Bioavailability of Heavy Metals in Soils from a City with Fe-Nb-REE Mineral Development

Baotou, a city renowned for its steel production and rare earth resource development, is facing significant threats from heavy metal pollution in its urban soils. This study analyzed nine soil samples from urban Baotou to assess the bioavailability of heavy metals, including rare earth elements (REEs), As, and Pb, and evaluate the environmental risks posed by these elements in soils. The bioavailabilities of REEs, As, and Pb were quantitatively examined through in vitro simulated digestion experiments involving physiologically based extraction tests (PBET) and in vitro gastrointestinal (IVG) methods. The results indicate that the bioavailability of REEs is relatively low, with higher bioavailability observed in PBET tests compared to IVG tests. Overall, the REE bioavailability was lower than those of As and Pb, and the REE bioavailability was higher in the gastric phase than in the gastrointestinal phase. Speciation analysis revealed that approximately 80% of the REEs were present in the residual fraction. Meanwhile, the non-residual fractions were primarily composed of carbonate-bound, Fe/Mn oxides-bound, and strong organic matter-bound REEs, exchangeable REEs accounting for less than 0.5%.