Ionic strength and polyelectrolyte molecular weight effects on floc formation and growth in Taylor–Couette flows

Athena E. Metaxas, Vishal Panwar, Ruth L. Olson, Cari S. Dutcher

Research output: Contribution to journalArticlepeer-review

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.

Original languageEnglish (US)
Pages (from-to)1246-1257
JournalSoft Matter
Volume17
Issue number5
DOIs
StatePublished - 2021

Bibliographical note

Funding Information:
This work was partially supported by the National Science Foundation (NSF) through the University of Minnesota MRSEC under Award Numbers DMR-1420013 and DMR-2011401. Part of this work was carried out in the College of Science and Engineering Polymer Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program under Award Number DMR-1420013. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. A. M. was supported through a National Science Foundation Graduate Research Fellowship. R. O. was supported by a Research Experience for Undergraduates fellowship through the UMN MRSEC. The authors would like to thank Lisa Zeeb for designing the graphical abstract.

Publisher Copyright:
© The Royal Society of Chemistry 2020.

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