Ionic strength and polyelectrolyte molecular weight effects on floc formation and growth in Taylor–Couette flows. Issue 5 (10th December 2020)

• Record Type: Journal Article
• Authors: Metaxas, Athena E.; Panwar, Vishal; Olson, Ruth L.; Dutcher, Cari S.
• Is Part Of: Soft matter. Volume 17:Issue 5(2021)
• Page Start: 1246
• Page End: 1257
• Publication Date: 2020-12-10
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/d0sm01517b
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : A Taylor–Couette cell capable of radial injection was used to study the effects of varying solution ionic strength and polyelectrolyte molecular weight on the polyelectrolyte-driven flocculation of bentonite suspensions. Abstract : Polyelectrolyte-driven flocculation of suspended particulate in solution is an important process in a variety of industrial processes such as drinking water treatment and composite material synthesis. Flocculation depends on a wide variety of physicochemical and hydrodynamic properties, which affect floc size, growth rate, and floc morphology. Floc formation and growth behavior is explored here using two different molecular weights of a cationic polyacrylamide flocculant and anisotropic Na-bentonite clay particles under a variety of solution ionic strengths. A Taylor–Couette cell with radial injection capabilities was used to study the effects of solution ionic strength and polyelectrolyte molecular weight on floc size, growth rate, and floc morphology during the flocculation process with a constant global velocity gradient. The floc size generally decreased with increasing ionic strength whereas the floc growth rate initially increased then decreased. This likely occurred due to charge screening effects, where increased bentonite aggregate size and a less expanded polyelectrolyte conformation at higher ionic strengths results in a decreased ability for the polyelectrolyte to bridge multiple bentonite aggregates. The densification of bentonite aggregates at higher ionic strengths resulted in floc morphologies that were more resistant to shear-induced breakage. With the exceptions of optimal dose concentration and dispersion coefficients, there were no clear differences in the floc growth rate behaviors for the two molecular weights studied. This work contributes to an improved understanding of the physicochemical complexities of polyelectrolyte-driven flocculation that can inform dosing requirements for more efficient industrial operations.