Determination of the Isotopic Composition of Aqueous Mercury in a Paddy Ecosystem Using Diffusive Gradients in Thin Films

Abstract

Measuring the isotopic composition of Hg in natural waters is challenging due to the ultratrace level of aqueous Hg (ng L–1). At least 5 ng of Hg mass is required for Hg isotopic analysis. Given the low Hg concentration in natural waters, a large volume of water (>10 L) is typically needed. The conventional grab sampling method is time-consuming, laborious, and prone to contamination during transportation and preconcentration steps. In this study, a DGT (diffusive gradients in thin films) method based on aminopropyl and mercaptopropyl bi-functionalized SBA-15 nanoparticles was developed and extended to determine the concentration and isotopic composition of aqueous Hg for the first time. The results of laboratory analysis showed that Hg adsorption by DGT induces ∼ −0.2‰ mass-dependent fractionation (MDF) and little mass-independent fractionation (MIF). The magnitude of MDF exhibits a dependence on the diffusion-layer thickness of DGT. Since Hg-MDF can occur in a broad range of environmental processes, monitoring the δ202Hg of aqueous Hg using the DGT method should be performed with caution. Field results show consistent MIF signatures (Δ199Hg) between the DGT and conventional grab sampling method. The developed DGT method serves as a passive sampling method that effectively characterizes the MIF of Hg in waters to understand the biogeochemical cycle of Hg at contaminated sites.

Using Mercury Stable Isotopes to Quantify Directional Soil–Atmosphere Hg(0) Exchanges in Rice Paddy Ecosystems: Implications for Hg(0) Emissions to the Atmosphere from Land Surfaces

“…The results showed that the Hg concentration in the overlying water before sampling in batches 2–8 was 6.3 ± 0.4 ng L –1 (1SD, n = 7), which was comparable ( p > 0.05) to the Hg concentration in the initial overlying water (6.9 ± 1.6 ng L –1 ). Similarly, the Hg isotopic composition (δ 202 Hg = −0.71 ± 0.14‰, Δ 199 Hg = 0.33 ± 0.03‰, 2SD, n = 7) of the overlying water before sampling in batches 2–8 was also not significantly ( p > 0.05) different from the Hg isotopic composition (δ 202 Hg = −0.67 ± 0.21‰, Δ 199 Hg = 0.29 ± 0.38‰) at the beginning. The isotopic signatures of the product Hg(0) during the in situ Hg(II) photoreduction and abiotic dark reduction processes are shown in Table S2.…”

“…The average Hg isotopic compositions of the overlying water (δ 202 Hg = −0.67 ± 0.21‰, Δ 199 Hg = 0.29 ± 0.38‰) in the investigated rice paddy during the same sampling periods were reported by Yin et al . The enrichment factor for microbial reduction (ε 202 Hg = −1.51 ± 0.24‰, E 199 Hg = 0) was determined in Section .…”

“…As shown in Figure S3, in situ photoreduction and abiotic dark reduction of Hg­(II) were performed in the field from 10th to 13th of July, 2023. To eliminate the effects of microorganisms, cells in the overlying water of the rice paddy were removed using a 0.22 μm filter. ,, The Hg concentration in the filtered overlying water was 6.9 ± 1.6 ng L –1 , and the average Hg isotopic composition was δ 202 Hg = −0.67 ± 0.21‰, Δ 199 Hg = 0.29 ± 0.38‰ (2SD) . Prior to the experiment, the filtered overlying water was divided into two precleaned polyethylene (PE) boxes (∼30 L) with the water level above the edge of the DFC to isolate ambient air.…”