Investigation of the effect of polyelectrolyte structure and type on the electrokinetics and flocculation behavior of bentonite dispersions

Abstract

The influence of surface charge type and density, and molecular weight of cationic, anionic and amphoteric polyacrylamide (PAM) on the electrokinetics, adsorption and flocculation behavior of highly stable bentonite dispersions is investigated. The results showed that PAM structure has a great influence on the dispersion turbidity and zeta potential. The reduction in the dispersion turbidity and negative zeta potential is considerably greater in the presence of cationic PAMs than anionic and amphoteric PAMs, due to an increased adsorbed layer thickness of cationic PAMs. In addition, the adsorption studies showed a higher bentonite surface affinity for cationic PAMs than anionic and amphoteric PAMs. Amphoteric PAMs exhibit both cationic and anionic behaviors and their strength and relative number of sites varies depending on surface charge density. Hence, the amphoteric PAMs adsorption isotherms were located in a region between cationic and anionic PAMs. Furthermore, the floc size and adsorption results suggested strong dependence on PAM surface charge density, type and molecular weight. Generally, at the optimum concentration, larger flocs are produced using cationic PAMs than anionic or amphoteric PAMs. It is also observed that the floc sizes increased with increasing cationic surface charge density but decreased with increasing anionic surface charge density. Molecular weight was also an essential factor influencing floc sizes. According to the floc size results, increasing the molecular weight of anionic and cationic PAMs increased the floc size. In conclusion, cationic PAM FO 4800 SH (very high charge density, high molecular weight) was determined as the best flocculant among the investigated PAMs. Overall, this work has been very successful in establishing the relationship between electrokinetics behavior and the produced flocs for the flocculated bentonite dispersions.