Thermodynamics and kinetics of reactions involving vanadium in natural systems: Accumulation of vanadium in sedimentary rocks
Thermodynamics and kinetics of reactions involving vanadium in natural systems: Accumulation of vanadium in sedimentary rocks
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DOI：
10.1016/0016-7037(92)90217-7
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发表时间：
1992-04
影响因子：
5
通讯作者：
R. Wanty;M. Goldhaber
中科院分区：
文献类型：
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作者：
R. Wanty;M. Goldhaber
水和固体V物种的热力学数据的一个重要的审查，以评估自然条件下的V的溶解，运输和沉淀。重点是V化学的实验研究结果，特别是那些实验条件接近自然界中发现的。在可能的情况下，获得298.15K、1 atm（1.01325 bar）和零离子强度的参考条件的数据或将其校正为参考条件。钒[IV]（VIV）和钒[V]（VV）是自然界中最易溶解的钒形式，它们与氟化物、硫酸盐和草酸盐的络合物在氧化条件下可以增加V的溶解度，由于氧化还原行为对理解天然V的化学性质至关重要，因此研究了VIV还原为VIIIH 2S的动力学。虽然从热力学数据预测H2S能够将VIV还原为VIII，但该反应尚未在实验中得到证实。实验在自然界中可能存在的温度（45°C）、pH（3.6-6.8）、离子强度（0.05-0.1 m）和V浓度（9.8-240 μ mol）的条件下进行。由于反应非常缓慢，因此使用了超过自然条件的H2S浓度（8.1 × 10− 4至0.41 atm）。结果表明，在各种条件下，VIV被还原为VIII。该速率随pH值的增加而增加，但不受离子强度的明显影响（如在所有情况下用作支持电解质的KCl的浓度所表示的）。在反应开始之前，有一个诱导期，其长度随着KCl浓度的增加或pH值的降低而增加。通过数值方法模拟反应机理的尝试未能产生令人满意的结果拟合，表明部分反应级数，复杂的机理，热力学和动力学研究的结果被应用于理解钒矿床的成因，如那些通常发现于科罗拉多高原。这些砂岩矿床中的钒主要以还原氧化态VIII存在。由于VIII羟基氧化物的不溶性，很可能是一种更氧化形式的V（[IV]或[V]）被运送到矿化部位，V在原地被还原，随后沉淀。一个可能的还原剂是硫化氢，黄铁矿同生的V矿物文件的存在下，在矿化过程中的硫化氢。这里描述的实验表明，H2S可以将VIV还原为VIII，从而导致这些沉积物的形成。
A critical review of thermodynamic data for aqueous and solid V species is presented to evaluate dissolution, transport, and precipitation of V under natural conditions. Emphasis is given to results of experimental studies of V chemistry, especially those for which the experimental conditions are near those found in nature. Where possible, data are obtained for or corrected to the reference conditions of 298.15K, 1 atm (1.01325 bar) and zero ionic strength. Vanadium [IV] (VIV) and vanadium[V] (VV) are the most soluble forms of V in nature, and their complexes with fluoride, sulfate, and oxalate may act to increase V solubility under oxidizing conditions.Because redox behavior is of fundamental importance to understanding natural V chemistry, the kinetics of reduction of VIVto VIIIH2S were studied. Although H2S is predicted from thermodynamic data to be capable of reducing VIVto VIII, this reaction has not been demonstrated experimentally. Experiments were carried out under conditions of temperature (45°C), pH (3.6–6.8), ionic strength (0.05–0.1 m), and V concentrations (9.8–240 μmolar) likely to be found in nature. Because the reaction is very slow, H2S concentrations in excess of natural conditions were used (8.1 × 10−4to 0.41 atm). The results show that VIVis reduced to VIIIunder a variety of conditions. The rate increases with increasing pH, but is not appreciably affected by ionic strength (as represented by the concentration of KCl, which was used as the supporting electrolyte in all cases). Prior to initiation of the reaction, there is an induction period, the length of which increases with increasing KCl concentration or decreasing pH. Attempts to model the reaction mechanism by numerical methods have failed to produce a satisfying fit of the results, indicating partial reaction orders, a complex mechanism, or involvement of a variety of intermediate species.The results of the thermodynamic and kinetic studies were applied to understanding the genesis of V deposits such as those commonly found on the Colorado Plateau. Vanadium in these sandstone-hosted deposits is present mostly in the reduced oxidation state, VIII. Because of the insolubility of VIIIoxyhydroxides, it is likely that a more oxidized form of V (either [IV] or [V]) was transported to the site of mineralization, and that the V was reducedin situand subsequently precipitated. A probable reductant is hydrogen sulfide; the presence of pyrite cogenetic with the V minerals documents the presence of H2S during mineralization. The experiments described here show that H2S could have reduced VIVto VIII, and thus led to the formation of these deposits.