Cadmium adsorption on iron oxides in the presence of alkaline-earth elements

The influence was studied of the alkaline-earth elements magnesium, calcium, strontium, and barium, at concentrations representative of soil solutions and groundwaters, on cadmium sorption onto iron oxides. Emphasis was placed on the Cd-Ca binary system. In binary-element sorption tests, competition was observed primarily in Cd-Ca binary mixtures; the extent of competition increased with increasing Ca concentration. In single-element experiments changes in ionic strength affected Ca sorption, implying that a portion of the Ca surface complexes was outer sphere. A triple-layer model, using inner- and outer-sphere complexation reactions, and a nonelectrostatic model incorporating only one adsorption plane, both adequately represented the adsorption of elements and predicted Cd-Ca competition.