Stabilization and destabilization of hematite suspensions by neutral and anionic polyacrylamide

Polyacrylamide (PAM) and its hydrolysis products are widely used flocculants in water treatment. These relatively flexible polymers may also serve as models for certain natural organic matter rich in carboxyl groups in aquatic systems. We studied the aggregation kinetics of hematite in the presence of neutral and hydrolyzed polyacrylamide (HPAM) at low hydrolysis ratios (τ=0.4–8.2%). Electrophoretic mobility measurements revealed that HPAM, even at very low degree of hydrolysis (0.4% and 2.5%), could reverse the surface charge of hematite. However, no flocculation was observed for neutral and HPAM with τ=0.4 and 2.5%, even at the zero point charge (ZPC). This clearly demonstrates the strong steric repulsion between the polymer layers whose hydrodynamic thickness was comparable to the particle size. For polymer chains containing more charged groups (⩾6.1%), flocculation indeed took place. The maximum rate occurred near the ZPC, indicating the importance of the charge neutralization mechanism. There was no indication of the involvement of polymer bridging in the present systems