Describing the adsorption of phosphate, citrate and selenite on a variable-charge mineral surface

Abstract

The effects were measured of varying the pH and the concentration of adsorbing ion on adsorption of phosphate, citrate and selenite by goethite and on the charge conveyed to the surface. The ability of the model of Bowden et al. to describe these effects was investigated. The observed effects were closely described by the model, provided it was modified to permit the adsorbed ions to reside in a plane between the surface and the diffuse layer. The model requires that individual ionic species be considered. For phosphate and selenite, the divalent ion appeared to be the only ion adsorbed whereas for citrate it was the trivalent ion. The model also requires that adsorption depends on the electrostatic potential in the plane of adsorption. This potential decreases with increasing pH. Thus the effects of pH on adsorption were explained by changes in this potential, together with changes in the proportion of the ionic species present. Because adsorption made the surface more negative, it also decreased the electrostatic potential in the plane of adsorption. This made further adsorption more difficult, and as a result, adsorption at a constant pH did not follow the Langmuir equation. The model showed that the increase in negative charge as a result of adsorption was partly balanced by an uptake of protons by the surface. This was most marked at near-neutral pH and as a result the net charge per adsorbed ion was least.