Enhancing the flocculation of stable bentonite suspension using hybrid system of polyelectrolytes and NADES

Abstract

In this study, the influence of a hybrid coagulation/flocculation process on the electrokinetic of highly stable bentonite suspension has been examined. Choline chloride (ChCl) based natural deep eutectic solvent (NADES) was utilized as coagulant coupled with polyacrylamide (PAM) with different charge types and densities used as a flocculant. Turbidity, zeta potential, and floc size analysis were used to evaluate the changes in the flocculation efficiency of hybrid systems relative to the single NADES and PAM systems. The study revealed a clear impact on the flocculation behavior of cationic and anionic PAMs in the presence of NADES. The addition of NADES as a coagulant aid reversed the flocculation activities of the PAMs due to the presence of positively charged NADES that enhances the adsorption of the anionic polyacrylamide (APAM) over the cationic polyacrylamide (CPAM). Low flocculation efficiency was observed for the hybrid systems of NADES/CPAM as indicated by the highly positive zeta potential (ζ>+25mV) and small flocs (D50<50μm). Hybrid systems of NADES/ APAMs, on the other hand, significantly improved the flocculation efficiency by achieving a zeta potential within the instability region (−10mV<ζ<+10mV) and relatively large flocs D50>50μm. Aside from the charge type, the molecular weight (MW) and charge density (CD) of the PAM had a significant impact on the flocculation behavior of bentonite suspension. High PAM adversely impacted the flocculation parameters for APAMs. Therefore, a hybrid system of NADES/ AN 923 SH at an APAM dosage of 15mgL−1 with residual turbidity of 0.3 NTU, ζ-potential of −3.9mV and D50 of 70.4μm was the optimum system with the most significant enhancements compared to single systems.