Enrichment and Controlling Factors of Thallium in Carbonate Rock Weathering Profile in Southwestern Guizhou, China

<p indent="0mm"> Surface enrichment of thallium (Tl) in soil, due to weathering of carbonate rocks, poses a potential risk to human health through the food chain. This study analysized of major and trace elements, mineral composition, and the geochemical fractions of Tl in a carbonate rock weathering profile in southwestern Guizhou, aiming to preliminarily reveal the migration of thallium and controlling factors during the weathering process of carbonate rocks. The research findings indicate that: (1) Carbonate rock weathered soil exhibits good geochemical inheritance with the underlying bedrock, demonstrating characteristics of in-situ weathering of carbonate rocks. The weathering of carbonate rocks results in Tl enrichment in soil with contents in the bedrock and profile soil ranging from <sc>1.66 mg/kg</sc> to 42.5~74.40 mg/kg, respectively. Tl content in the profile investigated was significantly higher than the average background Tl content in Chinese soils <sc>(1 mg/kg).</sc> (2) During weathering, with the bedrock (calcite) dissolution, Tl in the acid-soluble phase was released as the carbonate minerals hydrolyze (τ_Tl,Zr= –0.73~–0.56). The residual acid-insoluble phase further weathered into soil, resulting in Tl enrichment in soils. (3) After the weathering of carbonate rocks, soil Tl primarily occurred in residual form (60.4%) and crystalline iron-manganese-bound form (29.2%). The residual Tl showed a significant negative correlation with K₂O and Rb (<italic>p</italic>=0.05, <italic>R</italic>²= –0.938 and –0.919, respectively), primarily occurring in K-containing minerals (illite and kaolinite). The crystalline iron-manganese-bound Tl correlated with TFe₂O₃ (<italic>p</italic>=0.05, <italic>R</italic>²=0.882), indicating a possible association with its adsorption to TFe₂O₃. Scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS) revealed that Tl and S had similar distribution patterns, being present on the surfaces of fine particles <sc>(~5 μm)</sc> uniformly dispersed with Fe and O. (4) The adsorption capacity of soil minerals and iron-manganese oxides for Tl is indirectly influenced by soil pH.