Thallium contamination, health risk assessment and source apportionment in common vegetables

As an element with well-known toxicity, excessive thallium (Tl) in farmland soils may threaten food security and induce extreme risks to human health. Identification of key contamination sources is a prerequisite for remediation technologies. This study aims to examine the contamination level, health risks and source apportionment of Tl in common vegetables from typical farmlands distributed over a densely populated residential area in a pyrite mine city, which has been exploiting Tl-bearing pyrite minerals for over 50 years. Results showed excessive Tl levels in most of the vegetables (0.16-20.33 mg/kg) and alarming health risks induced by the vegetables via the food chain. Source apportionment of Tl contamination in vegetables was then evaluated by using the Pb isotope fingerprinting technique. Both vegetables and soils were characterized by overall low ²⁰⁶Pb/²⁰⁷Pb ratios. This indicated that a significant contribution may be ascribed to anthropogenic activities for pyrite deposit exploitation, whose raw material and slags were featured with lower ²⁰⁶Pb/²⁰⁷Pb ratios. Further calculations by the binary mixing model suggested that pyrite mining and smelting activities contributed 54-88% to the thallium contamination in vegetables. The results highlighted that Pb isotope tracing is a suitable technique for source apportionment of Tl contamination in vegetables and that contamination from pyrite mining/smelting activities urges authorities to initiate proper practices of remediation.