Particle size-dependent bioaccessibility of chromium in smelting soils: assessment by multi-method in vitro...

The closure or relocation of many factories has led to numerous abandoned sites with varying levels of potentially toxic element (PTE) enrichment pollution, damaging the surrounding environment. This study used three in vitro simulation methods (PBET, SBRC, and IVG) to systematically assess the impact of soil particle size on the bioaccessibility of chromium (Cr) in contaminated soils from a typical metal smelting site in Hunan Province, south-central China. Results showed that the highest total Cr concentration in the samples exceeded the background value of Hunan soil (67.0 mg/kg) by 132.54 times, with Cr(VI) making up 17.0% (1510 mg/kg). Soil particle size has a significant impact on Cr bioaccessibility by regulating PTE concentration, valence states, and physicochemical properties, including pH and soil organic matter (SOM). Under simulated gastrointestinal conditions, the < 75 µm particle fraction showed the highest chromium concentration and bioaccessibility, reaching 8880 mg/kg and 27.34%, respectively, notably higher than in coarser particles (75-125 µm, 125-250 µm). Among the three methods, the SBRC method was more conservative in the assessment. Correlation analysis revealed a strong positive relationship (r > 0.85, P < 0.05) between Cr/Cr(VI) concentrations and bioaccessibility in the < 75 µm fraction. This study highlights the importance of further refining soil particle size fractions to accurately assess chromium exposure risks at smelting sites.