Novel methodology for the study of mercury methylation and reduction in sediments and water using ¹⁹⁷Hg radiotracer

Abstract

Mercury tracers are powerful tools that can be used to study mercury transformations in environmental systems, particularly mercury methylation, demethylation and reduction in sediments and water. However, mercury transformation studies using tracers can be subject to error, especially when used to assess methylation potential. The organic mercury extracted can be as low as 0.01% of the endogenous labeled mercury, and artefacts and contamination present during methylmercury (MeHg) extraction processes can cause interference. Solvent extraction methods based on the use of either KBr/H₂SO₄ or HCl were evaluated in freshwater sediments using ¹⁹⁷Hg radiotracer. Values obtained for the ¹⁹⁷Hg tracer in the organic phase were up to 25-fold higher when HCl was used, which is due to the coextraction of ¹⁹⁷Hg²⁺ into the organic phase during MeHg extraction. Evaluations of the production of MeHg gave similar results with both MeHg extraction procedures, but due to the higher Hg²⁺ contamination of the controls, the uncertainty in the determination was higher when HCl was used. The Hg²⁺ contamination of controls in the HCl extraction method showed a nonlinear correlation with the humic acid content of sediment pore water. Therefore, use of the KBr/H₂SO₄ method is recommended, since it is free from these interferences. ¹⁹⁷Hg radiotracer (T _1/2 = 2.673 d) has a production rate that is about 50 times higher than that of ²⁰³Hg (T _1/2 = 46.595 d), the most frequently used mercury radiotracer. Hence it is possible to obtain a similar level of performance to ²⁰³Hg when it is used it in short-term experiments and produced by the irradiation of ¹⁹⁶Hg with thermal neutrons, using mercury targets with the natural isotopic composition. However, if the 0.15% natural abundance of the ¹⁹⁶Hg isotope is increased, the specific activity of the ¹⁹⁷Hg tracer can be significantly improved. In the present work, ¹⁹⁷Hg tracer was produced from mercury 51.58% enriched in the ¹⁹⁶Hg isotope, and a 340-fold increase in specific activity with respect to natural mercury targets was obtained. When this high specific activity tracer is employed, mercury methylation and reduction experiments with minimum mercury additions are feasible. Tracer recovery in methylation experiments (associated with Me¹⁹⁷Hg production from ¹⁹⁷Hg²⁺ spike, but also with Hg²⁺ contamination and Me¹⁹⁷Hg artefacts) with marine sediments was about 0.005% g⁻¹ WS (WS: wet sediment) after 20 h incubation with mercury additions of 0.05 ng g⁻¹ WS, which is far below natural mercury levels. In this case, the amount of Hg²⁺ reduced to Hg⁰ (expressed as the percent ¹⁹⁷Hg⁰ recovered with respect to the ¹⁹⁷Hg²⁺ added) varied from 0.13 to 1.6% g⁻¹ WS. Me¹⁹⁷Hg production from ¹⁹⁷Hg²⁺ spike after 20 h of incubation of freshwater sediment ranged from 0.02 to 0.13% g⁻¹ WS with mercury additions of 2.5 ng g⁻¹ WS, which is also far below natural levels. ¹⁹⁷Hg⁰ recoveries were low, 0.0058 ± 0.0013% g⁻¹ WS, but showed good reproducibility in five replicates. Me¹⁹⁷Hg production from ¹⁹⁷Hg²⁺ spiked in freshwater samples ranged from 0.1 to 0.3% over a period of three days with mercury additions of 10 ng L⁻¹. A detection limit of 0.05% for Me¹⁹⁷Hg production from ¹⁹⁷Hg²⁺ spike was obtained in seawater in a 25 h incubation experiment with mercury additions of 12 ng L⁻¹.