Predicting divalent metal sorption to hydrous Al, Fe, and Mn oxides

Intraparticle diffusion in microporous amorphous oxides of aluminum, iron, and manganese affects contaminant mobility and bioavailability in soils and sediments. This sorption is a lengthy process, as such, predictive methods to assess thermodynamic and transport parameters would be useful. Based on enthalpies observed in recent work, adsorption of Zn, Cd, and Sr to amorphous oxides is a physical type of reaction where the metal ions retain their waters of hydration. Consequently the adsorbate-surface interactions are a function of electrostatic forces of attraction. Accordingly, knowing the hydrated radius and the hydration number of a metal cation, a correlation is used to predict enthalpy and hence affinity. Using the resulting enthalpy and the Polanyi relation, the activation energy was evaluated for Ni and Ca. This Polanyi relationship reveals that for a given metal the activation energies with respect to these oxides are comparable. Additionally, metals of the same periodic group appear to form similar sorption complexes with a particular oxide and therefore have an equivalent Polanyi constant, α. Assuming a sinusoidal function describes the surface potential along the oxide surface, the surface diffusivity was predicted from the site activation theory. In this work, the predicted sorption parameters proved to be equivalent to experimental ones given the associated errors.