Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai, 519082, China.
Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai, 519082, China.
Chemosphere. 2024 Jan;348:140754. doi: 10.1016/j.chemosphere.2023.140754. Epub 2023 Nov 21.
The claystone-based Tamusu area in the Bayingebi Basin, Inner Mongolia, is preselected as a China's high-level radioactive waste (HLRW) repository site. This study investigated the redox reactions of U(VI)/UO on Tamusu claystone. Five Tamusu claystone samples collected from boreholes Tzk1 and Tzk2 at different depths were used for batch experiments at pH ∼5.0, ∼7.0, and ∼9.0. These claystones contain considerable amounts of organic matters and Fe-containing minerals such as pyrite, fluorannite, and ankerite. Results showed that aqueous U(VI) could be partially reduced to U(IV) and/or U(V)-containing precipitates (UO, UO, etc.) by these Tamusu claystones, and the reaction is more favorable under acidic condition. We proposed that leaching of the structural Fe followed by surface adsorption and interface reaction, is the primary mechanism responsible for U(VI) reduction. Under alkaline condition, organic matters might dominate the partial reduction of aqueous U(VI). Besides, the phosphorus-containing spots on Tamusu claystone surfaces are the reactive sites for U aggregation, implying the possible formation of U(VI)- and/or U(IV)-phosphate minerals. It is important to note that, due to the presence of minor Fe in Tamusu claystones, the high-purity UO could undergo partial oxidation to UO and/or UO. Therefore, insoluble UO (0 < x ≤ 0.67) is proposed to be the most thermodynamically stable form in Tamusu claystone. This study enhances our comprehension of the essential geochemical processes of uranium in claystone surroundings, but also offers crucial information for the safety evaluation of China's HLRW repository.
内蒙古巴音戈壁盆地的塔木苏黏土岩地区被预选为中国高放废物（HLRW）处置库场址。本研究调查了塔木苏黏土岩中 U(VI)/UO 的氧化还原反应。在 pH 值约为 5.0、7.0 和 9.0 条件下，使用来自 Tzk1 和 Tzk2 钻孔不同深度的五个塔木苏黏土岩样本进行了批实验。这些黏土岩含有大量的有机物质和含铁矿物，如黄铁矿、氟磷钙矿和铁白云石。结果表明，水溶液中的 U(VI)可部分被这些塔木苏黏土岩还原为 U(IV)和/或 U(V)含沉淀（UO、UO 等），且在酸性条件下反应更有利。我们提出，结构 Fe 的浸出，随后是表面吸附和界面反应，是 U(VI)还原的主要机制。在碱性条件下，有机物质可能主导水溶液 U(VI)的部分还原。此外，塔木苏黏土岩表面的含磷斑点是 U 聚集的反应位点，这意味着可能形成 U(VI)和/或 U(IV)磷酸盐矿物。需要注意的是，由于塔木苏黏土岩中存在少量的 Fe，高纯度 UO 可能会部分氧化为 UO 和/或 UO。因此，不溶性 UO（0<x≤0.67）被认为是塔木苏黏土岩中最热力学稳定的形式。本研究提高了我们对黏土岩环境中铀的基本地球化学过程的认识，同时也为中国高放废物处置库的安全评估提供了重要信息。
