Effect of electrolytes on electrokinetics and flocculation behavior of bentonite-polyacrylamide dispersions

In this study, the influence of electrolytes on bentonite dispersions flocculated with different polyacrylamides varying in charge type, density and molecular weights has been investigated. The presence of electrolytes was found to induce profound impact on the flocculation behavior of bentonite-polyacrylamide dispersions, which was analyzed by means of residual turbidity, zeta potential and floc size distribution tests. Electrolytes enhanced the adsorption of anionic polyacrylamide chains on negative bentonite surface, while hindering adsorption of cationic polyacrylamides. Electric double layer compression and mediated flocculation by metal cations were the principle mechanisms involved in the improvement of flocculation behavior. High molecular weight and charge density variants greatly reduced the turbidity and negative zeta potential of dispersions for anionic polyacrylamides. Furthermore, charge density of metal ions was found to be a determining factor. Divalent salts were more efficient for flocculating dispersions using anionic polyacrylamides, while monovalent salts were preferable for cationic polyacrylamides owing to electrostatic repulsions. Amphoteric polyacrylamides displayed similar results based on their anionic/cationic nature. Similar observations were recorded for floc size measurements. High charge density variants of anionic polyacrylamides in the presence of divalent salts like CaCl₂ produced large flocs. On the other hand, no significant changes in floc sizes were observed for cationic polyacrylamides in the presence of salts. Overall, this study successfully determines the effect of electrolytes on the flocculation behavior of bentonite dispersions.