pH dependence of bentonite aggregate size and morphology on polymer-clay flocculation

Polymer-driven flocculation of suspended particulate is a commonly used technique in applications ranging from water treatment to composite material synthesis. When complex particulate is present, such as anisotropic clays, solution physiochemical effects on flocculation become nontrivial. Properties such as ionic strength and pH affect both the individual particulate aggregates themselves, as well as the polymer – particle flocculation event. Using bentonite, a common inorganic clay, and cationic polyacrylamide, we show here that the clay's aggregate morphology is a more direct control parameter of optimal polymer dose and final turbidity than zeta potential for aqueous bentonite systems. Solutions were studied over a pH range from 3 to 11, resulting in zeta potentials from −73 to −106 mV and bentonite area averaged particle sizes ranging from 2 to 4.5 μm. Flocculation performance is the same when bentonite aggregate morphology is the same, regardless of a change in zeta potential. Likewise, when bentonite aggregate morphology changes, flocculation performance also changes, regardless of the identical zeta potential. The optimal polymer dosing for flocculation also depends on aggregate structure. Aggregates formed by edge-face dominated structures formed at low pH require less polymeric dosing, whereas aggregates formed by face-face dominated structures formed at high pH require more polymer dosing. Finally, using laser scanning confocal microscopy, it was observed that when the aggregate size remains relatively constant, the internal floc structure also remains constant, regardless of pH or bentonite aggregate morphology. Therefore, it appears that initial clay aggregate morphology controls the extent of polymer adsorption, determining optimal polymer dose, while aggregate size controls the internal floc structure. This work sheds light on the complexities of polymer flocculation towards improving reagent polymer dosing and optimization for applications such as water purification.