Enrichment of cadmium and selenium in soil-crop system and associated probabilistic health risks in black shale areas

Selenium (Se) is the essential component of selenoenzymes and contributes to antioxidant defenses. The capability of Se to antagonize the toxicity of heavy metals makes it an essential trace element for human and plant health. Soils derived from black shales are naturally enriched with Se; however, these soils often contain high geological cadmium (Cd), due to the weathering of black shales rich in Cd and Se. Cadmium, as a known Group I carcinogen, could induce damage to various organs. This therefore poses a major challenge for safe cultivation of Se-rich land resources. In this study, a total of 247 paired soil-crop samples were collected from a typical farmland derived from black shales. The concentrations of Cd and Se in the samples were analyzed by inductively coupled plasma mass spectroscopy and atomic fluorescence spectrometry. Monte Carlo simulation was applied to evaluate potential health risks associated with Cd exposure. Cadmium was the critical pollutant in the study area, with the average value of 1.53 mg/kg. Moreover, both children and adults living in the area had a significant non-carcinogenic health risk. Additional health risk assessments revealed that diet was the main contributor for both children and adults among the four pathways (diet > soil ingestion > soil dermal adsorption > soil inhalation). Furthermore, our results revealed that leguminous vegetables and maize were ideal for this site due to their high Se and low Cd accumulation abilities. These findings provide support for adjusting planting structure by variety screening to mitigate the health risk induced by Cd.