In Situ Measurement of Thallium in Natural Waters by a Technique Based on Diffusive Gradients in Thin Films Containing a δ-MnO 2 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.