Molecular-scale mechanisms of distribution and isotopic fractionation of molybdenum between seawater and ferromanganese oxides
Molecular-scale mechanisms of distribution and isotopic fractionation of molybdenum between seawater and ferromanganese oxides
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DOI：
10.1016/j.gca.2011.07.022
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发表时间：
2011-10
影响因子：
5
通讯作者：
T. Kashiwabara;Y. Takahashi;M. Tanimizu;A. Usui
中科院分区：
文献类型：
--
作者：
T. Kashiwabara;Y. Takahashi;M. Tanimizu;A. Usui
海水和海洋锰铁氧化物中钼的分布对现代含氧海水中钼的浓度和同位素组成有很大影响。为了揭示钼在铁锰氧化物上的吸附化学，我们进行了(I)用L3和K边XAFS详细分析了钼在δ-MnO_2、水合铁锰氧化物和氢化锰铁表面的结构，以及(Ii)在不同pH值、离子强度和钼浓度范围内钼在δ-MnO_2和水合铁锰氧化物上的吸附实验。X-射线光电子能谱分析表明，Mo在δ-MnO2上形成扭曲的八面体(OH)内球状络合物，而在亚铁氢氧化铁上形成四面体(TD)外球络合物。在水生锰铁氧化物中，Mo的主要寄主相为δ-MnO2。这些结构信息与Mo在吸附实验中的宏观行为相一致，可以用δ-MnO_2上的平衡吸附反应来解释现代含氧海水中Mo的浓度。此外，以前的研究发现海水和锰铁氧化物之间的钼同位素分馏很大，这可以用MoO_42-和δ-MnO_2相上吸附物种的结构差异来解释。与之相反，钼同位素在水铁矿上的分馏较小，这是由于在吸附过程中钼的局域结构变化不大。在pH=8，I=0.70M(NaNO3)时，还研究了晶态Fe(水合氧化物)、针铁矿和赤铁矿上吸附的Mo物种的结构。我们的XAFS分析表明，Mo在两种矿物上都形成了内球络合物：针铁矿为TD边共享(46%)和OH双角共享(54%)，赤铁矿为TD双角共享(14%)和OH边共享(86%)。这些结构信息与非晶态高铁水合物和δ-MnO_2的结构信息相结合，与前人报道的钼的吸附同位素分馏的大小有很好的相关性：钼表面络合物中OH物种的比例或它们的扭曲大小按OH值的顺序变大，这一趋势与同位素分馏的大小一致。在与以往文献报道的各种氧化物表面钼物种比较的基础上，讨论了影响钼表面络合物结构的化学因素。氧化物中阳离子的水解常数logKOH(或氧化物表面的酸性，PZC)与钼表面络合物的吸附方式(内球或外球)有很好的相关性。此外，Mo物种从Td到OH的对称性变化被认为是由氧化物表面特定位置上形成的内球络合物所驱动的。
The distribution of Mo between seawater and marine ferromanganese oxides has great impacts on concentration and isotopic composition of Mo in modern oxic seawater. To reveal the adsorption chemistry of Mo to ferromanganese oxides, we performed (i) detailed structural analyses of Mo surface complexes on δ-MnO2, ferrihydrite, and hydrogenetic ferromanganese oxides by L3- and K-edge XAFS, and (ii) adsorption experiments of Mo to δ-MnO2and ferrihydrite over a wide range of pHs, ionic strengths, and Mo concentrations. XAFS analyses revealed that Mo forms distorted octahedral (Oh) inner-sphere complexes on δ-MnO2whereas it forms a tetrahedral (Td) outer-sphere complex on ferrihydrite. In the hydrogenetic ferromanganese oxides, the dominant host phase of Mo was revealed to be δ-MnO2. These structural information are consistent with the macroscopic behaviors of Mo in adsorption experiments, and Mo concentration in modern oxic seawater can be explained by the equilibrium adsorption reaction on δ-MnO2. In addition, the large isotopic fractionation of Mo between seawater and ferromanganese oxides detected in previous studies can be explained by the structural difference between MoO42- and adsorbed species on the δ-MnO2phase in ferromanganese oxides. In contrast, smaller fractionation of Mo isotopes on ferrihydrite is due to little change in the Mo local structures during its adsorption to ferrihydrite. The structures of Mo species adsorbed on crystalline Fe (oxyhydr)oxides, goethite, and hematite were also investigated at pH 8 and I=0.70M (NaNO3). Our XAFS analyses revealed that Mo forms inner-sphere complexes on both minerals: Td edge-sharing (46%) and Oh double corner-sharing (54%) for goethite, and Td double corner-sharing (14%) and Oh edge-sharing (86%) for hematite. These structural information, combined with those for amorphous ferrihydrite and δ-MnO2, show the excellent correlation with the magnitude of adsorptive isotopic fractionation of Mo reported in previous studies: the proportion of Oh species or their magnitude of distortion in Mo surface complexes become larger in the order of ferrihydrite<goethite<hematite<δ-MnO2, a trend identical to the magnitude of isotopic fractionation. Based on the comparison with previous reports for Mo surface species on various oxides, the chemical factors that affect Mo surface complex structures were also discussed. The hydrolysis constant of cation in oxides, logKOH(or the acidity of the oxide surfaces, PZC) is well correlated with the mode of attachment (inner- or outer-sphere) of Mo surface complexes. Furthermore, the symmetric change in Mo species from Td to Oh is suggested to be driven by the formation of inner-sphere complexes on specific sites of the oxide surfaces.