Effects of nano-confinement on Zn(II) adsorption to nanoporous silica

Nanopores (at least one dimension ≤100 nm and no dimension <1 nm) are commonly observed features of grain coatings and primary silicate minerals and can contribute substantially to the total surface area available for chemical reactions. Despite the ubiquity of nanopores in natural and synthetic porous media, a generalizable framework for evaluating the effects of nanopore confinement on both fluid and sorbate properties across the entire nanoscale has yet to emerge. As a result, we have a poor understanding of the effect of pore size on molecular-scale processes controlling nano-confinement phenomena in systems in which mineral surfaces are in contact with aqueous solutions. To address these knowledge gaps, we carried out batch experiments designed to reveal the molecular-level details of adsorption reaction products and attendant isotope fractionation of aqueous Zn(II) for synthetic, variably porous, amorphous silica (SiO_2(am)) particles with average pore diameters ranging from 10 nm to 330 nm. Shell-by-shell fitting of Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra reveals that Zn(II) (1) adsorbs as dominantly monodentate, corner-sharing complexes to silicate tetrahedra of all SiO_2(am) particles examined, regardless of pore size, (2) is tetrahedrally coordinated by oxygen atoms in nanoporous silica with small pore sizes (≤10 nm), and (3) is present as a mixture of tetrahedrally and octahedrally coordinated surface complexes in silica with larger pore sizes (>10 nm). In addition, at low Zn(II) surface coverages (<0.9 μmol m⁻²) on macroporous SiO_2(am), Zn(II) forms a mixture of octahedrally and tetrahedrally coordinated surface complexes, whereas tetrahedrally coordinated surface complexes predominate at higher Zn(II) coverages (>0.9 μmol m⁻²). Although molecular-level differences in Zn(II) adsorption complexes are apparent with respect to SiO_2(am) pore size, we observed no quantifiable differences in attendant isotope fractionation or adsorption edge structure for Zn(II) uptake on nanoporous SiO_2(am) surfaces. These results suggest that the differences in binding energy of tetrahedrally versus octahedrally coordinated Zn(II) complexes at the same site on SiO_2(am) are small. Comparison of EXAFS spectra to the results of surface complexation modeling (SCM) of experimental Zn(II) uptake data indicates that monodentate Zn(II) adsorption on the SiO_2(am) surface releases two protons through binding one site and passivating an adjacent site, which accounts for the modification of surface charge and surface complex structure during adsorption of Zn(II) cations. This study shows that adsorption of Zn(II) in variably porous SiO_2(am) results in differences in surface complex geometry as pore size decreases and as surface coverage of Zn(II) increases. Although surface complexation modeling of Zn(II) adsorption edges on macroporous and nanoporous SiO_2(am) does not independently identify these molecular-level differences, combination of SCM and molecular-level observations reveals unique insight into the overall adsorption reaction and neighboring surface site reactivity.