Thallium-rich rust scales in drinkable water distribution systems: A case study from northern Tuscany, Italy

Following the detection of a severe thallium contamination of the drinkable water from the public distribution system of Valdicastello Carducci-Pietrasanta (northern Tuscany, Italy), and the identification of the source of contamination in the Molini di Sant'Anna spring (average Tl content c approximate to 15 mu g L-1), the replacement of the contaminated water with a virtually Tl-free one (Tl < 0.10 mu g L-1) caused an increase in TI concentration in the drinkable water. This suggested that the pipeline interior had become a secondary source of Tl contamination, promoting its mineralogical and geochemical study. Rust scales samples taken from several pipeline segments, as well as leaching products obtained from these samples, were investigated through scanning electron microscopy, Xray fluorescence chemical analyses, inductively coupled plasma - mass spectrometry, X-ray diffraction, and Xray absorption spectroscopy. Thallium-rich rust scales (up to 53 wt% Tl) have been found only in pipeline samples taken downstream the water treatment plant, whereas the sample taken upstream contains much less Tl (similar to 90 mu g g(-1)). The Tl-rich nature of such scales is related to the occurrence of nano-and micro-spherules of TI2O3 and less abundant nanocrystalline pm-sized encrustaTlons of TlCI. Leaching experiments on Tl-rich rust scales indicate that a fracTlon of the available Tl is easily dissolved in tap water; X-ray absorpTlon spectroscopy suggests that monovalent thallium occurs in water equilibrated with the rust scales, probably related to the dissoluTlon of TlCI encrustaTlons. Therefore, Tl dissolved as Tl+ only in the water from the Molini di Sant'Anna spring was parTlally removed through oxidaTlve precipitaTlon of Tl2O3 and precipitaTlon of TlCI. This highlights the criTlcal role played by the addiTlon of chlorine-based oxidants in water treatment plants that could favour the deposiTlon of Tl-rich coaTlngs within the pipelines, giving rise to unexpected secondary sources of contaminaTlon. (C) 2017 Elsevier B.V. All rights reserved.