Formation of Stable Uranium(VI) Colloidal Nanoparticles in Conditions Relevant to Radioactive Waste Disposal
Formation of Stable Uranium(VI) Colloidal Nanoparticles in Conditions Relevant to Radioactive Waste Disposal
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DOI：
10.1021/la502832j
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发表时间：
2014-12-09
期刊：
影响因子：
3.9
通讯作者：
Shaw, Samuel
中科院分区：
文献类型：
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作者：
Bots, Pieter;Morris, Katherine;Shaw, Samuel
深地质处置是高活度放射性废物处置的首选途径。许多地质处置设施的设计在其工程设计中包括水泥。从长远来看，地下水与水泥和废物的相互作用将形成一股超碱性沥滤液（pH值10-13），在这些极端条件下，需要限制放射性核素的行为，以尽量减少废物对环境的危害。对于铀，许多放射性废物的关键组成部分，热力学模型预测，在高pH值，U（VI）的溶解度将非常低（nM或更低），并通过与固相碱金属和碱土金属铀酸盐的平衡控制。然而，在这些条件下，U（VI）胶体的形成可能会增强U（VI）的流动性，并且表征U（VI）胶体的形成和中期稳定性的潜力对于我们理解废物处理中的U行为非常重要。考虑到这一点，我们采用了传统的地球化学和显微镜技术结合同步加速器为基础的原位和非原位X射线技术（小角X射线散射和X射线吸收光谱（XAS）），以表征胶体U（VI）纳米粒子在合成水泥浸出液（pH > 13）含有4.2-252 μ M U（VI）。结果表明，在42 μ M U（VI）的水泥沥滤液中，胶体在数小时内形成，并保持稳定数年。胶体由1.5-1.8 nm的纳米颗粒组成，部分形成20-60 nm的聚集体。使用XAS和电子显微镜，我们能够确定胶体纳米颗粒具有克拉克石（钠-铀）型晶体结构。所提出的结果有明确的和迄今未被认识到的影响U（VI）在水泥环境中的流动性，特别是与核废料的地质处置。
The favored pathway for disposal of higher activity radioactive wastes is via deep geological disposal. Many geological disposal facility designs include cement in their engineering design. Over the long term, interaction of groundwater with the cement and waste will form a plume of a hyperalkaline leachate (pH 10-13), and the behavior of radionuclides needs to be constrained under these extreme conditions to minimize the environmental hazard from the wastes. For uranium, a key component of many radioactive wastes, thermodynamic modeling predicts that, at high pH, U(VI) solubility will be very low (nM or lower) and controlled by equilibrium with solid phase alkali and alkaline-earth uranates. However, the formation of U(VI) colloids could potentially enhance the mobility of U(VI) under these conditions, and characterizing the potential for formation and medium-term stability of U(VI) colloids is important in underpinning our understanding of U behavior in waste disposal. Reflecting this, we applied conventional geochemical and microscopy techniques combined with synchrotron based in situ and ex situ X-ray techniques (small-angle X-ray scattering and X-ray adsorption spectroscopy (XAS)) to characterize colloidal U(VI) nanoparticles in a synthetic cement leachate (pH > 13) containing 4.2-252 mu M U(VI). The results show that in cement leachates with 42 mu M U(VI), colloids formed within hours and remained stable for several years. The colloids consisted of 1.5-1.8 nm nanoparticles with a proportion forming 20-60 nm aggregates. Using XAS and electron microscopy, we were able to determine that the colloidal nanoparticles had a clarkeite (sodium-uranate)-type crystallographic structure. The presented results have clear and hitherto unrecognized implications for the mobility of U(VI) in cementitious environments, in particular those associated with the geological disposal of nuclear waste.