Electrokinetic study of specific adsorption of cations on synthetic goethite

The sign of the electrokinetic potential of goethite in the presence of 10−4 mol dm−3 of cetyltrimethylammonium bromide is positive over the entire pH range. The same concentration of La cations induces a shift in the IEP of goethite to high pH by one pH unit. The influence of divalent metal cations (Zn, Ca) on the IEP of goethite is negligible. Probably the adsorption of one divalent metal cation on goethite is accompanied by release of two protons. Thus, the net electric charge within the slipping plane is not affected by the adsorption and this explains the absence of a shift in the IEP.