Thallium retention in sulfide-rich mine wastes associated with jarosite formation

Thallium (Tl) is an emerging contaminant of concern in mining-impacted environments because of its high toxicity and mobility, yet the mineralogical processes controlling its retention during mine-waste weathering remain poorly constrained. The sulfide-rich mine wastes investigated here contain up to 200 mg kg^-1 Tl and abundant secondary jarosite-group minerals. We combined synchrotron-based µXRF, µXRD, and Tl L₃-edge XANES analyses to determine Tl distribution, speciation, and mode of occurrence. Elemental mapping shows that Tl is concentrated in discrete particles (∼70-110 µm) rather than being diffusely distributed throughout the mine waste. Tl L₃-edge XANES analyses indicate that Tl occurs predominantly as Tl(I) (95-100% according to LCF results), with no detectable contributions from Tl(III) species. Linear combination fitting and cluster-based analysis, supported by µXRF and µXRD data, reveals systematic compositional zoning from Tl-rich jarosite cores toward Tl-poorer jarosite rims. Lattice expansion observed by µXRD, together with the spatial association of Tl with As, Se, and Ni, is consistent with possible trace-element co-hosting and/or structural incorporation within Tl-bearing jarosite particles. Overall, our results demonstrate that Tl is retained predominantly through structural incorporation into jarosite-type minerals and highlight the importance of these phases as secondary Tl hosts in sulfide-rich mine wastes.