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 (Gamma=0.2 to 3.3 µmol/m(2)) at pH values of 7.0 to 8.2. Over equilibration times of 15-111 h, we find that at low sorption densities (Gamma=0.2-1.1 µmol/m(2)) Zn(II) forms predominantly inner-sphere bidentate surface complexes with AlO(6) polyhedra, whereas at higher sorption densities (Gamma=1.5 to 3.5 µmol/m(2)), 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 identical withAl(OH(a))(OH(b))+Zn (H(2)O)(6)(2+)--> identical withAl(OH(a)') (OH(b)')Zn(OH(c)')(OH(d)'+4H(2)O+zH(+), where identical withAl(OH(a))(OH(b)) represents edge-sharing sites of Al(O,OH,OH(2))(6) octahedra to which Zn(O,OH,OH(2))(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/m(2)), 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 Gamma-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.