Thallium Long-Term Fate from Rock-Deposit to Soil: The Jas Roux Sulfosalt Natural Analogue

Inorganic contaminant release resulting from mining activities can impact surrounding ecosystems. Ores formed by primary sulfide minerals produce sulfuric acid after mineral oxidation, which is the driving force of metal release. Yet secondary metal sulfates may form and play a crucial role in controlling the metal fate. In the case of thallium (Tl), it has been shown that in natural Tl-rich sulfide deposits and those found in mining areas, Tl can be trapped by Tl-jarosite (Tl-rich iron sulfate) and dorallcharite (TlFe₃(SO₄)₂(OH)₆). Our Tl speciation characterization results have generated novel insight into the long-term behavior of this metal derived from a unique natural hotspot: the Jas Roux site (France). The biogeochemical cycle of the soil ecosystems of Jas Roux dates back almost 15000 years ago and has now reached a steady state. A chemical gradient was found in soils across the toposequence underlying the Jas Roux outcrop. X-ray absorption spectroscopy revealed that Tl was mainly present in secondary minerals at the top of the studied zone. Oxidative dissolution of Tl-rich sulfide minerals and pyrite accounts for the presence of Tl-jarosite in soils, either by direct formation in soils or by gravity erosion from the outcrop. The Tl-jarosite quantity was found to decrease from the top to the bottom of the toposequence, probably due to sulfate leaching. Released Tl likely adsorbed on phyllosilicates such as Illite or muscovite, and a fraction of Tl was found to have oxidized into Tl(III) along the toposequence.