Bioaccessibility of light rare earth elements in soil: distribution and relationships with speciation and soil...

The widespread use of light rare earth elements (LREEs) has raised concerns regarding their potential health risks. However, the bioaccessibility of LREEs in soil, a crucial factor in assessing human exposure risk, remains poorly understood. This study investigated the bioaccessibility of four LREEs (Sc, La, Ce and Sm) in three soil types using an in vitro digestion model. Furthermore, we examined the qualitative and quantitative relationships between LREE bioaccessibility, speciation, and soil properties using correlation analysis and General Linear Mix Model (GLMM), respectively. The results reveal that the total concentrations of individual LREEs ranged from 2.8 to 190.2 mg/kg, with the residual fraction predominantly accounting for more than 60 % of the total LREEs. Additionally, the gastric bioaccessibility of Sc, La, Ce and Sm ranged from 3.7 % to 11.6 %, while intestinal bioaccessibility is decreased from 1.1 % to 6.4 %. Correlation analysis revealed significant positive relationships between the intestinal bioaccessibility of all four LREEs and the speciation binding to Fe/Mn oxides, as well as significantly negative correlations with soil pH, Ca content, and moisture. Additionally, GLMM model can effectively described LREE bioaccessibility based on speciation distribution, with R² values ranging from 0.75 to 0.88. The importance analysis indicated that the speciation binding to Fe/Mn oxides had a greater influence on bioaccessibility than other speciation, while moisture content, pH, and Ca content were the key soil properties affecting bioaccessibility. These findings contribute to refine the assessment of exposure risks associated with LREEs in soil.