Mechanism of molybdenum removal from the sea and its concentration in black shales: EXAFS evidence
Mechanism of molybdenum removal from the sea and its concentration in black shales: EXAFS evidence
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DOI：
10.1016/0016-7037(96)00195-0
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发表时间：
1996-10-01
影响因子：
5
通讯作者：
Vaughan, DJ
中科院分区：
文献类型：
--
作者：
Helz, GR;Miller, CV;Vaughan, DJ
钼 K 边 EXAFS（扩展 X 射线吸收精细结构）光谱提供了有关高钼黑色页岩和沉积物中钼化学环境的新结构信息。发现了两种光谱类型。与中国页岩中开发的钼矿石相关的不太常见的一种是 MoS2 相，可能是 X 射线非晶态乔狄石。另一种与白垩纪深海沉积物和其他黑色页岩有关，其特征是 Mo-O 距离较短（1.69-1.71 埃），Mo-S 距离为 2.30-2.38 埃，在某些情况下具有第二壳层 Mo 和 Fe 相互作用，这表明一些 Mo 存在于富含过渡金属的相中。合成无定形材料的 EXAFS 光谱是通过用 Fe(II)、FeOOH 和腐殖酸从 HS 溶液中清除 Mo 来制备的，表明第二种光谱类型来自主要以两种形式存在的 Mo。一种是紧凑的 Mo-Fe-S“立方烷”型化合物，Mo-S 距离类似于 2.36 埃，Mo-Fe 距离类似于 2.66 埃，而另一种可能是包含一些 Mo-O 双键（类似于 1.69 埃）的有机形式。实验室产品是通过用腐殖酸从硫化物溶液中清除溶解的钼而制备的，产生的光谱与在页岩和沉积物中观察到的第二种光谱类型非常相似，包括 Mo-Fe 相互作用的意外迹象。钼 L 边光谱表明沉积物和页岩的平均氧化态位于 IV 和 VI 之间。这项工作证明了 EXAFS 在获取含有 X 射线无定形成分的天然材料的结构信息方面的优点，这些信息击败了传统的矿物学表征。考虑了这些发现对从含硫天然水中清除 Mo 的影响。我们引入了地球化学开关的概念，其中 HS- 将 Mo 的海洋行为从保守元素转变为粒子反应元素。 HS-开关的动作点计算为a(HS)- = 10(-3.6) - 10(-4.3)。当 a(HS)- 接近作用点时，Mo 会与含有过渡金属（例如 Fe）的颗粒发生反应。我们推测硫醇，包括腐殖酸结合的硫醇基团，也会改变 Mo 的行为。与以前的想法相反，我们的 Mo 清除模型不强调从 Mo(VI) 还原到 Mo(V) 作为清除的初始步骤的作用；相反，我们强调 Mo 通过 S 桥与过渡金属和有机分子形成共价键的容易性。
Molybdenum K-edge EXAFS (extended X-ray absorption fine structure) spectra yield new structural information about the chemical environment of Mo in high-Mo black shales and sediments. Two spectral types are found. The less common one, associated with Mo ores developed in shale in China, is that of a MoS2 phase, possibly X-ray amorphous jordisite. The other, associated with Cretaceous deep sea sediments and with other black shales, is characterized by short Mo-O distances (1.69-1.71 Angstrom) by Mo-S distances of 2.30-2.38 Angstrom, and in some cases by second shell Mo and Fe interactions, which suggests that some Mo resides in transition metal-rich phases. EXAFS spectra of synthetic amorphous materials, prepared by scavenging Mo from HS solutions with Fe(II), FeOOH, and humic acid, suggest that the second spectral type arises from Mo present chiefly in two forms. One is a compact, Mo-Fe-S ''cubane'' type compound with Mo-S distances of similar to 2.36 Angstrom and Mo-Fe distances of similar to 2.66 Angstrom, while the other is probably an organic form containing some Mo-O double bonds (similar to 1.69 Angstrom). Laboratory products, that were prepared by scavenging dissolved Mo from sulfidic solutions with humic acid, yield spectra quite similar to the second spectral type observed in shales and sediments, including unexpected indications of Mo-Fe interactions. Molybdenum L-edge spectra indicate that the mean oxidation state in the sediments and shales lies between IV and VI. This work demonstrates the merit of EXAFS for obtaining structural information on natural materials containing X-ray amorphous components which defeat conventional mineralogical characterization.The implications of these findings regarding Mo scavenging from sulfidic natural waters are considered. We introduce the concept of a geochemical switch, in which HS- transforms the marine behavior of Mo from that of a conservative element to that of a particle reactive element. The action point of the HS- switch is calculated to be, a(HS)- = 10(-3.6) - 10(-4.3). When a(HS)- approaches the action point, Mo becomes reactive to particles containing transition metals (e.g., Fe). We conjecture that thiols, including humic-bound thiol groups, also switch Mo behavior. In contrast to previous ideas, our model for Mo scavenging deemphasizes the role of reduction from Mo(VI) to Mo(V) as the initial step in scavenging; instead, we emphasize the ease with which Mo forms covalent bonds to transition metals and organic molecules via S bridges.