Analysis of Total Cr and Cr(Ⅵ) in Soils with High Chromium Contamination and Pollution Distribution of Chromium Residue Site

Legacy chromium residue stockpiles act as critical point sources of soil chromium (Cr) contamination. For soils with high-level and highly heterogeneous chromium pollution, the analysis of total Cr faces several bottlenecks, including insufficient scientific support for selecting suitable pretreatment methods and ambiguous applicability of X-ray fluorescence spectrometry (XRF) for rapid on-site screening of total Cr concentrations, which hinders precise investigation and remediation of contaminated sites. To clarify the selection of pretreatment methods for total Cr measurement in highly chromium-contaminated soils and establish an efficient and accurate hierarchical detection strategy, this study took a typical legacy chromium residue stockpile as the research object. Two pretreatment methods, namely open acid digestion and closed pressure acid digestion, were compared via inductively coupled plasma-optical emission spectrometry (ICP-OES) for total Cr quantification. The results demonstrated that open acid digestion produced volatile chromyl chloride (CrO₂Cl₂) from Cr(Ⅵ), leading to systematic error in total Cr measurements. The relative error of certified reference material GBW07101 reached −21.8% using open acid digestion, while only 6.7% for closed pressure acid digestion, confirming closed pressure acid digestion as the optimal pretreatment technique for accurate total Cr quantification in highly chromium-polluted soils. Furthermore, based on the benchmark data obtained from closed pressure acid digestion combined with ICP-OES, the quantitative performances of wavelength dispersive X-ray fluorescence spectrometry (WDXRF) and energy dispersive X-ray fluorescence spectrometry (EDXRF) for total Cr at different concentration levels were evaluated, and the spatial distribution characteristics of total Cr and Cr(Ⅵ) across the site were characterized. The main findings are as follows: (1) 4000 mg/kg serves as the critical threshold of total Cr concentration. WDXRF delivers superior quantitative accuracy below this threshold, whereas EDXRF performs better at concentrations above 4000 mg/kg; (2) The total Cr concentrations of soil samples collected from the target site ranged from 68 to 14833 mg/kg, showing a gradient decreasing trend outward from the core of the residue stockpile. The total Cr content in core-area soils exceeded the standard limit by up to 59 times, and the maximum Cr(Ⅵ) concentration was 3030.0 mg/kg, posing prominent environmental pollution risks. This study clarifies the theoretical basis and applicable conditions of analytical methods for highly chromium-contaminated soils and establishes a hierarchical detection workflow, providing systematic technical support for pollution control and remediation of similar legacy chromium residue stockpiles.