Significant Mass-dependent Fractionation Research Articles

Impact of forest fire on the mercury stable isotope composition in litter and soil in the Amazon

Mercury (Hg) emissions from forest fires, especially tropical forests such as the Amazonian forest, were shown to contribute significantly to the atmospheric mercury budget, but new methods are still necessary to improve the traceability and to reduce the great uncertainties related to this emission source. Recent studies have shown that the combustion process can result in Hg stable isotope fractionation that allows tracking coal combustion Hg emissions, as influenced by different factors such as combustion temperature. The main goal of the present study was, therefore, to investigate for the first time the potential of Hg stable isotopes to trace forest fire Hg emissions and pathways. More specifically, small-scale and a large scale prescribed forest fire experiments were conducted in the Brazilian Amazonian forest to study the impact of fire severity on Hg isotopic composition of litter, soil, and ash samples and associated Hg isotope fractionation pathways. In the small-scale experiment, no difference was found in the mercury isotopic composition of the samples collected before and after burning. In contrast, the larger-scale experiment resulted in significant mass dependent fractionation (MDF δ202Hg) in soils and ash suggesting that higher combustion temperature influence Hg isotopic fractionation with the emission of lighter Hg isotopes to the atmosphere and enrichment with heavier Hg in ashes. As for coal combustion, mass independent fractionation was not observed. To our knowledge, these results are the first to highlight the potential of forest fires to cause Hg isotopic fractionation, depending on the fire severity. The results also allowed to establish an isotopic fingerprint for tropical forest fire Hg emissions that corresponds to a mixture of litter and soil Hg isotopic composition (resulting atmospheric δ202Hg, Δ200Hg and Δ199Hg were −1.79 ± 0.24‰, −0.05 ± 0.04‰ and −0.45 ± 0.12‰, respectively).

Aquatic methylmercury is a significant subsidy for terrestrial songbirds: Evidence from the odd mass-independent fractionation of mercury isotopes

In contrast to aquatic food chains, knowledge of the origins and transfer of mercury (Hg) and methylmercury (MeHg) in terrestrial food chains is relatively limited, especially in songbirds. We collected soil, rice plants, aquatic and terrestrial invertebrates, small wild fish, and resident songbird feathers from an Hg-contaminated rice paddy ecosystem for an analysis of stable Hg isotopes to clarify the sources of Hg and its transfer in songbirds and their prey. Significant mass-dependent fractionation (MDF, δ202Hg), but no mass-independent fractionation (MIF, ∆199Hg) occurred in the trophic transfers in terrestrial food chains. Piscivorous, granivorous, and frugivorous songbirds and aquatic invertebrates were all characterized by elevated Δ199Hg values. The estimated MeHg isotopic compositions obtained using linear fitting and a binary mixing model explained both the terrestrial and aquatic origins of MeHg in the terrestrial food chains. We found that MeHg from aquatic habitats is an important subsidy for terrestrial songbirds, even those that feed mainly on seeds, fruits, or cereals. The results show that MIF of the MeHg isotope is a reliable tool to reveal MeHg sources in songbirds. Because the MeHg isotopic compositions was calculated with a binary mixing model or directly estimated from the high proportions of MeHg, compound-specific isotope analysis of Hg would be more useful for the interpretation of the Hg sources, and is highly recommended for future studies.

Fractionation of mercury stable isotopes in lichens

Bio-monitoring of mercury (Hg) in air using transplanted and in-situ lichens was conducted at three locations in Slovenia: (I) the town of Idrija in the area of the former Hg mine, where Hg contamination is well known; (II) Anhovo, a settlement with a cement production plant, which is a source of Hg contamination, and (III) Pokljuka, a part of a national park. Lichens from Pokljuka were transplanted to different sites and sampled four times—once per season, from January 2020 to February 2021. Lichens were set on tree branches, fences, and under cover, allowing them to be exposed to different environmental conditions (e.g., light and rain). The in-situ lichens were sampled at the beginning and the end of the sampling period. The highest concentrations were in the Idrija area, which was consistent with previous research. Significant mass-dependent fractionation has been observed in transplanted lichens during summer period. The δ202Hg changed from −3.0‰ in winter to −1.0‰ in summer and dropped again to the same value in winter the following year. This trend was observed in all samples, except those from the most polluted Idrija sampling site, which was in the vicinity of the former Hg ore-smelting plant. This was likely due to large amounts of Hg originating from polluted soil close to the former smelting plant with a distinct isotopic fingerprint in this local area. The Δ199Hg in transplanted lichens ranged from −0.5‰ to −0.1‰ and showed no seasonal trends. These findings imply that seasonality, particularly in summer months, may affect the isotopic fractionation of Hg and should be considered in the sampling design and data interpretation. This trend was thus described in lichens for the first time. The mechanism behind such change is not yet fully understood.

Denitrification devices in urban boilers change mercury isotope fractionation signatures of coal combustion products.

The installation rate of denitrification devices is accelerating in Chinese urban boilers. Previous studies on pulverized coal-fired boilers without denitrification devices showed that combustion products containing mainly oxidized mercury (Hg) preferably enriched lighter Hg isotopes than feed coals. However, the magnitude of this enrichment becomes less pronounced if denitrification devices are installed. The underlying Hg isotope fractionation mechanisms are still unclear. In this study, three types of urban boilers (two pulverized coal-fired boilers, one circulating fluidized bed boiler and one municipal waste incinerator boiler) all installed with denitrification devices were measured for Hg isotope compositions of their feed fuels and corresponding combustion products. We observed little mass independent fractionation but very significant mass dependent fractionation (MDF) between feed fuels and combustion products. The fly ash and desulfurization products both enriched heavier Hg isotopes than feed coals in three coal-fired boilers, and the enrichment of heavy Hg isotopes increased with sequential removal of combustion products in all boilers. Different from previously suggested kinetic MDF for gaseous Hg0(g)→HgII(g) and gaseous HgII(g)→particulate HgII(p) in coal combustion flue gases, we propose an equilibrium MDF for Hg0(g)↔HgII(g) followed by a kinetic MDF for HgII(g)→HgII(p). This equilibrium MDF most likely occurs during Hg0(g) oxidation in denitrification devices, which enriches heavy Hg isotopes in oxidized products (HgII(g) and HgII(p)) that are then sequestrated in fly ash and desulfurization products. The paradigm shift of MDF in boilers with denitrification devices was further verified by parallel Hg isotope measurement in urban atmosphere particulates. Our study clearly demonstrates that modern coal-fired boilers with denitrification devices have a quite different MDF compared to traditional boilers without denitrification devices. This has important implications for estimating isotope signatures of urban boiler Hg emissions, and for isotope tracing of anthropogenic Hg emissions.

Mercury isotope signatures of a pre-calciner cement plant in Southwest China

Characterizing the composition of mercury (Hg) isotopes in the atmospheric emissions of cement plants is critical to understand the global circulation of Hg because large quantities of Hg are released from this source annually. A pre-calciner cement plant in Guizhou Province in Southwest China was selected to investigate the mass dependent fractionation (MDF) and mass independent fractionation (MIF) of Hg in the entire production process and the speciated Hg isotope composition in stack gas. Significant MDF and insignificant MIF were observed in this cement plant. Different raw/correction materials have δ202Hg signals ranging from -1.68 to -2.19‰. Raw meal is featured with lighter Hg (δ202Hg = -2.83 ± 0.18‰) as results of Hg circulation and accumulation during the clinker production. Cement products possess negative δ202Hg values (-1.98 ± 0.02‰) due to the input of light δ202Hg isotopes through additives/retarder limestone, and fly ash and gypsum from coal-fired power plant (CFPPs). Speciated Hg isotopes in the stack gas of the kiln tail and kiln head show no significant differences, and δ202Hg and Δ199Hg in the discharged flue gas averaged at -2.03 ± 0. 31‰ and -0.03 ± 0.07‰, respectively, which has negative δ202Hg characteristics with other anthropogenic sources.

Mercury sources in a subterranean spontaneous combustion area.

Mercury is a toxic, persistent, and mobile contaminant. Coal spontaneous combustion are widely distributed in the world and releases a great deal of Hg. Identifying the burning coal seam is crucial for quickly extinguishing a coalfield fire. Mercury isotopes can be effective for identifying burning coal seams and beneficial for combating coal spontaneous combustion. In this study, Hg isotopic ratios of coal, topsoil, dustfall, sand, coal fire sponges (CFS), and n-topsoil (topsoil near the CFS) from coal fire area No. 9 in the Wuda coalfield were determined using multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Analysis of the correlation coefficients between the δ202Hg and Hg concentrations and the low-temperature ashes indicate that the higher mineral concentration in coal seam No. 9 not only increases the Hg concentration but also leads to more positive δ202Hg values compared to those for coal seam No. 10. By analyzing the Hg isotope characterizations in coal seam No. 9 and No. 10, we determined that Hg isotope characterizations can be useful for discriminating different coal seam Hg values in a coalfield. Significant mass-dependent fractionation (MDF) occur in the coal burning. The fractionation effect of burning and absorption process can play a key role in the δ202Hg more negative of ground surface samples. If Hg isotopes is added, the effect of coal-fire monitoring may be better. In addition, these finding could be used to better understand the transport and cycling of Hg.

Effect of air pollution control devices on mercury isotopic fractionation in coal-fired power plants

Coal combustion is an important source of anthropogenic mercury (Hg) emissions. Air pollution control devices (APCDs) are widely used in coal-fired power plants (CFPPs) to reduce pollutant emissions including Hg. To gain insight into the Hg isotopic fractionation produced by APCDs, the Hg isotopic compositions of feed coal (FC) samples and combustion byproducts from 7 CFPP utility boilers in Hebei Province of China were studied. Significant mass-dependent fractionation (MDF) was observed in combustion byproducts when comparing with FC samples, while mass-independent fractionation (MIF) was not identified. Compared to FC samples, no significant difference in bottom ash (BA, δ202HgBA-FC = 0.08 ± 0.16‰, 1SD, n = 3) samples while light isotopes were enriched in fly ash (FA, δ202HgFA-FC = −0.38 ± 0.75‰, 1SD, n = 7) samples. Boilers lacking of a selective catalytic reducer (SCR) unit produced gypsum (GY) samples with negative MDF signatures compared to FC samples (δ202HgGY-FC, no SCR = −0.22 ± 0.10‰, 1SD, n = 3). In contrast, boilers equipped with a SCR unit produced GY samples with higher MDF signatures (δ202HgGY-FC, SCR = 0.54 ± 0.40‰, 1SD, n = 3), probably as a result of the oxidation of Hg0 in the SCR unit. Calculated isotopic signatures of gaseous Hg in the stack emission (SE, δ202HgSE-FC = 0.03 ± 0.20‰, 1SD, n = 7) had no significant difference from that of FC samples. The results suggested distinct isotopic compositions should be fully considered during tracing Hg pollution via Hg isotopic signatures, especially when solid produced by CFPPs were involved.

Stable isotope composition of mercury forms in flue gases from a typical coal-fired power plant, Inner Mongolia, northern China

Mercury forms emitted from coal combustion via air pollution control devices are speculated to carry different Hg isotope signatures. Yet, their Hg isotope composition is still not reported. Here, we present the first onsite Hg isotope data for gaseous elemental Hg (GEM) and gaseous oxidized Hg (GOM) of flue gases from a typical lignite-fired power plant (CFPP). Significant mass dependent fractionation (MDF) and insignificant mass independent fractionation (MIF) are observed between feed coal and coal combustion products. As compared to feed coal (δ202Hg=−2.04±0.25‰), bottom ash, GEM and GOM in flue gases before and after wet flue gas desulfurization system significantly enrich heavy Hg isotopes by 0.7–2.6‰ in δ202Hg, while fly ash, desulfurization gypsum and waste water show slight but insignificant enrichment of light Hg isotopes. GEM is significantly enriched heavy Hg isotopes compared to GOM and Hg in fly ash. Our observations verify the previous speculation on Hg isotope fractionation mechanism in CFPPs, and suggest a kinetically-controlled mass dependent Hg isotope fractionation during transformation of Hg forms in flue gases. Finally, our data are compared to Hg isotope compositions of atmospheric Hg pools, suggesting that coal combustion Hg emission is likely an important atmospheric Hg contributor.

Concentrations and isotopic variability of mercury in sulfide minerals from the Jinding Zn-Pb deposit, Southwest China

Mercury (Hg) isotopes have been proven as a useful tracer in understanding sources and biogeochemical processes of Hg in the environment. However, the use of this tracer has not yet been explored in economic geology. This paper investigates the concentrations and isotopic compositions of Hg in sulfide minerals from the Jinding deposit, the largest Zn-Pb deposit in China. Total mercury concentration (HgT) is highly variable: with the highest in sphalerite (472–1010ng·g−1), intermediate concentrations in pyrite (195–342ng·g−1) and the lowest in galena (65–310ng·g−1). The variation was likely due to the fact that Hg2+ can more readily substitute for Zn2+ than for Fe2+ and Pb2+, but an influence of different parental fluids on the isotopic composition of the sulfide minerals cannot be excluded. An overall range of δ202Hg from −3.17 to −0.57‰ is observed in the sulfides. Samples from the early stage feature the enrichments of light Hg isotopes, with δ202Hg ranging from −3.17 to –1.59‰, suggesting significant mass-dependent fractionation during the transport and/or deposition of Hg. However, the volatilization of aqueous Hg(0) during boiling of hydrothermal fluids was likely the most important process causing the observed fractionation. Relatively higher δ202Hg values (−1.84 to −0.57‰) of the late stage samples indicate that the Hg was rarely fractionated from its sources. Additionally, small but significant mass-independent fractionations are measured for the deposit with Δ199Hg ranging from −0.06 to 0.10‰, indicating that the Hg may have been derived from the sedimentary rocks of the Lanping Basin. Finally, we conclude that Hg isotopes have the potential to be a new tracer of sources of ore-forming materials, as well as pathways of fluid evolution in hydrothermal deposits.

Mercury isotope fractionation in waters and sediments of the Murray Brook mine watershed (New Brunswick, Canada): Tracing mercury contamination and transformation

Mercury isotope fractionation was investigated in different environmental compartments of a mining-impacted ecosystem, the watershed of the Murray Brook deposit in northern New Brunswick, Canada. Mercury contaminated ground water, surface water as well as sediment and samples of suspended particulate matter were collected alongside Gossan Creek downstream of mine tailings and analyzed for mercury isotopic composition. While leachates from the tailings and ground water samples showed a uniform and similar mercury isotopic signature confirming the predominant role of the mining residues in the contamination of the surrounding environment, samples collected from the surface flow system provided evidence of significant mass-dependent fractionation of mercury with distance from the source. As dissolved mercury contents steadily decreased from 31.1 to 1.4μg Hg L-1 with distance from the headwaters of the stream, δ202Hg in the creek water progressively increased resulting in a fractionation of ~0.9‰ over 1.6km. As a result, mercury isotopic ratios in the creek appeared to closely follow a Rayleigh model assuming a constant fractionation factor α(reactant/product) of 1.00038. Stream bed sediments also demonstrated an enrichment of heavy isotopes along the creek with an isotopic shift of ~1‰ over the distance of Gossan Creek (3100m). Although mercury losses from the water column result partly from partitioning to the sediments, measured mercury isotopic ratios differences between dissolved and solid phases were likely small and this study did not observe significant fractionation due to partitioning processes. Results rather suggested that fractionation observed in the surface water system of Gossan Creek is most likely the result of a combined effect of reduction and volatilisation processes, the lighter isotopes being preferentially reduced and evaporated from the water column. Additional sediment samples collected along reaches of Eighteen Mile Brook and the Upsalquitch River that receive water from Gossan Creek, and from tributaries unaffected by the mine contamination were analyzed for mercury isotopic composition. Data obtained from those sediments clearly demonstrated that mercury derived from mining activities is isotopically different from mercury naturally present in the area. Measured isotopic differences can ultimately be employed to trace sources of mercury, notwithstanding potential fractionation caused by natural transformation processes.

Separation of magnesium from meteorites and terrestrial silicate rocks for high-precision isotopic analysis using multiple collector-inductively coupled plasma-mass spectrometry

We report a modified procedure for separating Mg and Al from meteorites and terrestrial igneous rocks for high-precision analysis of Mg isotopes with multiple collector-inductively coupled plasma-mass spectrometry. The separating procedure was carried out in a single ion-exchange column filled with AG50W-X12 resin, and Mg was eluted with 1 M HNO3, followed by Al eluted with 4M HNO3. The modified procedure efficiently eliminates most matrix elements (except for Ni, Co, and Cu) with a recovery yield of Mg > 99%. Measurements of Ni-, Co-, and Cu-bearing simulation solutions revealed no detectable matrix effects. However, test runs demonstrated significant mass-dependent fractionation during the chromatographic process; consequently, a high recovery yield of Mg (>99%) is required to limit the deviation to less than 0.05‰. Furthermore, analysis of Mg standard solution with a wide range of concentrations demonstrated negligible deviation for samples with concentrations of 0.3–2.5 μg ml−1Mg relative to Mg standard solution concentrations of 1 μg ml−1Mg. The total procedure bank is less than 2 ng. The long-term reproducibility of instrumental measurements of Mg isotopes is ±0.05‰ for δ25Mg, ±0.10‰ for δ26Mg, and ±0.06‰ for δ26Mg* (2 SD, n = 211), based on analyzing five pure Mg standard solutions. Analysis of three chondrites and seven igneous rock standards showed ranges of δ26Mg(DSM3) from 0.02‰ to 0.30‰ for the former and from 0.03‰ to 0.30‰ for the latter. This method is simple and actionable.

Formation of spinel-, hibonite-rich inclusions found in CM2 carbonaceous chondrites

We report petrography, mineral chemistry, bulk chemistry, and bulk isotopic compositions of a suite of 40 spinel-rich inclusions from the Murchison (CM2) carbonaceous chondrite. Seven types of inclusions have been identified based on mineral assemblage: spinel-hibonite-perovskite; spinelperovskite- pyroxene; spinel-perovskite-melilite; spinel-hibonite-perovskite-melilite; spinel-hibonite; spinel-pyroxene; and spinel-melilite-anorthite. Hibonite-bearing inclusions have Ti-poor spinel compared to the hibonite-free ones, and spinel-hibonite-perovskite inclusions have the highest average bulk TiO 2 contents (7.8 wt%). The bulk CaO/Al 2 O 3 ratios of the inclusions range from 0.005 to 0.21, well below the solar value of 0.79. Hibonite-, spinel-rich inclusions consist of phases that are not predicted by condensation calculations to coexist; in the equilibrium sequence, hibonite is followed by melilite, which is followed by spinel. Therefore, hibonite-melilite or melilite-spinel inclusions should be dominant instead. One explanation for the .missing melilite. is that it condensed as expected, but was lost due to evaporation of Mg and Ca during heating and melting of spherule precursors. If this theory were correct, melilite-poor spherules would have isotopically heavy Mg and Ca, assuming Rayleigh fractionation accompanied evaporation. Except for one inclusion with F Mg = 4.3 ± 2.6./amu and another with isotopically light Ca (F Ca = -3.4 ± 2.0./amu), however, all the inclusions we analyzed have normal isotopic compositions within their 2σ uncertainties. Thus, we found no evidence for significant mass-dependent fractionation. Conditions necessary for non-Rayleigh evaporation are unlikely if not unrealistic, and our preferred explanation for the general lack of melilite among hibonite-, spinel-bearing inclusions is kinetic inhibition of melilite condensation relative to spinel. Because of similarities between the crystal structures of hibonite and spinel, it should be easier for spinel than for melilite to form from hibonite.