U(VI) behaviour in hyperalkaline calcite systems
U(VI) behaviour in hyperalkaline calcite systems
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超碱性方解石体系中的 U(VI) 行为
DOI：
10.1016/j.gca.2014.09.043
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发表时间：
2015
影响因子：
5
通讯作者：
Smith K
中科院分区：
文献类型：
--
作者：
Smith K
研究了U（VI）浓度范围（5.27 × 10−5μM ~ 42.0 μM）和两个高pH值体系（新的和旧的合成水泥浸出液）中U（VI）在超碱性流体/方解石体系中的行为。这些系统被选为代表年轻的（pH值13.3）和老阶段（pH值10.5）的沥滤液演变在水泥地质处置设施。批吸附实验，建模，扩展的X射线吸收精细结构光谱，电子显微镜，小角X射线散射和发光光谱被用来定义的研究系统中的U（VI）的形态。在最低浓度（5.27 × 10−5μ M 232 U（VI））下，观察到旧水泥和新水泥沥滤物都有显著的U去除，并使用一级动力学吸附建模方法成功地模拟了这一情况。在较新的水泥沥滤液中，在较高浓度（>4.20 μM）下，U（VI）在18个月内未与方解石表面发生相互作用。小角X射线散射技术表明，在高U浓度（42.0 μM）下，18个月后，U（VI）以胶体形式存在，与方解石表面几乎没有相互作用，由半径分别为7.6 ± 1.1和217 ± 24 μ m的初级和聚集颗粒组成。在旧水泥浸出液中，发光光谱法确定了U（VI）在方解石上的两个表面结合位点：在含有0.21 μM U（VI）的系统中，确定了类似钙钛矿的Ca 2UO 2（CO 3）3表面复合物;在较高的U（VI）浓度（0.42 μM）下，确定了第二个未确定配位的结合位点。在老水泥沥滤液中U（VI）浓度升高（>2.10 μM）时，地球化学数据和发光光谱都表明，表面介导的沉淀控制了U（VI）的行为。使用聚焦离子束磨机来产生穿过U（VI）沉淀物-方解石界面的部分。该部分的透射电子显微镜图像显示，方解石表面涂覆有纳米晶体，含U相。在U（VI）浓度为4.20 μM时形成的U沉淀的选区电子衍射图像与形成的尿酸钙一致。在较高浓度（约21.0 μM）下的XAS光谱表明形成了第二个U（VI）相，可能是氢氧化铀酰相。这些结果表明，在新的水泥浸出液中，除非U（VI）浓度很低（5.27 × 10−5μM），否则U（VI）不会与方解石表面反应。在较高的浓度，形态计算表明，U（VI）是显着过饱和和实验观察证实，它存在于一个胶体的形式，与矿物表面只有微弱的相互作用。在旧的水泥渗滤液系统在低浓度批次吸附和发光数据表明，U（VI）的去除是由表面络合机制驱动。然而，在较高浓度下，光谱方法表明表面络合和表面介导的沉淀的组合负责所观察到的去除。总体而言，U（VI）的行为在超碱性方解石系统是不同的，在circumneutral pH条件下：在高pH值和任何但低U（VI）浓度，表面介导的沉淀机制发生，这是在相反的circumneutral pH条件下，U（VI）表面络合反应往往占主导地位。
The behaviour of U(VI) in hyperalkaline fluid/calcite systems was studied over a range of U(VI) concentrations (5.27 × 10−5μM to 42.0 μM) and in two high pH systems, young and old synthetic cement leachate in batch sorption experiments. These systems were selected to be representative of young- (pH 13.3) and old-stage (pH 10.5) leachate evolution within a cementitious geological disposal facility. Batch sorption experiments, modelling, extended X-ray absorption fine structure spectroscopy, electron microscopy, small angle X-ray scattering and luminescence spectroscopy were used to define the speciation of U(VI) across the systems of study. At the lowest concentrations (5.27 × 10−5μM232U(VI)) significant U removal was observed for both old and young cement leachates, and this was successfully modelled using a first order kinetic adsorption modelling approach. At higher concentrations (>4.20 μM) in the young cement leachate, U(VI) showed no interaction with the calcite surface over an 18 month period. Small angle X-ray scattering techniques indicated that at high U concentrations (42.0 μM) and after 18 months, the U(VI) was present in a colloidal form which had little interaction with the calcite surface and consisted of both primary and aggregated particles with a radius of 7.6 ± 1.1 and 217 ± 24 Å, respectively. In the old cement leachate, luminescence spectroscopy identified two surface binding sites for U(VI) on calcite: in the system with 0.21 μM U(VI), a liebigite-like Ca2UO2(CO3)3surface complex was identified; at higher U(VI) concentrations (0.42 μM), a second binding site of undetermined coordination was identified. At elevated U(VI) concentrations (>2.10 μM) in old cement leachate, both geochemical data and luminescence spectroscopy suggested that surface mediated precipitation was controlling U(VI) behaviour. A focused ion beam mill was used to create a section across the U(VI) precipitate–calcite interface. Transmission electron microscope images of the section revealed that the calcite surface was coated with a nano crystalline, U containing phase. Selected area electron diffraction images of the U precipitate which was formed at a U(VI) concentration of 4.20 μM were consistent with the formation of calcium uranate. XAS spectroscopy at higher concentrations (⩾21.0 μM) suggested the formation of a second U(VI) phase, possibly a uranyl oxyhydroxide phase.These results indicated that in the young cement leachate, U(VI) did not react with the calcite surface unless U(VI) concentrations were very low (5.27 × 10−5μM). At higher concentrations, speciation calculations suggested that U(VI) was significantly oversaturated and experimental observations confirmed it existed in a colloidal form that interacted with the mineral surface only weakly. In the old cement leachate systems at low concentrations batch sorption and luminescence data suggested that U(VI) removal was being driven by a surface complexation mechanism. However, at higher concentrations, spectroscopic methods suggest a combination of both surface complexation and surface mediated precipitation was responsible for the observed removal. Overall, U(VI) behaviour in hyperalkaline calcite systems is distinct from that at circumneutral pH conditions: at high pH and anything but low U(VI) concentrations, a surface mediated precipitation mechanism occurs; this is in contrast to circumneutral pH conditions where U(VI) surface complexation reactions tend to dominate.
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