Constraining the effect of pore size on sorption is a critical step in assessing the impact of nano-confinement on porosity evolution, reactive transport, and contaminant sequestration in natural environments. Adsorption and precipitation occur at mineral-water interfaces, often within nanopores (1-100 nm), that contribute significantly to the total specific surface area of rocks and soils. 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 intrinsically with pore size, and the molecular-scale mechanisms controlling nano-confinement phenomena in environmentally relevant systems (i.e., mineral surfaces in aqueous solutions). Moreover, recent research has presented seemingly contrary observations of preferential precipitation within nanopores, but a unifying theory has yet to be deciphered. Because adsorption is a precursor to heterogeneous precipitation, we have employed synchrotron-based characterization of batch adsorption and precipitation experiments to examine the underlying causes of nano-confinement altered sorption patterns. Specifically, we studied model systems of variably porous silica (i.e., 14 nm-300 nm pores and nonporous) and aqueous Ni and Zn to probe sorption reactions on nanopore surfaces. X-ray absorption spectroscopy of adsorption complexes reveals that Zn adsorbed at surface coverages &lt;0.8 μmol/m2 in silica macropores and on silica macro-particles dominantly exhibits octahedral coordination, as opposed to tetrahedral coordination in silica nanopores. Tetrahedral coordination is also characteristic of many Zn minerals. Precipitation experiments display enhanced precipitation of metal (i.e., Zn and Ni) (hydroxy)carbonates in silica nanopores, which we attribute to nano-confinement altered adsorption complexes.

Adsorption and precipitation of Zn and Ni in nanoporous silica

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.