Persistent thallium enrichment and its high ecological risks developed from historical carbonaceous Hg-Tl mining waste.

Thallium (Tl) is a priority pollutant with high biotoxicity and has been of great concern worldwide in recent years. The former Lanmuchang Hg-Tl mining site in southwest China is a hotspot of multiple metal(loid)s pollution that previously caused large-scale chronic Tl poisoning, mainly resulting from carbonaceous Tl-bearing mining waste. However, arable land destroyed by historical mining wastes persists at high ecological risks decades after reclamation, but little is known about the solid phase partitioning and species of Tl during soil formation of underlying mining wastes as potential Tl sources. In this study, a representative reclaimed soil profile (100 cm depth) was selected in the lowlands to explore the geochemical cycling and environmental fate of Tl in mining waste-derived subsoil. The Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analysis revealed an unexpected enrichment of Mn (2920-7250 mg/kg) and Tl (205-769 mg/kg) in the mining waste-derived subsoil. Results from BCR sequential extraction, X-ray Photoelectron Spectroscopy (XPS), and Electron Probe Microanalyses (EPMA) indicate that high Tl loading Mn oxide particulates (up to 15,712 ppm Tl) dominate the sequestration of Tl in the subsoil via oxidation-complexation and have a high potential for migration to both topsoil and groundwater. In addition, insights from microbial fossils and Fe-metabolizing bacteria closely related to Tl indicated that Fe (hydr)oxide particulates showing high Tl levels (up to 3865 mg/kg) point to biomineralization. Detailed mineralogical investigations revealed that hematite-siderite syngenetic particulates could serve as a promising mineralogical proxy for redox oscillations under periodic flooding and recorded the frequent groundwater level fluctuations experienced in the probed profile. Despite the potential for long-term preservation of high Tl loading Fe/Mn (hydr)oxides under HCO-rich groundwater conditions in karst areas, the reductive release of Tl will be inevitable during flooding, implying that underlying carbonaceous mining waste will pose persistent and severe hazards to the ecosystem.