Strontium complexation in aqueous solutions and silicate glasses: Insights from high energy-resolution fluorescence detection X-ray spectroscopy and ab-initio modeling
Strontium complexation in aqueous solutions and silicate glasses: Insights from high energy-resolution fluorescence detection X-ray spectroscopy and ab-initio modeling
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DOI：
10.1016/j.gca.2014.07.010
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发表时间：
2014-10
影响因子：
5
通讯作者：
M. Borchert;M. Wilke;C. Schmidt;K. Kvashnina;S. Jahn
中科院分区：
文献类型：
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作者：
M. Borchert;M. Wilke;C. Schmidt;K. Kvashnina;S. Jahn
尽管流体和熔体化学成分对微量元素锶、钡、镧、钇的流体-熔体分配有很大的影响，但在高温高压下，对其在硅酸盐饱和流体中的形态研究还不够充分。本文应用高能分辨荧光检测-X射线吸收光谱(HERFD-XAS)研究了锶在晶体模型化合物、硅酸盐玻璃和水溶液中的局域环境。在温度为780℃、压力为80 0℃、压力为∼80 0 0 Mpa的条件下，原位获得了SrCl2和Sr(OH)2水溶液以及3种含有硅酸盐组分的水溶液的sRK边HERFDXAS光谱，并用近边结构有限差分法进行了理论计算.这种方法在许多晶体模型化合物中得到了验证。对于硅酸盐玻璃和水溶液(SrCl2和Sr(OH)2)，观察到了小团簇。在过铝玻璃或过碱玻璃中，对称或扭曲的SrO6团簇描述了锶的络合作用。然而，小的静态团簇似乎并不完全适合于解释高温下水溶液中动态变化的原子排列。因此，我们进行了从头算分子动力学模拟，并将其作为X射线吸收光谱模型的输入。对模拟结果的分析表明，在氯化物和氢氧化物溶液中，[SrCl(H2O)6]+和Sr(OH)2(H2O)4分别是最可能的络合物。对富硅酸盐流体的光谱分析表明，熔体和流体的组成都对锶的络合有很大的影响。对于富硅酸盐流体，当流体中(Na++K)/Cl和(Si++Al)/(Na++K)比值较低时，形成锶-氯络合物，而当比值较高时，则有利于氢氧化锶和可能的硅酸盐络合物(类似于硅酸盐玻璃中的络合物)的形成。我们的X射线光谱结果解释了以前非原位研究中测量的流体-熔体分配对熔体组成的依赖关系，并强调了除氯(硅酸盐和铝硅酸盐)以外的组分在高温和压力下控制流体-熔体系统中金属形态的重要性。
Although fluid–melt partitioning of trace elements like Sr, Ba, La, and Y is known to be strongly influenced by the fluid and melt chemical composition, their speciation in silicate-saturated fluids is studied insufficiently at high temperatures and pressures. Here, high energy-resolution fluorescence detection–X-ray absorption spectroscopy (HERFD–XAS) has been applied to investigate the local environment of strontium in crystalline model compounds, silicate glasses, and aqueous solutions. Acquisition of Sr K-edge HERFD–XAS spectra of aqueous solutions of SrCl2and Sr(OH)2, and three aqueous fluids with dissolved silicate components was done in situ at temperatures to 780 °C and pressures to ∼800 MPa using hydrothermal diamond-anvil cells.Experiments were complemented by theoretical spectroscopy calculations using the finite difference method near edge structure (FDMNES) code. This approach was validated for a number of crystalline model compounds. For the silicate glasses and aqueous solutions (SrCl2and Sr(OH)2), small clusters were examined. Either symmetric or distorted SrO6clusters were found to describe Sr complexation in peraluminous or peralkaline glasses. However, small ‘static’ clusters seem not to be fully suited to account for the dynamically changing atomic arrangements in aqueous solutions at high temperature. Therefore,ab-initiomolecular dynamics simulations were performed and used as input for modeling of X-ray absorption spectra. Analyses of these simulations indicated [SrCl(H2O)6]+and Sr(OH)2(H2O)4as the most likely complexes in the chloride and hydroxide solutions, respectively.Analysis of the spectra of the silicate-rich fluids shows that both melt and fluid composition strongly influence Sr complexation. For the silicate-rich fluids, formation of Sr–Cl complexes occurs at low (Na + K)/Cl and (Si + Al)/(Na + K) ratios in the fluid, whereas Sr hydroxide and possibly silicate complexes (similar to those in the silicate glass) are favored at higher ratios. Our X-ray spectroscopic results offer an explanation for the dependence of fluid–melt partitioning of Sr on melt composition measured in previous ex situ studies, and highlight the importance of components other than chloride (silicate and aluminosilicate) in controlling metal speciation in fluid–melt systems at high temperatures and pressures.