GEOCHEMICAL IMPLICATIONS OF NANOMETER-SCALE PORES IN NATURAL MATERIALS

Nanopores are ubiquitous in porous geologic media and constitute an integral part of total porosity of rocks. Existing data indicate that the contribution of nanopores to the total surface area in geologic materials can be very high, probably over 90 percents. To clarify the effect of nanopore confinement, acid-base titration and metal adsorption experiments were performed on both nanoporous alumina and alumina particles under various chemical conditions. The experiments have demonstrated that the nano-scale confinement has a significant effect, most likely via the overlap of the electric double layer, on ion sorption onto nanopore surfaces. Under the same chemical conditions, the surface charge per mass on nanoporous alumina was as much as 45 times higher than that on alumina particles. The nanopore confinement leads to a shift of ion sorption edges and enhances ion sorption on nanopore surfaces for both cations and anions. As a result, trace elements in natural systems tend to be preferentially enriched in nanopores. This effect cannot be adequately modeled by existing surface complexation models. To understand the state of water in nanopores, the experiment of water adsorption under various relative-humidity conditions were performed in combination with H magic-angle spinning nuclear magnetic resonance (MAS NMR) and Fourier transform infrared (FTIR) spectroscopic analyses. The experimental results show that water molecules in nanopores form more rigid structures than those in bulk solutions. The observed nanopore confinement effects will shed a light on our understanding many fundamental geochemical issues such as the bioavailability of contaminants and the formation of ore deposits.