Zeta potential and viscosity of colloidal silica suspensions: Effect of seawater salts, pH, flocculant, and shear rate

Abstract

The effect of various seawater electrolytes on the zeta potential and viscosity of silica suspensions has been studied as a function of electrolyte concentration, pH, flocculant and shear rate. Salts were alkali and alkaline-earth metal chlorides. The magnitude of the negative zeta potential of silica as a function of pH for monovalent cations follows Li+>Na+>K+>Cs+ implying that adsorption follows Cs+>K+>Na+>Li+. In the presence of divalent cations, the magnitude of the negative zeta potential as a function of pH decreases greatly following Mg2+>Ca2+, implying that adsorption follows Ca2+>Mg2+. Regarding the flocculant in solution, molecular simulations indicate that repulsion between anionic acrylate units causes the polymer to adopt expanded tertiary conformations, however with cations this repulsion is shielded causing the polymer to fold into balled-up conformations. The adsorption of cations on anionic flocculant chains follows inverse series than silica, i.e., Li+>Na+>K+>Cs+ and Mg2+<Ca2+. The different ability of silica and flocculant to adsorb cations leads to force fields that create three-dimensional networks of particles with resistance depending on the cation. The viscosity of silica suspensions formulated at 0.5M salt and pH 7 in anionic flocculant follows KI>KCl>NaCl>MgCl2> water, and in cationic flocculant follows KI<KCl<NaCl<MgCl2< water. In anionic flocculant, the counterion is the same for silica and flocculant leading to strong particle-flocculant interaction and suspensions with high viscosity. The resistance of the particle networks formed are weakened only slightly as the shear stress increases In cationic flocculant, the cation is the counterion of silica while the anion is the counterion of flocculant, their ionic atmospheres are different giving rise to weak particle-flocculant interaction and low viscosity suspensions. The resistance of such networks is increasingly weakened as the shear rate increases. The higher the pH, the higher the repulsion, and the lower the viscosities of the suspensions formulated with either flocculant.