• Thallium
• speciation
• HPLC ICP MS
• Acid mine drainage

The analysis of chemical speciation has an important role in environmental protection since availability, fate and toxicity of a number of potentially toxic elements depend on their chemical form. In particular, thallium has been identified as an environmental significant element, being more toxic to humans than many heavy metals including Hg, Pb and Cd. Thallium can occur in the aqueous phase and sediments in the state of oxidation Tl(I) and Tl(III). Tl(I) is the more thermodynamically stable species under the typical conditions for natural waters. Italian Regulations pose a concentration limit of 2 µg/l for total Tl in groundwater, and do not distinguish among the different Tl species, although some studies highlight a higher toxicity for Tl(III). Thallium concentrations limits are 1 mg/kg and 10 mg/kg (on a dry basis) in soils for residential and industrial use, respectively.
This Thesis is focused on the study of occurrence and oxidation state of thallium in waters and sediments sampled in a watershed impacted by past mining, characterized by high thallium concentrations in orebodies.
Thallium concentration and speciation were determined at the Hydro Sciences Laboratory of CNRS of Montpellier (France) by High Pressure Liquid Chromatography coupled with Inductively Coupled Plasma Mass Spectrometry (HPLC-ICP-MS). In addition, major ion and trace elements analysis on waters samples were obtained by Ion Chromatography (IC) and ICP-MS at the Department of Earth Sciences of the University of Pisa. Thallium speciation analysis and some trace element abundance were also done at the Institute for Chemistry of OrganoMetallic Compounds (CNR) of Pisa, for a comparison. The results indicate a Tl concentration in the range between 375 and 923 μg/l in acid mine drainages, 9.11-157 μg/l in spring and surface waters, and 58.6-570 μg/g in sediments at the drainages outflow and stream. It was found that in waters thallium occurs mainly in the monovalent form Tl(I), even if in some cases a significant amount of Tl(III) was detected. In particular, Tl(I) characterizes acid mine drainages, while Tl(III) with a maximum concentration of 91.7 μg/l (up to the 40% of the total amount) has been measured in surface waters in two different stations along the stream course. This observation poses the question of the possible mechanisms for Tl(I) oxidation processes in these settings. This point requires additional investigations. Satisfactory results for Tl speciation in sediments were not obtained, since only a small fraction of the total Tl was actually extracted by both DTPA and H3PO4 for speciation analysis. However, despite not quantitative, speciation results seem to indicate the occurrence of Tl(I) and Tl(III) in different amounts in sediments. Possible oxidation reactions involve the role of manganese oxides.
Finally, the role of changing the oxygenation state of waters on thallium speciation was preliminary investigated by microcosm experiments on samples consisting of sediment and water. The results indicate that, in both oxic and anoxic conditions, Tl(I) and Tl(III) concentrations increased as a function of incubation time, although Tl(III) presents a slight increase. The observed increase of Tl(I) and Tl(III) concentrations probably indicates the migration of species from sediment to the aqueous phase.