The ability of a model to describe quantitatively the adsorption of copper, lead and zinc on goethite was tested. The model was able to describe both the increase in adsorption of metals with increasing pH and the release of protons that accompanied adsorption. For copper and zinc, MeOH+ were the adsorbing species. Part of the increase in adsorption with increasing pH arose because these ions increase in concentration with pH. For lead, Pb2+ ions appeared to be adsorbed, and in this case the increase in adsorption with pH was entirely due to the decreased charge on the surface and thus decreased repulsion. For all three metals, the MeCl+ ions also appeared to be adsorbed, and this explained higher adsorption from chloride solution than from nitrate solutions. The observed release of protons accompanying adsorption was matched by moving the plane of adsorption of metal ions relative to that of the H+ and OH- ions. When the two planes were close, the proton release was almost equivalent to the charge on the metal ions; as the distance increased, proton release decreased.

Barrow, N., Bowden, J., Posner, A., & Quirk, J. (1981). Describing the adsorption of copper, zinc and lead on a variable charge mineral surface. Soil Research, 19(3), 309.