Methodology for Determining Point of Zero Salt Effect of Clays in Terms of Surface Charge Properties

Surface charge properties of colloids can be studied from their point of zero charge (PZC) measurements. The point of zero salt effect (PZSE), a category of PZC, indicates the pH value at which the effect of electrolyte concentration on a pH-dependent charge is zero. In the present study, it was mathematically deduced that for clay minerals, the rate change of pH and zeta potential with ionic concentration approaches zero at a pH value corresponding to the PZSE of the soil. The procedure is based on applying the constraint of electroneutrality to the charge-balance equation for colloids in equilibrium with an electrolyte. The theoretical observations were corroborated experimentally by obtaining the pH and zeta potential variations of three mineralogically diverse soils and comparing the values with the standard salt titration results. The study identifies PZSE from pH measurement as a more suitable approach for soil exhibiting variable charge. In the case of highly reactive soils, PZSE measurement using a divalent electrolyte medium yielded comparable results. In view of the obtained results, the proposed method can be an alternative for PZSE determination of clay minerals.