Abstract
Adsorption by nanoporous media is critically involved in many fundamental geological and geochemical processes including chemical weathering, element migration and enrichment, environmental pollution, etc. Yet, the adsorption behavior of metal ions on nanoporous materials has not been systematically investigated. In this study, MCM-41 material with a monodisperse pore size (4.4 nm) and a large BET specific surface area (839 m2/g) was hydrothermally prepared and used as a model silica adsorbent to study the adsorption characteristics of Cu2+ as a representative metal ion. The Cu2+ adsorption capacity was found to increase with increasing suspension pH in the range from 3 to 5 and to decrease in the presence of NaNO3. At 25 °C, pH = 5, and a solid-to-liquid ratio of 5 g/L, the adsorption capacity was determined to be 0.29 mg/g, which can be converted to a dimensionless partition coefficient of 45, indicating a strong enriching effect of nanoporous silica. The adsorption isotherm and kinetic data were fitted to several commonly used thermodynamic, kinetic, and diffusion models. The adsorption mechanism was also studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and synchrotron-based X-ray absorption spectroscopy. The results suggest that Cu2+ ion adsorption is an entropy-driven endothermal process, possibly involving both outer-sphere and inner-sphere complexes.
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