Effect of competitive adsorption of Zn2+ on the flocculation of lauryl sulfate micelles by Al3+

In micellar flocculation of lauryl sulfate micelles with solutions containing Zn2+ and Al3+ cations, the charge ratios polyvalent cation/micellar surfactant may be greater than 1 above [Zn2+] = 0.031 M. This is unlike the case of flocculation of lauryl sulfate micelles with Al3+ or Al3+/Na+ solutions, where the charge ratio Al/sodium dodecyl sulfate in the floc is lower than or nearly equal to 1. Only below [Zn2+] = 0.031 M may the localized adsorption model be used to describe the binding of cations to micelles. The apparent charge inversion may be explained by assuming that a flocculating micelle drags the ions contained within its hydrodynamic radius. A reduction of the Stern and diffuse layer radii under increasing electrolyte concentrations may collapse these within the hydrodynamic radius of the micelle, allowing the hydrodynamic radius to reach even into the bulk solution. If all the solution included within the hydrodynamic radius is dragged into the floc, this would explain the apparent charge inversion. Such a mechanical effect would explain the observed enhancement of pollutant removal by adsorptive micellar flocculation in the presence of Zn2+. Binding of phthalic acid is shown as an example.