SR DIFFUSION AND REACTION WITHIN FE OXIDES - EVALUATION OF THE RATE-LIMITING MECHANISM FOR SORPTION

Sorption studies were conducted to study intraparticle diffusion of st rontium in amorphous iron oxide. Oxides of manganese and especially ir on often control contaminant sorption in the subsurface environment. I n the two-step sorption of inorganics by oxide minerals, the first ste p is a fast, reversible reaction between the bulk aqueous phase and th e adsorbent external surface sites. The second step is a slow process wherein the contaminant at the surface slowly diffuses into the interi or of the oxide particle. During intraparticle transport, the contamin ant diffuses through micropores along interior surface sites. This con figurational surface diffusion is the rate-controlling mechanism in th e sorption process. Experimental studies were used to determine adsorb ent and sorption characteristics. Ferrihydrite characteristics evaluat ed were porosity, pore size distribution, site density, size, and shap e. Isotherm and constant boundary condition studies were used to obser ve the sorption process at the external and internal surface sites, re spectively. By fitting the diffusion model to experimental data, a val ue of 4 x 10(-13) cm(2)/s was found for Sr surface diffusivity. (C) 19 95 Academic Press, Inc.