Modeling Cd and Zn sorption to hydrous metal oxides

The mobility and bioavailability of Cd and Zn in soils and sediments are af fected by contaminant distribution mechanisms. One important process is sor ption to hydrous aluminum, iron, and manganese oxides, which are ubiquitous in soils and sediments as both discrete particles and coatings and exhibit a high affinity for these metals. Mechanistic models are required for accu rately assessing risks to populations and in the long-term management of co ntaminated soils and sediments. This research demonstrates intraparticle di ffusion is the rate-limiting step in the sorption of Cd and Zn to microporo us oxides. Furthermore, as much as 90% of the total sorption sites on the o xides reside on the micropore walls. Because longterm experiments require a lengthy period of time, predictive methods would be useful for determining surface diffusivities. Theoretically, surface diffusivities can be predict ed from site activation theory, which is based on the random walk model whe re atoms or molecules vibrate at localized sites along the surface. Once th e vibrating ion has sufficient energy, it will jump to a neighboring site. For a given metal, the associated activation energy was observed to be equi valent for all three oxides; in an effort to predict this energy, a correla tion is presented between the adsorption enthalpy and the adsorbate hydrate d radius, for each oxide, the Polanyi constant (alpha) that relates adsorpt ion enthalpy and activation energy was equivalent for the transition metals studied.