Lability, bioaccessibility, and ecological and health risks of anthropogenic toxic heavy metals in the arid calcareous soil around a nonferrous metal smelting area

Abstract

Lability and bioaccessibility of anthropogenic toxic heavy metals in arid calcareous soils are critical to understand their ecological and health risks. This study examined toxic heavy metal speciation in the calcareous soil contaminated by nonferrous metal smelting. Results demonstrated that approximately 70 years’ nonferrous metal smelting and mining in Baiyin led to significant contamination of nearby soil down to about 200 cm depth by cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn), with more serious contamination in the downwind areas of smelting or mining. More than half of Cd, Cu, Pb, and Zn in the soil was present in the labile fractions while more than 75% of cobalt (Co), chromium (Cr), nickel (Ni), and vanadium (V) was present in the residual fraction. Carbonate minerals in this calcareous soil play an important role in the labile fractions, with approximate 25% of Cd and Pb and 15% of Cu and Zn bound in carbonates. Bioaccessible Cd, Cu, Pb, and Zn in the soil were approximately 49.8%, 29.4%, 12.2%, and 33.8% in gastric phase and 13.5%, 15.9%, 4.3%, and 9.1% in intestinal phase of their total concentrations, respectively. Therefore, Cd and Zn were removed from gastric solution to a greater extent than Cu and Pb by neutral intestine environment. However, bioaccessible Co, Cr, Ni, and V in the soil were less than 3% of their total concentrations. Bioaccessibility of these metals but Cu in this calcareous soil was significantly lower than that for the acidic Ultisols and Alfisols in U.S. The concentrations of Cd, Cu, Pb, Zn, and Ni in each chemical and bioaccessible forms were significantly correlated linearly with their total concentrations in the calcareous soil, while only residual concentration was significantly correlated with the total concentration for Co, Cr, and V. These linear slopes showed that relative lability and bioaccessibility increased for Cd, but decreased for Cu, Pb, and Zn with the increase in their total concentrations in the calcareous soil. Direct oral soil ingestion would not pose a non-carcinogenic health risk to local children. However, very high potential ecological risk would be caused by these metals in the soil. These results provide improved insights into the biogeochemical processes of anthropogenic toxic heavy metals in the arid calcareous soils worldwide.