Mercury Isotope Fractionation during the Exchange of Hg(0) between the Atmosphere and Land Surfaces: Implications for Hg(0) Exchange Processes and Controls

Abstract

Atmosphere–surface exchange of elemental mercury (Hg(0)) is a vital component in global Hg cycling; however, Hg isotope fractionation remains largely unknown. Here, we report Hg isotope fractionation during air–surface exchange from terrestrial surfaces at sites of background (two) and urban (two) character and at five sites contaminated by Hg mining. Atmospheric Hg(0) deposition to soils followed kinetic isotope fractionation with a mass-dependent (MDF) enrichment factor of −4.32‰, and negligible mass-independent fractionation (MIF). Net Hg(0) emission generated average MDF enrichment factors (ε202Hg) of −0.91, −0.59, 1.64, and −0.42‰ and average MIF enrichment factors (E 199Hg) of 0.07, −0.20, −0.14, and 0.21‰ for urban, background, and Hg mining soils and cinnabar tailing, respectively. Positive correlations between ε202Hg and ambient Hg(0) concentration indicate that the co-occurring Hg(0) deposition (accounting for 10–39%) in a regime of net soil emission grows with ambient Hg(0). The MIF of Hg(0) emission from soils (E 199Hg range −0.27 to 0.14‰, n = 8) appears to be overall controlled by the photochemical reduction of kinetically constrained Hg­(II) bonded to O ligands in background soils, while S ligands may have been more important in Hg mining area soils. In contrast, the small positive MIF of Hg(0) emission from cinnabar ore tailing (mean E 199Hg = 0.21‰) was likely controlled by abiotic nonphotochemical reduction and liquid Hg(0) evaporation. This research provides critical observational constraints on understanding the Hg(0) isotope signatures released from and deposited to terrestrial surfaces and highlight stable Hg isotopes as a powerful tool for resolving atmosphere–surface exchange processes.

Post-depositional Hg transformation processes in an open boreal peatland constrained by Hg stable isotopes

“…Light-driven Hg II reduction in systems with DOC is several orders of magnitude faster than dark reduction in the presence of DOC 30,68 . Our suggestion of photoreduction of rainwater Hg II is in line with isotopic data from agricultural soils 69 , in which O/N functional groups were suggested to be involved in the initial complexation of Hg II after deposition. As a consequence of the weaker Hg-O/N bond, rates of Hg II reduction are greater than when Hg II forms complexes with the chemically much stronger bonding thiol functional groups 64,68 .…”

“…The dominance of net plant uptake of atmospheric Hg 0 over net rainfall Hg II deposition to the vegetated surface of peatlands might be partially due to Hg II in precipitation being more easily available for photoreduction and subsequent evasion back to the atmosphere than the Hg II associated with plant tissues. The relative importance of photoreduction of Hg II in rainfall can be explained by the abundance of organic R-O/N functional groups at peat and vegetation surfaces providing a weaker bond to Hg II to compete with the reduction process, during the time window required for Hg II to re-arrange to stronger bonding RS functional groups 30,64,69 . Once bound to the stronger RS functional groups, Hg II is less susceptible to reduction.…”

Mercury Isotope Variations in Lake Sediment Cores in Response to Direct Mercury Emissions from Non-Ferrous Metal Smelters and Legacy Mercury Remobilization

“…We suggest that Hg in recent sediments of near-field lakes was primarily derived from tailing fugitive dust, which is expected to have comparable Hg isotope compositions as fed ores (i.e., no significant Hg isotope fractionation during mechanical separation). , We attribute the negative shifts in δ 202 Hg values in recent sediments from the middle- and far-field lakes to increasing contributions from the re-emission of legacy Hg from tailing deposits and contaminated soils. Re-emission of Hg involves reduction and volatilization processes, which are expected to enrich light Hg isotopes in the emitted gaseous elemental Hg(0). − , …”

“…Re-emission of Hg involves reduction and volatilization processes, which are expected to enrich light Hg isotopes in the emitted gaseous elemental Hg(0). [28][29][30]46 Uptake of atmospheric Hg(0) by vegetation further enriches light Hg isotopes in vegetation, which are then transferred to soils. 47−50 The soils and vegetation in lake catchments have been reported to mainly exhibit a significant enrichment of light Hg isotopes.…”

Changes in Atmospheric Gaseous Elemental Mercury Concentrations and Isotopic Compositions at Mt. Changbai During 2015–2021 and Mt. Ailao During 2017–2021 in China

“…Wang et al, 2016). Isotopic compositions of GEM net emissions from terrestrial surfaces were recently investigated, which showed lower δ 202 Hg (median: −1.13‰, IQR: −2.00-−0.47‰) and Δ 199 Hg (median: −0.13‰, IQR: −0.26-−0.02‰), and similar Δ 200 Hg (median: 0.02‰, IQR: −0.01-0.03‰) as compared with those of anthropogenic emissions (Figure 5; Table S3 in Supporting Information S1) (Yuan et al, 2019(Yuan et al, , 2021Zhu et al, 2022). Thus, decreasing surface emissions could also lead to decreases in GEM concentrations and Δ 200 Hg and increases in δ 202 Hg values, but would cause insignificant Δ 199 Hg shift because the GEM Δ 199 Hg at MCB and MAL in earlier years were close to that emitted from land surfaces.…”