Gypsum precipitation enhanced by electrokinetic method and porewater chemistry in compacted montmorillonite
Gypsum precipitation enhanced by electrokinetic method and porewater chemistry in compacted montmorillonite
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动电法和孔隙水化学在压实蒙脱土中促进石膏沉淀
DOI：
10.1016/j.clay.2018.05.011
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发表时间：
2018
影响因子：
5.6
通讯作者：
Tanaka Shingo
中科院分区：
文献类型：
--
作者：
Matsuda Hiroki;Iwamoto Hiroki;Meigo Shin-ichiro;Takeshita Hayato;Maekawa Fujio;Tanaka Shingo
了解膨润土中的化学反应对于完成放射性废物地质处置的可靠安全评估至关重要。在本研究中，通过使用电动方法增强的石膏沉淀反应，探索压实的钠/钙混合蒙脱石中的孔隙水化学和钙反应性。结果表明，CaSO 4以石膏形式稀疏地沉淀在300 μm以下的团聚体中，并生长成蒙脱石结构。实验发现，在干密度为1.0 kg/dm ~ 3、用0.5 M Na_2SO_4溶液饱和的条件下，Na/Ca混合蒙脱石在石膏沉淀中的钙离子当量分数XCa在XCa = 0.1 ~ 0.25之间。石膏沉淀实验后的试样的饱和指数进行了评估的基础上，包括阴离子排斥和阳离子交换（Gains-Thomas选择性系数，KGT）之间的蒙脱石和颗粒间的溶液，使用PHREEQC代码的模型。从沉淀实验中得到的石膏沉淀区域与饱和指数大于零的区域吻合良好，当假定KGT = 200时，这远大于分批实验中报道的KGT = 4.9。在这项研究中隐含的KGT值表明，受限溶液的热力学性质是不同于稀释或散装溶液。KGT偏差的原因尚不清楚，但当KGT值足够大时，石膏的沉淀倾向可以用蒙脱石与颗粒间溶液的阴离子排斥和阳离子交换模型来解释。
An understanding of chemical reactions in bentonite is important in completing a reliable safety assessment of the geological disposal of radioactive waste. In the present study, porewater chemistry and calcium reactivity in a compacted Na/Ca-mixed montmorillonite is explored through a gypsum precipitation reaction enhanced using an electrokinetic method. Results showed that CaSO4was sparsely precipitated as gypsum in up to 300-μm aggregates, and grew into the montmorillonite texture. A threshold ionic equivalent fraction of calcium, XCa, of the Na/Ca-mixed montmorillonite in the gypsum precipitation was experimentally found to be within a range between XCa= 0.1 and XCa= 0.25 under a dry density of 1.0 kg/dm3saturated with 0.5 M Na2SO4solution. The saturation indices of gypsum for specimens following the precipitation experiments were evaluated based on a model including anion exclusion and cation exchange (Gains-Thomas selectivity coefficients,KGT) between the montmorillonite and interparticle solution using the PHREEQC code. Gypsum precipitation regions obtained from the precipitation experiments are in good agreement with regions in which the saturation indices are greater than zero when assumingKGT= 200, which is much greater than theKGT= 4.9 reported from the batch experiment. The greaterKGTvalue implied in this study indicates that the thermodynamic properties of confined solutions are different from those of diluted or bulk solutions. The reason for theKGTdeviation remains unresolved; however, the precipitation tendency of gypsum may be explained by a model including anion exclusion and cation exchange between the montmorillonite and the interparticle solution when an adequateKGTvalue is used in the calculation.
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