Solid-phase distribution and mobility of thallium in mining-metallurgical residues: Environmental hazard implications

Thallium (Tl) and its compounds are non-essential and highly toxic for living organisms, even at low concentrations. In this paper, we analyzed the presence and geochemical distribution of Tl in different mining-metallurgical and sediment samples collected from several mining zones of Mexico. A modified BCR sequential extraction procedure was also applied to the samples to investigate the geochemical behavior and potential environmental risk of Tl according to types of ore deposit and mineral processing method applied. Results revealed the presence of Tl in the majority of the mining-metallurgical samples, with labile concentrations reaching up to values of 184.4 mg kg^-1, well above the environmental standards. A comparison of Tl partitioning in different samples showed that Tl was usually found associated with labile fractions instead of entrapped in the environmentally-passive residual fraction. Specifically, high levels of Tl were extracted from the exchangeable/acid-extractable and poorly-crystalline reducible fractions, suggesting its association with both soluble and amorphous Fe-Mn oxyhydroxides, respectively. Besides, Tl was also frequently found associated with the crystalline reducible fraction, presumably bonded to manganese oxides and jarosite-like minerals. Lastly, little amounts of Tl were extracted from the oxidizable fraction. Considering the fractionation of Tl in these mining-metallurgical samples, they may pose a significant environmental hazard. This study provides useful insights into the potential sources of Tl pollution in Mexico and emphasizes the need for further research to determine the extent of its impact and to develop effective remediation protocols to protect the environment from Tl toxicity.