School of Environmental Studies , China University of Geosciences , 388 Lumo Rd. , Wuhan , Hubei 430074 , People's Republic of China.
Department of Earth and Environmental Sciences , University of Waterloo , 200 University Ave. W. , Waterloo , ON N2L 3G1 , Canada.
Anal Chem. 2019 Apr 16;91(8):5142-5149. doi: 10.1021/acs.analchem.8b05718. Epub 2019 Apr 1.
Redox mapping of solid-phase particles has been used for speciation mapping of near-surface materials or within grains through the use of thin-sections without depth information. Here, a procedure is presented for data collection and processing of depth-dependent redox mapping within solid particles using confocal micro-X-ray fluorescence imaging (CMXRFI). The procedure was applied to a biochar particle that was reacted with Cr(VI)-spiked water. The total Cr distribution was first obtained at an above-edge energy of the K-edge, and showed that Cr was primarily distributed near the surface of the particle. Redox mapping was conducted at 33 representative energies and linear combination fitting (LCF) was performed for the 33 data points from each pixel. The results indicate Cr(III) is the primary species with fractions ranging from 0.6 to 1 and that this fraction is greater in the interior pixels of the particle than at the surface; in contrast, the Cr(VI) fraction is greater at the surface than for interior pixels. The results likely indicate Cr(VI) was first adsorbed and diffused into the biochar, and then reduced to Cr(III). With more Cr(VI) adsorption and the exceedance of the reduction potential of the biochar, remaining Cr(VI) was accumulated on the surface. The redox mapping method was validated by micro-XANES (X-ray absorption near-edge structure) and XPS (X-ray photoelectron spectroscopy) results. This demonstration indicates the developed method combined with CMXRFI can be used to delineate the distribution of different oxidation states of an element within an intact particle or layer.
固相颗粒的氧化还原图谱已经被用于通过使用没有深度信息的薄片来对近表面材料或颗粒内的物质进行形态分析。在这里，提出了一种使用共聚焦微 X 射线荧光成像（CMXRFI）对固相颗粒内的深度依赖氧化还原图谱进行数据收集和处理的程序。该程序应用于与 Cr(VI) 污染水反应的生物炭颗粒。首先在 K 边的上边缘能量处获得总 Cr 分布，结果表明 Cr 主要分布在颗粒表面附近。在 33 个代表性能量点进行了氧化还原图谱绘制，并对每个像素的 33 个数据点进行了线性组合拟合（LCF）。结果表明，Cr(III)是主要物种，分数范围为 0.6 至 1，并且该分数在颗粒的内部像素中大于表面；相比之下，Cr(VI)分数在表面大于内部像素。结果可能表明 Cr(VI)首先被吸附并扩散到生物炭中，然后被还原为 Cr(III)。随着更多的 Cr(VI)吸附和生物炭还原电位的超过，剩余的 Cr(VI)在表面上积累。氧化还原图谱方法通过微 XANES（X 射线吸收近边结构）和 XPS（X 射线光电子能谱）结果进行了验证。该演示表明，开发的方法与 CMXRFI 结合使用可以用于描绘元素的不同氧化态在完整颗粒或层内的分布。
