Complexation of Zr and Hf monomers in supercritical aqueous solutions: Insights from ab initio molecular dynamics simulations
Complexation of Zr and Hf monomers in supercritical aqueous solutions: Insights from ab initio molecular dynamics simulations
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DOI：
10.1016/j.chemgeo.2014.10.012
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发表时间：
2015-12
期刊：
影响因子：
3.9
通讯作者：
S. Jahn;J. Dubrail;M. Wilke
中科院分区：
文献类型：
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作者：
S. Jahn;J. Dubrail;M. Wilke
如现场观察和最近的溶解度测量所示，某些地质流体在高压P和高温T下对高场强元素（HFSE）的增强动员最有可能与流体相中复杂地层的类型有关。然而，只有少数situwin实验研究已进行到目前为止，以限制HFSE形态在超临界水相流体。在这里，我们补充这些调查abinitomolecular动力学模拟探索Zr 4+和Hf 4+单体在水溶液中的络合在T = 1000 K和P <$1 GPa。无论流体的类型（碱性，中性，酸性）Zr和Hf似乎更喜欢八面体的协调。通过系统地改变流体组成和配位阴离子，研究了[（Zr，Hf）Clk（OH）m（H2O）n]4 −k−m（k= 0-6，m= 2-6，n= 0-4）配合物的结构.对于在模拟的时间尺度上稳定的每个络合物，计算理论X射线吸收光谱并与实验数据进行比较。根据这个比较，HCl溶液中最可能的络合物是[（Zr，Hf）Cl 3（Cl，OH）2（H2O）]−。实验和理论XANES光谱之间的差异存在于NaOH溶液的情况下，表明HFSE形态超出单体在这个系统中。电荷中性的[（Zr，Hf）（OH）4（H2O）1-2]配合物在HFSE浓度很低的情况下可能是中性流体中的主要物种，在所有研究的物种中显示出最低配位（5.2（1））。最后，锆和铪在水溶液中在环境和超临界条件下的络合的差异和可能形成的更复杂的低聚物物种进行了讨论。
The enhanced mobilization of high field strength elements (HFSE) by certain geological fluids at high pressures,P, and high temperatures,T, as indicated by field observations and recent solubility measurements is most likely related to the type of complex formation in the fluid phase. However, only a fewin situexperimental studies have been performed so far to constrain HFSE speciation in supercritical aqueous fluids. Here, we complement these investigations byab initiomolecular dynamics simulations to explore the complexation of Zr4 +and Hf4 +monomers in aqueous solutions atT= 1000 K andP∼ 1 GPa. Regardless of the type of fluid (basic, neutral, acidic) both Zr and Hf seem to prefer an octahedral coordination. By systematically changing fluid composition and coordinating anions, the structure of various [(Zr, Hf)Clk(OH)m(H2O)n]4 −k−m(k= 0–6,m= 2–6,n= 0–4) complexes is investigated. For each complex that is stable on the time scale of the simulation, theoretical X-ray absorption spectra are calculated and compared to experimental data. From this comparison, the most likely complexes in HCl solutions are [(Zr, Hf)Cl3(Cl, OH)2(H2O)]−. The differences between experimental and theoretical XANES spectra present in the case of NaOH solutions indicate HFSE speciation beyond monomers in this system. Charge-neutral [(Zr, Hf)(OH)4(H2O)1–2] complexes, which may be the dominant species in neutral fluids at very low HFSE concentrations, show the lowest coordination (5.2(1)) of all investigated species. Finally, differences in the complexation of Zr and Hf in aqueous solutions at ambient and supercritical conditions and the possible formation of more complex oligomeric species are discussed.