Stable isotope fractionation of thallium as novel evidence for its geochemical transfer during lead-zinc smelting activities

Thallium (Tl) is a highly toxic trace metal. Lead (Pb)-zinc (Zn) smelting, which is a pillar industry in various countries, is regarded as one of the dominant anthropogenic sources of Tl contamination in the environment. In this study, thallium isotope data have been evaluated for raw material and a set of industrial wastes produced at different stages of Pb-Zn smelting in a representative large facility located by the North River, South China, in order to capture Tl isotope signatures of such typical anthropogenic origin for laying the foundation of tracking Tl pollution. Large variations in Tl isotopic compositions of raw Pb-Zn ores and solid smelting wastes produced along the process chain were observed. The epsilon 205Tl values of raw Pb-Zn ores and return fines are -0.87 +/- 0.26 and -1.0 +/- 0.17, respectively, contrasted by increasingly more negative values for electrostatic precipitator dust (epsilon 205Tl = -2.03 +/- 0.14), lime neutralizing slag (epsilon 205Tl = -2.36 +/- 0.18), and acid sludge (epsilon 205Tl = -4.62 +/- 0.76). The heaviest epsilon 205Tl (1.12 +/- 0.51) was found in clinker. These results show that isotopic fractionation occurs during the smelting processes. Obviously, the lighter Tl isotope is enriched in the vapor phase (-3.75 epsilon 205Tl units). Further XPS and STEM-EDS analyses show that Tl isotope fractionation conforms to the Rayleigh fractionation model, and adsorption of 205Tl onto hematite (Fe2O3) may play an important role in the enrichment of the heavier Tl isotope. The findings demonstrate that Tl isotope analysis is a robust tool to aid our understanding of Tl behavior in smelting processes and to provide a basis for source apportionment of Tl contaminations. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Thallium isotope analysis of industrial wastes in lead-zinc smelting processes shows significant variations in isotopic compositions, indicating isotopic fractionation occurs during smelting. Lighter thallium isotopes are enriched in the vapor phase, while heavier isotopes are adsorbed onto hematite. Thallium isotope analysis is proven to be a robust tool to aid understanding of thallium behavior in smelting processes.