High geological background concentrations of As and Cd in karstic soils may not contribute to greater risks to human health via rice consumption

High geological background concentrations of toxic metal(loid)s arsenic (As) and cadmium (Cd) from natural enrichment in soils of karst regions have attracted much attention. In this study, paired soil-rice samples were collected from karst and non-karst regions in Guangxi, China to assess the potential risks of metal(loid) transfer from soil to rice grains, and rice grains to humans. Our results indicate that the karstic soils had greater As (25.7 vs. 12.4 mg·kg^-1) and Cd (2.12 vs. 1.04 mg·kg^-1) contents than those in non-karstic soils. However, metal(loid) transfer from soil to rice grains (ratio of rice grains to soil content) of As and Cd was 40 and 49% lower in karst regions, which may relate to their 42 and 61% lower HNO₃-extractable As and CaCl₂-extractable Cd, resulting in similar As/Cd contents in karstic and non-karstic rice grains. In vitro assay using a modified physiologically-based extraction test showed that karstic rice grains had a lower As/Cd bioaccessibility than non-karstic grains, which can be attributed to their ~50% greater P content, which negatively correlated with As/Cd bioaccessibility. Additionally, karstic rice grains had 39% greater phytate and exhibited 45% and 9.4% lower As and Cd bioaccessibility in the gastric phase with phytate supplement at 0.6%. Our work indicates that despite the greater As/Cd contents in karstic soils, the risks of As/Cd transfer from soil to rice grains as well as their exposure risks to humans via rice consumption may not be greater than non-karst regions.