Np(V) sorption and solubility in high pH calcite systems
Np(V) sorption and solubility in high pH calcite systems
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高 pH 方解石系统中的 Np(V) 吸附和溶解度
DOI：
10.1016/j.chemgeo.2018.06.016
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发表时间：
2018
期刊：
影响因子：
3.9
通讯作者：
Smith K
中科院分区：
文献类型：
--
作者：
Smith K
在 CO2 控制气氛下，研究了两种合成的高 pH 水泥浸出液中不同镎浓度（1.62×10−3μM–1.62μM）的碱性含方解石系统中的 Np(V) 行为。水泥渗滤液代表了较旧的（pH10.5，Ca2+）和较新的（pH13.3，Na+，K+，Ca2+）水泥地质处置设施中预期的条件。结合批量吸附和溶解度实验、X 射线吸收光谱和地球化学建模来研究这些系统，以描述 Np 行为。方解石平衡的旧水泥浸出液和新水泥浸出液（OCL 和 YCL）中的 Np(V) 溶解度分别为 9.7 和 0.084μM。在 OCL 系统中，这与控制溶解度的 Np(V)O2OH(am) 相一致。然而，这一阶段并不能解释 YCL 系统中观察到的非常低的 Np(V) 溶解度。通过一系列不同 Ca2+(aq) 浓度的 pH13.3 溶解度实验进一步探讨了这种不一致性。这些实验表明，在 pH13.3 下，Np(V) 溶解度随着 Ca2+ 浓度的增加而降低，证实 Ca2+ 是 YCL 系统中 Np 溶解度的关键控制因素。 42.2μM Np(V) 实验中沉淀物的 X 射线吸收近边结构光谱证实，Np(V) 二氧化烯基物种占主导地位。地球化学和扩展 X 射线吸收精细结构数据都支持了这一点，这表明含钙的 Np(V) 氢氧化物相正在控制溶解度。在 YCL 系统中，在一定范围的 Np 浓度和固液比范围内观察到 Np(V) 对方解石的吸附。表面络合和/或沉淀的结合可能是在这些系统中观察到的 Np(V) 与方解石反应的原因。在 OCL 吸附实验中，在一定 Np 浓度范围内，方解石的 Np(V) 吸附取决于固液比，这与单核表面络合物的形成一致。所有系统均表现出缓慢的吸附动力学，反应时间需要数周才能达到表观平衡。这可以通过方解石表面的缓慢重结晶和/或 Np(V) 胶体物质的存在来解释。总体而言，这些数据为与中水平放射性废物处置相关的碱性条件下的 Np(V) 和锕系元素 (V) 行为提供了有价值的新见解。
Np(V) behaviour in alkaline, calcite containing systems was studied over a range of neptunium concentrations (1.62 × 10−3μM–1.62 μM) in two synthetic, high pH, cement leachates under a CO2controlled atmosphere. The cement leachates were representative of conditions expected in an older (pH 10.5, Ca2+) and younger (pH 13.3, Na+, K+, Ca2+) cementitious geological disposal facility. These systems were studied using a combination of batch sorption and solubility experiments, X-ray absorption spectroscopy, and geochemical modelling to describe Np behaviour. Np(V) solubility in calcite equilibrated old and young cement leachates (OCL and YCL) was 9.7 and 0.084 μM, respectively. In the OCL system, this was consistent with a Np(V)O2OH(am)phase controlling solubility. However, this phase did not explain the very low Np(V) solubility observed in the YCL system. This inconsistency was explored further with a range of pH 13.3 solubility experiments with and variable Ca2+(aq)concentrations. These experiments showed that at pH 13.3, Np(V) solubility decreased with increasing Ca2+concentration confirming that Ca2+was a critical control on Np solubility in the YCL systems. X-ray absorption near-edge structure spectroscopy on the precipitate from the 42.2 μM Np(V) experiment confirmed that a Np(V) dioxygenyl species was dominant. This was supported by both geochemical and extended X-ray absorption fine structure data, which suggested a calcium containing Np(V) hydroxide phase was controlling solubility. In YCL systems, sorption of Np(V) to calcite was observed across a range of Np concentrations and solid to solution ratios. A combination of both surface complexation and/or precipitation was likely responsible for the observed Np(V) reaction with calcite in these systems. In the OCL sorption experiments, Np(V) sorption to calcite across a range of Np concentrations was dependent on the solid to solution ratio which is consistent with the formation of a mono-nuclear surface complex. All systems demonstrated slow sorption kinetics, with reaction times of weeks needed to reach apparent equilibrium. This could be explained by slow recrystallisation of the calcite surface and/or the presence of Np(V) colloidal species. Overall, these data provide valuable new insights into Np(V) and actinide(V) behaviour in alkaline conditions of relevance to the disposal of intermediate level radioactive wastes.
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