Iron oxide formation from FeCl2 solutions in the presence of uranyl (UO22+) cations and carbonate rich media
Iron oxide formation from FeCl2 solutions in the presence of uranyl (UO22+) cations and carbonate rich media
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DOI：
10.1016/j.gca.2015.02.038
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发表时间：
2015-06-01
影响因子：
5
通讯作者：
Graefe, Markus
中科院分区：
文献类型：
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作者：
Doornbusch, Brodie;Bunney, Karl;Graefe, Markus
针铁矿 (α-FeOOH) 此前已被研究为许多潜在有毒金属（例如镉、铅、铜）的热力学稳定储存库。然而，对铀（U 作为铀酰，UO22+）替代 Fe 的研究很少，并且铀与针铁矿结构的最终结合已被多矿物 Fe 系统的形成所掩盖，而这些系统似乎都参与了 U 的封存。在本研究中，我们研究了在七种标称铀酰浓度（0.2-4 摩尔%，U/(U + Fe) x 100）存在下由 FeCl2 形成针铁矿的过程，并使用定量 X 射线衍射 (QXRD)、UL-III 边缘 (17,166 eV) 处的 X 射线吸收精细结构 (XAFS) 光谱、漫反射红外傅里叶对固体进行了表征 变换（DRIFT）光谱测定和酸溶解的一致性。我们的研究结果表明，U 确实（尽管很少）融入到针铁矿结构中。从 XRD 图谱的 Rietveld 模型确定的晶胞体积作为 U 含量的函数线性增加，这可归因于晶胞长度 a 和 c 的线性增加。 U 对 Fe 的最高取代率为 0.48 摩尔%，然而，大多数含 U 针铁矿样品的取代水平约为 0.2 摩尔%，这与之前的发现非常一致。 DRIFT 谱显示对称 Fe-O 和不对称 Fe-OH 拉伸模式（分别是 tau-O 和 tau-OH 谱带）向较低频率的转变，根据 Hooks 类比，只有当较重的原子取代 Fe（即 U）时，才会发生这种转变。酸溶解结果的一致性表明 U 释放分数大于相应的 Fe 释放到溶液中的分数，这表明 U 总体上更易溶解。然而，在两种合成物（2 和 4 的初始 U 摩尔百分比）中，溶解是一致的。在选定的样品子集上收集的 XAFS 数据显示，随着铀掺入和酸萃取水平的提高，铀酰部分消失了。在样品 S6 中，扩展 XAFS 数据的非线性最小二乘拟合表明，U 原子周围的配位环境被 O/OH 原子占据六倍，并进一步由下一个最近的 Fe 邻居配位，这些邻居的距离是没有 U 取代的针铁矿中正常 Fe-Fe 距离的 1.063 +/- 0.008 倍。尽管掺入量较低，但与针铁矿结合的 U 在酸性溶液中难以解吸/溶解，因此需要进一步研究使用氧化铁作为 U 吸收池的可能性。Crown 版权所有 (C) 2015 由 Elsevier Ltd 出版。保留所有权利。
The mineral goethite (alpha-FeOOH) has previously been investigated as a thermodynamically stable repository for many potentially toxic metals (e.g., Cd, Pb, Cu). The substitution of uranium (U as uranyl, UO22+) for Fe, however, has been studied sparingly, and conclusive uranium incorporation into the goethite structure has been obscured by formation of ploymineralic Fe systems that all appeared involved in U's sequestration. In this study, we investigated the formation of goethite from FeCl2 in the presence of seven nominal uranyl concentrations (0.2-4 mole percent, U/(U + Fe) x 100) and characterised the solids using quantitative X-ray diffraction (QXRD), X-ray absorption fine structure (XAFS) spectroscopy at the U L-III edge (17,166 eV), diffuse reflectance infrared Fourier Transform (DRIFT) spectrometry and congruency of acid dissolution. Our findings show that U does indeed, however sparingly, incorporate into the goethite structure. The unit cell volume, ascertained from Rietveld models of XRD patterns, increased linearly as a function of U content, which could be ascribed to a linear increase of the unit cell length a and c. The highest U-for-Fe substitution was 0.48 mole percent, however, most U-containing goethite samples showed substitution levels around 0.2 mole percent, which was in good agreement with previous findings. DRIFT spectra showed a shift of the symmetric Fe-O and asymmetric Fe-OH stretch modes (tau-O and tau-OH bands, respectively) to lower frequency, which by Hooks analogy, can only occur if a heavier atom substitutes for Fe, i.e., U. The congruency of acid dissolution results showed that fractional U release was greater than the corresponding fractional Fe release into solution, suggesting that U was overall more soluble. In two of the synthates, however, (initial U mole percent of 2 and 4), the dissolution was congruent. XAFS data collected on a selected subset of samples showed the disappearance of the uranyl moiety with higher levels of U incorporation and acid extraction. In sample S6, non-linear least-square fits of the extended XAFS data demonstrated that the coordination environment around U atoms was sixfold occupied by O/OH atoms and was further coordinated by next nearest Fe neighbours that are 1.063 +/- 0.008 times inflated in distance to the normal Fe-Fe distances in goethite without U substitution. Despite low levels of incorporation, U bound by goethite was recalcitrant to desorption/dissolution in increasingly acidic solutions thus warranting further research into the possibility of using iron oxides as a sink for U. Crown copyright (C) 2015 Published by Elsevier Ltd. All rights reserved.