Abstract
Nano-scale spatial confinement can alter chemistry at mineral–water interfaces. These nano-scale confinement effects can lead to anomalous fate and transport behavior of aqueous metal species. When a fluid resides in a nanoporous environments (pore size under 100 nm), the observed density, surface tension, and dielectric constant diverge from those measured in the bulk. To evaluate the impact of nano-scale confinement on the adsorption of copper (Cu2+), we performed batch adsorption studies using mesoporous silica. Mesoporous silica with the narrow distribution of pore diameters (SBA-15; 8, 6, and 4 nm pore diameters) was chosen since the silanol functional groups are typical to surface environments. Batch adsorption isotherms were fit with adsorption models (Langmuir, Freundlich, and Dubinin–Radushkevich) and adsorption kinetic data were fit to a pseudo-first-order reaction model. We found that with decreasing pore size, the maximum surface area-normalized uptake of Cu2+ increased. The pseudo-first-order kinetic model demonstrates that the adsorption is faster as the pore size decreases from 8 to 4 nm. We attribute these effects to the deviations in fundamental water properties as pore diameter decreases. In particular, these effects are most notable in SBA-15 with a 4-nm pore where the changes in water properties may be responsible for the enhanced Cu mobility, and therefore, faster Cu adsorption kinetics.
Highlights
Developing a better understanding of the chemistry at mineral surfaces is critical to predict the fate and transport of chemical species in soils, sediments, and in porous rocks [1]
Examples of these deviations in water properties include a decrease in the dielectric constant [7, 8], density, and surface tension when the pore size is less than 5 nm [9]
To evaluate the effects of nano-scale confinement with environmentally relevant reactive surfaces, we focused on unmodified mesoporous silica to present the first study, to our knowledge, that directly investigated the impact of pore size on the C u2+ adsorption kinetics and uptake
Summary
Developing a better understanding of the chemistry at mineral surfaces is critical to predict the fate and transport of chemical species in soils, sediments, and in porous rocks [1]. Nano-scale confined domains are ubiquitous in the environment and exist in tight rocks (i.e. shale), where large fraction of pores are in the nano-scale range [2, 3] These domains have been recognized as an important factor in the selective permeability of tight rocks, a phenomenon largely attributed to the compressed electrical double layer at the water–solid interface as the pores decrease in size [2]. That with decreasing pore size, the fundamental properties of water change, which is expected to affect the adsorption of ions at mineral–water interfaces Examples of these deviations in water properties include a decrease in the dielectric constant (as much as a 50% decrease with a 1.2 nm pore) [7, 8], density, and surface tension when the pore size is less than 5 nm [9]. These deviations are largely attributed to the high ratio of mineral-surfacebound (structured) water in nano-scale confined systems relative to bulk water [9]
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