Mercury isotope fractionation during precipitation of metacinnabar (β-HgS) and montroydite (HgO)

Legacy Mercury Re-emission and Subsurface Migration at Contaminated Sites Constrained by Hg Isotopes and Chemical Speciation

The re-emission and subsurface migration of legacy mercury (Hg) are not well understood due to limited knowledge of the driving processes. To investigate these processes at a decommissioned chlor-alkali plant, we used mercury stable isotopes and chemical speciation analysis. The isotopic composition of volatilized Hg(0) was lighter compared to the bulk total Hg (THg) pool in salt-sludge and adjacent surface soil with mean ε202HgHg(0)-THg values of -3.29 and -2.35‰, respectively. Hg(0) exhibited dichotomous directions (E199HgHg(0)-THg = 0.17 and -0.16‰) of mass-independent fractionation (MIF) depending on the substrate from which it was emitted. We suggest that the positive MIF enrichment during Hg(0) re-emission from salt-sludge was overall controlled by the photoreduction of Hg(II) primarily ligated by Cl- and/or the evaporation of liquid Hg(0). In contrast, O-bonded Hg(II) species were more important in the adjacent surface soils. The migration of Hg from salt-sludge to subsurface soil associated with selective Hg(II) partitioning and speciation transformation resulted in deep soils depleted in heavy isotopes (δ202Hg = -2.5‰) and slightly enriched in odd isotopes (Δ199Hg = 0.1‰). When tracing sources using Hg isotopes, it is important to exercise caution, particularly when dealing with mobilized Hg, as this fraction represents only a small portion of the sources.

Nano-mineral assemblages in mercury- and silver-contaminated soils: records of sequestration, transformation, and release of mercury- and silver-bearing nanoparticles

Mercury-bearing nano-mineral assemblages (Hg-NMAs) are chemically and mineralogically heterogeneous, micrometer-sized aggregates of nanoparticles (NPs) found in contaminated soils and sediments. Although these NMAs control sequestration and release of Hg that is a global contaminant, our understanding is limited with respect to the conditions of different types of Hg-NMAs, the diversity of its minerals, the size distribution of its NPs and whether mineral replacement and alteration reactions in these NMAs result in the release of Hg-bearing NPs. For this purpose, Hg-NMAs in four sediment samples from the Guanajuato Mining District (GMD) in Mexico, a region that was polluted by Hg and silver (Ag) due to historical mining involving Hg amalgamation, are characterized at the micro- and nanoscale. Microscale examinations with SEM show that the majority of Hg-NMAs occurs in mineral surface coatings (MSC) and fillings in fractures within quartz grains and are enriched in Hg and sulfur (S) relative to Ag, and in S and selenium (Se) relative to chloride (Cl). Examinations at the nanoscale show that Hg-NMAs contain (a) residuals of the patio process such as amalgam phases and elemental Ag; (b) associations of Hg- and Ag-sulfide NPs with pyrite and marcasite; (c) associations of Hg- and Ag-sulfide NPs with goethite and clay minerals along the rims of the MSC. The latter minerals replaced the Fe-Si-rich matrix at high-water rock ratios most likely due to an increase in porosity during flooding of the Pastita River. Consequently, the rims are depleted in Hg-Ag-sulfide NPs relative to the unaltered Fe-Si-rich matrices indicating that changes in the physiochemical conditions of soils and sediments in the GMD can result in the release of Hg-Ag-bearing NPs. In this context, this study discusses whether release and dissolution of Hg-Ag-bearing NPs contribute to the recently observed elevated gaseous elemental Hg concentrations in the soil, interstitial air and ambient air, and to the fate and effects of Hg in local aquatic environments.

Mercury Isotopes in Deep-Sea Epibenthic Biota Suggest Limited Hg Transfer from Photosynthetic to Chemosynthetic Food Webs

Deep oceans receive mercury (Hg) from upper oceans, sediment diagenesis, and submarine volcanism; meanwhile, sinking particles shuttle Hg to marine sediments. Recent studies showed that Hg in the trench fauna mostly originated from monomethylmercury (MMHg) of the upper marine photosynthetic food webs. Yet, Hg sources in the deep-sea chemosynthetic food webs are still uncertain. Here, we report Hg concentrations and stable isotopic compositions of indigenous biota living at hydrothermal fields of the Indian Ocean Ridge and a cold seep of the South China Sea along with hydrothermal sulfide deposits. We find that Hg is highly enriched in hydrothermal sulfides, which correlated with varying Hg concentrations in inhabited biota. Both the hydrothermal and cold seep biota have small fractions (<10%) of Hg as MMHg and slightly positive Δ¹⁹⁹Hg values. These Δ¹⁹⁹Hg values are slightly higher than those in near-field sulfides but are 1 order of magnitude lower than the trench counterparts. We suggest that deep-sea chemosynthetic food webs mainly assimilate Hg from ambient seawater/sediments and hydrothermal fluids formed by percolated seawater through magmatic/mantle rocks. The MMHg transfer from photosynthetic to chemosynthetic food webs is likely limited. The contrasting Hg sources between chemosynthetic and trench food webs highlight Hg isotopes as promising tools to trace the deep-sea Hg biogeochemical cycle.

Imaging and micro-invasive analyses of black stains on the passepartout of Codex Atlanticus Folio 843 by Leonardo da Vinci

This paper accounts for the diagnostic campaign aimed at understanding the phenomenon of black stains appeared on the passepartout close to the margins of Folio 843 of Leonardo da Vinci's Codex Atlanticus. Previous studies excluded microbiological deterioration processes. The study is based on a multi-analytical approach, including non-invasive imaging measurements of the folio, micro-imaging and synchrotron spectroscopy investigations of passepartout fragments at different magnifications and spectral ranges. Photoluminescence hyperspectral and lifetime imaging highlighted that black stains are not composed of fluorescent materials. μATR-FTIR imaging of fragments from the passepartout revealed the presence of a mixture of starch and PVAc glues localized only in the stained areas close to the margin of the folio. FE-SEM observations showed that the dark stains are localized inside cavities formed among cellulose fibers, where an accumulation of inorganic roundish particles (∅100-200 nm in diameter size), composed of Hg and S, was detected. Finally, by employing synchrotron μXRF, μXANES and HR-XRD analyses it was possible to identify these particles as metacinnabar (β-HgS). Further research is needed to assess the chemical process leading to the metacinnabar formation in the controlled conservation condition of Leonardo's Codex.

Mercury migration to surface water from remediated mine waste and impacts of rainfall in a karst area - Evidence from Hg isotopes

Mine waste (MW) in historical mercury (Hg) mining areas continuously emits Hg into local environment, including aquatic ecosystems. Tracing Hg migration process from MW and determining its relative contribution to Hg pollution is critical for understanding the environmental impact of MW remediation. In this study, we combined data of Hg concentration, speciation, and isotope to address this issue in the Wanshan Hg mining area in southwest China. We found that rainfall can elevate Hg concentrations in river water and control the partitioning and transport of Hg in karst fissure zones through changing the hydrological conditions. A consistently large offset of δ²⁰²Hg (1.24‰) was observed between dissolved Hg (DHg) and particulate Hg (PHg) in surface water during the low-flow period (LFP), which may have been related to the relatively stable hydrologic conditions and unique geological background (karst fissure zones) of the karst region (KR). Results from the ternary Hg isotopic mixing model showed that, despite an order of magnitude reduction in Hg concentration and flux in river water after remediation, the remediated MW is still a significant source of Hg pollution to local aquatic ecosystems, accounting for 49.3 ± 11.9% and 37.8 ± 11.8% of river DHg in high flow period (HFP) and LFP, respectively. This study provides new insights into Hg migration and transportation in aquatic ecosystem and pollution source apportionment in Hg polluted area, which can be used for making polices for future remediation actions.

Understanding the risks of mercury sulfide nanoparticles in the environment: Formation, presence, and environmental behaviors

Mercury (Hg) could be microbially methylated to the bioaccumulative neurotoxin methylmercury (MeHg), raising health concerns. Understanding the methylation of various Hg species is thus critical in predicting the MeHg risk. Among the known Hg species, mercury sulfide (HgS) is the largest Hg reservoir in the lithosphere and has long been considered to be highly inert. However, with advances in the analytical methods of nanoparticles, HgS nanoparticles (HgS NPs) have recently been detected in various environmental matrices or organisms. Furthermore, pioneering laboratory studies have reported the high bioavailability of HgS NPs. The formation, presence, and transformation (e.g., methylation) of HgS NPs are intricately related to several environmental factors, especially dissolved organic matter (DOM). The complexity of the behavior of HgS NPs and the heterogeneity of DOM prevent us from comprehensively understanding and predicting the risk of HgS NPs. To reveal the role of HgS NPs in Hg biogeochemical cycling, research needs should focus on the following aspects: the formation pathways, the presence, and the environmental behaviors of HgS NPs impacted by the dominant influential factor of DOM. We thus summarized the latest progress in these aspects and proposed future research priorities, e.g., developing the detection techniques of HgS NPs and probing HgS NPs in various matrices, further exploring the interactions between DOM and HgS NPs. Besides, as most of the previous studies were conducted in laboratories, our current knowledge should be further refreshed through field observations, which would help to gain better insights into predicting the Hg risks in natural environment.

Isotopic and Spectroscopic Investigation of Mercury Accumulation in Houttuynia cordata Colonizing Historically Contaminated Soil

Houttuynia cordata Thunb (H. cordata) is a native vegetable colonizing mercury (Hg) mining sites in the southwest of China; it can accumulate high Hg concentrations in the rhizomes and roots (edible sections), and thus consumption of H. cordata represents an important Hg exposure source to human. Here, we studied the spatial distribution, chemical speciation, and stable isotope compositions of Hg in the soil-H. cordata system at the Wuchuan Hg mining region in China, aiming to provide essential knowledge for assessing Hg risks and managing the transfer of Hg from soils to plants and agricultural systems. Mercury was mainly compartmentalized in the outlayer (periderm) of the underground tissues, with little Hg being translocated to the vascular bundle of the stem. Mercury presented as Hg-thiolates (94% ± 8%), with minor fractional amount of nanoparticulate β-HgS (β-HgS_NP, 15% ± 4%), in the roots and rhizomes. Analysis of Hg stable isotope ratios showed that cysteine-extractable soil Hg pool (δ²⁰²Hg_cys), root and rhizome Hg (δ²⁰²Hg_root, δ²⁰²Hg_rhizome) were isotopically lighter than Hg in the bulk soils. A significant positive correlation between δ²⁰²Hg_cys and δ²⁰²Hg_root was observed, suggesting that cysteine-extractable soil Hg pool was an important Hg source to H. cordata. The slightly positive Δ¹⁹⁹Hg value in the plant (Δ¹⁹⁹Hg_root = 0.07 ± 0.07‰, 2SD, n = 21; Δ¹⁹⁹Hg_rhizome = 0.06 ± 0.06‰, 2SD, n = 22) indicated that minor Hg was sourced from the surface water. Our results are important to assess the risks of Hg in H. cordata, and to develop sustainable methods to manage the transfer of Hg from soils to agricultural systems.

In Situ Photochemical Transformation of Hg Species and Associated Isotopic Fractionation in the Water Column of High-Altitude Lakes from the Bolivian Altiplano

Photochemical reactions are major pathways for the removal of Hg species from aquatic ecosystems, lowering the concentration of monomethylmercury (MMHg) and its bioaccumulation in foodwebs. Here, we investigated the rates and environmental drivers of MMHg photodegradation and inorganic Hg (IHg) photoreduction in waters of two high-altitude lakes from the Bolivian Altiplano representing meso- to eutrophic conditions. We incubated three contrasting waters in situ at two depths after adding Hg-enriched isotopic species to derive rate constants. We found that transformations mostly occurred in subsurface waters exposed to UV radiation and were mainly modulated by the dissolved organic matter (DOM) level. In parallel, we incubated the same waters after the addition of low concentrations of natural MMHg and followed the stable isotope composition of the remaining Hg species by compound-specific isotope analysis allowing the determination of enrichment factors and mass-independent fractionation (MIF) slopes (Δ¹⁹⁹Hg/Δ²⁰¹Hg) during in situ MMHg photodegradation in natural waters. We found that MIF enrichment factors potentially range from -11 to -19‰ and average -14.3 ± 0.6‰ (1 SE). The MIF slope diverged depending on the DOM level, ranging from 1.24 ± 0.03 to 1.34 ± 0.02 for the low and high DOM waters, respectively, and matched the MMHg MIF slope recorded in fish from the same lake. Our in situ results thus reveal (i) a relatively similar extent of Hg isotopic fractionation during MMHg photodegradation among contrasted natural waters and compared to previous laboratory experiments and (ii) that the MMHg MIF recorded in fish is characteristic for the MMHg bonding environment. They will enable a better assessment of the extent and conditions conducive to MMHg photodegradation in aquatic ecosystems.

Safe, accurate, and precise sulfur isotope analyses of arsenides, sulfarsenides, and arsenic and mercury sulfides by conversion to barium sulfate before EA/IRMS

The stable isotope ratios of sulfur (δ34S relative to Vienna Cañon Diablo Troilite) in sulfates and sulfides determined by elemental analysis and isotope ratio mass spectrometry (EA/IRMS) have been proven to be a remarkable tool for studies of the (bio)geochemical sulfur cycles in modern and ancient environments. However, the use of EA/IRMS to measure δ34S in arsenides and sulfarsenides may not be straightforward. This difficulty can lead to potential health and environmental hazards in the workplace and analytical problems such as instrument contamination, memory effects, and a non-matrix-matched standardization of δ34S measurements with suitable reference materials. To overcome these practical and analytical challenges, we developed a procedure for sulfur isotope analysis of arsenides, which can also be safely used for EA/IRMS analysis of arsenic sulfides (i.e., realgar, orpiment, arsenopyrite, and arsenian pyrite), and mercury sulfides (cinnabar). The sulfur dioxide produced from off-line EA combustion was trapped in an aqueous barium chloride solution in a leak-free system and precipitated as barium sulfate after quantitative oxidation of hydrogen sulfite by hydrogen peroxide. The derived barium sulfate was analyzed by conventional EA/IRMS, which bracketed the δ34S values of the samples with three international sulfate reference materials. The protocol (BaSO4-EA/IRMS) was validated by analyses of reference materials and laboratory standards of sulfate and sulfides and achieved accuracy and precision comparable with those of direct EA/IRMS. The δ34S values determined by BaSO4-EA/IRMS in sulfides (arsenopyrite, arsenic, and mercury sulfides) samples from different origins were comparable to those obtained by EA/IRMS, and no sulfur isotope fractionations were introduced during sample preparation. We report the first sulfur isotope data of arsenides obtained by BaSO4-EA/IRMS.

Graphical abstract

Coprecipitation of Mercury from Natural Iodine-Containing Seawater for Accurate Isotope Measurement

Oceans play a key role in the global mercury (Hg) cycle, but studies on Hg isotopes in seawater are rare due to the extremely low Hg concentration and the lack of a good preconcentration method. Here, we introduce a new coprecipitation method for separating and preconcentrating Hg from seawater for accurate isotope measurement. The coprecipitation was achieved by sequential addition of 0.5 mL of 0.5 M CuSO₄, 1 mL of 0.5 M Na₂S, and 1 mL of 0.5 M CuSO₄ reagents, which allowed for quantitatively precipitating Hg from up to 10 L of seawater. The protocol was validated by testing synthetic solutions with varying Hg and iodide (I^-) concentrations and by comparing the reaction times of various reagents added. The method resulted in a quantitative recovery of 98 ± 12% (n = 32, two standard deviations, 2 SD) and a relatively low procedure blank (103 pg of Hg, n = 8). The precipitates were filtrated and analyzed for Hg isotopes. Repeated measurements of synthetic seawaters spiked with certificated standard materials (NIST 3133 and 3177) using the entire method gave identical Hg isotope ratios with near-quantitative Hg recovery, indicating no isotope fractionation during preconcentration. A total of six nearshore seawater samples from the Yellow Sea and the Bohai Sea (China) were analyzed using the coprecipitation method. The data showed a large fractionation of Hg isotopes and revealed the possible impact of both atmospheric and anthropogenic inputs to the coastal seawater Hg budget, implying the potential application of this method in studying marine Hg systematics and global Hg cycling.

Mass-Independent Fractionation of Mercury Isotopes during Photoreduction of Soot Particle Bound Hg(II)

Soot and mercury (Hg) are two notorious air pollutants, and the fate and transport of Hg may be affected by soot at various scales in the environment as soot may be both a carrier and a reactant for active Hg species. This study was designed to quantify photoreduction of Hg(II) in the presence of soot and the associated Hg isotope fractionation under both atmospheric aerosol and aqueous conditions (water-saturated). Photoreduction experiments were conducted with diesel soot particulate matter under controlled temperature and relative humidity (RH) conditions using a flow-through semibatch reactor system. Mass-dependent fractionation resulted in the enrichment of heavier Hg isotopes in the remaining Hg(II) with enrichment factors (ε²⁰²Hg) of 1.48 ± 0.02‰ (±2 standard deviation) to 1.75 ± 0.05‰ for aerosol-phase reactions (RH 28-68%) and from 1.26 ± 0.11 to 1.50 ± 0.04‰ for aqueous-phase reactions. Positive odd mass-independent fractionation (MIF) was observed in aqueous-phase reactions, resulting in Δ¹⁹⁹Hg values for reactant Hg(II) as high as 5.29‰, but negative odd-MIF occurred in aerosol-phase reactions, in which Δ¹⁹⁹Hg values of reactant Hg(II) varied from -1.02 to 0‰. The average ratio of Δ¹⁹⁹Hg/Δ²⁰¹Hg (1.1) indicated that under all conditions, MIF was dominated by magnetic isotope effects during photoreduction of Hg(II). Increasing RH resulted in higher reduction rates but lower extents of negative MIF in the aerosol-phase experiments, suggesting that the reduction of soot particle-bound Hg(II) was responsible for the observed negative odd-MIF. Our results suggest that mass-independent Hg isotope fractionation during Hg(II) photoreduction varies with soot aerosol water content and that Hg-stable isotope ratios may be used to understand the transformational histories of aerosol-bound Hg(II) in the environment.

Use of sequential extraction and mercury stable isotope analysis to assess remobilization of sediment-bound legacy mercury

The goal of this project was to assess how anthropogenic legacy mercury (Hg) retained in streambed sediment may be remobilized to stream water. To do this, we performed sequential extractions and Hg isotope analyses on streambed sediment collected along the length of East Fork Poplar Creek, a point-source contaminated stream in Oak Ridge, Tennessee, USA. Legacy Hg within streambed sediment appears to have been isotopically fractionated by equilibrium isotope effects driven by isotope exchange between co-existing Hg(0) and Hg(ii) species, potentially over-printing fractionation patterns that would have been imparted by kinetic redox reactions. Weakly-bound and recalcitrant sediment Hg pools were isotopically similar to one another, suggesting that small amounts of recalcitrant Hg may be released and then rapidly and weakly re-adsorbed onto the sediment. This weakly-bound Hg pool appears to contribute dissolved Hg to the hyporheic pore water, which may subsequently enter the surface flow. The isotopic composition of the organically-bound sediment Hg pools, as well as biofilm and suspended particulates, converged with that of the weakly-bound and recalcitrant sediment Hg pools along the flow path. This appears to be indicative of both physical mixing with streambed sediment and the transfer of weakly-bound sediment Hg into biofilm and suspended particulates, followed by re-incorporation into the organically-bound sediment Hg pool. Overall, these results provide evidence that legacy Hg in the streambed is remobilized, enters the stream water as dissolved Hg, and may be incorporated into streambed biofilm, which constitutes a basal resource within the stream ecosystem.

Mercury isotope signatures in sediments and marine organisms as tracers of historical industrial pollution

Isotopic composition of mercury (Hg) in marine organisms and sediment cores was used to identify sources and reconstruct historical trends of contamination in the coastal-marine area of Rosignano Solvay (Italy), affected by Hg pollution from a chlor-alkali plant on the near land. Sediments show a wide range of Hg concentration and Hg isotope signatures. Particularly, coupled Hg concentration and δ²⁰²Hg values trace inputs from different sources. The two depth-profiles clearly indicate three distinct periods: "pre-industrial" (before 1941), "industrial" (between 1941 and 2007) and "post-industrial" (after 2007) ages. This is also corroborated by sediment chronology, using ²¹⁰Pb dating method, validated through ¹³⁷Cs. Marine organisms are characterized by Hg isotope signatures comparable to "post-industrial" surface sediments. Notably, specimens of Mullus spp. evidence isotope composition comparable to the "industrial" sediments, thus suggesting a still active role of those sediments as source of Hg for the benthic fish compartment. The small amount of MIF and the Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio recorded in organisms are reasonably consistent with limited processes of MMHg demethylation in the water column.

Variation in the mercury concentration and stable isotope composition of atmospheric total suspended particles in Beijing, China

We investigated the effect of the temporary ban of local industrial activities during the Asia-Pacific Economic Cooperation (APEC) summit (4th-14th Nov 2014) in Beijing, China on total suspended particulate mercury (Hg_TSP) concentrations and isotope compositions. We measured Hg concentrations and isotope ratios in 33 TSP samples from central Beijing, including 21 samples collected from Jun 2012 to Apr 2014, and 12 samples collected from 14th Oct-19th Nov 2014. Volumetric concentrations of both TSP and Hg_TSP during the APEC summit were a factor of 2 lower than during the pre-APEC period, indicating substantial reductions in total particulate matter and Hg_TSP as a result of emissions controls. However, mass-normalized concentrations and mercury isotope ratios of Hg_TSP did not vary significantly between samples collected before, during, or after the APEC summit. These results show that local emissions are important sources of particle bound mercury (PBM) in Beijing and that their control can be used to immediately lower the volumetric concentration of Hg_TSP. They also indicate that a similarly complex mixture of sources contributed to PBM in Beijing before and during emissions controls were put in place and that PBM concentrations in Beijing are primarily controlled by emissions and secondarily by photoreduction.

Mercury Isotope Fractionation in the Subsurface of a Hg(II) Chloride-Contaminated Industrial Legacy Site

To understand the transformations of mercury (Hg) species in the subsurface of a HgCl₂-contaminated former industrial site in southwest Germany, Hg isotope analysis was combined with an investigation of Hg forms by a four-step sequential extraction protocol (SEP) and pyrolytic thermodesorption. Data from two soil cores revealed that the initial HgCl₂ was partly reduced to metallic Hg(0) and that Hg forms of different mobility and oxidation state coexist in the subsurface. The most contaminated sample (K2-8, 802 mg kg^-1 Hg) had a bulk δ²⁰²Hg value of around -0.43 ± 0.06‰ (2SD), similar to published average values for industrial Hg sources. Other sample signatures varied significantly with depth and between SEP pools. The most Hg-rich samples contained mixtures of Hg(0) and Hg(II) phases, and the water-extractable, mobile Hg pool exhibited heavy δ²⁰²Hg values of up to +0.18‰. Sequential water extracts revealed slow dissolution kinetics of mobile Hg pools, continuously releasing isotopically heavy Hg into solution. This was further corroborated by heavy δ²⁰²Hg values of groundwater samples. Our results demonstrate that the Hg isotope signature of an industrial contamination source can be significantly altered during the transformations of Hg species in the subsurface, which complicates source tracing applications but offers the possibility of using Hg isotopes as process tracers in contaminated subsurface systems.

High-precision isotopic analysis sheds new light on mercury metabolism in long-finned pilot whales (Globicephala melas)

Whales accumulate mercury (Hg), but do not seem to show immediate evidence of toxic effects. Analysis of different tissues (liver, kidney, muscle) and biofluids (blood, milk) from a pod of stranded long-finned pilot whales (Globicephala melas) showed accumulation of Hg as a function of age, with a significant decrease in the MeHg fraction. Isotopic analysis revealed remarkable differences between juvenile and adult whales. During the first period of life, Hg in the liver became isotopically lighter (δ²⁰²Hg decreased) with a strongly decreasing methylmercury (MeHg) fraction. We suggest this is due to preferential demethylation of MeHg with the lighter Hg isotopes and transport of MeHg to less sensitive organs, such as the muscles. Also changes in diet, with high MeHg intake in utero and during lactation, followed by increasing consumption of solid food contribute to this behavior. Interestingly, this trend in δ²⁰²Hg is reversed for livers of adult whales (increasing δ²⁰²Hg value), accompanied by a progressive decrease of δ²⁰²Hg in muscle at older ages. These total Hg (THg) isotopic trends suggest changes in the Hg metabolism of the long-finned pilot whales, development of (a) detoxification mechanism(s) (e.g., though the formation of HgSe particles), and Hg redistribution across the different organs.

An investigation of mercury sources in the Puyango-Tumbes River: Using stable Hg isotopes to characterize transboundary Hg pollution

Mercury (Hg) concentrations and stable isotopes along with other trace metals were examined in environmental samples from Ecuador and Peru's shared Puyango-Tumbes River in order to determine the extent to which artisanal- and small-scale gold mining (ASGM) in Portovelo-Zaruma, Ecuador contributes to Hg pollution in the downstream aquatic ecosystem. Prior studies investigated the relationship between ASGM activities and downstream Hg pollution relying primarily on Hg concentration data. In this study, Hg isotopes revealed an isotopically heavy Hg signature with negligible mass independent fractionation (MIF) in downstream sediments, which was consistent with the signature observed in the ASGM source endmember. This signature was traced as far as ∼120 km downstream of Portovelo-Zaruma, demonstrating that Hg stable isotopes can be used as a tool to fingerprint and trace sources of Hg over vast distances in freshwater environments. The success of Hg isotopes as a source tracer in fresh waters is largely due to the particle-reactive nature of Hg. Furthermore, the magnitude and extent of downstream Hg, lead, copper and zinc contamination coupled with the Hg isotopes suggest that it is unlikely that the smaller artisanal-scale activities, which do not use cyanidation, are responsible for the pollution. More likely it is the scale of ores processed and the cyanide leaching, which can release other metals and enhance Hg transport, used during small-scale gold mining that is responsible. Thus, although artisanal- and small-scale gold mining occur in tandem in Portovelo-Zaruma, a distinction should be made between these two activities.

In vivo fractionation of mercury isotopes in tissues of a mammalian carnivore (Neovison vison)

The use of isotope ratios to trace Hg contamination sources in environmental compartments is now generally accepted. However, for biota and especially for mammals, it is still unknown if and/or how Hg isotopes fractionate in vivo and which tissue is most representative of the source(s) of contamination. We measured fractionation of Hg in mink (Neovison vison) tissues (fur, brain, blood, liver, kidney) collected during a controlled feeding experiment where captive mink were fed differing amounts of methylmercury. There was no significant effect of dietary MeHg concentrations on Hg fractionation in most tissues. Net fractionation of Hg, i.e., fractionation corrected for diet (δ²⁰²Hg_tissue-δ²⁰²Hg_diet) was observed in all tissues with the greatest net fractionation occurring in the mink liver (-1.39‰) and kidney (-0.95‰). Less net fractionation, occurred in the brain (-0.12‰), blood (0.38‰) and fur (0.30‰). In the absence of brain tissue, fur is a suitable proxy which is readily obtainable and can be non-lethally collected. In these mink, it appears that biochemical processes such as demethylation, contribute to significant fractionation of Hg in the liver and kidney, but not as much in the brain and fur, where transport of Hg via thiol-containing complexes may be more important.

Factors Affecting Mercury Stable Isotopic Distribution in Piscivorous Fish of the Laurentian Great Lakes

Identifying the sources of methylmercury (MeHg) and tracing the transformations of mercury (Hg) in the aquatic food web are important components of effective strategies for managing current and legacy Hg sources. In our previous work, we measured stable isotopes of Hg (δ²⁰²Hg, Δ¹⁹⁹Hg, and Δ²⁰⁰Hg) in the Laurentian Great Lakes and estimated source contributions of Hg to bottom sediment. Here, we identify isotopically distinct Hg signatures for Great Lakes trout ( Salvelinus namaycush) and walleye ( Sander vitreus), driven by both food-web and water-quality characteristics. Fish contain high values for odd-isotope mass independent fractionation (MIF) with averages ranging from 2.50 (western Lake Erie) to 6.18‰ (Lake Superior) in Δ¹⁹⁹Hg. The large range in odd-MIF reflects variability in the depth of the euphotic zone, where Hg is most likely incorporated into the food web. Even-isotope MIF (Δ²⁰⁰Hg), a potential tracer for Hg from precipitation, appears both disconnected from lake sedimentary sources and comparable in fish among the five lakes. We suggest that similar to the open ocean, water-column methylation also occurs in the Great Lakes, possibly transforming recently deposited atmospheric Hg deposition. We conclude that the degree of photochemical processing of Hg is controlled by phytoplankton uptake rather than by dissolved organic carbon quantity among lakes.

Thallium isotopes in metallurgical wastes/contaminated soils: A novel tool to trace metal source and behavior

Thallium (Tl) concentration and isotope data have been recorded for contaminated soils and a set of industrial wastes that were produced within different stages of Zn ore mining and metallurgical processing of Zn-rich materials. Despite large differences in Tl levels of the waste materials (1-500mgkg^-1), generally small changes in ε²⁰⁵Tl values have been observed. However, isotopically lighter Tl was recorded in fly ash (ε²⁰⁵Tl∼-4.1) than in slag (ε²⁰⁵Tl∼-3.3), implying partial isotope fractionation during material processing. Thallium isotope compositions in the studied soils reflected the Tl contamination (ε²⁰⁵Tl∼-3.8), despite the fact that the major pollution period ended more than 30 years ago. Therefore, we assume that former industrial Tl inputs into soils, if significant, can potentially be traced using the isotope tracing method. We also suggest that the isotope redistributions occurred in some soil (subsurface) horizons, with Tl being isotopically heavier than the pollution source, due to specific sorption and/or precipitation processes, which complicates the discrimination of primary Tl. Thallium isotope analysis proved to be a promising tool to aid our understanding of Tl behavior within the smelting process, as well as its post-depositional dynamics in the environmental systems (soils).

Methylmercury degradation and exposure pathways in streams and wetlands impacted by historical mining

Monomethyl mercury (MMHg) and total mercury (THg) concentrations and Hg stable isotope ratios (δ(202)Hg and Δ(199)Hg) were measured in sediment and aquatic organisms from Cache Creek (California Coast Range) and Yolo Bypass (Sacramento Valley). Cache Creek sediment had a large range in THg (87 to 3870ng/g) and δ(202)Hg (-1.69 to -0.20‰) reflecting the heterogeneity of Hg mining sources in sediment. The δ(202)Hg of Yolo Bypass wetland sediment suggests a mixture of high and low THg sediment sources. Relationships between %MMHg (the percent ratio of MMHg to THg) and Hg isotope values (δ(202)Hg and Δ(199)Hg) in fish and macroinvertebrates were used to identify and estimate the isotopic composition of MMHg. Deviation from linear relationships was found between %MMHg and Hg isotope values, which is indicative of the bioaccumulation of isotopically distinct pools of MMHg. The isotopic composition of pre-photodegraded MMHg (i.e., subtracting fractionation from photochemical reactions) was estimated and contrasting relationships were observed between the estimated δ(202)Hg of pre-photodegraded MMHg and sediment IHg. Cache Creek had mass dependent fractionation (MDF; δ(202)Hg) of at least -0.4‰ whereas Yolo Bypass had MDF of +0.2 to +0.5‰. This result supports the hypothesis that Hg isotope fractionation between IHg and MMHg observed in rivers (-MDF) is unique compared to +MDF observed in non-flowing water environments such as wetlands, lakes, and the coastal ocean.

Mercury isotope signatures of seawater discharged from a coal-fired power plant equipped with a seawater flue gas desulfurization system

Seawater flue gas desulfurization (SFGD) systems are commonly used to remove acidic SO2 from the flue gas with alkaline seawater in many coastal coal-fired power plants in China. However, large amount of mercury (Hg) originated from coal is also transferred into seawater during the desulfurization (De-SO2) process. This research investigated Hg isotopes in seawater discharged from a coastal plant equipped with a SFGD system for the first time. Suspended particles of inorganic minerals, carbon residuals and sulfides are enriched in heavy Hg isotopes during the De-SO2 process. δ(202)Hg of particulate mercury (PHg) gradually decreased from -0.30‰ to -1.53‰ in study sea area as the distance from the point of discharge increased. The results revealed that physical mixing of contaminated De-SO2 seawater and uncontaminated fresh seawater caused a change in isotopic composition of PHg isotopes in the discharging area; and suggested that both De-SO2 seawater and local background contributed to PHg. The impacted sea area predicted with isotopic tracing technique was much larger than that resulted from a simple comparison of pollutant concentration. It was the first attempt to apply mercury isotopic composition signatures with two-component mixing model to trace the mercury pollution and its influence in seawater. The results could be beneficial to the coal-fired plants with SFGD systems to assess and control Hg pollution in sea area.

Isotopic Composition of Inorganic Mercury and Methylmercury Downstream of a Historical Gold Mining Region

We measured total mercury (THg) and monomethyl mercury (MMHg) concentrations and mercury (Hg) isotopic compositions in sediment and aquatic organisms from the Yuba River (California, USA) to identify Hg sources and biogeochemical transformations downstream of a historical gold mining region. Sediment THg concentrations and δ(202)Hg decreased from the upper Yuba Fan to the lower Yuba Fan and the Feather River. These results are consistent with the release of Hg during gold mining followed by downstream mixing and dilution. The Hg isotopic composition of Yuba Fan sediment (δ(202)Hg = -0.38 ± 0.17‰ and Δ(199)Hg = 0.04 ± 0.03‰; mean ± 1 SD, n = 7) provides a fingerprint of inorganic Hg (IHg) that could be methylated locally or after transport downstream. The isotopic composition of MMHg in the Yuba River food web was estimated using biota with a range of %MMHg (the percent of THg present as MMHg) and compared to IHg in sediment, algae, and the food web. The estimated δ(202)Hg of MMHg prior to photodegradation (-1.29 to -1.07‰) was lower than that of IHg and we suggest this is due to mass-dependent fractionation (MDF) of up to -0.9‰ between IHg and MMHg. This result is in contrast to net positive MDF (+0.4 to +0.8‰) previously observed in lakes, estuaries, coastal oceans, and forests. We hypothesize that this unique relationship could be due to differences in the extent or pathway of biotic MMHg degradation in stream environments.

Isotopic study of mercury sources and transfer between a freshwater lake and adjacent forest food web

Studies of monomethylmercury (MMHg) sources and biogeochemical pathways have been extensive in aquatic ecosystems, but limited in forest ecosystems. Increasing evidence suggests that there is significant mercury (Hg) exchange between aquatic and forest ecosystems. We use Hg stable isotope ratios (δ(202)Hg and Δ(199)Hg) to investigate the relative importance of MMHg sources and assess Hg transfer pathways between Douglas Lake and adjacent forests located at the University of Michigan Biological Station, USA. We characterize Hg isotopic compositions of basal resources and use linear regression of % MMHg versus δ(202)Hg and Δ(199)Hg to estimate Hg isotope values for inorganic mercury (IHg) and MMHg in the aquatic and adjacent forest food webs. In the aquatic ecosystem, we found that lake sediment represents a mixture of IHg pools deposited via watershed runoff and precipitation. The δ(202)Hg and Δ(199)Hg values estimated for IHg are consistent with other studies that measured forest floor in temperate forests. The Δ(199)Hg value estimated for MMHg in the aquatic food web indicates that MMHg is subjected to ~20% photochemical degradation prior to bioaccumulation. In the forest ecosystem, we found a significant negative relationship between total Hg and δ(202)Hg and Δ(199)Hg of soil collected at multiple distances from the lakeshore and lake sediment. This suggests that IHg input from watershed runoff provides an important Hg transfer pathway between the forest and aquatic ecosystems. We measured Δ(199)Hg values for high trophic level insects and compared these insects at multiple distances perpendicular to the lake shoreline. The Δ(199)Hg values correspond to the % canopy cover suggesting that forest MMHg is subjected to varying extents of photochemical degradation and the extent may be controlled by sunlight. Our study demonstrates that the use of Hg isotopes adds important new insight into the relative importance of MMHg sources and complex Hg transfer pathways across ecosystem boundaries.