Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy
Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy
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DOI：
10.1016/s0016-7037(02)00842-6
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发表时间：
2002-07
影响因子：
5
通讯作者：
R. Dähn;A. Scheidegger;A. Manceau;M. Schlegel;B. Baeyens;M. Bradbury;M. Morales
中科院分区：
文献类型：
--
作者：
R. Dähn;A. Scheidegger;A. Manceau;M. Schlegel;B. Baeyens;M. Bradbury;M. Morales
采用湿化学动力学、粉末和偏振扩展x射线吸收精细结构（EXAFS和P-EXAFS）光谱相结合的方法研究了在pH为8、高离子强度（0.2 M Ca(NO3)2）、初始Ni浓度为660 μM、固体浓度为5.3 g/L条件下蒙脱土对Ni的吸附机理。约20%的Ni在前24小时内被吸附；之后，Ni的吸收速度减慢，反应206 d后，溶液中Ni的浓度仍保持在初始浓度的12%。湿膏体在1、14、90和206 d采集的粉末EXAFS光谱显示，在~ 3.08 Å处存在Ni-Ni对，其数量随时间逐渐增加。结果被解释为Ni相的成核，具有α-Ni-氢氧化物或Ni-层状硅酸盐样局部结构。后一种可能性通过记录在与湿样品相同条件下制备的高度织构的自支撑蒙脱土薄膜的P-EXAFS光谱得到证实，并平衡了14 d。通过定量织构分析，单个粘土颗粒的c*轴在薄膜平面上的取向分布为32.8°全宽。用该值从织构效应上修正P-EXAFS测定的Ni和Si最近邻有效数。Ni原子在3.08 Å面内被2.6±0.5 Ni原子包围，在3.26 Å面外被4.2±0.5 Si原子包围。这些结构参数，以及Ni和Si壳的取向和角度依赖关系，有力地支持了镍层状硅酸盐的形成，其层与蒙脱土层平行。本文首次记录了层状硅酸盐对蒙脱土金属吸收的新形成，具有重要的地球化学意义，因为这种二八面体蒙脱石在环境中绝大多数存在。所产生的吸附微量金属在疏溶的叶状硅酸盐结构中的固存可能会持久地减少它们在地球表面和近地表的迁移和生物利用度。
Wet chemistry kinetics and powder and polarized extended X-ray absorption fine structure (EXAFS and P-EXAFS) spectroscopy were combined to investigate the mechanism of Ni uptake on montmorillonite, at pH 8, high ionic strength (0.2 M Ca(NO3)2), initial Ni concentration of 660 μM, and solid concentration of 5.3 g/L. Approximately 20% of Ni sorbed within the first 24 h; thereafter, the Ni uptake rate slowed, and 12% of the initial Ni concentration remained in solution after 206 d of reaction time. Powder EXAFS spectra collected on wet pastes at 1, 14, 90, and 206 d showed the presence of Ni-Ni pairs at ∼3.08 Å in an amount that gradually increased with time. Results were interpreted by the nucleation of a Ni phase having either an α-Ni-hydroxide– or a Ni-phyllosilicate–like local structure. The latter possibility was confirmed by recording P-EXAFS spectra of a highly textured, self-supporting montmorillonite film prepared in the same conditions as the wet samples and equilibrated for 14 d. The orientation distribution of the c*-axes of individual clay particles off the film plane, as measured by quantitative texture analysis, was 32.8° full width at half maximum, and this value was used to correct from texture effect the effective numbers of Ni and Si nearest neighbors determined by P-EXAFS. Ni atoms were found to be surrounded by 2.6 ± 0.5 Ni atoms at 3.08 Å in the in-plane direction and by 4.2 ± 0.5 Si atoms at 3.26 Å in the out-of-plane direction. These structural parameters, but also the orientation and angular dependence of the Ni and Si shells, strongly support the formation of a Ni phyllosilicate having its layers parallel to the montmorillonite layers. The neoformation of a phyllosilicate on metal uptake on montmorillonite, documented herein for the first time, has important geochemical implications because this dioctahedral smectite is overwhelmingly present in the environment. The resulting sequestration of sorbed trace metals in sparingly soluble phyllosilicate structure may durably decrease their migration and bioavailability at the Earth’s surface and near surface.