Joey Nelson, John Bargar, Gordon E. Brown, Jr., Kate Maher

Stanford University

Nanopores (<100 nm diameter pores) contribute significantly to the total specific surface area of rocks and soils, and synthetic nanoporous substrates have been proposed as a useful material for environmental remediation. Within these nanopores, the physiochemical processes of sorption and transport differ from those within macropores. Despite the ubiquity of confined spaces in natural and industrial porous media, we lack an understanding of how the effects of nanopore confinement vary with pore size, and the molecular-scale mechanisms controlling nano-confinement phenomena in environmentally relevant systems (i.e., mineral surfaces in aqueous solutions).

We studied the simplified Zn-silica-H₂O model system to probe sorption reactions on nanopore surfaces that commonly influence the fate of contaminants in complex geologic contexts. Batch equilibrium adsorption experiments were performed and the molecular configuration of Zn adsorption complexes were observed with XAS. Shell-by-shell fitting of EXAFS spectra reveals that Zn adsorbed at a surface coverage of ~0.2 µmol/m2 in silica nanopores exhibits tetrahedral coordination as opposed to octahedral coordination in silica macropores and on silica macro-particles. With increasing surface coverage, tetrahedral complexes dominate on all pore-size surfaces. This difference in Zn coordination may be the result of complexation at different surface sites and/or alteration of the hydration shell of Zn under nano-confinement. Confinement effects within nanopores represent an emerging frontier in geochemistry, and pursuit of molecular-scale understanding will contribute to more accurate modeling of reactive transport in natural porous media and to the use of nanoporous materials to sequester contaminants.