A Comparison of Strontium Sorption to Hydrous Aluminum, Iron, and Manganese Oxides

Strontium sorption to hydrous aluminum (HAO) and manganese (HMO) oxides was studied using macroscopic studies on short- and long-term bases. These studies were compared with previous results for strontium sorption to hydrous ferric oxide (HFO). Sorption to these microporous oxides can be described as a two-step process: a rapid and reversible adsorption reaction to the external surface including the macropores is followed by a slow surface diffusion along the micropore walls of the oxides. Strontium adsorption is a physical type of reaction, where the sorbate retains its hydration sheath. Because the relationship between the amount sorbed and the bulk aqueous concentration was linear (over six orders of magnitude), the range of surface sites can be described with one average type of site. Thermodynamically, this adsorption is an entropy-driven endothermic reaction. Transient studies revealed surface diffusion coefficients ranged from 10(-12) to 10(-11) cm(2)/s; these diffusivities were compared to theoretical diffusivities using site activation theory. Results observed in these studies are consistent with those of HFO where for all the oxides, the energy barrier between sites is approximately equivalent. The long-term studies revealed as much as 90% of the total sites were located on the internal surfaces of the hydrous oxides. Therefore, current transport models must include the contribution from intraparticle diffusion to accurately describe the migration of Sr in the aquatic environments. Copyright 1999 Academic Press.