Effects of pH and Ca exchange on the structure and redox state of synthetic Na-birnessite
Effects of pH and Ca exchange on the structure and redox state of synthetic Na-birnessite
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DOI：
10.2138/am-2020-7112
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发表时间：
2021-01-01
影响因子：
3.1
通讯作者：
Ilton, Eugene S.
中科院分区：
文献类型：
--
作者：
Elmi, Chiara;Post, Jeffrey E.;Ilton, Eugene S.
类水钠锰矿是表层土壤和沉积物中最常见的锰氧化物之一，它们调节着重要的环境过程(如生物地球化学循环、重金属限制)，并具有新的技术应用(如水氧化催化)。在生物和非生物水钠长石中，钙是主要的层间阳离子，特别是当它们与碳酸盐形成时。本研究研究了在pH值为2~7.5的条件下，通过阳离子交换与合成的钠矿进行阳离子交换而制备的一系列合成钙钠钠矿类似物的结构。用粉末X射线衍射和Rietveld精细化、傅里叶变换红外光谱、拉曼光谱、X射线光电子能谱、扫描和透射电子显微镜对钙交换水钠铝石相进行了表征。在pH值大于3的情况下合成的所有样品都显示出相似的三斜结构，其晶胞参数几乎相同。在pH值为2和3的条件下交换的样品得到了六方结构，或六方相和三斜相的混合物。Rietveld结构修正和X射线光电子能谱分析表明，Na与Ca的交换引发了部分Mn3+的还原，在八面体中产生了层间Mn2+和空位。本研究中所描述的三斜钙水钠石和六方钙水钠石结构分别不同于钠水钠石和氢水钠石。因此，模拟天然富钙水钠长石的X射线吸收光谱，通过混合Na-和H-水钠长石端元，将不能得到真实结构的准确表示。
Birnessite-like minerals are among the most common Mn oxides in surficial soils and sediments, and they mediate important environmental processes (e.g., biogeochemical cycles, heavy metal confinement) and have novel technological applications (e.g., water oxidation catalysis). Ca is the dominant interlayer cation in both biotic and abiotic birnessites, especially when they form in association with carbonates. The current study investigated the structures of a series of synthetic Ca-birnessite analogs prepared by cation-exchange with synthetic Na-birnessite at pH values from 2 to 7.5. The resulting Ca-exchanged birnessite phases were characterized using powder X-ray diffraction and Rietveld refinement, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning and transmission electron microscopy. All samples synthesized at pH values greater than 3 exhibited a similar triclinic structure with nearly identical unit-cell parameters. The samples exchanged at pH 2 and 3 yielded hexagonal structures, or mixtures of hexagonal and triclinic phases. Rietveld structure refinement and X-ray photoelectron spectroscopy showed that exchange of Na by Ca triggered reduction of some Mn3+, generating interlayer Mn2+ and vacancies in the octahedral layers. The triclinic and hexagonal Ca-birnessite structures described in this study were distinct from Na- and H-birnessite, respectively. Therefore, modeling X-ray absorption spectra of natural Ca-rich birnessites through mixing of Na- and H-birnessite end-members will not yield an accurate representation of the true structure.