Mercury speciation, bioavailability and risk assessment on soil–rice systems from a watershed impacted by abandoned Hg mine-waste tailings

Mercury (Hg) is a global pollutant and can be accumulated in the food chain, posing exposure risks to humans. In this study, rice plants and corresponding rhizosphere soil samples were collected from a watershed of the Wawu River Basin that is heavily impacted by historic Hg mining and retorting activities. Total mercury (THg) and methylmercury (MeHg) in rice grains, as well as other tissues and soil samples, were measured. Five soil Hg fractions, as well as soil parameters, were also determined. The results show that the average concentrations of THg and MeHg in rice grains were 14 ± 7.0 μg kg−1 and 7.2 ± 4.0 μg kg−1. Soil organic-bound (Hg-o) and strong complex-bound (Hg-s) were the main Hg fractions, accounting for 44% of the total. To estimate the Hg–ligand interaction in the soils, soil -N/Hg (R = 0.451, p < 0.05), -S/Hg (R = 0.372, p < 0.1), and -OM/Hg ratio (R = 0.320, p < 0.5) with MeHgsoil were observed with significant positive correlations, indicating that the formation of Hg–OM, Hg–N–OM or Hg–S–OM complexes could prevent Hg(II) from methylation in soils. The significant positive correlations of the -N/Hg ratio, -S/Hg ratio and -OM/Hg ratio with MeHg in rice tissues suggested that Hg methylation and MeHg demethylation occurred throughout the rice paddy ecosystem. The estimated MeHg daily intake (EDI) was 0.075 ± 0.041 μg kg−1 bw d−1 and was lower than the RfD level of 0.1 μg kg−1 bw d−1 recommended by the US EPA. However, approximately 29% of the hazardous index (HI) of MeHg in grain exceeded 1, posing a potential threat to local populations, particularly pregnant women and children.