength when the contribution of electrostatic attraction is reduced because of charge screening by salt. Discrepancies in quantity between the adsorbed and the polymer predicted from the monolayer surface coverage theory for flocculation rate were apparent. The polymer adsorption kinetics and its conformation were important variables in determining particle flocculation in this study, more than the quantity of adsorbed polymer.