EXAFS investigation of Ni(Ⅱ) sorption at the palygorskite-solution interface: New insights into surface-induced precipitation phenomena
EXAFS investigation of Ni(Ⅱ) sorption at the palygorskite-solution interface: New insights into surface-induced precipitation phenomena
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DOI：
10.1016/j.gca.2021.09.012
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发表时间：
2021-12
影响因子：
5
通讯作者：
Xi Mo;Matthew G. Siebecker;Wenxian Gou;Wei Li
中科院分区：
文献类型：
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作者：
Xi Mo;Matthew G. Siebecker;Wenxian Gou;Wei Li
矿物/水界面的吸附反应控制着水相地球化学环境中痕量金属的去向和迁移。虽然表面络合理论已经建立，但对矿物/水界面的表面诱导沉淀的理解仍然有限。在本研究中，我们采用宏观批量吸附研究、透射电子显微镜(TEM)、漫反射光谱(DRS)和扩展X射线吸收精细结构(EXAFS)光谱相结合的方法来解释凹凸棒石/溶液界面上镍的吸附机理。与广泛关注氧化物和层状粘土矿物的研究相比，链状粘土矿物(如凹凸棒石和海泡石)的研究较少。因此，利用链状粘土矿物进行的新的研究可以为金属隔离机理提供新的见解，并改进平衡模型。变温动力学实验表明，凹凸棒石在pH为6.0时吸附Ni的活化反应能(Ea)为35.1kJ·−-1，显著低于pH为7.5时的活化反应能(Ea=约102.1 kJ·−-1)。这些值表明低pH时为吸附反应，高pH时为沉淀反应。EXAFS分析证实，在pH为6以下，低离子强度(If=0.01M)的外球表面络合物和高离子强度(If=100.1M)的内球表面络合是主要的Ni吸附。在pH 7.5以上，EXAFS数据表明形成了镍的层状硅酸盐沉淀，与其线性吸附等温线相一致。一个重要的发现是，在很低的Ni浓度(∼0.07 mm)下可以形成沉淀，其吸附密度为Γ=0.09μ摩尔/m−2，相当于凹凸棒石的单层覆盖率为0.3%。相反，在如此低的镍浓度下，γ-Al_2O_3上没有形成表面析出物。我们认为，镍可以直接在凹凸棒石表面析出，这一过程我们称之为“连续成核”。这在力学上不同于传统的γ-Al_2O_3表面吸附-聚合过程，我们称之为“阶段性成核”。进一步的测试表明，镍在海泡石上的吸附等温线也呈线形，海泡石是一种结构和化学组成与坡缕石相似的粘土矿物，表明两种链状层状硅酸盐的反应活性相似。这不同于镍与氧化铝和蒙脱石反应的朗缪尔型吸附等温线。这项研究的结果不仅改善了目前对矿物/水界面金属隔离和表面沉淀的理解，而且揭示了具有链状结构的粘土矿物与层状结构的粘土矿物表面反应性的巨大差异。
Sorption reactions at the mineral/water interface control the fate and transport of trace metals in aqueous geochemical environments. While surface complexation theory is well established, understanding of surface induced precipitation at mineral/water interfaces is still limited. In this research, we employed a combination of macroscopic batch sorption studies, transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), and extended X-ray absorption fine structure (EXAFS) spectroscopy to elucidate the sorption mechanisms of nickel (Ni) at palygorskite/solution interfaces. Compared to extensive studies focused on oxides and layer-structured clay minerals, research on chain-type clay minerals (e.g., palygorskite and sepiolite) is sparse. Thus, novel investigations using chain-type clay minerals can provide new insights into the metal sequestration mechanisms and improve equilibrium modeling. In this work, temperature-dependent kinetic experiments demonstrated that the activation reaction energy (Ea) for Ni sorption on palygorskite at pH 6.0 is 35.1 kJ mol−1, significantly lower than that of pH 7.5 (Ea= 102.1 kJ mol−1). These values indicated an adsorption reaction at low pH and a precipitation reaction at high pH. EXAFS analysis confirmed that below pH 6, Ni sorption was dominated by outer-sphere surface complexes at low ionic strength (I = 0.01 M) and inner-sphere surface complexation at high ionic strength (I = 0.1 M). Above pH 7.5, EXAFS data suggested the formation of Ni phyllosilicate precipitates, in agreement with its linear sorption isotherm. An important finding is that the precipitates can form at very low Ni concentrations (∼0.07 mM), with the sorption density of Γ = 0.09 μmol m−2, which corresponds to a 0.3% monolayer coverage for palygorskite. In contrast, at this low level of Ni concentration, surface precipitates did not form on γ-Al2O3. We propose that Ni can be directly induced to precipitate on the palygorskite surface in a process we term “continuous nucleation”. This is mechanistically different from the traditional adsorption-to-polymerization process on the γ-Al2O3surface, which we term “staged nucleation”. Further testing revealed that a linear-shaped isotherm was also observed for Ni sorption on sepiolite, a clay mineral with similar structure and chemical composition to palygorskite, indicating a similar reactivity for both chain-structure phyllosilicates. This is distinct from the Langmuir-type sorption isotherm observed for Ni reaction with Al oxides and montmorillonite. The findings presented in this study improve not only the current understanding of metal sequestration and surface precipitation at mineral/water interfaces but also shed light on the large differences in the surface reactivities of clay minerals with chain structure versus layered structure.