MODELING THE RATE OF CADMIUM AND SELENITE ADSORPTION ON MICROPOROUS AND MESOPOROUS TRANSITION ALUMINAS

The rates of cadmium and selenite uptake by porous aluminas were studi ed using three porous transition aluminas. The three adsorbents differ ed in size and pore structure, CP-5 and CP-100 being the smallest and largest particles, respectively, both exhibiting some microporosity, a nd C-33 being intermediate-size, mesoporous particles. The rate data w ere interpreted with a diffusion model, assuming solute diffusion in a sphere from limited volume. The diffusion model was in fair agreement with the rate data, suggesting that cadmium and selenite uptake is co ntrolled by intraparticle mass transfer. Solute uptake by the smaller particles (CP-5) was considerably faster than uptake by the larger par ticles (CP-100). The measured apparent diffusivities for both adsorbat es and all adsorbents were orders of magnitude lower than bulk aqueous diffusivities, in accordance with expectations for highly retarded so rption. The measured effective diffusivities were substantially lower than aqueous molecular diffusivities, suggesting the presence of stron g hindrance effects in these microporous adsorbents.