Accurate bioaccessibility assessment of soil heavy metals by combining their speciation and in vitro model with human gut microbiota

The bioaccessibility of Zn, Ni, Cd, and Cu is a critical parameter for accurately assessing the human health risk associated with oral exposure to soils contaminated with heavy metals. However, most studies have evaluated the bioaccessibility of these metals without considering the impact of metallic chemical fractionations and human gut microbiota, which are important factors affecting the uptake of heavy metals. In this study, the bioaccessibility of Zn, Ni, Cd, and Cu in soils was measured in an improved in vitro model. The metallic chemical fractionations and bioaccessibility of heavy metals in the sterilized colon phase were also investigated. The results showed that the bioaccessibility of all four heavy metals was below 70%, indicating that relying solely on total metal concentration for assessing human health risks would result in overestimation. The exchangeable, bound to carbonates, and bound to iron and manganese fractions (F123) of heavy metals were found to be close to their bioaccessibility. Furthermore, the bioaccessibility of heavy metals in the gastric phase was strongly and positively correlated with their F123 values (r of Zn, Ni, and Cu were 0.80, 0.57, and 0.68, respectively), highlighting the close connection between metallic chemical fractionation and bioaccessibility. The bioaccessibility of Ni and Cd was found to be higher in the sterile colon phase compared to the active colon phase, whereas Cu showed lower bioaccessibility in the sterile colon phase. The significant differences in heavy metal bioaccessibility between the active and sterile colon suggested that human gut microbiota in vitro played a key role in the adsorption and desorption processes of heavy metals in the intestine. These findings underscore the importance of taking into account both metallic chemical fractionation and human gut microbiota in vitro when assessing the bioaccessibility of heavy metals and their associated health risks.