CADMIUM ADSORPTION ON IRON-OXIDES IN THE PRESENCE OF ALKALINE-EARTH ELEMENTS

Cadmium sorption on Fe2O3.H2O(am) in the presence of alkaline-earth cations was investigated with emphasis on the Cd-Ca binary system. In binary-element sorption experiments with Ca, Mg, Sr, and Ba, competition was observed primarily in Cd-Ca binary mixtures, which were studied at three ionic strengths (0.5, 0.1, and 0.005 M). The extent of competition increased with increasing Ca concentration. In single-element experiments changes in ionic strength (between 0.005 and 0.5 M) did not influence Cd sorption but did affect Ca sorption, implying that a portion of the Ca surface complexes were outer sphere. The triple-layer model (TLM), using inner- and outer-sphere complexation reactions, and the nonelectrostatic model (NEM), incorporating only one adsorption plane, both adequately represented the adsorption of the individual elements and predicted the Cd-Ca competition at ionic strengths of 0.5 and 0.1 M. Neither model predicted the competition at 0.005 M. The NEM better simulated the shape of the measured Cd-Ca adsorption edge at all ionic strengths. The combined experimental and modeling results suggest that Cd-Ca competitive adsorption on Fe2O3.H2O(am) occurs via mass action on mutually accessible surface sites.