Trainor TP, Brown GE, Parks GA
Surface & Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California, 94305-2115
J Colloid Interface Sci. 2000 Nov 15;231(2):359-372. doi: 10.1006/jcis.2000.7111.
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. Copyright 2000 Academic Press.
采用XAFS光谱研究了高比表面积氧化铝粉末（Linde - A）对水溶液中Zn(II)的吸附产物，研究了在pH值为7.0至8.2时，Zn(II)吸附密度（Γ = 0.2至3.3 μmol/m²）的函数关系。在15 - 111小时的平衡时间内，我们发现，在低吸附密度（Γ = 0.2 - 1.1 μmol/m²）下，Zn(II)主要与AlO₆多面体形成内球双齿表面络合物，而在较高吸附密度（Γ = 1.5至3.5 μmol/m²）下，我们发现形成了具有水滑石型局部结构的混合金属Zn(II)-Al(III)氢氧化物共沉淀的证据。这些结论基于对在环境条件下对湿样品收集的EXAFS光谱得出的第一和第二近邻原子间距离的分析。在低吸附密度下，吸附机制涉及从六配位的Zn-六水合溶液络合物（平均Zn - O距离为2.07 Å）转变为四配位的表面络合物（平均Zn - O距离为1.97 Å），如反应式Al(OH(a))(OH(b)) + Zn (H₂O)₆²⁺ → Al(OH(a)') (OH(b)')Zn(OH(c)')(OH(d)' + 4H₂O + zH⁺所示，其中Al(OH(a))(OH(b))代表Al(O,OH,OH₂)₆八面体的边共享位点，Zn(O,OH,OH₂)₄以双齿方式键合于此。与该反应一致的质子释放（z = a - a' + b - b' + 4 - c' - d'），并且根据键价分析，当忽略吸附的Zn(II)络合物的水解时，质子释放量在0至2 H⁺/Zn(II)范围内。这种解释表明质子释放可能强烈依赖于表面羟基的配位化学。在较高吸附密度（1.5至3.5 μmol/m²）下，EXAFS光谱的傅里叶变换中的高振幅第二壳层特征表明形成了具有水滑石状局部结构的三维混合金属共沉淀。硝酸根阴离子可能满足Al(III)-Zn(II)氢氧化物层的正层电荷，其中Zn/Al比在1 : 1至2 : 1范围内。我们对较高Γ值吸附样品的结果表明，类水滑石型Zn相可能是天然或含可溶性氧化铝化合物的催化体系中Zn(II)的重要汇。版权所有2000年学术出版社。
