A Method for Redox Mapping by Confocal Micro-X-ray Fluorescence Imaging: Using Chromium Species in a Biochar Particle as an Example

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.