Isotope investigation of mercury sources in a creek impacted by multiple anthropogenic activities

An anodic stripping voltammetric approach for total mercury determination in sea sponges from the Niger Delta region of Nigeria

Mercury pollution from ongoing crude oil refining and waste disposal activities threatens aquatic ecosystems and human health in the Niger Delta. Mercury monitoring exercise in this region is challenging due to the high cost of traditional instruments and the complexity of marine samples. This research presents a novel analytical method using differential pulse anodic stripping voltammetry (DPASV) with a glassy carbon electrode (GCE) to determine mercury levels in sea sponges from the Niger Delta. Using a 2.36 M HCl + 2.4 M NaCl supporting electrolyte, -0.6 V deposition potential, and 300 s deposition time, average mercury levels were found to be 0.98 mg kg-1, 0.63 mg kg-1 and 0.42 mg kg-1 for Ibiotirem, Kaa and Samanga, respectively. The result showed that the Niger Delta is polluted, and remediation efforts are necessary. Furthermore, the DPASV method could be used for routine mercury monitoring as it is cost-effective, user-friendly, and highly sensitive.

Environmental forensics approach to source investigation in a mercury contaminated river: Insights from mercury stable isotopes

Environmental forensics approach was applied to assess the efficacy of mercury (Hg) stable isotopes for source screening and decision-making in the Hyeongsan River, South Korea. Four Hg contamination scenarios were identified- atmospheric Hg emissions from a steel manufacturing industry, upstream riverine Hg transport, and industrial Hg releases and historical landfill collapse from Gumu Creek. The absence of significant Hg isotope difference between the Hyeongsan River sediments (δ202Hg; -0.46 ± 0.17‰, Δ199Hg; -0.04 ± 0.06‰) and the Gumu Creek sediment (δ202Hg; -0.39 ± 0.26‰, Δ199Hg; -0.04 ± 0.03‰) confirm that Hg source is originated from Gumu Creek. The heterogeneous Hg distribution throughout Gumu Creek and statistically similar Hg isotope ratios between Gumu Creek and solid waste cores from the landfill suggests that the landfill collapse is the dominant source to the Hyeongsan-Gumu system. Present Hg releases is also possible given the elevated and matching Δ199Hg between some riverine sediments and wastewater sampled from the landfill. The ternary mixing model estimates that the landfill collapse and wastewater releases contribute 61 ± 25 % and 22 ± 11 %, and the regional background, reflecting terrestrial runoff using deep sediment cores, explain 17 ± 24 % of Hg to the riverine sediment. We suggest that Hg isotopes can be used for routine source screening in areas where Hg sources are unknown.

Current trends and challenges in the analysis of marine environmental contaminants by isotope ratio mass spectrometry

An increasing number of organic and inorganic pollutants are being detected in the marine environment, posing a severe threat to the ecosystem and human health, even in trace concentrations. Isotope ratio mass spectrometry (IRMS) is one of the critical methods for determining the origin and fate of environmental pollutants and characterising their transformation processes. It has been used for a relatively long time for ecological monitoring of some well-studied industrial hydrocarbons at contaminated sites. However, the method still faces many analytical challenges. This review provides a comprehensive overview of recent technical advances concerning IRMS analysis of various contaminants and discusses typical pitfalls encountered in marine environment analysis. Particular attention is given to the study of sampling techniques and sample preparation for examination, often the keys to successful research given the complexity of marine matrices and the diverse and numerous nature of contaminants. Prospects for developing IRMS to monitor pollution sources and pollutant transformation in the marine environment are outlined.

Reconsidering mercury sources and exposure pathways to bivalves: Insights from mercury stable isotopes

Identifying mercury (Hg) sources and exposure pathways to bivalves, particularly in relation to sediment, is important for expanding the utility of bivalves as a monitoring organism for sediment quality. Here we use Hg isotope ratios to decipher Hg sources accumulated into bivalves by conducting field studies and in situ experiments. In the first part of this study, we characterized Hg isotope ratios in individual geochemical fractions of riverine sediment, contaminated by liquid Hg in South Korea (Hyeongsan River; HS). Asian clams (Corbicula fluminea) were then deployed at the contaminated sites to evaluate the isotopic turnover. Over the two-month period, the isotope ratios of the clams shifted toward the labile/exchangeable Hg pools (F1, F2 fractions) of the sediment. Conversely, in the control site where sediment Hg is low, we observed similar Hg isotope ratios between Asian clams and the samples of precipitation and dissolved phase of water column. In East Fork Poplar Creek, (Oak Ridge) U.S., Asian clams also displayed similar Hg isotope ratios with the dissolved phase of water column, which have undergone substantial in-stream processing or input from Hg-contaminated groundwater from the hyporheic zones and riparian tributary during high hydrologic flow seasons. Our study demonstrates that the dissolved Hg phases within the water column, whether originating via sediment diffusion or derived externally, act as the primary source and exposure pathways to bivalves. The results of our study also shed new light to the prior Hg isotope measurement in bivalves collected from estuarine, lake, and coastal systems, which showed significant isotopic deviation from bulk sediment. The fact that bivalves are sensitive to in situ and external dissolved Hg phases provides additional insight into the existing biomonitoring program, which uses bivalves as a bioindicator for sediment quality.

Depth-dependent microbial communities potentially mediating mercury methylation and various geochemical processes in anthropogenically affected sediments

Metal contamination and other geochemical alterations affect microbial composition and functional activities, disturbing natural biogeochemical cycles. Therefore, it is essential to understand the influences of multi-metal and geochemical interactions on microbial communities. This work investigated the distributions of total mercury (THg), methylmercury (MeHg), and trace metals in the anthropogenically affected sediment. The microbial communities and functional genes profiles were further determined to explore their association with Hg-methylation and geochemical features. The levels of THg and MeHg in sediment cores ranged between 10 and 40 mg/kg and 0.01-0.16 mg/kg, respectively, with an increasing trend toward bottom horizons. The major metals present at all depths were Al, Fe, Mn, and Zn. The enrichment and contamination indices confirmed that the trace metals were highly enriched in the anthropogenically affected sediment. Various functional genes were detected in all strata, indicating the presence of active microbial metabolic processes. The microbial community profiles revealed that the phyla Proteobacteria, Bacteroidetes, Bathyarchaeota, and Euryarchaeota, and the genera Thauera, Woeseia, Methanomethylovorans, and Methanosarcina were the dominant microbes. Correlating major taxa with geochemical variables inferred that sediment geochemistry substantially affects microbial community and biogeochemical cycles. Furthermore, archaeal methanogens and the bacterial phyla Chloroflexi and Firmicutes may play crucial roles in enhancing MeHg levels. Overall, these findings shed new light on the microbial communities potentially involved in Hg-methylation process and other biogeochemical cycles.

Tracking the multiple Hg sources in sediments in a typical river-lake basin by isotope compositions and mixing models

In this study, total mercury (THg) contents and Hg isotope compositions in sediments were investigated in the Lianxi River, Zijiang River and South Dongting Lake to identify and quantify multiple Hg sources and evaluate the Hg environmental processes. The THg contents, δ202Hg and Δ199Hg values in sediments were 48.22 ∼ 4284.32 µg/kg, - 1.33 ∼ 0.04‰ and - 0.25 ∼ 0.03‰, respectively. Relatively distinct Hg isotope characteristics of sediments were presented in the Lianxi River, Zijiang River and South Dongting Lake, indicating the dominant Hg sources considerably varied in these regions. Source apportionment based on MixSIAR proved that Hg in sediments mainly originated from industrial activities, and the ternary mixing model concluded non-ferrous metal smelting was the dominant industrial Hg contributor in the Lianxi River. Compared with the Lianxi River, the relative contribution of Hg in sediments from industrial activities significantly decreased, while the relative contributions of Hg from background releases significantly increased in the Zijiang River and South Dongting Lake. Nonetheless, the contribution of industrial Hg in this study area deserves more attention. These results are conducive to further manage Hg pollution.

Mercury stable isotopes in the ocean: Analytical methods, cycling, and application as tracers

Mercury (Hg) has seven stable isotopes that can be utilized to trace the sources of Hg and evaluate the importance of transport and transformation processes in the cycling of Hg in the environment. The ocean is an integral part of the Earth and plays an important role in the global mercury cycle. However, there is a lack of a systematic review of Hg stable isotopes in marine environments. This review is divided into four sections: a) advances in Hg stable isotope analysis, b) the isotope ratios of Hg in various marine environmental matrices (seawater, sediment, and organisms), c) processes governing stable Hg isotope ratios in the ocean, and d) application of Hg stable isotopes to understand biotic uptake and migration. Mercury isotopes have provided much useful information on marine Hg cycling that cannot be given by Hg concentrations alone. This includes (i) sources of Hg in coastal or estuarine environments, (ii) transformation pathways and mechanisms of different forms of Hg in marine environments, (iii) trophic levels and feeding guilds of marine fish, and (iv) migration/habitat changes of marine fish. With the improvement of methods for seawater Hg isotope analysis (especially species-specific methods) and the measurement of Hg isotope fractionation during natural biogeochemical processes in the ocean, Hg stable isotopes will advance our understanding of the marine Hg cycle in the future, e.g., mercury exchange at the sea-atmosphere interface and seawater-sediment interface, contributions of different water masses to Hg in the ocean, fractionation mechanisms of Hg and MeHg transformation in seawater.

Mercury contamination of sediments in an open coastal area of the Hupo Basin, East Sea, Korea

Mercury (Hg) contamination in open coastal areas has attracted public concern regarding safe fish consumption and management of the coastal environment, especially in areas of accidental Hg spills on inland coasts. This study investigated the temporal and spatial distribution of Hg in sediments of Youngil Bay and the Hupo Basin, East Sea, Korea; it also discussed the sources and transport of anthropogenic Hg. Hg hot spots were found in the northern Hupo Basin (elevated by 2-3×) and the river mouth area in Youngil Bay (elevated by approximately 70×). The river mouth contamination resulted from the destruction of a dam impounding landfill waste, while the basin contamination was attributed to atmospheric deposition and Hg enrichment associated with increased organic carbon concentrations driven by high biological production in the coastal upwelling area. Spilled Hg was transported to open coastal areas up to 36.6°N.

Roles of plant-associated microorganisms in regulating the fate of Hg in croplands: A perspective on potential pathways in maintaining sustainable agriculture

In heavy metal-contaminated croplands, plant-associated microorganisms play important roles in the adaptation of crops to heavy metals. Plant-associated microbes can interact with Hg and stimulate plant resistance to Hg toxicity, which is crucial for impeding Hg accumulation along the food chain. The roles of rhizosphere microorganisms for the improvement of plant growth and Hg resistance have drawn great research attention. However, the interactions among plant-endophyte-Hg have been neglected although they might be important for in vivo Hg detoxification. In this study, we systematically summarized 1) the roles of plant-associated microorganisms in Hg detoxification and plant growth, 2) Hg methylation and demethylation driven by plant-associated microbes, 3) the relationships between plant-associated microbes and Hg biogeochemical cycling. The possible mechanisms underlying crop-endophyte-Hg interactions were discussed, although limited studies on this aspect are available to date. The challenges and perspectives of plant-endophytes in dampening Hg phytotoxicity and controlling Hg accumulation in croplands were proposed on the basis of the present knowledge. Taken together, this work provides evidence for further understanding the interactions between soil-plant-endophyte-Hg systems and as well as new interpretations and perspectives into regulating the fate of Hg in croplands.

Mercury in sediment reflecting the intensive coal mining activities: Evidence from stable mercury isotopes and Bayesian mixing model analysis

Severe environmental issues are caused by long-term coal mining activities; however, the process of mercury (Hg) response in mining subsidence area sediments (MSAS) is still unclear, and direct evidence showing the relationship between Hg accumulation mechanism in sediments and mining activities is lacking. In this study, the characteristics of total mercury (THg) content in MSAS were investigated. Moreover, Hg isotopes were obtained to determine the main sources and environmental process of mercury in MSAS, and a MixSIAR mixing model was first used to estimate the potential Hg sources. The THg content ranged from 27.5 to 113.9 ng/g, with a mean of 65.8 ± 29.4 ng/g, exceeding the local soil background value (19.7 ng/g). The Hg in MSAS was affected by clay and organic matter. The Δ199Hg and Δ201Hg in the sediments varied from - 0.05-0.05‰ (mean: -0.01 ± 0.03‰) and - 0.07-0.01‰ (mean: -0.02 ± 0.03‰), respectively, with the fitting results suggesting that a photochemical reaction occurred in some of the Hg in the sediments prior to deposition. The results of the MixSIAR mixing model revealed that the Hg in MSAS was mainly derived from gangue, soil erosion, coal, fly ash, and feed, and their corresponding percentage contribution was 51.5 ± 9.6%, 23.8 ± 13.1%, 13.9 ± 7.9%, 8.1 ± 5.4%, and 3.1 ± 1.4%, respectively. Hg isotopes can be used to trace the transport and transformation of environmental pollutants, and this may provide an important reference for the assessment and prevention of Hg pollution in typical areas such as coal mining and coal-fired.

Stable Isotope Ratios of Mercury in Commercially Available Thermometers and Fluorescent Tubes

Five stable isotope ratios of mercury (¹⁹⁹Hg/¹⁹⁸Hg, ²⁰⁰Hg/¹⁹⁸Hg, ²⁰¹Hg/¹⁹⁸Hg, ²⁰²Hg/¹⁹⁸Hg, and ²⁰⁴Hg/¹⁹⁸Hg) in commercially available thermometers and fluorescent tubes were analyzed to characterize their potential anthropogenic emission source to landfills, manufacturing factories, and our daily lives. The results for the liquid metal mercury yielded from the thermometers showed similar mass-independent fractionation values to those in the literature. The analysis of fluorescent tubes resulted in that more than 96% of mercury in the fluorescent tubes was found in the adsorbed state, and up to 3.5% of mercury was in the gas-phase. Unique mass-independent isotope fractionation values were found in the gaseous and adsorbed mercury in the fluorescent tubes. This fractionation is distinct from other emission sources and systematic; therefore, it can potentially be used to fingerprint mercury in fluorescent tubes in environmental samples.