Confinement in Nanopores at the Oxide/Water Interface: Modification of Alumina Adsorption Properties

Abstract

There is limited knowledge on the influence of the pore size on surface phenomena (adsorption, dissolution, precipitation, etc.) at the oxide/water interface and a better understanding of the space confinement in nanoscale pores should have practical implications in different areas, such as transport of contaminants in the environment or heterogeneous catalyst preparation, to name a few. To investigate the modifications of the oxide adsorption properties at the oxide/water interface in a confined environment, the surface acidobasic and ion adsorption properties of six different aluminas (5 porous commercial aluminas with pore diameters ranging from 25 to 200 A and 1 non-porous alumina) were determined by means of acid-base titration and Ni(II) adsorption. It is shown that the confinement has a moderate impact on the alumina adsorption capacity because all materials have similar surface charging behaviours and ion saturation coverages. However, a confined geometry has a much larger impact on the ion adsorption constants, which decrease drastically when the average pore diameter decreases below 200 A. These results are discussed in terms of nanoscale pore space confinement.

U(VI) and Sr(II) batch sorption and diffusion kinetics into mesoporous silica (MCM-41)

“…In the absence of the U precipitation events, most of the U and Sr removed from solution occurred within 24 hours of exposure, which is consistent with previous work on metal sorption rates into mesoporous materials; for example, Ni diffuses completely within mesoporous alumina within 5 minutes (Baca et al, 2008;van de Water et al, 2005). Although the surface chemistry of alumina differs from silica, MCM-41 has long channels absent in other mesoporous materials.…”

Environmental Applications of Engineered Materials with Nanoconfinement

“…These porous adsorbents with spatial nanoconfinement have demonstrated significant effects on the behavior of sorbate and sorbent compared to their bulk counterparts, resulting in a great variation in their kinetic and equilibrium behaviors. These include directly changing (i) the hydration sphere of the sorbate, − (ii) the coordination state with host atoms , or other guest molecules/ions, (iii) noncovalent binding with the host, and (iv) the structural patterns of the adsorbent , or by indirectly changing the adsorption process by modulating the surface chemistry of sorbents toward a preferred (i) phase or (ii) spin state …”

Nanogeochemistry: Nanostructures and their Reactivity in Natural Systems

“…This is probably the reason for the observed reduction in [Ni(en) 3 ] 2 + sorption on alumina with decreasing pore sizes (Baca et al 2008 ). In this case, the size -exclusion effect may become important and, as a consequence, large molecules may not be able to access nanopores.…”