Continuous Flow-Double Purge and Trap Method for Preconcentrating Mercury in Large Volumes of Seawater for Stable Isotope Analysis.

Mercury (Hg) isotopes provide a useful tool to understand Hg sources and processes in the environment. The Hg isotopic composition of seawater remains poorly constrained due to the lack of an efficient method to process large volumes of low-Hg-concentration seawater samples. Here, we develop a continuous flow-double purge and trap device for the in situ preconcentration of Hg in seawater. This method yielded a good Hg recovery of 91.7 ± 3.3% (n = 4, 1SD) for spiked seawater samples and gave reasonably similar Hg isotope ratios of NIST 8610, indicating a limited matrix effect and limited Hg isotope fractionation during processing of seawater. NIST 8610 δ202Hg (-0.55 ± 0.09‰, n = 4, 1SD) and Δ199Hg (0.07 ± 0.02‰, n = 4, 1SD) were similar to previously published data. The method was successfully applied to seawater collected from the Xiamen Bay and the South China Sea. The seawater samples showed a Hg recovery of 91.6 ± 5.4% (n = 12, 1SD). Seawater Δ199Hg (-0.04 ± 0.05‰, n = 7, 1SD) in the Xiamen Bay was different from seawater Δ199Hg (0.05 ± 0.07‰, n = 5, 1SD) in the South China Sea, which implies distinct Hg sources to coastal and open ocean areas and highlights the robustness of our method in understanding the Hg isotopic composition of seawater.

The Hg behaviors in mangrove ecosystems revealed by Hg stable isotopes: a case study of Maowei mangrove.

As one of the most productive marine ecosystems in the tropics and subtropics, mangroves are an important part of the global mercury (Hg) cycling. The environmental processes and effects of Hg in mangroves are complex and affect human Hg exposure, and it is crucial to understand Hg behaviors in the mangrove ecosystem. However, clarifying Hg behaviors in the mangrove ecosystem remains difficult because of an insufficient understanding of the dominant pathways. In this study, measurements of mercury (Hg) concentration and isotope ratios in sediment and plant tissues from a mangrove wetland were used to investigate Hg isotope fractionation in mangrove plants and sediments. Spatial patterns in Hg concentration and isotope signatures indicate that Hg re-emission in the sediment was suppressed by mangrove plants. The ratio of Δ199Hg/Δ201Hg was 0.93 for all sediments, indicating that Hg mass-independent fractionation in the mangrove ecosystem was primarily affected by photoreduction, while the ratios of Δ199Hg/Δ201Hg and Δ199Hg/δ202Hg for plant tissues suggested that natural organic matter reduction of Hg(II) was occurred in the plants. The distinct positive Δ199Hg values found in mangrove plants were supposed to be the results of the unique physiological characteristics of mangroves. The exterior Hg sources from atmosphere and seawater emphasize the role of mangrove ecosystems in the global Hg biogeochemistry. Our study highlights the distinct Hg isotope signatures in the mangrove from that in forests and indicates unique Hg behaviors in the mangrove ecosystem.

Distribution of mercury isotope signatures in Yundang Lagoon, Xiamen, China, after long-term interventions.

Isotope signatures of mercury (Hg) were determined for Hg fractions in seawater, sediments, porewaters, core sediments and fish from the Yundang Lagoon, Xiamen, China. Sequential extraction was used to extract Hg fractions in sediments and the purge-trap method was used to preconcentrate Hg in seawater. A large variation in mass dependent fractionation (δ202Hg: -2.50‰ to -0.36‰) was observed in the lagoon. Seawater and fish samples showed positive mass-independent fractionation (Δ199Hg: -0.06‰-0.45‰), while most of sediment and porewater samples displayed insignificant mass-independent fractionation (Δ199Hg: -0.10‰-0.07‰). Ancillary parameters (total organic carbon, sulfide, pH, Eh, water content and grain size) were also measured in the sediments to investigate correlations with Hg isotopes. Three sources (domestic sewage, sediments and atmospheric deposition) were identified as the main sources of Hg in the lagoon seawater. Photochemical reaction was the main process causing isotope fractionation in seawater. Through Hg partitioning and deposition, light isotopes were enriched from dissolved Hg to particulate Hg, then to sediments, and then to porewaters. Finally, Hg isotope signatures were used to identify the Hg sources and fractionation processes in core sediments from different depths. Our results demonstrate that Hg isotopes are powerful tools for tracing Hg sources and arriving at a better understanding of Hg biogeochemical cycling in the lagoon after long-term interventions.