U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study
U(VI) sorption on montmorillonite in the absence and presence of carbonate: A macroscopic and microscopic study
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DOI：
10.1016/j.gca.2012.04.017
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发表时间：
2012-09-15
影响因子：
5
通讯作者：
Bradbury, M. H.
中科院分区：
文献类型：
--
作者：
Marques Fernandes, Maria;Baeyens, B.;Bradbury, M. H.
通过不同方法的组合研究了在不存在和存在碳酸盐的情况下U（VI）在蒙脱石（Na-SWy-1）上的吸附机理：宏观吸附实验、使用2位点质子解非静电表面络合和阳离子交换模型的表面络合模型和扩展X射线吸收精细结构（EXAFS）光谱。在不存在碳酸盐的情况下，在固定离子强度（0.1 M NaClO4）下，在U（VI）痕量浓度（类似于9 × 10（-8）M）下作为pH值的函数，以及在固定pH值（5、6.8和8）下作为U（VI）浓度（类似于10（-7）-10（-4）M）的函数，进行U（VI）吸附测量。在碳酸盐的存在下，实验在与大气pCO（2）和1、3和5 mM NaHCO3平衡的情况下进行。在不存在碳酸盐的情况下，通过考虑以下表面物种的形成来模拟痕量浓度下的pH依赖性吸附测量，所述表面物种在强位点上相当于（SOUO2+）-O-S、相当于（SOUO2OH0）-O-S、相当于（SOUO2）-O-S（OH）（2）（-）和相当于（SOUO2）-O-S（OH）（3）（2-）。从等温线推断，在弱位上形成了等价于（SOUO 2+）-O-W 1和等价于（SOUO 2 OH 0）-O-W 1。强中心上的两个额外的表面络合物，相当于（SOUO2 CO3-）-O-S和相当于（SOUO2）-O-S（CO3）（2）（3-）和一个弱中心上的表面络合物，（SOUO2 CO3-）-O-W1，是重现在碳酸盐存在下获得的吸附数据所必需的。EXAFS测量不允许验证在蒙脱石表面上形成三元铀酰-碳酸盐络合物。然而，所获得的拟合结果，即赤道氧壳层的分裂，一个Si/Al壳层近似于3.09埃，一个Si/Al壳层近似于3.29埃，或一个Fe壳层近似于3.42埃，清楚地表明，在给定的实验条件下，（pH，U（VI）负载）U（VI）通过与铝八面体和/或硅四面体结合在蒙脱石边缘位点上形成内球表面络合物。(C)2012爱思唯尔有限公司保留所有权利。
The mechanism of U(VI) sorption on montmorillonite (Na-SWy-1) in the absence and presence of carbonate was investigated through a combination of different approaches: macroscopic sorption experiments, surface complexation modelling using the 2 Site Protolysis Non Electrostatic Surface Complexation and Cation Exchange sorption model and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. U(VI) sorption measurements were performed in the absence of carbonate at fixed ionic strength (0.1 M NaClO4) as a function of pH at U(VI) trace concentration (similar to 9 x 10(-8) M) and as a function of U(VI) concentration (similar to 10(-7) -10(-4) M) at a fixed pH (5, 6.8 and 8). In the presence of carbonate, experiments were carried out in equilibrium with atmospheric pCO(2) and in 1, 3 and 5 mM NaHCO3. The pH dependent sorption measurements at trace concentration in the absence of carbonate were modelled by considering the formation of the following surface species, equivalent to(SOUO2+)-O-S, equivalent to(SOUO2OH0)-O-S, equivalent to(SOUO2)-O-S(OH)(2)(-) and equivalent to(SOUO2)-O-S(OH)(3)(2-) on the strong sites. From the isotherms the formation of equivalent to(SOUO2+)-O-W1 and equivalent to(SOUO2OH0)-O-W1 on the weak sites was inferred. Two additional surface complexes on the strong sites, equivalent to(SOUO2CO3-)-O-S and equivalent to(SOUO2)-O-S(CO3)(2)(3-) and one surface complex on the weak sites, (SOUO2CO3-)-O-W1, were necessary to reproduce the sorption data obtained in the presence of carbonate. The EXAFS measurements did not allow to verify the formation of ternary uranyl-carbonate complexes on the montmorillonite surface. However, the obtained fit results, i.e. splitting of the equatorial oxygen shell, one Si/Al shell at similar to 3.09 angstrom and one Si/Al at similar to 3.29 angstrom or one Fe shell at similar to 3.42 angstrom, clearly indicate that under the given experimental conditions (pH, U(VI) loading) U(VI) forms inner-sphere surface complexes on montmorillonite edge sites via binding to aluminum octahedra and/or silicon tetrahedra. (C) 2012 Elsevier Ltd. All rights reserved.