Adsorption and Precipitation of Aqueous Zn(II) on Alumina Powders

The products of aqueous Zn(II) sorption on high-surface-area alumina powders (Linde-A) have been studied using XAFS spectroscopy as a function of Zn(II) sorption density (Γ=0.2 to 3.3 μmol/m2) at pH values of 7.0 to 8.2. Over equilibration times of 15–111 h, we find that at low sorption densities (Γ=0.2–1.1 μmol/m2) Zn(II) forms predominantly inner-sphere bidentate surface complexes with AlO6 polyhedra, whereas at higher sorption densities (Γ=1.5 to 3.5 μmol/m2), we find evidence for the formation of a mixed-metal Zn(II)–Al(III) hydroxide coprecipitate with a hydrotalcite-type local structure. These conclusions are based on an analysis of first- and second-neighbor interatomic distances derived from EXAFS spectra collected under ambient conditions on wet samples. At low sorption densities the sorption mechanism involves a transformation from six-coordinated Zn–hexaaquo solution complexes (with an average Zn–O distance of 2.07 Å) to four-coordinated surface complexes (with an average Zn–O distance of 1.97 Å) as described by the reaction ≡Al(OHa)(OHb)+Zn (H2O)62+→≡Al(OHa′) (OHb′)Zn(OHc′)(OHd′+4H2O+zH+, where ≡Al(OHa)(OHb) represents edge-sharing sites of Al(O,OH,OH2)6 octahedra to which Zn(O,OH,OH2)4 bonds in a bidentate fashion. The proton release consistent with this reaction (z=a-a′+b-b′+4-c′-d′), and with bond valence analysis falls in the range of 0 to 2 H+/Zn(II) when hydrolysis of the adsorbed Zn(II) complex is neglected. This interpretation suggests that proton release is likely a strong function of the coordination chemistry of the surface hydroxyl groups. At higher sorption densities (1.5 to 3.5 μmol/m2), a high-amplitude, second-shell feature in the Fourier transform of the EXAFS spectra indicates the formation of a three-dimensional mixed-metal coprecipitate, with a hydrotalcite-like local structure. Nitrate anions presumably satisfy the positive layer charge of the Al(III)–Zn(II) hydroxide layers in which the Zn/Al ratio falls in the range of 1 : 1 to 2 : 1. Our results for the higher Γ-value sorption samples suggest that Zn–hydrotalcite-like phases may be a significant sink for Zn(II) in natural or catalytic systems containing soluble alumina compounds.