A thermodynamic adsorption/entrapment model for selenium(IV) coprecipitation with calcite
A thermodynamic adsorption/entrapment model for selenium(IV) coprecipitation with calcite
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DOI：
10.1016/j.gca.2014.02.044
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发表时间：
2014-06
影响因子：
5
通讯作者：
F. Heberling;V. Vinograd;R. Polly;J. Gale;S. Heck;J. Rothe;D. Bosbach;H. Geckeis;B. Winkler
中科院分区：
文献类型：
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作者：
F. Heberling;V. Vinograd;R. Polly;J. Gale;S. Heck;J. Rothe;D. Bosbach;H. Geckeis;B. Winkler
硒是一种与环境相关的微量元素，而放射性同位素79 Se在核废物处置安全方面特别令人关注。氧化硒物种是相对可溶的，并且在常见矿物表面仅显示弱吸附。然而，硒在地圈中的一种可能的吸附机制是硒（IV）（亚硒酸盐，SeO 32 −）与方解石（CaCO 3）的结构结合。在本研究中，我们通过一系列实验和计算方法研究了亚硒酸盐与方解石之间的相互作用，旨在量化标准条件下亚硒酸盐与方解石的结合。我们进一步试图描述的热力学亚硒酸盐掺杂的方解石，和亚硒酸盐与方解石共沉淀的结构进行了研究，采用硒K边EXAFS（各向同性和偏振相关）和结果进行了比较，密度泛函理论（DFT）计算。这些研究证实了亚硒酸盐通过碳酸盐取代亚硒酸盐进入方解石的结构，导致形成Ca（SeO 3）X（CO 3）（1−X）固溶体。低过饱和度下的共沉淀实验表明，随着接触溶液中亚硒酸盐与碳酸盐比例的增加，固体中亚硒酸盐与碳酸盐的比例线性增加。这种关系可以在假设方解石和虚拟CaSeO 3端元之间存在理想混合的情况下进行描述，其标准吉布斯自由能（G 0（CaSeO3_exp）= −953 ± 6 kJ/mol，log 10（KSP（CaSeO3_exp））= −6.7 ± 1.0）由从稀亨利定律域到X（CaSeO 3）= 1的过量自由能的线性外推定义。与此实验结果相反，DFT和力场计算预测虚拟体相CaSeO 3端元的稳定性明显降低，可溶性更高：G 0（CaSeO 3体相）= −912 ± 10 kJ/mol，log 10（KSP（CaSeO3_bulk））= 0.5 ± 1.7。这个概念是基于这样的想法，即实验值−953 ± 6 kJ/mol反映了表面层内CaSeO 3的吉布斯自由能，而从原子计算获得的值反映了整体热力学性质。在稳态条件下进行的共沉淀实验中，这些值之间的差异由过饱和度补偿。因此，如果体CaCO 3和CaSeO 3端元的吉布斯自由能被替换为表面端元的吉布斯自由能，共沉淀实验仍然可以在平衡热力学的形式主义内处理。这一概念导致了一些重要的后果，这可以通过实验和理论测试。我们表明，亚硒酸盐在方解石表面的吸附和亚硒酸盐与方解石在过饱和条件下的共沉淀可以用相同的分配系数来描述。这意味着共沉淀可以被看作是一系列的吸附和截留事件。另一方面，我们的文石重结晶实验表明，在近平衡条件下，方解石的生长被抑制在亚硒酸盐的存在。与这些观察结果相一致，我们的DFT计算表明，碳酸盐取代亚硒酸盐是积极的表面比内bulk.The整个实验和原子模拟结果的结论，方解石-CaSeO 3固溶体只能连续增长，如果水溶液是相对于本体固溶体过饱和。在这些条件下，亚硒酸盐与方解石在一个分区共沉淀。
Selenium is an environmentally relevant trace element, while the radioisotope79Se is of particular concern in the context of nuclear waste disposal safety. Oxidized selenium species are relatively soluble and show only weak adsorption at common mineral surfaces. However, a possible sorption mechanism for selenium in the geosphere is the structural incorporation of selenium(IV) (selenite, SeO32−) into calcite (CaCO3).In this study we investigate the interactions between selenite and calcite by a series of experimental and computational methods with the aim to quantify selenite incorporation into calcite at standard conditions. We further seek to describe the thermodynamics of selenite-doped calcite, and selenite coprecipitation with calcite.The structure of the incorporated species is investigated using Se K-edge EXAFS (isotropic and polarization dependent) and results are compared to density functional theory (DFT) calculations. These investigations confirm structural incorporation of selenite into calcite by the substitution of carbonate for selenite, leading to the formation of a Ca(SeO3)X(CO3)(1−X)solid solution.Coprecipitation experiments at low supersaturation indicate a linear increase of the selenite to carbonate ratio in the solid with the increase of the selenite to carbonate ratio in the contact solution. This relationship can be described under the assumption of an ideal mixing between calcite and a virtual CaSeO3endmember, whose standard Gibbs free energy (G0(CaSeO3_exp) = −953 ± 6 kJ/mol, log10(KSP(CaSeO3_exp)) = −6.7 ± 1.0) is defined by linear extrapolation of the excess free energy from the dilute Henry’s law domain toX(CaSeO3) = 1. In contrast to this experimental result, DFT and force field calculations predict the virtual bulk CaSeO3endmember to be significantly less stable and more soluble: G0(CaSeO3 bulk) = −912 ± 10 kJ/mol and log10(KSP(CaSeO3_bulk)) = 0.5 ± 1.7.To explain this discrepancy we introduce a thermodynamic adsorption/entrapment concept. This concept is based on the idea that the experimental value of −953 ± 6 kJ/mol reflects the Gibbs free energy of CaSeO3within the surface layer, while the value obtained from atomistic calculations reflects bulk thermodynamic properties. In coprecipitation experiments performed at steady-state conditions the difference between these values is compensated by the supersaturation. Thus, if the Gibbs free energies of the bulk CaCO3and CaSeO3endmembers are substituted with the Gibbs free energies of the surface endmembers, the coprecipitation experiment can still be treated within the formalism of equilibrium thermodynamics. This concept leads to a number of important consequences, which can be tested both experimentally and theoretically.We show that selenite adsorption at the calcite surface and selenite coprecipitation with calcite under supersaturated conditions can be described with the same partition coefficient. This implies that the coprecipitation can be viewed as a sequence of adsorption and entrapment events. On the other hand, our aragonite recrystallization experiments show that at near equilibrium conditions the calcite growth is inhibited in the presence of selenite. Consistent with these observations, our DFT calculations show that the substitution of carbonate for selenite is energetically more favorable at the surface than inside the bulk.The whole set of the experimental and atomistic simulation results leads to the conclusion that the calcite–CaSeO3solid solution can only grow continuously if the aqueous solution is supersaturated with respect to the bulk solid solution. Under these conditions selenite coprecipitates with calcite at a partition …