Modeling Cd and Zn sorption to hydrous metal oxides

The mobility and bioavailability of Cd and Zn in soils and sediments are affected by contaminant distribution mechanisms. One important process is sorption 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 accurately assessing risks to populations and in the long-term management of contaminated soils and sediments. This research demonstrates intraparticle diffusion is the rate-limiting step in the sorption of Cd and Zn to microporous oxides. Furthermore, as much as 90% of the total sorption sites on the oxides reside on the micropore walls. Because long-term experiments require a lengthy period of time, predictive methods would be useful for determining surface diffusivities. Theoretically, surface diffusivities can be predicted from site activation theory, which is based on the random walk model where atoms or molecules vibrate at localized sites along the surface. Once the vibrating ion has sufficient energy, it will jump to a neighboring site. For a given metal, the associated activation energy was observed to be equivalent for all three oxides; in an effort to predict this energy, a correlation is presented between the adsorption enthalpy and the adsorbate hydrated radius. For each oxide, the Polanyi constant (α) that relates adsorption enthalpy and activation energy was equivalent for the transition metals studied.