Wavelength-Dispersive XRF Analysis of Total Cr and the Effects of Cr Speciation in High-Chromium Soils

With the surge in industrial use of chromium (Cr) and continuous intensification of pollution emission, Cr levels in soils across numerous regions have risen significantly. X-ray fluorescence spectrometry (XRF) offers advantages including simple sample preparation, simultaneous multi-element determination, and a broad analytical concentration range. However, when determining Cr-polluted soils, this method faces multiple challenges such as insufficient concentration gradients in certified reference materials (CRMs), mineral effects, particle size effects, and absorption-enhancement effects, all of which can impact the accuracy of Cr and multi-element determinations. These limitations are addressed by preparing 24 artificial standard samples across gradient concentrations via spiking low-Cr soil CRMs (GBW series) with K₂Cr₂O₇ (Cr⁶⁺) and CrCl₃·6H₂O (Cr³⁺). Integrating these artificial standards with national soil CRMs for calibration curve development effectively compensates for the inadequate concentration coverage of existing CRMs, making the calibration range more suitable for the detection needs of high Cr-polluted soils. Experimental verification demonstrates the influence of trivalent chromium (Cr³⁺) and hexavalent chromium (Cr⁶⁺) on analytical results in terms of measurement angle, resolution, and sensitivity. There are slight differences in the measurement angles for Cr³⁺ and Cr⁶⁺, but the count difference for the same sample measured at their respective characteristic peak angles is negligible. No significant differences are observed in resolution and sensitivity. When applying this method to determine CRMs, the relative error (RE) is less than 10%, and the relative standard deviation (RSD) is less than 5%. Furthermore, analysis of actual soil samples from a Cr residue site and its surrounding areas in China shows that the RE of the Cr determination results are within 10% compared to those obtained by ICP-OES. Wavelength-dispersive XRF (WDXRF) analysis reveals a distinct difference in the peak angles between Cr³⁺and Cr⁶⁺, corresponding to an energy shift of approximately 0.39 eV, with the peak energy of Cr⁶⁺ being lower than that of Cr³⁺. This is consistent with the energy shift values reported in the literature, demonstrating the potential of this method for Cr speciation analysis in soils. The proposed method not only accurately determines the total Cr content in soil but also simultaneously analyzes major and minor elements such as Na, Mg, Al, and Si, thereby expanding the detection scope of soil metal elements. It is suitable for the quantitative multi-element analysis of various soil samples, especially for the accurate quantification of multiple elements in highly Cr-contaminated soils, establishing a more comprehensive methodological system for the precise multi-element determination in high-Cr soil environments. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202504010075.