Previous studies have indicated that acid-washed pyrite is inert toward aqueous U(VI) under most pH conditions at ambient temperature, even though insoluble UO is the thermodynamically predicted product for the reduction of U(VI) by pyrite. Considering the exothermic nature of nuclear waste, the interaction between uranyl nitrate/acetate and natural pyrite was studied at temperatures ranging from 25 °C to 85 °C and at pH values between ∼4.0 and ∼9.5, to simulate the scenarios encountered in a high-level radioactive waste repository. The results revealed that the reactivity of pyrite toward aqueous U(VI) significantly increased with rising temperature, and the reaction could be described by a pseudo-first-order kinetic equation. Activation energies were calculated to be 79.4 ± 10.6 and 45.7 ± 3.8 kJ⋅mol for the reduction of uranyl nitrate, and 78.2 ± 5.2 and 42.2 ± 5.8 kJ⋅mol for the reduction of uranyl acetate, at pH values of ∼4.5 and ∼5.0, respectively. These values indicate that the reaction is controlled by the surface chemical processes. In addition, the complete reduction of aqueous U(VI) to UO product was first observed for the reactions at pH ∼4.0 to ∼5.5 when the temperature was ≥75 °C. Conversely, non-stoichiometric UO(s) (0 < x ≤ 0.67) was found at lower temperatures within the same pH range, and also at 85 °C with a reaction pH of ∼9.5. Moreover, the ratio of reduced U(IV/V) on pyrite surfaces increased gradually over time, indicating that reaction time played a significant role in the reduction products. The findings are essential not only for the safety assessment of the high-level radioactive waste repository, but also for understanding the metallogenic mechanism of U(IV)-bearing ore deposits under the relevant anaerobic and hydrothermal conditions.

中文翻译：

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天然黄铁矿随温度还原水溶液 U(VI) 的动力学研究


先前的研究表明，酸洗黄铁矿在环境温度下的大多数 pH 条件下对含水 U(VI) 呈惰性，尽管不溶性 UO 是黄铁矿还原 U(VI) 的热力学预测产物。考虑到核废料的放热性质，在 25°C 至 85°C 的温度范围和 ∼4.0 至 ∼9.5 的 pH 值范围内研究了硝酸铀酰/乙酸双氧铀与天然黄铁矿之间的相互作用，以模拟在高温下遇到的情况。级放射性废物处置库。结果表明，黄铁矿对U(VI)水溶液的反应活性随着温度的升高而显着增加，反应可用准一级动力学方程描述。 pH 值~4.5 和~5.0 时，硝酸铀酰还原的活化能计算为 79.4 ± 10.6 和 45.7 ± 3.8 kJ·mol，乙酸铀酰还原的活化能为 78.2 ± 5.2 和 42.2 ± 5.8 kJ·mol ， 分别。这些值表明反应是由表面化学过程控制的。此外，当温度≥75°C时，首次观察到在pH~4.0至~5.5的反应中水性U(VI)完全还原为UO产物。相反，在相同 pH 范围内的较低温度下，以及在 85 °C 反应 pH 值约为 9.5 时，发现非化学计量 UO(s) (0 < x ≤ 0.67)。此外，黄铁矿表面还原U(IV/V)的比例随着时间的推移逐渐增加，表明反应时间对还原产物起着重要作用。该研究结果不仅对于高放废物处置库的安全评估具有重要意义，而且对于了解相关厌氧和热液条件下含铀矿床的成矿机制也具有重要意义。