Interfacial chemistry in geologic nanopores

Abstract:

Global-scale cycling of chemical species is driven by molecular-scale reactions that occur within nano-scale pores in soils and sedimentary rocks. Nanoporous sedimentary silicates comprise a quarter of all continental landmass. Therefore, understanding the physicochemical properties of silica-water interfaces is key to predicting the fate and transport of chemical species. While the chemistry of unconfined silica-water interfaces is understood reasonably well, it becomes unpredictable when the interface is confined, like in the case of nanometer-scale pores within sedimentary rocks. We assessed the reactivity of such spatially confined silica-water interfaces using trivalent lanthanide ions. The ionic radii of lanthanides decrease with increasing mass, while the hydration energy increases across the series. We use these systematic variations to quantify how nanoconfinement affects the energetics of adsorption, overall uptake, and coordination environment around adsorbed species. We use synthetic silicas with controlled and narrow distribution of pore diameters in the 4nm to 8nm range and non-porous amorphous silica. The heats of adsorption are quantified in situ using flow-microcalorimetry. The local coordination environment for lanthanides is quantified using synchrotron-based X-ray absorption fine structure spectroscopy. Cluster-based density functional theory calculations with varying dielectric constants are performed to estimate nanoconfinement effects in homogeneous systems. The cumulative results indicate inner-sphere adsorption, with heats of adsorption and coordination environments being affected by nanoconfinement.

This material is based upon work supported by the U.S. DOE, Office of Science, Office of Basic Energy Sciences. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.