Environmental context. The aqueous geochemistry of thallium is not well known in comparison with cadmium and lead, although it is more highly toxic, and at the same time has a wide range of industrial applications. A database allowing us to reliably predict the speciation and solubility of thallium in various environments in low temperature systems would be invaluable in providing some understanding of thallium’s mobilisation and mitigation. We propose here such a thermodynamic database based on critical reviews.

Abstract. Thallium is a highly toxic element, and at the same time it has a wide range of applications in industry. Therefore, it is important to know its speciation and solubility under low temperature conditions. This study expands the thermodynamic database of the first paper of this series on the aqueous geochemistry of thallium by providing the formation constants of some important thallium complexes, including TlEDTA^3–, TlOx^– (Ox: oxalate), TlSuc^– (Suc: succinate), TlMal^– (Mal: malonate) and TlHPO₄^–. This study also recommends the solubility product constant of TlCl(s) as 10^–3.65. The combined database allows us to model reliably the speciation and solubility of thallium in the Earth surface environments. As an example, the speciation and solubility of thallium in soil solutions are presented based on thermodynamic calculations.