Thallium isotopic fractionation in industrial process of pyrite smelting and environmental implications

•Tl isotopic fractionation was firstly observed in industrial pyrite smelting process.•Large Tl isotopic variations were detected during the smelting process.•Tl isotope fractionation may be due to Rayleigh fractionation and adsorption process.•Tl isotopic compositions provide a new way to trace Tl pollution process. Thallium (Tl) is typical rare element with severe toxicity comparable to Hg and Pb. To track Tl pollution, isotopic fractionation of Tl was evaluated during pyrite smelting for sulfuric acid production. Large variations in Tl isotope compositions were observed among the pyrite ore (PO) and its four different smelting wastes. The starting raw PO had an ε205Tl value of +1.28. The fluidized-bed furnace slag generated by high-temperature smelting had the heaviest ε205Tl (+16.24) in the system. Meanwhile, the boiler fly ash (ε205Tl = +8.34), cyclone fly ash (ε205Tl = +2.17), and electrostatic precipitation fly ash (ε205Tl = –1.10), with decreasing grain sizes during the treatment processes, were characterized by elevated levels of Tl contents and substantial enrichment in the light Tl isotopes relative to the furnace slag. Further calculation and high-resolution transmission electron microscopy indicated that Tl isotope fractionation could be governed by both Rayleigh-type fractionation and adsorption of volatilized Tl by particles of various grain sizes. According to the substantial differences in the PO from its smelting wastes and the measurement precision of isotopic fractionation, it is suggested that Tl isotopes can serve as a new tool for tracing pollution of Tl.