Specific adsorption of nickel and ζ potential of silica at various solid-to-liquid ratios

The electrokinetic curves of silica obtained at given total concentrations of heavy metal cations depend on the solid-to-liquid ratio; namely, at low solid-to-liquid ratios the sign of the ζ potential of silica is reversed to positive at relatively low concentrations of heavy metal cations, while at higher solid-to-liquid ratios the electrokinetic curves are not substantially influenced at much higher concentrations. The surface complexation model fails to properly reflect the Ni effect on the course of electrokinetic curves of silica. The course of electrokinetic curves [positions of the isoelectric point(s) and of the maxima in mobility] is correlated with a linear combination of the surface charge density in the absence of heavy metal cations at given ionic strength and the adsorption density of Ni.