Ionic strength dependence of aggregate size and morphology on polymer-clay flocculation

Polymer-driven flocculation of suspended particles is a critical process for many applications, including composite materials synthesis, paper manufacturing, and water treatment. However, the role of solution physicochemical properties on the polymer-particle assembly dynamics is nontrivial, particularly for non-spherical, polydisperse particulates such as natural clays. In this work, we study the effects of ionic strength and aggregate size and structure on the polymer behavior and flocculation performance with anisotropic bentonite clay particles. Using jar tests, laser diffraction, confocal microscopy, and X-ray diffraction, we demonstrate that for smectite clay particles, the final floc structure is largely informed by ionic-strength driven changes to the initial clay aggregate size and surface structure. With increasing bentonite aggregate size, a transition from a networked to a patched polymer − aggregate floc structure is observed, independent of ionic strength during flocculation. Solutions were studied over four orders of magnitude of NaCl ionic strength. Initial, pre-flocculated bentonite aggregate sizes ranging from 0.015 μm to 15 μm were used, produced by tuning the initial solution ionic strength. Bentonite aggregate size and structure were measured with laser and X-ray diffraction, internal floc structure was visualized using confocal microscopy, and macro floc structure (fractal dimension) was measured with optical imaging. These results shed new light on the fundamental complexity of ionic strength dependence and the importance of the aggregate size and structure of the initial dispersion, in determining optimal reagent dosing and structure control for flocculation of anisotropic clays.