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    可变价核素在塔木素黏土岩合成地下水中的溶解度与种态分布

    Investigation on Solubility and Speciation of Valence-Variable Radionuclides in Synthetic Tamusu Claystone Groundwater

      摘要
    • 摘要: 塔木素黏土岩是我国高放废物地质处置库黏土岩场址的候选围岩之一。以内蒙古塔木素地区Tzk1钻孔542 m深（Tzk1-542）和Tzk2钻孔516 m深（Tzk2-516）黏土岩岩芯样品为研究对象，采用浸泡平衡法合成了pH分别为9.45和8.24的地下水并分析了其组分。Tzk1-542和Tzk2-516岩芯样品的Fe2+和总Fe质量比分别为78.36%和91.92%，据此计算得到其氧化还原电势（Eh）分别为-169.4 mV和11.5 mV。利用PHREEQC程序、OECD/NEA最新热力学数据及碳酸铀酰与Mg/Ca配合物的热力学参数，计算了U、79Se、99Tc和Np在合成地下水中的溶解度及种态分布情况。结果表明，在假设地下水氧气分压不变、pH为5.0~11.0的条件下，U、Se、Tc、Np在Tzk1-542合成地下水中的溶解度范围分别为10-11~10-8、10-16~10-9、10-14~10-12、10-18~10-17 mol/L，在Tzk2-516合成地下水中的溶解度范围分别为10-14~10-5、10-14~10-5、10-11~10-4、10-18~10-17 mol/L，两组合成地下水中U、Se、Tc和Np的溶解态均主要以中性分子或阴离子形式存在。鉴于黏土岩地下水一般呈碱性，固定Tzk1-542和Tzk2-516合成地下水的pH分别为9.45和8.24，Eh在-169.4~169.4 mV范围变化时，Tzk1-542合成地下水中U、Se、Tc、Np的溶解度范围分别为10-10~2.28×10-4、10-12~10-3、10-14~10-2、10-18~10-12 mol/L，其溶解态主要以UO2(CO3)4-3、CaUO2(CO3)2-3、SeO2-3、HSeO-3、TcO-4、NpO2OH等带负电或中性种态的形式存在；Tzk2-516合成地下水中U、Se、Tc、Np的溶解度范围分别为10-11~10-5、10-11~10-3、10-16~10-2、10-19~10-13 mol/L，其溶解态主要以Ca2UO2(CO3)3、CaUO2(CO3)2-3、SeO2-3、HSeO-3、TcO-4、NpO2OH等形式存在。本工作仅对均相溶液体系进行模拟，针对地下水-岩体系的实际情况，仍需进一步开展相关核素的实验研究。

       

      Abstract: Tamusu claystone is one of the preselected host rocks for geological disposal of high-level radioactive waste in China. In this work, two claystone samples collected from borehole Tzk1 at the depth of 542 m(Tzk1-542) and borehole Tzk2 at the depth of 516 m (Tzk2-516) of Tamusu site in Inner Mongolia were used to synthesize groundwater by soaking-balance method. The pH values of the synthetic groundwater were 9.45 and 8.24, respectively, and their components were analysed. Based on the Fe2+ and total Fe mass ratios of 78.36% and 91.92% in Tzk1-542 and Tzk2-516, the Eh values of -169.4 mV and 11.5 mV were proposed, respectively. In light of the obtained Eh, the solubility and speciation of U, 99Tc, 79Se and Np were calculated using PHREEQC with the latest OECD/NEA thermodynamic data and the thermodynamic data of uranyl carbonate complexes with Mg and Ca. When the pH range is 5.0-11.0 and the partial pressure of oxygen remains unchanged, the results show that the solubility of U, Se, Tc and Np in Tzk1-542 groundwater are 10-11-10-8, 10-16-10-9, 10-14-10-12 and 10-18-10-17 mol/L, respectively, while in Tzk2-516 groundwater they are 10-14-10-5, 10-14-10-5, 10-11-10-4 and 10-18-10-17 mol/L, respectively. The dissolved U, Se,  Tc and Np in both groundwaters are mainly in the form of neutral molecules or anions. The claystone groundwater is generally under alkaline condition. When the pH is fixed(at 9.45 and 8.24 for Tzk1-542 and Tzk2-516, respectively) and the Eh varies from -169.4 mV to 169.4 mV, the solubility ranges of U, Se, Tc and Np in groundwater Tzk1-542 are 10-10-2.28×10-4, 10-12-10-3, 10-14-10-2 and 10-18-10-12 mol/L, respectively. The dissolved U, Se, Tc and Np mainly exist as negatively charged or neutral species such as UO2(CO3)4-3, CaUO2(CO3)2-3, SeO2-3, HSeO-3, TcO-4, NpO2OH, etc. For groundwater Tzk2-516, the solubility ranges of U, Se, Tc and Np are 10-11-10-5, 10-11-10-3, 10-16-10-2 and 10-19-10-13 mol/L, respectively. The dissolved U, Se, Tc and Np mainly exist as Ca2UO2(CO3)3, CaUO2(CO3)2-3, SeO2-3, HSeO-3, TcO-4, NpO2OH, etc. This work provides the simulation results of the homogeneous solution system. Further experimental works in combination with the real groundwaterwater condition for the related radionuclides are needed.

       

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