Characterization of rare earth elements (REEs) associated with suspended particles in deep granitic groundwater and their post-closure behavior from a simulated underground facility
Characterization of rare earth elements (REEs) associated with suspended particles in deep granitic groundwater and their post-closure behavior from a simulated underground facility
复制标题
DOI：
10.1016/j.apgeochem.2017.04.016
复制
发表时间：
2017-07-01
影响因子：
3.4
通讯作者：
Kato, Toshihiro
中科院分区：
文献类型：
--
作者：
Iwatsuki, Teruki;Munemoto, Takashi;Kato, Toshihiro
了解放射性核素在地下深处的迁移过程对于放射性废物地质处置的安全评估是必不可少的。本文研究了日本米祖伊地下研究实验室（URL）深部花岗岩地下水和500 m深的封闭巷道中悬浮颗粒中稀土元素的行为，作为地下废物处置设施周围放射性核素行为的模拟研究。天然地下水中悬浮颗粒由硅酸盐矿物组成，包括粘土、氢氧化铁、方解石硫化物和有机物。地下水中约有10%-60%的稀土元素与悬浮颗粒物有关。此外，连续的化学提取分析膜过滤器上捕获的悬浮物表明，稀土元素主要吸附在深层花岗岩地下水中的碳酸盐颗粒。地下水中的碳酸盐颗粒最有可能来源于方解石等过饱和碳酸盐矿物的原位沉淀。例如，当方解石饱和指数显示平衡（SI = 0）时，与悬浮颗粒相关联的镧的浓度最高，并且随着方解石饱和指数的增加而降低。由于深部花岗岩地下水一般处于方解石饱和平衡状态，研究结果表明，方解石颗粒不可避免地存在于地下水中，并优先吸附稀土元素，封闭漂积物中的水和悬浮颗粒物中稀土元素浓度明显低于周围地下水中的浓度。热力学计算结果表明，在封闭期，封闭冰碛物中溶解的稀土碳酸盐复合体减少。这些复合物可能已被吸收或共同沉淀在巷道壁上的喷射混凝土内。在封闭的漂移，ZnS颗粒和大量的有机悬浮液，作为人工悬浮颗粒，除了在天然地下水中观察到的颗粒。一个连续的化学提取分析表明，稀土元素主要吸附在碳酸盐颗粒，而不是在大量的有机颗粒。有机悬浮物对稀土元素的吸附量可能很小，或者稀土元素可能优先吸附在方解石颗粒或喷浆混凝土上，而不是有机质上。方解石颗粒在封闭巷道胶结条件下的赋存状态随时间由大颗粒悬浮体向相对小颗粒聚集体转变。碳酸盐颗粒携带的稀土元素的沉淀可能是悬浮稀土元素浓度降低的原因之一。水泥基材料的使用将产生环境条件，在这种环境条件下，稀土元素在地下设施内和周围基本上是固定的。(C)2017作者爱思唯尔有限公司出版
Understanding radionuclide transport processes deep underground is indispensable for safety assessments of the geological disposal of radioactive wastes. This study investigated the behavior of rare earth elements (REEs) associated with suspended particles in deep granitic groundwater and in a sealed drift at a depth of 500 m in the Mizunami Underground Research Laboratory (URL) in Japan, as an analogue study of radionuclide behavior around an underground waste disposal facility.Particles suspended in natural groundwater consist of silicate minerals including clays, and iron hydroxide, calcite, sulfide, and organic substances. Approximately 10%-60% of REEs in groundwater are associated with suspended particles. Furthermore, sequential chemical extraction analyses of the suspended materials trapped on membrane filters suggests that REEs are primarily sorbed onto carbonate particles in deep granitic groundwater. Carbonate particles in groundwater are most likely derived from in situ precipitation of supersaturated carbonate minerals such as calcite. The concentration of Lanthanum associated with suspended particles, as an example, is highest when the calcite saturation index shows equilibrium (SI = 0) and decreases as the calcite saturation index increase. Because deep granitic groundwater is generally in equilibrium with the saturated condition for calcite, the results of this study suggest that calcite particles inevitably exist in groundwater and preferentially sorb REEs.The REE concentrations in the water and suspended particles from the closed drift are significantly lower compared with concentrations in the surrounding groundwater. Thermodynamic calculations show that the dissolved REE carbonate complexes in the closed drift decreased in the drift closure period. These complexes may have been absorbed or co-precipitated within the shotcrete on the drift wall. In the closed drift, ZnS particles and large amounts of organic suspensions, as artificial suspended particles, were observed in addition to the particles observed in natural groundwater. A sequential chemical extraction analysis indicates that REEs are primarily sorbed onto the carbonate particles and not on the large quantity of organic particles. There may be little REE adsorption onto organic suspensions, or REEs may have preferentially adsorbed onto calcite particles or shotcrete rather than organic matter. The occurrence of calcite particles under the cementitious conditions of the closed drift changes from the large suspended particles to aggregates of relatively small particles with time. Precipitation of carbonate particle borne REEs is probably one of reasons for the decrease in the suspended REEs concentrations. The usage of cement based materials would generate environmental conditions in which REEs are fundamentally immobile in and around the underground facilities. (C) 2017 The Authors. Published by Elsevier Ltd.