Adsorption of metal ions onto goethite: Single-adsorbate and competitive systems

Adsorption of anthropogenically released toxic metals such as Ni and Zn to goethite effects their mobility and bioavailability in aquatic environments. In this research sorption studies were conducted to understand competitive adsorption of environmentally important metals such as Ni, Zn, and Ca onto the goethite surface. Adsorption edges conducted as a function of ionic strength suggest that these metals are chemisorbed to the goethite surface. Furthermore, the adsorption affinity follows the order of the inverse of the hydrated radii multiplied by the number of waters in the primary solvation shell: Zn>Ni>Ca. Isotherm studies revealed a linear relation between the amount of metal adsorbed and the aqueous bulk phase concentrations, where site saturation was obtained by reducing the goethite concentration to 0.1 g l^-1. Accordingly the single-site Langmuir model provided a good fit; equilibrium constants were found to be independent of pH indicative of one type of adsorption reaction. The equilibrium constants for both transition metals (Ni and Zn) were greater than that of Ca, suggesting that transition metals have a greater affinity for the surface. Analyses of site densities revealed two types of sites on the surface of goethite: high affinity ones to which transition metals bind, and low affinity sites that comprise 100× that of the high affinity ones. From the isotherm studies, it appears that only the alkaline earth metals such as Ca adsorb to this lower affinity site. The single-site Langmuir model was able to accurately describe adsorption competition between Ni and Zn for the goethite surface. In contrast, no competitive effects were observed in Ni-Ca and Zn-Ca binary systems.