In Situ Measurement of Thallium in Natural Waters by a Technique Based on Diffusive Gradients in Thin Films Containing a δ-MnO2 Gel Layer

Thallium (Tl) has been identified as a priority contaminant because of its severe toxic effects. Exact measurement of Tl is a challenge because it is difficult to avoid altering the element’s chemical speciation during sampling, transport, and storage. In situ measurement may be a good choice. Based on the in situ technique of diffusive gradients in thin films (DGT), new DGT devices equipped with novel laboratory-synthesized manganese oxide (δ-MnO 2 ) binding gels were developed and systematically validated for the measurement of Tl, including Tl(I) and Tl(III) species, in water. Comparison between Chelex binding gel and δ-MnO 2 gel on the uptake kinetics of Tl demonstrated that δ-MnO 2 binding gels could adsorb Tl rapidly and effectively. Removal of Tl from the δ-MnO 2 gels was achieved by use of 1 mol·L –1 oxalic acid, yielding elution efficiencies of 1.0 for Tl(I) and 0.86 for TI(III). Theoretical responses from DGT devices loaded with δ-MnO 2 gel (δ-MnO 2 -DGT) were obtained irrespective of pH (4–9) and ionic strength (0.1–200 mmol·L –1 NaNO 3 ). δ-MnO 2 -DGT showed good potential for long-term monitoring of Tl due to its high adsorption capacity of 27.1 μg·cm –2 and the stable performance of δ-MnO 2 binding gel kept in solution, containing only 10 mmol·L –1 NaNO 3, for at least 117 days. Field deployment trials confirmed that δ-MnO 2 -DGT can accurately measure the time-averaged concentrations of Tl in fluvial watercourses. In summary, the newly developed δ-MnO 2 -DGT technique shows potential for environmental monitoring and biogeochemical investigation of Tl in waters.