Describing the effects of electrolyte on adsorption of phosphate by a variable charge surface

The effects were measured of varying the concentration of sodium chloride and of calcium chloride on adsorption of phosphate by goethite. The ability of the model of Bowden et al, to describe these effects was investigated. At low pH, increasing the concentration of either salt decreased adsorption. As the pH was increased, a point was reached at which there was no effect of salt on adsorption. Beyond that point, increasing the concentration of salt increased adsorption. For calcium chloride, adsorption was greater than for sodium chloride of equal ionic strength, but observations were limited to below pH 6 because octo-calcium phosphate precipitated at higher pH. The effects were closely described by the model. They were ascribed to differences in the distribution of electrolyte ions near the surface, with the result that the electrostatic potential in the plane of adsorption was changed. The direction of the effect differs on either side of the point of zero charge of the phosphated surface. The greater adsorption in calcium chloride was ascribed partly to the greater valency of the cation, and partly to a specific affinity of calcium ions for the surface.