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 stu dies were compared with previous results for strontium sorption to hydrous ferric oxide (HFO). Sorption to these microporous oxides can be described a s a two-step process: a rapid and reversible adsorption reaction to the ext ernal surface including the macropores is followed by a slow surface diffus ion along the micropore walls of the oxides. Strontium adsorption is a phys ical type of reaction, where the sorbate retains its hydration sheath. Beca use the relationship between the amount sorbed and the bulk aqueous concent ration was linear (over six orders of magnitude), the range of surface site s can be described with one average type of site. Thermodynamically, this a dsorption is an entropy-driven endothermic reaction. Transient studies reve aled 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 th ose of HFO where for all the oxides, the energy barrier between sites is ap proximately equivalent. The long-term studies revealed as much as 90% of th e total sites were located on the internal surfaces of the hydrous oxides. Therefore, current transport models must include the contribution from intr aparticle diffusion to accurately describe the migration of Sr in the aquat ic environments. (C) 1999 Academic Press.