Cadmium and selenite adsorption on porous aluminum oxides: Equilibrium, rate of uptake, and spectroscopic studies.

摘要

Studying inorganic ion uptake by porous oxides is important for our understanding of the fate and transport of toxic inorganics in surface and subsurface environments and for the design of engineered removal processes. Adsorption of inorganics on high-surface-area oxide adsorbents could be used for wastewater and contaminated groundwater treatment, but the process could be mass transfer limited. To investigate sorption processes and mass transfer limitations in porous oxide particles, equilibrium, rate of uptake, and spectroscopic studies of cadmium and selenite adsorption on porous, high-surface-area transition aluminas (ALCOA CP-5, C-33, and CP-100) were conducted. CP-5 and CP-100 were similar microporous transition aluminas (BET surface areas 200 and 170 m{dollar}sp2{dollar}/g), with Sauter mean diameters 8 and 60 {dollar}mu{dollar}m, respectively. C-33 was a more crystalline, mesoporous transition alumina with 110 m{dollar}sp2{dollar}/g surface area and 34 {dollar}mu{dollar}m Sauter mean diameter.; Equilibrium studies confirmed the high sorption capacity of porous adsorbents. The more disordered forms (CP-5 and CP-100) had higher sorption capacity than C-33. The equilibrium experiments were modeled with the triple-layer surface complexation model as implemented in the computer code HYDRAQL. The model fit was good considering the range of sorbate/sorbent ratios studied. The usefulness of the model as a design tool was demonstrated.; The rate of uptake, a function of particle size, pore structure, and pH-dependent sorption distribution coefficient, was in fair agreement with an intraparticle diffusion model. Equilibration required from a few hours (CP-5 and C-33 adsorbents) to a few days (CP-100). Apparent diffusivities, from 4.0 {dollar}times{dollar} 10{dollar}sp{lcub}-16{rcub}{dollar} to 1.0 {dollar}times{dollar} 10{dollar}sp{lcub}-15{rcub}{dollar} m{dollar}sp2{dollar}/s for cadmium adsorption and from 1.5 {dollar}times{dollar} 10{dollar}sp{lcub}-15{rcub}{dollar} to 5.0 {dollar}times{dollar} 10{dollar}sp{lcub}-14{rcub}{dollar} m{dollar}sp2{dollar}/s for selenite adsorption, suggest severe ion retardation due to strong sorption.; Spectroscopic studies (X-ray Photoelectron Spectroscopy, XPS and X-ray Absorption Spectroscopy, XAS) were consistent with intraparticle ion diffusion and the formation of mononuclear sorption complexes, irrespective of sorbent.