Thallium speciation and transformations: pathways, fates and risks

Thallium (Tl) is highly toxic, and its environmental behavior is governed by speciation and interfacial redox. In most circumneutral waters, dissolved Tl remains near the aqueous reference state (Tl(I)), whereas oxidized Tl is expressed mainly as solid-associated Tl(III) on redox-active mineral surfaces, most prominently vacancy-rich Mn oxides. Ligands (halides, carbonate, and DOM) tune free-ion activity without overturning the Tl(I) reference pattern, while Fe/Mn oxides, clays, and sulfide phases partition Tl between mobile and retained stocks. Biological processes reshape these outcomes by generating reactive minerals, providing reductants/oxidants, and supplying biosorptive matrices, leading to strong biotic-abiotic coupling. This review links static speciation, interfacial pathways, and moderators (pH, site density/coverage, K⁺/NH₄⁺, ionic strength/halides, DOM, and prior redox state) to explain when Tl is mobile versus sequestered. We highlight priorities for in situ, species-resolved measurements, kinetic constraints under realistic chemistries, and scalable interface representations for risk appraisal and predictive modeling.