Sr diffusion and reaction within Fe oxides: Evaluation of the rate-limiting mechanism for sorption

Sorption studies were conducted to study intraparticle diffusion of strontium in amorphous iron oxide. Oxides of manganese and especially iron often control contaminant sorption in the subsurface environment. In the two-step sorption of inorganics by oxide minerals, the first step is a fast, reversible reaction between the bulk aqueous phase and the adsorbent external surface sites. The second step is a slow process wherein the contaminant at the surface slowly diffuses into the interior of the oxide particle. During intraparticle transport, the contaminant diffuses through micropores along interior surface sites. This configurational surface diffusion is the rate-controlling mechanism in the sorption process. Experimental studies were used to determine adsorbent and sorption characteristics. Ferrihydrite characteristics evaluated were porosity, pore size distribution, site density, size, and shape. Isotherm and constant boundary condition studies were used to observe the sorption process at the external and internal surface sites, respectively. By fitting the diffusion model to experimental data, a value of 4 × 10^-13 cm²/s was found for Sr surface diffusivity.