Increasing chloride concentration causes retention of mercury in melted Arctic snow due to changes in photoreduction kinetics

A ratiometric fluorescent probe with a large Stokes shift for rapid and sensitive detection of Hg2+ in environmental water samples and its applications in living cells and zebrafish

Detection of highly toxic mercury ions (Hg2+) in actual environmental and biological samples is of significant importance for protecting environment and human health. In this paper, a new ratiometric fluorescent probe BTIA was designed and synthesized from 3-pinone based on Internal Charge Transfer (ICT) mechanism. BTIA could selectively recognize Hg2+ over other competitive analytes with short reaction time (5 s), distinct ratiometric response, strong anti-interference ability, large Stokes shift (200 nm), and low detection limit (2.36 × 10-7 M). Furthermore, BTIA was applicable for detecting Hg2+ in actual water samples and it also performed an excellent imaging capability in living RAW264.7 cells, zebrafish and onion tissue.

Salinity and total suspended solids control mercury speciation in a tidal river: Comparisons with a photochemical mercury model

Daytime volatilization of gaseous elemental mercury (Hg(0)aq) is a significant mechanism for mercury removal from aquatic systems and potentially limits the production and bioaccumulation of methylmercury. Changes in incoming solar radiation (in the ultraviolet range), dissolved organic matter, salinity, and total suspended particles were investigated concurrently with several mercury species (Hg(0)aq, dissolved total mercury (THg), easily reducible mercury (ERM), and mercury associated with total suspended solids (THgTSS)) during daylight hours near the mouth of a hypertidal river. There were no predictable temporal changes observed for Hg(0)aq in unfiltered surface water. Hg(0)aq ranged from 0 to 12 pg L-1, THg ranged from 0 to 492 pg L-1, ERM ranged from 13 to 381 pg L-1, and THgTSS ranged from <1.58 ng g-1 to 261.32 ng g-1. The range of Hg(0)aq predicted by the empirical model was similar to measured ERM concentrations, but it was shown that ERM did not significantly predict in-situ photoreducible Hg(II) (Hg(II)RED). Production of Hg(0)aq appears to largely be suppressed by suspended solids, which limits ultraviolet radiation transmission through surface water. Comparison of these results to an empirical model developed for this site to predict Hg(0)aq indicates that significantly more mercury is available for photoreduction near the mouth of the tidal river, and that Hg(II) will likely photoreduce quickly when TSS levels decrease with ocean mixing.

Modeling the Mercury Cycle in the Sea Ice Environment: A Buffer between the Polar Atmosphere and Ocean

Sea ice (including overlying snow) is a dynamic interface between the atmosphere and the ocean, influencing the mercury (Hg) cycling in polar oceans. However, a large-scale and process-based model for the Hg cycle in the sea ice environment is lacking, hampering our understanding of regional Hg budget and critical processes. Here, we develop a comprehensive model for the Hg cycle at the ocean-sea ice-atmosphere interface with constraints from observational polar cryospheric data. We find that seasonal patterns of average total Hg (THg) in snow are governed by snow thermodynamics and deposition, peaking in springtime (Arctic: 5.9 ng/L; Antarctic: 5.3 ng/L) and minimizing during ice formation (Arctic: 1.0 ng/L, Antarctic: 0.5 ng/L). Arctic and Antarctic sea ice exhibited THg concentration peaks in summer (0.25 ng/L) and spring (0.28 ng/L), respectively, governed by different snow Hg transmission pathways. Antarctic snow-ice formation facilitates Hg transfer to sea ice during spring, while in the Arctic, snow Hg is primarily moved through snowmelt. Overall, first-year sea ice acts as a buffer, receiving atmospheric Hg during ice growth and releasing it to the ocean in summer, influencing polar atmospheric and seawater Hg concentrations. Our model can assess climate change effects on polar Hg cycles and evaluate the Minamata Convention's effectiveness for Arctic populations.

Mercury photoreduction and photooxidation kinetics in estuarine water: Effects of salinity and dissolved organic matter

Net photoreduction of divalent mercury (Hg(II)) and volatilization of photoreduction products (i.e., elemental mercury (Hg(0))/dissolved gaseous mercury (DGM)) is a mechanism by which mercury burdens in ecosystems are lessened. The effects of salinity on mercury photoreactions were investigated while controlling the concentration of DOM (>1 kDa) using natural surface water from the tidal Jijuktu'kwejk (Cornwallis River) and processed with a tangential ultrafiltration-dilution technique. Pseudo first-order rate constants in estuarine water salinity dilutions ranged between 0.22 h^-1 and 0.73 h^-1. The amount of mercury available for photoreduction (Hg(II)_RED) ranged between 67.2 and 265.9 pg. Pseudo first-order rate constants decreased with increasing salinity treatments (0-13.5 g L^-1), with minimal change in rate constants occurring in higher salinity treatments (e.g. 20.3 or 26.8 g L^-1), while Hg(II)_RED increased with salinity. In lower salinity treatments, DOM was more photoactive. Taken together, results suggest changes in the mercury photoreduction mechanism from DOM-bound electron transfer to photochemically produced secondary reduction products with increasing salinity. Experiments examining photooxidation showed decreases in Hg (0) with longer exposure time, suggesting transformation of Hg(II)_RED into a non-reducible form. This research highlights the importance of salinity and DOM interactions in estuarine surface water and their effects on mercury photochemistry.

Mercury isotope evidence for Arctic summertime re-emission of mercury from the cryosphere

During Arctic springtime, halogen radicals oxidize atmospheric elemental mercury (Hg⁰), which deposits to the cryosphere. This is followed by a summertime atmospheric Hg⁰ peak that is thought to result mostly from terrestrial Hg inputs to the Arctic Ocean, followed by photoreduction and emission to air. The large terrestrial Hg contribution to the Arctic Ocean and global atmosphere has raised concern over the potential release of permafrost Hg, via rivers and coastal erosion, with Arctic warming. Here we investigate Hg isotope variability of Arctic atmospheric, marine, and terrestrial Hg. We observe highly characteristic Hg isotope signatures during the summertime peak that reflect re-emission of Hg deposited to the cryosphere during spring. Air mass back trajectories support a cryospheric Hg emission source but no major terrestrial source. This implies that terrestrial Hg inputs to the Arctic Ocean remain in the marine ecosystem, without substantial loss to the global atmosphere, but with possible effects on food webs.

Arctic warming thaws permafrost, leading to enhanced soil mercury transport to the Arctic Ocean. Mercury isotope signatures in arctic rivers, ocean and atmosphere suggest that permafrost mercury is buried in marine sediment and not emitted to the global atmosphere

Climate change and mercury in the Arctic: Abiotic interactions

Dramatic environmental shifts are occuring throughout the Arctic from climate change, with consequences for the cycling of mercury (Hg). This review summarizes the latest science on how climate change is influencing Hg transport and biogeochemical cycling in Arctic terrestrial, freshwater and marine ecosystems. As environmental changes in the Arctic continue to accelerate, a clearer picture is emerging of the profound shifts in the climate and cryosphere, and their connections to Hg cycling. Modeling results suggest climate influences seasonal and interannual variability of atmospheric Hg deposition. The clearest evidence of current climate change effects is for Hg transport from terrestrial catchments, where widespread permafrost thaw, glacier melt and coastal erosion are increasing the export of Hg to downstream environments. Recent estimates suggest Arctic permafrost is a large global reservoir of Hg, which is vulnerable to degradation with climate warming, although the fate of permafrost soil Hg is unclear. The increasing development of thermokarst features, the formation and expansion of thaw lakes, and increased soil erosion in terrestrial landscapes are increasing river transport of particulate-bound Hg and altering conditions for aquatic Hg transformations. Greater organic matter transport may also be influencing the downstream transport and fate of Hg. More severe and frequent wildfires within the Arctic and across boreal regions may be contributing to the atmospheric pool of Hg. Climate change influences on Hg biogeochemical cycling remain poorly understood. Seasonal evasion and retention of inorganic Hg may be altered by reduced sea-ice cover and higher chloride content in snow. Experimental evidence indicates warmer temperatures enhance methylmercury production in ocean and lake sediments as well as in tundra soils. Improved geographic coverage of measurements and modeling approaches are needed to better evaluate net effects of climate change and long-term implications for Hg contamination in the Arctic.

Rates and Dynamics of Mercury Isotope Exchange between Dissolved Elemental Hg(0) and Hg(II) Bound to Organic and Inorganic Ligands

Mercury (Hg) isotope exchange is a common process in biogeochemical transformations of Hg in the environment, but it is unclear whether and at what rates dissolved elemental Hg(0)_aq may exchange with divalent Hg(II) bound to various organic and inorganic ligands in water. Using enriched stable isotopes, we investigated the rates and dynamics of isotope exchange between ²⁰²Hg(0)_aq and ²⁰¹Hg(II) bound to organic and inorganic ligands with varying chemical structures and binding affinities. Time-dependent exchange reactions were followed by isotope compositional changes using both inductively coupled plasma mass spectrometry and Zeeman cold vapor atomic absorption spectrometry. Rapid, spontaneous isotope exchange (<1 h) was observed between ²⁰²Hg(0)_aq and ²⁰¹Hg(II) bound to chloride (Cl^-), ethylenediaminetetraacetate (EDTA), and thiols, such as cysteine (CYS), glutathione (GSH), and 2,3-dimercaptopropanesulfonic acid (DMPS) at a thiol ligand-to-Hg(II) molar ratio of 1:1. Without external reductants or oxidants, the exchange resulted in transfer of two electrons and redistribution of Hg isotopes bound to the ligand but no net changes of chemical species in the system. However, an increase in the ligand-to-Hg(II) ratio decreased the exchange rates due to the formation of 2:1 or higher thiol:Hg(II) chelated complexes, but had no effects on exchange rates with ²⁰¹Hg(II) bound to EDTA or Cl^-. The exchange between ²⁰²Hg(0)_aq and ²⁰¹Hg(II) bound to dissolved organic matter (DOM) showed an initially rapid followed by a slower exchange rate, likely resulting from Hg(II) complexation with both low- and high-affinity binding functional groups on DOM (e.g., carboxylates vs bidentate thiolates). These results demonstrate that Hg(0)_aq readily exchanges with Hg(II) bound to various ligands and highlight the importance of considering exchange reactions in experimental enriched Hg isotope tracer studies or in natural abundance Hg isotope studies in environmental matrices.

Recent advances in understanding and measurement of Hg in the environment: Surface-atmosphere exchange of gaseous elemental mercury (Hg0)

The atmosphere is the major transport pathway for distribution of mercury (Hg) globally. Gaseous elemental mercury (GEM, hereafter Hg⁰) is the predominant form in both anthropogenic and natural emissions. Evaluation of the efficacy of reductions in emissions set by the UN's Minamata Convention (UN-MC) is critically dependent on the knowledge of the dynamics of the global Hg cycle. Of these dynamics including e.g. red-ox reactions, methylation-demethylation and dry-wet deposition, poorly constrained atmosphere-surface Hg⁰ fluxes especially limit predictability of the timescales of its global biogeochemical cycle. This review focuses on Hg⁰ flux field observational studies, namely the theory, applications, strengths, and limitations of the various experimental methodologies applied to gauge the exchange flux and decipher active sub-processes. We present an in-depth review, a comprehensive literature synthesis, and methodological and instrumentation advances for terrestrial and marine Hg⁰ flux studies in recent years. In particular, we outline the theory of a wide range of measurement techniques and detail the operational protocols. Today, the most frequently used measurement techniques to determine the net Hg⁰ flux (>95% of the published flux data) are dynamic flux chambers for small-scale and micrometeorological approaches for large-scale measurements. Furthermore, top-down approaches based on Hg⁰ concentration measurements have been applied as tools to better constrain Hg emissions as an independent way to e.g. challenge emission inventories. This review is an up-dated, thoroughly revised edition of Sommar et al. 2013 (DOI: 10.1080/10643389.2012.671733). To the tabulation of >100 cited flux studies 1988-2009 given in the former publication, we have here listed corresponding studies published during the last decade with a few exceptions (2008-2019). During that decade, Hg stable isotope ratios of samples involved in atmosphere-terrestrial interaction is at hand and provide in combination with concentration and/or flux measurements novel constraints to quantitatively and qualitatively assess the bi-directional Hg⁰ flux. Recent efforts in the development of relaxed eddy accumulation and eddy covariance Hg⁰ flux methods bear the potential to facilitate long-term, ecosystem-scale flux measurements to reduce the prevailing large uncertainties in Hg⁰ flux estimates. Standardization of methods for Hg⁰ flux measurements is crucial to investigate how land-use change and how climate warming impact ecosystem-specific Hg⁰ sink-source characteristics and to validate frequently applied model parameterizations describing the regional and global scale Hg cycle.