Thallium contamination in farmlands and common vegetables in a pyrite mining city and potential health risks

Thallium (Tl) is a trace metal of severe toxicity. Its health concerns via consumption of contaminated vegetables have often been overlooked or underestimated. This study was designed to gain insight into the actual level and distribution characteristics of Tl and metal (loid)s (Pb, Cd, Cr, Sb, Mn, Cu, Zn, Ni, and Co) in agricultural soils and common vegetables cultivated in different zones (upstream, midstream, and downstream) of a densely populated residential area in a typical mine city, which has been open-pit exploiting Tl-bearing pyrite minerals since 1960s. The results show that most of the agricultural soils exhibit contaminated levels of Tl, with Tl contents (upstream: 1.35–4.31 mg/kg, midstream: 2.43–5.19 mg/kg, and downstream: 0.65–2.33 mg/kg) mostly exceeding the maximum permissible level (MPL) for agricultural land use (1 mg/kg). Sequential extraction procedure indicates that even Tl is predominantly retained in the residual fraction, significant levels of Tl are still present in the geochemically mobile fractions. Besides, metals like Cu, Cd, Mn, and Co are mostly distributed in the labile fractions. Almost all metal (loid)s in edible parts of the vegetables exceed their corresponding MPL for consumption. The chronic daily intake (CDI) and hazard quotient (HQ) values calculated for inhabitants at different ages indicate non-negligible Tl risks via consumption of local vegetables, especially for children. Therefore, it is critical to establish effective measures for hazardous waste management and enforceable regulations in Tl-polluted area to mitigate potential severe impacts of Tl on human health through food chain.