Development of a new diffusive gradient in the thin film (DGT) method for the simultaneous measurement of CH3Hg+ and Hg2+

Abstract

In this study, a Tulsion® CH-95 binding gel-based diffusive gradient in the thin film (TCH DGT) method was developed for the simultaneous measurement of CH₃Hg⁺ and Hg²⁺. The binding kinetics of CH₃Hg⁺ and Hg²⁺ in a mixed solution indicated rapid uptake to the TCH gel. The Hg²⁺ exhibited slower absorption rates compared to the CH₃Hg⁺, which did not affect the DGT uptake overall. Stable elution efficiencies were obtained for CH₃Hg⁺ and Hg²⁺ using a mild reagent containing 0.1 M HCL and 2% thiourea. The diffusion coefficients of CH₃Hg⁺ and Hg²⁺ in agarose gel were calculated on the basis of the time-series deployment method and were 9.94 (×10⁻⁶ cm² s⁻¹, 25 °C) and 7.65 (×10⁻⁶ cm² s⁻¹, 25 °C), respectively. The DGT uptakes were independent of pH for CH₃Hg⁺ (pH 4.10–8.10) and Hg²⁺ (pH 4.10–10.03), with no effect of the ionic strengths between 0.1–1000 mM NaCl. The capacity of TCH DGT for CH₃Hg⁺ and Hg²⁺ according to linear and theoretical responses were calculated to be 33.97 μg cm⁻² and 30.34 μg cm⁻², respectively. The performance of this technique was further tested in sediment. Two distinct peaks of CH₃Hg⁺ and Hg²⁺ appeared at similar sediment depths, reflecting that the localized remobilization of CH₃Hg⁺ and Hg²⁺ may occur through similar mechanisms. This confirms that the TCH DGT is a useful in situ monitoring tool.