Stabilization and destabilization of hematite suspensions by neutral and anionic polyacrylamide

Polyacrylamide (PAM) and its hydrolysis products are widely used flocculant s in water treatment. These relatively flexible polymers may also serve as models for certain natural organic matter rich in carboxyl groups in aquati c systems. We studied the aggregation kinetics of hematite in the presence of neutral and hydrolyzed polyacrylamide (HPAM) at low hydrolysis ratios (t au=0.4-8.2%). Electrophoretic mobility measurements revealed that HPAM, eve n at very low degree of hydrolysis (0.4% and 2.5%), could reverse the surfa ce charge of hematite. However, no flocculation was observed for neutral an d HPAM with tau=0.4 and 2.5%, even at the zero point charge (ZPC). This cle arly demonstrates the strong steric repulsion between the polymer layers wh ose hydrodynamic thickness was comparable to the particle size. For polymer chains containing more charged groups (greater than or equal to 6.1%), flo cculation indeed took place. The maximum rate occurred near the ZPC, indica ting the importance of the charge neutralization mechanism. There was no in dication of the involvement of polymer bridging in the present systems. (C) 1999 Elsevier Science B.V. All rights reserved.