Multiple-site adsorption of Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide

Adsorption of Cd, Zn, Cu, and Pb onto amorphous iron oxyhydroxide was measured as a function of pH, metal ion concentration, and adsorbent concentration. For each metal, there is a narrow pH band where fractional adsorption increases from near nil to near 100%. For fixed adsorbent concentration, the pH region of the pH-adsorption edge is independent of total adsorbate concentration when adsorption density is less than 10−5.0, 10−3.7, and 10−2.3 moles adsorbate per mole Fe for adsorption of Cd, Cu, and Zn, respectively. At larger adsorption densities for these three metals and over the entire range of adsorption densities studied for Pb, the pH region of the adsorption edge becomes more alkaline as total adsorbate concentration increases. In no case did adsorption density attain a maximum, limiting value. The results suggest that the surface is composed of many groups of binding sites. The strength of binding between a given metal and the surface may vary by an order of magnitude or more from one site to another. At small adsorption densities all types of sites are available in excess, and adsorption can be described by the Langmuir isotherm. However, at higher adsorption densities, availability of the strongest binding sites decreases, leading to a decrease in the apparent adsorption equilibrium constant. This phenomenon occurs under conditions where only a few percent of all surface sites are occupied, and is inconsistent with available single-site models.