Abstract The kinetics of tungstate (i.e., WO42−) thiolation were investigated in experimental solutions buffered at different pH values and as a function of varying dissolved sulfide concentrations. Similar to molybdate (MoO42−), tungstate undergoes stepwise thiolation to tetrathiotungstate according to: W O x S 4 - x 2 - + H 2 S ( a q ) ↔ W O x - 1 S 5 - x 2 - + H 2 S ; 1 ≤ x ≤ 4. Under equilibrium conditions at standard conditions (298.15 K, 105 Pa) WO42− also converts to tetrathiotungate (WS42−) around 1.0 mM H2S(aq), reminiscent of the chemical switch for the MoO42− → MoS42− transition reported in the literature, but at nearly 100-fold higher H2S(aq) concentrations. The laboratory experiments show that thiotungstate formation is first order with respect to H2S concentration, and that the thiolation reactions are catalyzed by general Bronsted acids. Therefore, the high NH4+ and HCO3− concentrations employed in the experiments both favored WO42− thiolation. The experimental data were used to develop Bronsted relationships for the successive thiolation reactions (i.e., WO3S2− → WO2S22−, WO2S22− → WOS32−, WOS32− → WS42−) that allow the acid-catalyzed thiolation rates of WO42− to be estimated as a function of pH in natural waters. Reaction modeling of tungstate thiolation kinetics indicates that di- to trithiotungstate (WO2S22− → WOS32−) conversion and tri- to tetrathiomolybdate (MoOS32− → MoS42−) conversion may not be achieved in temporally variable sulfidic waters. In such environments, intermediate thioanions of W and Mo dominate with Mo exhibiting a higher degree of thiolation than that of W. As the partition coefficient of W into minerals is affected by its speciation in solution, its enrichment in minerals may change due to the level and changes in dissolved sulfide concentrations. Specifically, the partition coefficient of W decreases from oxic environments to sulfidic environments, suggesting W enrichment in sediment is likely to be a good tracer of redox conditions (i.e., oxic and sulfidic conditions) in the overlying waters. In comparison, the partition coefficient of Mo increases from seasonally sulfidic environments to permanently sulfidic environments, indicating that Mo enrichment in sediments is a good tracer of sulfidic conditions. In addition, the Mo/W concentration ratio in black shales shows the potential for identifying fluctuation of redox conditions from ca. 2500 Ma to ca. 0.1 Ma and is a potential proxy for tracking deep time sulfidic conditions. Thus, even though the geochemical behavior of W is different from Mo, W can nevertheless be a potential proxy for tracking changing redox conditions in the modern and ancient ocean.

中文翻译：

---

钨酸盐硫醇化反应动力学和沉积钼/钨富集的研究：对硫化水中钨形态的影响以及古氧化还原研究的可能应用

摘要 在不同 pH 值缓冲的实验溶液中研究了钨酸盐（即 WO42-）硫醇化的动力学，并作为不同溶解硫化物浓度的函数。与钼酸盐 (MoO42-) 类似，钨酸盐根据以下条件逐步硫醇化为四硫钨酸盐： WO x S 4 - x 2 - + H 2 S ( aq ) ↔ WO x - 1 S 5 - x 2 - + H 2 S ；1 ≤ x ≤ 4. 在标准条件 (298.15 K, 105 Pa) 的平衡条件下，WO42− 也会在 1.0 mM H2S(aq) 附近转化为四硫钨酸 (WS42−)，让人想起 MoO42− → MoS42− 转变的化学转换文献中报道，但 H2S(aq) 浓度高出近 100 倍。实验室实验表明，硫钨酸盐的形成与 H2S 浓度有关，并且硫醇化反应是由一般的布朗斯台德酸催化的。因此，实验中使用的高 NH4+ 和 HCO3- 浓度都支持 WO42- 硫醇化。实验数据用于开发连续硫醇化反应（即 WO3S2- → WO2S22-、WO2S22- → WOS32-、WOS32- → WS42-）的布朗斯台德关系，允许酸催化的 WO42- 硫醇化速率估计为天然水中 pH 值的函数。钨酸盐硫醇化动力学的反应模型表明，在随时间变化的硫化水中可能无法实现二至三硫代钨酸盐 (WO2S22- → WOS32-) 的转化和三至四硫代钼酸盐 (MoOS32- → MoS42-) 的转化。在这种环境中，W 和 Mo 的中间硫阴离子占主导地位，Mo 表现出比 W 更高的硫醇化程度。由于 W 在矿物中的分配系数受其在溶液中的形态影响，因此其在矿物中的富集可能会因溶解硫化物浓度的水平和变化而发生变化。具体而言，W 的分配系数从好氧环境到硫化环境降低，这表明沉积物中的 W 富集可能是上覆水域氧化还原条件（即，好氧和硫化条件）的良好示踪剂。相比之下，Mo 的分配系数从季节性硫化环境到永久硫化环境增加，表明沉积物中的 Mo 富集是硫化条件的良好示踪剂。此外，黑色页岩中的 Mo/W 浓度比显示了从大约 10 小时识别氧化还原条件波动的潜力。2500 毫安到大约 0. 1 Ma，是跟踪深度硫化条件的潜在代理。因此，即使 W 的地球化学行为与 Mo 不同，W 仍然可以作为跟踪现代和古代海洋中氧化还原条件变化的潜在代表。