Massey Michael S, Lezama-Pacheco Juan S, Michel F Marc, Fendorf Scott
Department of Environmental & Earth System Science, Stanford University, Stanford, California 94305, USA.
Environ Sci Process Impacts. 2014 Sep 20;16(9):2137-44. doi: 10.1039/c4em00148f. Epub 2014 Aug 15.
Uranium retention processes (adsorption, precipitation, and incorporation into host minerals) exert strong controls on U mobility in the environment, and understanding U retention is therefore crucial for predicting the migration of U within surface and groundwater. Uranium can be incorporated into Fe (hydr)oxides during Fe(ii)-induced transformation of ferrihydrite to goethite. However, ferrihydrite seldom exists as a pure phase within soils or sediments, and structural impurities such as Al alter its reactivity. The presence of Al in ferrihydrite, for example, decreases the rate of transformation to goethite, and thus may impact the retention pathway, or extent of retention, of U. Here, we investigate the extent and pathways of U(vi) retention on Al-ferrihydrite during Fe(ii)-induced transformation. Ferrihydrite containing 0%, 1%, 5%, 10%, and 20% Al was reacted with 10 μM U and 300 μM Fe(ii) in the presence of 0 mM and 4 mM Ca(2+) and 3.8 mM carbonate at pH 7.0. Solid reaction products were characterized using U L3-edge EXAFS spectroscopy to differentiate between adsorbed U and U incorporated into the goethite lattice. Uranium incorporation into Al-ferrihydrite declined from ∼70% of solid-phase U at 0% and 1% Al to ∼30% of solid phase U at 20% Al content. The decrease in U incorporation with increasing Al concentration was due to two main factors: (1) decreased transformation of ferrihydrite to goethite; and, (2) a decrease of the goethite lattice with increasing Al, making the lattice less compatible with large U atoms. However, uranium incorporation can occur even with an Al-substituted ferrihydrite precursor in the presence or absence of Ca(2+). The process of U incorporation into Al-goethite may therefore be a potential long-term sink of U in subsurface environments where Al-substituted iron oxides are common, albeit at lower levels of incorporation with increasing Al content.
铀的保留过程（吸附、沉淀以及掺入宿主矿物）对环境中铀的迁移性有很强的控制作用，因此了解铀的保留对于预测铀在地表水和地下水中的迁移至关重要。在由亚铁离子诱导水铁矿向针铁矿转变的过程中，铀可以掺入铁（氢）氧化物中。然而，水铁矿在土壤或沉积物中很少以纯相存在，诸如铝等结构杂质会改变其反应活性。例如，水铁矿中铝的存在会降低向针铁矿转变的速率，进而可能影响铀的保留途径或保留程度。在此，我们研究了在亚铁离子诱导转变过程中六价铀在铝水铁矿上的保留程度和途径。含有0%、1%、5%、10%和20%铝的水铁矿在pH值为7.0、存在0 mM和4 mM钙离子以及3.8 mM碳酸盐的情况下与10 μM铀和300 μM亚铁离子发生反应。使用铀L3边扩展X射线吸收精细结构光谱对固体反应产物进行表征，以区分吸附的铀和掺入针铁矿晶格中的铀。铀掺入铝水铁矿的比例从铝含量为0%和1%时固相铀的约70%下降到铝含量为20%时固相铀的约30%。随着铝浓度增加铀掺入量的减少主要归因于两个因素：（1）水铁矿向针铁矿的转变减少；（2）随着铝含量增加针铁矿晶格减小，使得晶格与大尺寸铀原子的相容性降低。然而，无论有无钙离子存在时即使有铝取代的水铁矿前驱体，铀掺入也可能发生。因此，在铝取代铁氧化物常见的地下环境中铀掺入铝针铁矿的过程可能是铀的一个潜在长期汇，尽管随着铝含量增加掺入水平较低。
