Nanogeochemistry: Nanostructures, emergent properties and their control on geochemical reactions and mass transfers

Nanopores are ubiquitous in porous geologic media and may account for >90% of total mineral surface areas. Surface chemistry, ion sorption, and the related geochemical reactions within nanopores can be significantly modified by a nanometer-scale space confinement. As the pore size is reduced to a few nanometers, the difference between surface acidity constants (ΔpK = pK2 − pK1) decreases, giving rise to a higher surface charge density on a nanopore surface than that on an unconfined mineral-water interface. The change in surface acidity constants results in a shift of ion sorption edges and enhances ion sorption on nanopore surfaces. Also, the water activity in a nanopore is greatly reduced, thus increasing the tendency for inner sphere complexation and mineral precipitation. All these effects combine to preferentially enrich trace elements in nanopores, as observed in both field and laboratory studies. The work reported here sheds new light on such fundamental geochemical issues as the irreversibility of ion sorption and desorption, the bioavailability of subsurface contaminants, and the enrichment of trace metals in ore deposits, as well as the kinetics of mineral dissolution and/or precipitation.

Nanogeochemistry: Geochemical reactions and mass transfers in nanopores

Removal of uranium from mining industry feed simulant solutions using trapped amidoxime functionality within a mesoporous imprinted polymer material

The synthesis of functional mesoporous materials

This paper presents a part of our work on understanding the effect of nanoscale pore space confinement on ion sorption by mesoporous materials Acid-base titration experiments were performed on both mesoporous alumina and alumina particles under various ionic strengths The point of zero charge (PZC) for mesoporous alumina was measured to be approximately 91, similar to that for nonmesoporous alumina materials, indicating that nanoscale pore space confinement does not have a significant effect on the PZC of pore surfaces However, for a given pH deviation from the PZC, (pH-PZC), the surface charge per mass on mesoporous alumina was as much as 45 times higher than that on alumina particles This difference cannot be fully explained by the surface area difference between the two materials Our titration data have demonstrated that nanoscale confinement has a significant effect, most likely via the overlap of the electric double layer (EDL), on ion sorption onto mesopore surfaces This effect cannot be adequately modeled by existing surface complexation models, which were developed mostly for an unconfined solid-water interface Our titration data have also indicated that the rate of ion uptake by mesoporous alumina is relatively slow, probably due to diffusion into mesopores, and complete equilibration for sorption could take 4-5 min A molecular simulation using a density functional theory was performed to calculate ion adsorption coefficients as a function of pore size The calculation has shown that as pore size is reduced to nanoscales (<10 nm), the adsorption coefficients of ions can vary by more than two orders of magnitude relative to those for unconfined interfaces The prediction is supported by our experimental data on Zn sorption onto mesoporous alumina Owing to their unique surface chemistry, mesoporous materials can potentially be used as effective ion adsorbents for separation processes and environmental cleanup

Interface chemistry of nanostructured materials: ion adsorption on mesoporous alumina.

Mesoporous titanosilicate sol-gel materials with different Silicon to titanium ratios were prepared by hydrothermal synthesis through the micellar template approach. The prepared samples have been characterized via infrared spectroscopy, nitrogen Sorption, powder X-ray diffraction, and acid-base titration. The uptake of uranyl ion (UOi\"^) by the samples was determined at pH = 5.10. The uranyl loading capacity of the pure mesoporous titania was determined to be the largest. This capacity is much higher than that of the corresponding microporous material. The increase of capacity is correlated to the large surface area induced by the Surfactant template synthetic method for mesoporous materials, increased concentrations of TiO\" surface functional ligands, and large pore openings of the mesoporous materials allowing for access to the inner pore surfaces. The adsorption kinetics of our mesoporous sorbents are much faster than those of the corresponding microporous materials. The adsorption equilibria can be reached in less than 15 min.

Investigation of Uranyl Adsorption on Mesoporous Titanium-based Sorbents

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Investigation of Uranyl Adsorption on Mesoporous Titanium-based Sorbents