Evaluating in vitro bioaccessible fractions of rare earth elements in soils by using a simplified bioaccessibility extraction test

Rare earth elements (REEs) are emerging contaminants; however, information on their bioaccessibility via soil ingestion and its implications for human health risk remains limited. The physiologically based extraction test (PBET) is commonly used to evaluate the bioaccessibility of potentially toxic elements in soils, but the simplified bioaccessibility extraction test (SBET) offers a streamlined in vitro bioaccessibility assessment. This study examined thirty-two surface soil (0–10 cm) samples from Taiwan for the bioaccessibility of REEs using PBET and SBET. Additionally, the tested soils were characterized by fractional REEs with the BCR sequential extraction. The bioaccessible concentration of REEs extracted by the SBET was 21.1 ± 15.7 mg kg⁻¹ with a mean bioaccessibility of 15.2 ± 9.62%. The SBET-based bioaccessibility positively and significantly correlated with that of the PBET (r = 0.85, p < 0.001). These labile pools were mainly from the acid-exchangeable and reducible fractions of REEs in the soils. The bioaccessible REEs, as measured by SBET and PBET, were predominantly retained by pedogenic iron (Fe) oxides in this study. However, the SBET procedure was time-efficient and low-cost compared with the PBET. Thus, the consistent targeting of the reducible fraction of REEs by both methods confirms the SBET as a reliable and promising alternative to the PBET for assessing bioaccessibility of soil REEs.