Demystifying the interfacial aquatic geochemistry of thallium(I): New and old data reveal just a regular cation

Abstract

In aquatic systems the group 13 element Tl can be both highly mobile and highly toxic. Despite these properties there are inadequate studies regarding controls on thallium's solubility and significant discrepancies exist particularly relating to the affinity of Tl+ for iron oxide surfaces. It is unclear whether these discrepancies relate to other enigmatic aspects of the geochemistry of Tl+, which has both lithophilic and chalcophilic character. The solubility of many trace cations is limited by sorption to iron oxides and an important principle in aquatic geochemistry is that cation affinity for iron oxides is strongly related to the cation's hydrolysis constant (logKMOH). The Tl+ cation has a low logKMOH of 10−13.2 and is expected to sorb weakly, hence the high mobility of Tl+, but reported sorption constants for Tl+ sorption by ferrihydrite vary by up to 4 orders of magnitude. In this study we have measured Tl+ sorption to ferrihydrite over a wide range of conditions and also re-examined the few previous studies to provide a consistent understanding and description of Tl+ sorption using the Diffuse Layer Model. In terms of both the kinetics and thermodynamics of Tl+ sorption, it was found that Tl+ behaved exactly as expected for a cation with a logKMOH of 10−13.2. This means that in general Tl+ sorption will only be appreciable at alkaline pH and is consistent with the observed high mobility of Tl+ in aquatic environments.