COMPETITIVE ADSORPTION OF CD, CU, ZN, AND PB ON AMORPHOUS IRON OXYHYDROXIDE

Competitive adsorption of Cd, Cu, Pb and Zn onto amorphous iron oxyhydroxide (Fe2O3 .cntdot. H2O (am)) was studied in systems containing pairs of metal adsorbates. The concentration of the more strongly binding metal was 10- to 100-fold that of the other metal in the system. The conditions were such that if the metals competed for the same group of surface sites, adsorption of the weaker-binding metal should have been significantly reduced when the 2nd metal was added to the system. Competitive interactions were minimal, indicating that many strong binding sites for 1 metal were not preferred binding sites for other metals. These results supported the hypothesis that the Fe2O3 .cntdot. H2O (am) surface consisted of several distinct groups of binding sites. A differential adsorption equilibrium constant was defined and related to the overall average adsorption equilibrium constant. Based on this relationship, the binding constants for a single metal to different surface sites ranged over at least 2 orders of magnitude. The differential constant could be used to assess competitive interactions. For instance, when 5 x 10-5 M Cu and 5 .times. 10-7 M Cd competed for sites on Fe2O3 .cntdot. H2O (am) (1 .times. 10-3 M FeT), < 2% of the adsorbed Cu bound to preferred Cd-binding sites. Failure to account accurately for competitive effects could lead to serious errors when modeling trace metal behavior in natural systems and engineering processes.