Nonionic polyacrylamide-clay interaction: Insight from sedimentation experiments and viscosity

Nonionic polyacrylamide (NPAM) is a water-soluble flocculant frequently used to immobilize toxic contaminants and for contaminant treatment. This study examines the influence of NPAM concentration (0–10 mg/L), cationic valence (NaCl or CaCl 2 solution), and clay mineralogy (kaolinitic and illitic clay) on the flocculation and post-settling behavior under water content ( w ) = 500–4000 %, using sedimentation and rheology experiments. The significant increase in settling velocity by NPAM treatment was observed, with a maximum of 92 % reduction in settling velocity. In addition, kaolinitic clay formed larger and faster-settling flocs than illitic clay, and CaCl 2 solution showed a stronger shear-thinning response than NaCl solution. The zero shear viscosity of NPAM-treated kaolinitic clay increased approximately 3-fold compared to untreated conditions, indicating strong intra-floc bonding of NPAM-treated kaolinitic clay. The high separation distance (up to 2.2 µm for kaolinitic clay) of stacked clay flocs at high NPAM concentration suggests that NPAM concentrations below 5 mg/L can be recommended for obtaining mechanical stability of settled clay after settling. Overall, these findings provide a comprehensive understanding of NPAM-clay interactions at varied cationic valence, clay type, and NPAM concentration under saline conditions.