Isotopic Fractionation and Source Appointment of Methylmercury and Inorganic Mercury in a Paddy Ecosystem

Transport and transformation of colloidal and particulate mercury in contaminated watershed

Submicron colloids ubiquitously present in aquatic environments and can facilitate long transport of absorbed contaminants. Impact of particle size distribution on mercury (Hg) mobility and transformation in the complex aqueous matrices is still unclear. In this study, we considered Hg mine wastes as a natural Hg releasing source to local rivers, and collected water samples from the source to the downstream during high and low flow periods. The water samples were analyzed for Hg morphology, concentration, speciation, and isotope to understand transport and transformation dynamics along the river flows. We found that visible Hg compounds observed by transmission electron microscopy were mainly bound to particles with size fractions of <0.05 and >0.45 μm in the upstream, while the proportion of Hg bound to particles with 0.05-0.45 μm only accounted for 20.0 ± 17.1 % of the total Hg (THg). With increasing distance from the mine waste pile in the downstream, the <50 nm size fraction Hg became the dominant from due to settling of large particles and remained constant throughout the whole river. The Hg isotope results also revealed that the <50 nm size fraction Hg could migrate steadily for long distances into the downstream. Most importantly, a significantly positive correlation was observed between the proportion of the <50 nm size fraction Hg to water THg and the proportion of methylmercury (MeHg) to water THg, indicating the <50 nm size fraction Hg as an important substrate for Hg methylation in the river. These results highlighted the pivotal role of the the <50 nm size particles as a significant reservoir for Hg in aquatic environment.

Sources and Transformation of Methylmercury in Paddy Water: Insights from Mercury Isotopes Collected by Diffusive Gradients in Thin Films

Despite concerns about methylmercury (MeHg) contamination in rice, the sources and transformation mechanisms of MeHg within paddy field water, the primary source of MeHg in rice, remain unclear. Determination of the isotopic composition of MeHg in paddy water is crucial to clarify these processes. However, there is a lack of sampling and analytical methods for quantifying MeHg isotopes in water samples. In this study, we use diffusive gradients in thin films (DGT) in situ to collect MeHg from paddy water to determine the concentration of MeHg and the associated isotopic composition. This technique enables high collection efficiency of aqueous MeHg with limited Hg isotope mass-dependent fractionation (∼- 0.2‰ δ202Hg) and mass-independent fractionation (<0.1‰ Δ199Hg). Field applications using the developed DGT method suggest that in situ methylation of soluble Hg(II) drives the generation of MeHg in paddy water. MeHg in overlying water exhibits a Δ199Hg/Δ201Hg ratio of 1.07 ± 0.09, indicating significant photoreduction of aqueous Hg(II) before methylation. The absence of photodemethylation Δ199Hg/Δ201Hg ratio (∼1.36) suggests limited MeHg demethylation in the overlying water. This study provides insights into the sources and transformation of MeHg in rice paddies and helps develop mitigation strategies to reduce MeHg exposure through rice consumption.

New insights into aqueous Hg(II) photoreduction from paddy field system to natural water: Gear effect of straw returning and soil tillage

Soil dissolved organic matter (SDOM) has a strong complex with divalent mercury (Hg(II)) and can affect the fate of aqueous Hg(II) photoreduction. However, little is known about the influence of straw returning and soil tillage on the composition of SDOM in paddy soil and Hg(II) photoreduction in paddy water. Here, we demonstrate that the combined drivers of long-term straw returning and tillage can result in higher degrees of aromatization, and the enrichment of oxygen-containing functional groups in surface SDOM. Hg(II) photoreduction under low Hg/DOC conditions is mainly constrained by the composition of SDOM, whereas solar radiation emerged as a dominant controlling factor associated with high ratio of Hg/DOC. By increasing the release of SDOM and mobility of Hg(II), reducing the stability of Hg(II)-SDOM complexes, and potentially enhancing generation of reactive intermediates, gear effect of straw returning and soil tillage significantly enhanced Hg(II) photoreduction in the presence of surface SDOM from 0-40 cm (maximum photoreduction percentage can reach 44.76 ± 2.24 %). Previous inventories of Hg(0) emissions from paddy field system may have overlooked or underestimated this critical process. Future modeling work should be carried out to evaluate the role of straw returning and soil tillage on global Hg cycle.

Mitigating Mercury Accumulation and Enhancing Methylmercury Degradation in Rice: Insights from Zinc-Mercury Antagonism at Molecular Levels

Zn as an essential element has the potential to protect against mercury (Hg) toxicity in rice. However, the antagonistic effects between Zn and Hg in rice and their mechanisms remain unknown. This study proposed a promising strategy for Zn application to mitigate Hg accumulation and toxicity in rice and revealed the underlying molecular mechanisms. The findings revealed that Zn supplementation significantly reduced the uptake and transportation of both IHg and MeHg in rice, thereby alleviating Hg phytotoxicity. In particular, Zn profoundly mitigated Hg-induced oxidative damage to rice, which was attributed to the redistribution of Hg and Zn in the root and Zn competing for binding sites on glutathione. The co-binding of Zn2+ and HgCH3+ within the same active sites of Zn transporters can promote the transfer of regions with a high charge density distribution at the highest occupied molecular orbital (HOMO) level. This process facilitates proton attack on the Hg-C bond, thereby enhancing MeHg demethylation in rice. By elucidating the molecular mechanisms of Zn, IHg, and MeHg interactions in rice, this study offers new insights for developing efficient strategies to mitigate Hg risks while boosting the Zn content in crops, thereby fortifying food safety.

Significant biomagnification of methylmercury in songbird nestlings through a rice-based food web: Insights from stable mercury isotopes

To elucidate the sources and transfer of mercury (Hg) in terrestrial food chains, particularly in heavily Hg-contaminated rice paddy ecosystems, we collected rice leaves, invertebrates, and Russet Sparrow nestlings from a clear food chain and analyzed the dietary compositions and potential Hg sources using stable Hg isotopes coupled with a Bayesian isotope mixing model (BIMM). Our findings indicated that MeHg exposure is dominant through the dietary route, with caterpillars, grasshoppers, and katydids being the main prey items, while the less provisioned spiders, dragonflies, and mantises contributed the most of the Hg to nestlings. We found minimal MIF but certain MDF in this terrestrial food chain and identified two distinct MeHg sources of dietary exposure and maternal transfer. We firstly found that the dietary route contributed substantially (almost tenfold) more MeHg to the nestlings than maternal transfer. These findings offer new insights into the integration of Hg from the dietary route and maternal transfers, enhancing our understanding of fluctuating Hg exposure risk during the nestling stage. Our study suggested that Hg isotopes combined with BIMM is an effective approach for tracing Hg sources in birds and for gaining in-depth insight into the trophic transfers and biomagnification of MeHg in food chains.

Mercury stable isotopes in seabirds in the Ebro Delta (NE Iberian Peninsula): Inter-specific and temporal differences

Mercury (Hg) is a global pollutant, which particularly affects aquatic ecosystems, both marine and freshwater. Top-predators depending on these environments, such as seabirds, are regarded as suitable bioindicators of Hg pollution. In the Ebro Delta (NE Iberian Peninsula), legacy Hg pollution from a chlor-alkali industry operating in Flix and located ca. 100 km upstream of the Ebro River mouth has been impacting the delta environment and the neighboring coastal area. Furthermore, levels of Hg in the biota of the Mediterranean Sea are known to be high compared to other marine areas. In this work we used a Hg stable isotopes approach in feathers to understand the processes leading to different Hg concentrations in three Laridae species breeding in sympatry in the area (Audouin's gull Ichthyaetus audouinii, black-headed gull Chroicocephalus ridibundus, common tern Sterna hirundo). These species have distinct trophic ecologies, exhibiting a differential use of marine resources and freshwater resources (i.e., rice paddies prey). Moreover, for Audouin's gull, in which in the Ebro Delta colony temporal differences in Hg levels were documented previously, we used Hg stable isotopes to understand the impact of anthropogenic activities on Hg levels in the colony over time. Hg stable isotopes differentiated the three Laridae species according to their trophic ecologies. Furthermore, for Audouin's gull we observed temporal variations in Hg isotopic signatures possibly owing to anthropogenic-derived pollution in the Ebro Delta. To the best of our knowledge this is the first time Hg stable isotopes have been reported in seabirds from the NW Mediterranean.

Unveiling the Sources and Transfer of Mercury in Forest Bird Food Chains Using Techniques of Vivo-Nest Video Recording and Stable Isotopes

Knowledge gaps in mercury (Hg) biomagnification in forest birds, especially in the most species-rich tropical and subtropical forests, limit our understanding of the ecological risks of Hg deposition to forest birds. This study aimed to quantify Hg bioaccumulation and transfer in the food chains of forest birds in a subtropical montane forest using a bird diet recorded by video and stable Hg isotope signals of biological and environmental samples. Results show that inorganic mercury (IHg) does not biomagnify along food chains, whereas methylmercury (MeHg) has trophic magnification factors of 7.4-8.1 for the basal resource-invertebrate-bird food chain. The video observations and MeHg mass balance model suggest that Niltava (Niltava sundara) nestlings ingest 78% of their MeHg from forest floor invertebrates, while Flycatcher (Eumyias thalassinus) nestlings ingest 59% from emergent aquatic invertebrates (which fly onto the canopy) and 40% from canopy invertebrates. The diet of Niltava nestlings contains 40% more MeHg than that of Flycatcher nestlings, resulting in a 60% higher MeHg concentration in their feather. Hg isotopic model shows that atmospheric Hg0 is the main Hg source in the forest bird food chains and contributes >68% in most organisms. However, three categories of canopy invertebrates receive ∼50% Hg from atmospheric Hg2+. Overall, we highlight the ecological risk of MeHg exposure for understory insectivorous birds caused by atmospheric Hg0 deposition and methylation on the forest floor.

New Insights into MeHg Accumulation in Rice (Oryza sativa L.): Evidence from Cysteine

The intake of methylmercury (MeHg)-contaminated rice poses immense health risks to rice consumers. However, the mechanisms of MeHg accumulation in rice plants are not entirely understood. The knowledge that the MeHg-Cysteine complex was dominant in polished rice proposed a hypothesis of co-transportation of MeHg and cysteine inside rice plants. This study was therefore designed to explore the MeHg accumulation processes in rice plants by investigating biogeochemical associations between MeHg and amino acids. Rice plants and underlying soils were collected from different Hg-contaminated sites in the Wanshan Hg mining area. The concentrations of both MeHg and cysteine in polished rice were higher than those in other rice tissues. A significant positive correlation between MeHg and cysteine in rice plants was found, especially in polished rice, indicating a close geochemical association between cysteine and MeHg. The translocation factor (TF) of cysteine showed behavior similar to that of the TF of MeHg, demonstrating that these two chemical species might share a similar transportation mechanism in rice plants. The accumulation of MeHg in rice plants may vary due to differences in the molar ratios of MeHg to cysteine and the presence of specific amino acid transporters. Our results suggest that cysteine plays a vital role in MeHg accumulation and transportation inside rice plants.

Insights into the reduction of methylmercury accumulation in rice grains through biochar application: Hg transformation, isotope fractionation, and transcriptomic analysis

Methylmercury (MeHg), a potent neurotoxin, easily moves from the soil into rice plants and subsequently accumulates within the grains. Although biochar can reduce MeHg accumulation in rice grains, the precise mechanism underlying biochar-mediated responses to mercury (Hg) stress, specifically regarding MeHg accumulation in rice, remains poorly understood. In the current study, we employed a 4% biochar amendment to remediate Hg-contaminated paddy soil, elucidate the impacts of biochar on MeHg accumulation through a comprehensive analysis involving Hg isotopic fractionation and transcriptomic analyses. The results demonstrated that biochar effectively lowered the levels of MeHg in paddy soils by decreasing bioavailable Hg and microbial Hg methylation. Furthermore, biochar reduced the uptake and translocation of MeHg in rice plants, ultimately leading to a reduction MeHg accumulation in rice grains. During the process of total mercury (THg) uptake, biochar induced a more pronounced negative isotope fractionation magnitude, whereas the effect was less pronounced during the upward transport of THg. Conversely, biochar caused a more pronounced positive isotope fractionation magnitude during the upward transport of MeHg. Transcriptomics analyses revealed that biochar altered the expression levels of genes associated with the metabolism of cysteine, glutathione, and metallothionein, cell wall biogenesis, and transport, which possibly enhance the sequestration of MeHg in rice roots. These findings provide novel insights into the effects of biochar application on Hg transformation and transport, highlighting its role in mitigating MeHg accumulation in rice.

Determination of the Isotopic Composition of Aqueous Mercury in a Paddy Ecosystem Using Diffusive Gradients in Thin Films

Measuring the isotopic composition of Hg in natural waters is challenging due to the ultratrace level of aqueous Hg (ng L-1). At least 5 ng of Hg mass is required for Hg isotopic analysis. Given the low Hg concentration in natural waters, a large volume of water (>10 L) is typically needed. The conventional grab sampling method is time-consuming, laborious, and prone to contamination during transportation and preconcentration steps. In this study, a DGT (diffusive gradients in thin films) method based on aminopropyl and mercaptopropyl bi-functionalized SBA-15 nanoparticles was developed and extended to determine the concentration and isotopic composition of aqueous Hg for the first time. The results of laboratory analysis showed that Hg adsorption by DGT induces ∼ -0.2‰ mass-dependent fractionation (MDF) and little mass-independent fractionation (MIF). The magnitude of MDF exhibits a dependence on the diffusion-layer thickness of DGT. Since Hg-MDF can occur in a broad range of environmental processes, monitoring the δ202Hg of aqueous Hg using the DGT method should be performed with caution. Field results show consistent MIF signatures (Δ199Hg) between the DGT and conventional grab sampling method. The developed DGT method serves as a passive sampling method that effectively characterizes the MIF of Hg in waters to understand the biogeochemical cycle of Hg at contaminated sites.

Oral and inhalation bioaccessibility of mercury in contaminated soils and potential health risk to the kidneys and neurodevelopment of children in Taiwan

Health risk assessments of exposure to mercury (Hg) from soils via ingestion and inhalation are indispensable for Taiwanese people living in the vicinity of Hg-contaminated sites. In this study, anthropogenic soils were collected from various polluted sources in Taiwan. In vitro oral and inhalation bioaccessible fractions of Hg were analyzed to avoid from overestimating the exposure risk. Discrepancies in oral and inhalation bioaccessible levels of Hg in soils were found using different in vitro assays with different pH levels and chemical compositions. The freshly contaminated soil (soil S7) polluted by chlor-alkali production activity sampled before the site was remediated had the highest total Hg concentration of 1346 mg/kg, with the highest oral bioaccessibility of 26.2% as analyzed by SW-846 Method 1340 and inhalation bioaccessibility of 30.5% as analyzed by modified Gamble's solution. The lesser extent of aging of Hg in soil S7 increased the Hg availability for humans, which was also found based on results of a sequential extraction procedure. Results of the hazard quotient showed that soil ingestion was the main pathway causing non-carcinogenic risks for children and adults. Children were also exposed to higher risks than were adults due to higher frequencies of hand-to-mouth behaviors and lower body weights. Furthermore, hazard index results adjusted for oral and inhalation bioaccessible Hg were lower than those obtained based on the total Hg content; however, an unacceptable value of the non-carcinogenic risk (> 1) for children living near soil S7 was still observed. This study suggests that children living near sites polluted for a short period of time may suffer potential renal effects regardless of the bioaccessibility. Our findings provide suggestions for decision makers on setting new strategies for managing risks of Hg-contaminated soils in Taiwan.

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.

Highly selective fluorescent probe for detecting mercury ions in water

Mercury ion (Hg²⁺) is a well-known toxic heavy metal. It has become one of the most significant environmental pollutants in the world because of its serious physiological toxicity, persistence, easy migration, and high bioconcentration. Thus, the development of methods for monitoring Hg²⁺ is indispensable. Herein, we have designed and synthesized a new fluorescent probe, TPH, for the detection of Hg²⁺ in the water environment. The TPH probe could quantitatively detect Hg²⁺ between 0 and 5 μM (LOD = 16 nM), with a linear range of 0-2.5 μM. In addition, the TPH probe was used to monitor Hg²⁺ in water samples successfully. Thus, this probe is suitable for monitoring Hg²⁺ in the actual water environment.

Mercury and Sulfur Redox Cycling Affect Methylmercury Levels in Rice Paddy Soils across a Contamination Gradient

Methylmercury (MeHg) contamination in rice via paddy soils is an emerging global environmental issue. An understanding of mercury (Hg) transformation processes in paddy soils is urgently needed in order to control Hg contamination of human food and related health impacts. Sulfur (S)-regulated Hg transformation is one important process that controls Hg cycling in agricultural fields. In this study, Hg transformation processes, such as methylation, demethylation, oxidation, and reduction, and their responses to S input (sulfate and thiosulfate) in paddy soils with a Hg contamination gradient were elucidated simultaneously using a multi-compound-specific isotope labeling technique (200HgII, Me198Hg, and 202Hg0). In addition to HgII methylation and MeHg demethylation, this study revealed that microbially mediated reduction of HgII, methylation of Hg0, and oxidative demethylation-reduction of MeHg occurred under dark conditions; these processes served to transform Hg between different species (Hg0, HgII, and MeHg) in flooded paddy soils. Rapid redox recycling of Hg species contributed to Hg speciation resetting, which promoted the transformation between Hg0 and MeHg by generating bioavailable HgII for fuel methylation. Sulfur input also likely affected the microbial community structure and functional profile of HgII methylators and, therefore, influenced HgII methylation. The findings of this study contribute to our understanding of Hg transformation processes in paddy soils and provide much-needed knowledge for assessing Hg risks in hydrological fluctuation-regulated ecosystems.

Plastispheres as hotspots of microbially-driven methylmercury production in paddy soils

Microplastics (MPs) as emerging contaminants have accumulated extensively in agricultural ecosystems and are known to exert important effects on biogeochemical processes. However, how MPs in paddy soils influence the conversion of mercury (Hg) to neurotoxic methylmercury (MeHg) remains poorly understood. Here, we evaluated the effects of MPs on Hg methylation and associated microbial communities in microcosms using two typical paddy soils in China (i.e., yellow and red soils). Results showed that the addition of MPs significantly increased MeHg production in both soils, which could be related to higher Hg methylation potential in the plastisphere than in the bulk soil. We found significant divergences in the community composition of Hg methylators between the plastisphere and the bulk soil. In addition, the plastisphere had higher proportions of Geobacterales in the yellow soil and Methanomicrobia in the red soil compared with the bulk soil, respectively; and plastisphere also had more densely connected microbial groups between non-Hg methylators and Hg methylators. These microbiota in the plastisphere are different from those in the bulk soil, which could partially account for their distinct MeHg production ability. Our findings suggest plastisphere as a unique biotope for MeHg production and provide new insights into the environment risks of MP accumulation in agricultural soils.

Effects of varying amounts of different biochars on mercury methylation in paddy soils and methylmercury accumulation in rice (Oryza sativa L.)

There is growing evidence for the potential of biochars (BCs) in remediating mercury-contaminated paddy soils, but the high doses commonly used in laboratory studies discourage BC application in practice. To address these difficulties, we compared the effects of varying amounts of BCs from different sources on the formation of methylmercury (MeHg) in soil and its accumulation in rice through microcosm and pot experiments. The addition of a wide range of added doses (0.3, 0.6, 1, 2, 4 and 5 %, w/w) of BCs derived from different biomass feedstocks (i.e., corn stalk, wheat straw, bamboo, oak and poplar) significantly decreased the fraction of ammonium thiosulfate ((NH₄)₂S₂O₃)-extractable MeHg in the soil, although the MeHg contents varied with BC types and doses during soil incubation. However, the extractable MeHg in the soil did not continuously decrease with increasing BC doses, especially at doses of >1 %, resulting in limited further reductions. Moreover, a relatively low application rate (0.3-0.6 %, w/w) of BCs (i.e., corn stalk, wheat straw and bamboo-derived BC), especially of bamboo-derived BCs, significantly decreased the MeHg levels (42-76 %) in rice grains (brown rice). Meanwhile, the extractable soil MeHg decreased (57-85 %), although the MeHg in the soil varied under BC amendment during rice cultivation. These results provide further evidence that applying BC produced from different raw carbon materials (e.g., lignocellulosic biomass) could effectively reduce MeHg accumulation in rice grains, possibly due to MeHg bioavailability reduction in the soil. Our results suggest the possibility of mitigating MeHg accumulation in rice with a low dose of BCs, with great potential for use in remediating moderately contaminated paddy soils.

Mercury Biogeochemical Cycle in Yanwuping Hg Mine and Source Apportionment by Hg Isotopes

Although mercury (Hg) mining activities in the Wanshan area have ceased, mine wastes remain the primary source of Hg pollution in the local environment. To prevent and control Hg pollution, it is crucial to estimate the contribution of Hg contamination from mine wastes. This study aimed to investigate Hg pollution in the mine wastes, river water, air, and paddy fields around the Yanwuping Mine and to quantify the pollution sources using the Hg isotopes approach. The Hg contamination at the study site was still severe, and the total Hg concentrations in the mine wastes ranged from 1.60 to 358 mg/kg. The binary mixing model showed that, concerning the relative contributions of the mine wastes to the river water, dissolved Hg and particulate Hg were 48.6% and 90.5%, respectively. The mine wastes directly contributed 89.3% to the river water Hg contamination, which was the main Hg pollution source in the surface water. The ternary mixing model showed that the contribution was highest from the river water to paddy soil and that the mean contribution was 46.3%. In addition to mine wastes, paddy soil is also impacted by domestic sources, with a boundary of 5.5 km to the river source. This study demonstrated that Hg isotopes can be used as an effective tool for tracing environmental Hg contamination in typical Hg-polluted areas.

Identification of sources and their potential health risk of potential toxic elements in soils from a mercury‑thallium polymetallic mining area in Southwest China: Insight from mercury isotopes and PMF model

Identification of potential toxic element (PTE) sources and their specific human health risk is critical to the management of PTEs in soils. In this study, multi-medium were collected from a mercury‑thallium polymetallic mining area in Southwestern China. Hg isotope technique together with positive matrix factorization (PMF) model was used to identify PTE sources and assess their source-oriented health risk. Results showed that among the studied PTEs, this study area presented high pollution of Hg, Tl and As, with higher concentrations than their corresponding background values of Guizhou province, yet their average concentrations in covering soils were significantly lower than those in the natural soils. The Tl in coix grains should also be paid more attention due to its high concentration. Both natural and covering soils had different Hg isotope composition with tailings, while sediments have similar Hg isotope fractionation with covering soils. According to the PMF model, three sources in both natural and covering soils were apportioned and Hg, Tl and As were mainly influenced by the historical mining activities, which also confirmed by their Hg isotope signatures. The contributions of historical mining activities accounted for 40 % and 20 % of the PTEs in natural and covering soils, respectively. The assessment of source-specific health risks suggested that the non-carcinogenic risk of Hg, Tl and As was much higher than other elements. Historical mining activities were regarded as the major contributor to health risks (79 % and 76 % for natural soils and 50 % and 59 % for covering soils, respectively). This indicated that the restoration of coveing soils indeed decreased the health risk in this study area. These findings thus highlight the importance of ongoing monitoring of covering soils in the polymetallic mining area, which is imperative for preferably assessing the health risk of PTEs in similar mining area worldwide.

Trace mercury migration and human exposure in typical mercury-emission areas by compound-specific stable isotope analysis

Anthropogenic mercury (Hg) emissions have increased significantly since the Industrial Revolution, resulting in severe health impacts to humans. The consumptions of fish and rice were primary human methylmercury (MeHg) exposure pathways in Asia. However, the lifecycle from anthropogenic Hg emissions to human MeHg exposure is not fully understood. In this study, a recently developed approach, termed MeHg Compound-Specific Isotope Analysis (CSIA), was employed to track lifecycle of Hg in four typical Hg-emission areas. Distinct Δ¹⁹⁹Hg of MeHg and inorganic Hg (IHg) were observed among rice, fish and hair. The Δ¹⁹⁹Hg of MeHg averaged at 0.07 ± 0.15 ‰, 0.80 ± 0.55 ‰ and 0.43 ± 0.29 ‰ in rice, fish and hair, respectively, while those of IHg averaged at - 0.08 ± 0.24 ‰, 0.85 ± 0.43 ‰ and - 0.28 ± 0.68 ‰. In paddy ecosystem, Δ¹⁹⁹Hg of MeHg in rice showed slightly positive shifts (∼0.2 ‰) from those of IHg, and comparable Δ¹⁹⁹Hg of IHg between rice grain and raw/processed materials (coal, Hg ore, gold ore and sphalerite) were observed. Simultaneously, it was proved that IHg in fish muscle was partially derived from in vivo demethylation of MeHg. By a binary model, we estimated the relative contributions of rice consumption to human MeHg exposure to be 84 ± 14 %, 58 ± 26 %, 52 ± 20 % and 34 ± 15 % on average in Hg mining area, gold mining area, zinc smelting area and coal-fired power plant area, respectively, and positive shifts of δ²⁰²Hg_MeHg from fish/rice to human hair occurred during human metabolic processes. Therefore, the CSIA approach can be an effective tool for tracking Hg biogeochemical cycle and human exposure, from which new scientific knowledge can be generated to support Hg pollution control policies and to protect human health.

Effect of peat and thiol-modified peat application on mercury (im)mobilization in mercury-polluted paddy soil

Mercury (Hg) pollution in paddy soil has gained special attention because methylmercury (MeHg) can accumulate in rice grains. Therefore, there is an urgent need to explore the remediation materials of mercury-polluted paddy soil. In this study, herbaceous peat (HP), peat moss (PM), and thiol-modified HP/PM (MHP/MPM) were selected to investigate the effects and probable mechanism of their application on Hg (im)mobilization in mercury-polluted paddy soil through pot experiments. The results showed that HP, PM, MHP and MPM addition increased MeHg concentrations in the soil, indicating that the addition of peat and thiol-modified peat might increase the exposure risk of MeHg in soil. The addition of HP could significantly decrease the total mercury (THg) and MeHg concentrations in rice, with average reduction efficiencies of 27.44% and 45.97%, respectively, while adding PM slightly increased the THg and MeHg concentrations in rice. In addition, the addition of MHP and MPM significantly decreased the bioavailable Hg concentrations in the soil and THg and MeHg concentrations in rice, with reduction efficiencies of rice THg and MeHg of 79.14∼93.14% and 82.72∼93.87%, respectively, indicating that thiol-modified peat had good remediation potential. The possible mechanism is that Hg can bind with thiols in MHP/MPM and form steady compounds in the soil, reducing Hg mobility in the soil and inhibiting its uptake by rice. Our study showed the potential value of HP, MHP and MPM addition for Hg remediation. Additionally, we must weigh the pros and cons when adding organic materials as remediation agents to mercury-polluted paddy soil.

Coupling of nitric acid digestion and anion-exchange resin separation for the determination of methylmercury isotopic composition within organisms

Isotope ratios of methylmercury (MeHg) within organisms can be used to identify sources of MeHg that have accumulated in food webs, but these isotopic compositions are masked in organisms at lower trophic levels by the presence of inorganic mercury (iHg). To facilitate measurement of MeHg isotope ratios in organisms, we developed a method of extracting and isolating MeHg from fish and aquatic invertebrates for compound-specific isotopic analysis involving nitric acid digestion, batch anion-exchange resin separation, and pre-concentration by purge and trap. Recovery of MeHg was quantified after each step in the procedure, and the average cumulative recovery of MeHg was 93.4 ± 2.9% (1 SD, n = 28) for biological reference materials and natural biota samples and 96.9 ± 1.8% (1 SD, n = 5) for aqueous MeHgCl standards. The amount of iHg impurities was also quantified after each step, and the average MeHg purity was 97.8 ± 4.3% (1 SD, n = 28) across all reference materials and natural biota samples after the final separation step. Measured MeHg isotopic compositions of reference materials agreed with literature values obtained using other MeHg separation techniques, and MeHg isotope ratios of aqueous standards, reference materials, and natural biota samples were reproducible. On average, the reproducibility associated with reference material process replicates (2 SD) was 0.10‰ for δ202MeHg and 0.04‰ for Δ199MeHg. This new method provides a streamlined, reliable technique that utilizes a single sample aliquot for MeHg concentration and isotopic analysis. This promotes a tight coupling between MeHg concentration, %MeHg, and Hg isotopic composition, which may be especially beneficial for studying complex food webs with multiple isotopically distinct sources of iHg and/or MeHg.

Methylmercury formation in biofilms of Geobacter sulfurreducens

Introduction

Mercury (Hg) is a major environmental pollutant that accumulates in biota predominantly in the form of methylmercury (MeHg). Surface-associated microbial communities (biofilms) represent an important source of MeHg in natural aquatic systems. In this work, we report MeHg formation in biofilms of the iron-reducing bacterium Geobacter sulfurreducens.

Methods

Biofilms were prepared in media with varied nutrient load for 3, 5, or 7 days, and their structural properties were characterized using confocal laser scanning microscopy, cryo-scanning electron microscopy and Fourier-transform infrared spectroscopy.

Results

Biofilms cultivated for 3 days with vitamins in the medium had the highest surface coverage, and they also contained abundant extracellular matrix. Using 3 and 7-days-old biofilms, we demonstrate that G. sulfurreducens biofilms prepared in media with various nutrient load produce MeHg, of which a significant portion is released to the surrounding medium. The Hg methylation rate constant determined in 6-h assays in a low-nutrient assay medium with 3-days-old biofilms was 3.9 ± 2.0 ∙ 10^-14  L ∙ cell^-1 ∙ h^-1, which is three to five times lower than the rates found in assays with planktonic cultures of G. sulfurreducens in this and previous studies. The fraction of MeHg of total Hg within the biofilms was, however, remarkably high (close to 50%), and medium/biofilm partitioning of inorganic Hg (Hg(II)) indicated low accumulation of Hg(II) in biofilms.

Discussion

These findings suggest a high Hg(II) methylation capacity of G. sulfurreducens biofilms and that Hg(II) transfer to the biofilm is the rate-limiting step for MeHg formation in this systems.

Effect of Iron Plaque on Gaseous Elemental Mercury Uptake, Translocation, and Volatilization by Paddy Rice

Paddy rice is a typical wetland plant species, and mercury (Hg) accumulation in this rice has received much attention over the last two decades. The role of root iron plaque on rice Hg accumulation is not well understood. The effects of iron plaque on Hg⁰ uptake, translocation, and volatilization in rice seedlings were investigated under hydroponic conditions using different rice genotypes. After induction of iron plaque on rice roots with pretreatment solutions containing 0, 15 and 30 mg Fe²⁺L^-1, rice seedlings were transplanted into specially designed airtight culture chambers, where roots were separated from the aerial parts and exposed to saturated Hg⁰ vapor. The results showed the following: (1) There were significant differences in the amount of iron plaque formed on the rice roots among the three genotypes. (2) A significant correlation was observed between the concentrations of Hg and Fe in the iron plaque of the root surface for the three genotypes (R² = 0.933, p < 0.01). (3) Iron plaque may act as a barrier for Hg⁰ behavior, i.e., inhibiting the process of Hg⁰ uptake and translocation from the rhizosphere.

Inorganic versus organic fertilizers: How do they lead to methylmercury accumulation in rice grains

Both inorganic and organic fertilizers are widely used to increase rice yield. However, these fertilizers are also found to aggravate mercury methylation and methylmercury (MeHg) accumulation in paddy fields. The aim of this study was to reveal the mechanisms of inorganic and organic fertilizers on MeHg accumulation in rice grains, which are not yet well understood. Potting cultures were conducted in which different fertilizers were applied to a paddy soil. The results showed that both inorganic and organic fertilizers increased MeHg concentrations rather than biological accumulation factors (BAFs) of MeHg in mature rice grains. Inorganic fertilizers, especially nitrogen fertilizer, enhanced the bioavailability of mercury and the relative amount Hg-methylating microbes and therefore intensified mercury methylation in paddy soil and MeHg accumulation in rice grains. Unlike inorganic fertilizers, organic matter (OM) in organic fertilizers was the main reason for the increase of MeHg concentrations in rice grains, and it also could immobilize Hg in soil when it was deeply degraded. The enhancement of MeHg concentrations in rice grains induced by inorganic fertilizers (5.18-41.69%) was significantly (p < 0.05) lower than that induced by organic fertilizers (80.49-106.86%). Inorganic fertilizers led to a larger increase (50.39-99.28%) in thousand-kernel weight than MeHg concentrations (5.18-41.69%), resulting in a dilution of MeHg concentrations in mature rice grains. Given the improvement of soil properties by organic fertilizer, increasing the proportion of inorganic fertilizer application may be a better option to alleviate MeHg accumulation in rice grains and guarantee the rice yield in the agricultural production.

Transcriptomic and physio-biochemical features in rice (Oryza sativa L.) in response to mercury stress

Mercury (Hg) is a toxic and nonessential element for organisms, and its contamination in the environment is a global concern. Previous research has shown that Hg stress may cause severe damage to rice roots; however, the transcriptomic changes in roots and physio-biochemical responses in leaves to different levels of Hg stress are not fully understood. In the present study, rice seedlings were exposed to 20, 80, and 160 μM HgCl₂ for three days in hydroponic experiments. The results showed that the majority of Hg was accumulated in rice roots after Hg exposure, and the 80- and 160-μM Hg stresses significantly increased the root-to-shoot translocation factors relative to 20-μM Hg stress, resulting in elevated Hg concentrations in rice shoots. Only the 160-μM Hg stress significantly inhibited root growth compared with the control, while photosynthesis capacity in leaves was significantly reduced under Hg stress. RNA transcriptome sequencing analyses of the roots showed that common responsive differentially expressed genes were strongly associated with glutathione metabolism, amino acid biosynthesis, and secondary metabolite metabolism, which may play significant roles in Hg accumulation by rice plants. Nine crucial genes identified by protein-protein interaction network analysis may be used as candidate target genes for further investigation of the detoxification mechanism, encoding proteins involved in jasmonic acid synthesis, sugar metabolism, allene oxide synthase, glutathione peroxidase, dismutase, and catalase. Furthermore, physio-biochemical analyses of the leaves indicated that higher production of reactive oxygen species was induced by Hg stress, while glutathione and antioxidant enzymes may play crucial roles in Hg detoxification. Our findings provide transcriptomic and physio-biochemical features of rice roots and shoots, which advance our understanding of the responsive and detoxification mechanisms in rice under different levels of Hg stress.

Mercury isotope fractionation during methylmercury transport and transformation: A review focusing on analytical method, fractionation characteristics, and its application

Methylmercury (MeHg), a potent neurotoxin, can be formed, migrated and transformed in environmental compartments, accompanying with unique mass-dependent and mass-independent fractionation of mercury (Hg). These Hg isotope fractionation signals have great potential to probe the transformation and transport of MeHg in aquatic environments. However, the majority of studies to date have focused on total Hg isotopic composition, with less attention to the isotopic fractionation of MeHg due to technical difficulties in analysis, which severely hinders the understanding of MeHg isotopic fractionation and its applications. This review a) evaluates the reported analytical methods for Hg isotopic composition of MeHg, including online and offline measurement techniques; b) summarizes the extent and characteristics of Hg isotopic fractionation during MeHg transport and transformation, focusing on methylation, demethylation, trophic transfer and internal metabolism; and c) briefly discusses several applications of MeHg isotopic fractionation signatures in estimating the extent of photodemethylation, tracing the source of Hg species, and diagnosing reaction mechanisms. Additionally, the existing problems and future directions in MeHg isotope fractionation are highlighted to improve the analytical protocol for Hg isotope fractionation and deepen our understanding of Hg isotope fractionation in the biogeochemical cycling of MeHg.

Reclaimable MoS2 Sponge Absorbent for Drinking Water Purification Driven by Solar Energy

With the fast development of modern industries, scarcity of freshwater resources caused by heavy metal pollution (i.e., Hg²⁺) has become a severe issue for human beings. Herein, a 3D-MoS₂ sponge as an excellent absorbent is fabricated for mercury removal due to its multidimensional adsorption pathways, which decreases the biomagnification effect of methylmercury in water bodies. Furthermore, a secondary water purification strategy is employed to harvest drinkable water with the exhausted adsorbents, thus alleviating the crisis of drinking water shortage. Compared to the conventional landfill treatment, the exhausted MoS₂ sponge absorbents are further functionalized with a poly(ethylene glycol) (PEG) layer to prevent the heavy metals from leaking and enhance the hydrophilicity for photothermal conversion. The fabricated evaporator displays excellent evaporation rates of ∼1.45 kg m^-2 h^-1 under sunlight irradiation and produces freshwater with Hg²⁺ under the WHO drinking water standard at 0.001 mg L^-1. These results not only assist in avoiding the biodeposition effect of mercury in water but also provide an environment-friendly strategy to recycle hazardous adsorbents for water purification.

Source Apportionment of Speciated Mercury in Chinese Rice Grain Using a High-Resolution Model

Rice grain consumption is a primary pathway of human mercury exposure. To trace the source of rice grain mercury in China, we developed a rice paddy mercury transport and transformation model with a grid resolution of 1 km × 1 km by using the unit cell mass conservation method. The simulated total mercury (THg) and methylmercury (MeHg) concentrations in Chinese rice grain ranged from 0.08 to 243.6 and 0.03 to 238.6 μg/kg, respectively, in 2017. Approximately, 81.3% of the national average rice grain THg concentration was due to atmospheric mercury deposition. However, soil heterogeneity, especially the variation in soil mercury, led to the wide rice grain THg distribution across grids. Approximately, 64.8% of the national average rice grain MeHg concentration was due to soil mercury. In situ methylation was the main pathway via which the rice grain MeHg concentration was increased. The coupled impact of high mercury input and methylation potential led to extremely high rice grain MeHg in partial grids among Guizhou province and junctions with surrounding provinces. The spatial variation in soil organic matter significantly impacted the methylation potential among grids, especially in Northeast China. Based on the high-resolution rice grain THg concentration, we identified 0.72% of grids as heavily polluted THg grids (rice grain THg > 20 μg/kg). These grids mainly corresponded to areas in which the human activities of nonferrous metal smelting, cement clinker production, and mercury and other metal mining were conducted. Thus, we recommended measures that are targeted at the control of heavy pollution of rice grain by THg according to the pollution sources. In addition, we observed a wide spatial variation range of MeHg to THg ratios not only in China but also in other regions of the world, which highlights the potential risk of rice intake.

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.

Solvent-Free Preparation of Tannic Acid Carbon Dots for Selective Detection of Ni2+ in the Environment

Carbon dots (CDs) are widely used nanomaterials that not only exhibit good biocompatibility and photostability, but also benefit from a simple preparation process and easy functionalization, making them promising for broad applications in the fields of heavy metal ion detection and optoelectronic devices. Based on the excellent optical properties of CDs and the current situation of increasing energy shortages, this paper selects the natural polyphenolic compound tannic acid (TA) found in biomass materials as the carbon source and innovatively adopts a simple and convenient solvent-free pyrolysis method without auxiliary reagents or solvents. The CDs with good water solubility and certain fluorescence properties were directly prepared under the condition of high temperature, and the obtained CDs exhibited blue fluorescence, and a high QY of 35.4% was obtained at 300 °C. The analysis and results demonstrate the selectivity of these CDs for the detection of various metal ion solutions. In particular, these CDs are sensitive to Ni²⁺ and can be used as fluorescent sensors for the efficient and sustainable detection of Ni²⁺, whereas previous sensors were often specific to Fe³⁺ and Hg²⁺. Thus, a new sensing technique has been developed for the detection of Ni²⁺ to achieve more sensitive and rapid detection.

Mercury pollution in China: implications on the implementation of the Minamata Convention

Mercury (Hg) is a toxic metal released into the environment through human activities and natural processes. Human activities have profoundly increased the amount of Hg in the atmosphere and altered its global cycling since the Industrial Revolution. Gaseous elemental Hg is the predominant form of Hg in the atmosphere, which can undergo long-range transport and atmospheric deposition into the aquatic systems. Hg deposition elevates the methylmercury (MeHg) level in fish through bioaccumulation and biomagnification, which poses a serious human health risk. Acute poisoning of MeHg can result in Minamata disease, while low-level long-term exposure in pregnant women can reduce the intelligence quotient of infants. After five sessions of intergovernmental negotiation, the Minamata Convention on mercury entered into force in August 2017 to protect human health and the environment from Hg pollution. Currently China contributes the largest quantity of Hg production, consumption, and emission globally. However, the status of Hg pollution in the environment in China and its associated health risk remains relatively unknown, which hinders the development of implementation plans of the Minamata Convention. In this paper, we provide a comprehensive review on the atmospheric release of Hg, distribution of air Hg concentration, human exposure to MeHg and health impacts caused by Hg pollution in China. Ongoing improvement of air pollution control measures is expected to further decrease anthropogenic Hg emissions in China. Air Hg concentrations in China are higher than the background values in the Northern Hemisphere, with spatial distribution largely influenced by anthropogenic emissions. Long-term observations of GEM in China show a decline in recent years. The net Hg transport outflow from China in 2013 is estimated to be 511 t year^-1, and ∼60% of such outflow is caused by natural surface Hg emissions. Hg concentrations in fish and rice in China are relatively low and therefore the associated risks of human Hg exposure are low. Future research needs and recommendations for the implementation of the Minamata Convention are also discussed in this paper.

Important Roles of Thiols in Methylmercury Uptake and Translocation by Rice Plants

The bioaccumulation of the neurotoxin methylmercury (MeHg) in rice is a significant concern due to its potential risk to humans. Thiols have been known to affect MeHg bioavailability in microorganisms, but how thiols influence MeHg accumulation in rice plants remains unknown. Here, we investigated effects of common low-molecular-weight thiols, including cysteine (Cys), glutathione (GSH), and penicillamine (PEN), on MeHg uptake and translocation by rice plants. Results show that rice roots can rapidly take up MeHg, and this process is influenced by the types and concentrations of thiols in the system. The presence of Cys facilitated MeHg uptake by roots and translocation to shoots, while GSH could only promote MeHg uptake, but not translocation, by roots. Conversely, PEN significantly inhibited MeHg uptake and translocation to shoots. Using labeled ¹³Cys assays, we also found that MeHg uptake was coupled with Cys accumulation in rice roots. Moreover, analyses of comparative transcriptomics revealed that key genes associated with metallothionein and SULTR transporter families may be involved in MeHg uptake. These findings provide new insights into the uptake and translocation of MeHg in rice plants and suggest potential roles of thiol attributes in affecting MeHg bioavailability and bioaccumulation in rice.

Use smaller size of straw to alleviate mercury methylation and accumulation induced by straw incorporation in paddy field

Straw sizes were found to affect the methylmercury (MeHg) accumulation in rice grains induced by straw incorporation. The mechanism behind, however, still remains unclear. Here, we incorporated rice straw in different sizes (powder, 2 cm and 5 cm) into a Hg-contaminated paddy soil. Our results showed that straw sizes regulated the release of different fractions of organic matter (OM) in straw residues and further Hg methylation in paddy soil. The easily degradable OM (EDOM) was a key driving factor that facilitated net Hg methylation, though it only occupied a small fraction (1.12-3.12%) of the soil OM. Powdered straw reduced the duration of net Hg methylation by 74.39% compared to 5 cm straw, resulting in a strong and rapid net Hg methylation in paddy soil before the rice flowering. After the release of EDOM, the humified OM dominated in paddy soil and bound to MeHg, leading to less MeHg being transported to rice grains during the grain filling. Powdered straw decreased MeHg accumulation by 25.32% in the mature rice grains compared with 5 cm straw. Our study suggests that straw powdering before incorporation provides a feasible pathway for reducing MeHg accumulation in rice grains induced by straw incorporation.

Unravelling the interactive effect of soil and atmospheric mercury influencing mercury distribution and accumulation in the soil-rice system

Mercury (Hg) accumulation in rice is an emerging health concern worldwide. However, sources and interactions responsible for Hg species accumulation in different rice tissues are still uncertain. Four experimental plots were carefully designed at an artisanal Hg mining site and a control site to evaluate the effect of atmospheric and soil Hg contents on Hg accumulation in rice. We showed that inorganic Hg (IHg) contents in rice tissues grown either in contaminated or control site soil (non-contaminated soil) were higher at Hg artisanal mining site than those at the control site. Elevated total gaseous mercury (TGM) levels in ambient air were the predominant source of IHg to rice at the Hg mining area. Methylmercury (MeHg) concentrations in rice plant tissues increased in proportionality with MeHg contents in paddy soil. Our results suggest that both atmosphere and soil Hg sources have been impacted the IHg accumulation in rice. Above ground rice tissues, grains, leaves, and stalk accumulated IHg from both atmosphere and soil to varying degrees. Nonetheless, the study also provides the first direct evidence that atmospheric Hg accumulated by above-ground rice tissues could be translocated to below-ground tissues (roots). However, the extent to which atmosphere or soil Hg contributes to IHg in rice tissues may vary with each source's concentration gradient at the given site. No evidence of in planta Hg methylation was found during the current study. Hence, paddy fields are potential MeHg production sites, whereas paddy soil is a unique MeHg accumulation source in rice plants. This study expands and clarifies the contribution of various sources involved in Hg accumulation in the soil rice system. The findings here provide the basis for future research strategies to deal with the global issue of Hg contaminated rice.

The underappreciated role of natural organic matter bond Hg(II) and nanoparticulate HgS as substrates for methylation in paddy soils across a Hg concentration gradient

Rice consumption is the major pathway for human methylmercury (MeHg) exposure in inland China, especially in mercury (Hg) contaminated regions. MeHg production, a microbially driven process, depends on both the chemical speciation of inorganic divalent mercury, Hg(II), that determines Hg bioavailability for methylation. Studies have shown that Hg(II) speciation in contaminated paddy soils is mostly controlled by natural organic matter and sulfide levels, which are typically thought to limit Hg mobility and bioavailability. Yet, high levels of MeHg are found in rice, calling for reconsideration of the nature of Hg species bioavailable to methylators in paddy soils. Here, we conducted incubation experiments using a multi-isotope tracer technique including ¹⁹⁸Hg(NO₃)₂, natural organic matter bond Hg(II) (NOM-¹⁹⁹Hg(II)), ferrous sulfide sorbed Hg(II) (≡FeS-²⁰⁰Hg(II)), and nanoparticulate mercuric sulfide (nano-²⁰²HgS), to investigate the relative importance of geochemically diverse yet relevant Hg(II) species on Hg methylation in paddy soils across a Hg concentration gradient. We show that methylation rates for all Hg(II) species tested decreased with increasing Hg concentrations, and that methylation rates using NOM-¹⁹⁹Hg(II) and nano-²⁰²HgS as substrates were similar or greater than rates obtained using the labile ¹⁹⁸Hg(NO₃)₂ substrate. ≡FeS-²⁰⁰Hg(II) yielded the lowest methylation rate in all sites, and thus the formation of FeS is likely a sink for labile ¹⁹⁸Hg(NO₃)₂ in sulfide-rich paddy soils. Moreover, the variability in the methylation data for a given site (1 to 5-fold variation depending on the Hg species) was smaller than what was observed across the Hg concentration gradient (10³-10⁴ fold variation between sites). These findings emphasize that at broad spatial scales, site-specific characteristics, such as microbial community structure, need to be taken into consideration, alongside the nature of the Hg substrate available for methylation, to determine net MeHg production. This study highlights the importance of developing site-specific strategies for remediating Hg pollution.

A new method of predicting the contribution of TGM to Hg in white rice: Using leaf THg and implications for Hg risk control in Wanshan Hg mine area

Rice plants accumulate Hg from the soil and ambient air, however, evaluating the contribution of Hg from these two sources remains challenging. Here, we proposed a practical method to predict the contribution of total gaseous mercury (TGM) to Hg in white rice in Wanshan Hg mine area (WMM). In this study, rice was planted in the same low-Hg soil at different sites of WMM with varying TGM levels. Comparing to the control sites at IG (Institute of Geochemistry, Guiyang), TGM is the dominant source of Hg in rice leaves and white rice at TB (Tianba) and ZJW (Zhangjiawan) sites of WMM. Subsequently, a good correlation between the Hg concentrations in rice leaves and the concentration contributions of TGM to Hg in white rice was obtained. Such a correlation enabled feasible quantification of the contribution of TGM to Hg in white rice collected from the Wanshan Hg mine. The contribution of TGM to Hg in white rice across the WMM area was also estimated, demonstrating that white rice receives 14-83% of Hg from the air. Considering the high contribution of TGM to Hg in white rice, we compared the relative health risks of Hg via inhalation and rice consumption and found that inhalation, rather than rice consumption, was the major pathway for bioaccessible Hg exposure in adults at high-TGM sites. This study provides new knowledge of Hg biogeochemistry in Hg-mining areas.

Compound-Specific Stable Isotope Analysis Provides New Insights for Tracking Human Monomethylmercury Exposure Sources

Monomethylmercury (MMHg) exposure can induce adverse neurodevelopmental effects in humans and is a global environmental health concern. Human exposure to MMHg occurs predominately through the consumption of fishery foods and rice in Asia, but it is challenging to quantify these two exposure sources. Here, we innovatively utilized MMHg compound-specific stable isotope analyses (MMHg-CSIA) of the hair to quantify the human MMHg sources in coastal and inland areas, where fishery foods and rice are routinely consumed. Our data showed that the fishery foods and rice end members had distinct Δ¹⁹⁹Hg_MMHg values in both coastal and inland areas. The Δ¹⁹⁹Hg_MMHg values of the human hair were comparable to those of fishery foods but not those of rice. Positive shifts in the δ²⁰²Hg_MMHg values of the hair from diet were observed in the study areas. Additionally, significant differences in δ²⁰²Hg versus Δ¹⁹⁹Hg were detected between MMHg and inorganic Hg (IHg) in the human hair but not in fishery foods and rice. A binary mixing model was developed to estimate the human MMHg exposures from fishery foods and rice using Δ¹⁹⁹Hg_MMHg data. The model results suggested that human MMHg exposures were dominated (>80%) by fishery food consumption and were less affected by rice consumption in both the coastal and inland areas. This study demonstrated that the MMHg-CSIA method can provide unique information for tracking human MMHg exposure sources by excluding the deviations from dietary surveys, individual MMHg absorption/demethylation efficiencies, and the confounding effects of IHg.

Stable mercury isotopes stored in Masson Pinus tree rings as atmospheric mercury archives

The accuracy of mercury (Hg) dendrochemistry has been questioned because significant knowledge gaps exist in understanding the Hg translocation and mobility in tree-ring. In this study, we evaluated Hg concentrations and isotopic profiles in the tree-ring at a Hg artisanal mining site and a control site with the documented local Hg production inventory. Results show that the Hg concentration accumulated in tree-ring fails to reconstruct the temporal trend of Hg production due to confounded tree physiological and environmental factors, specifically, the radial translocation and tree age effects occurring during the fast-growing period. The temporal profiles of δ²⁰²Hg exhibit pronounced tree-specific variabilities due to the complexity of Hg isotopic mass dependent fractionation during atmospheric Hg uptake and translocation in vegetation. The Hg odd-MIF (mass independent fractionation) profiles in tree-ring can reconstruct a decadal-scale temporal trend of the atmospheric Hg⁰ pollution level, and also be used as a tracer to distinguish the emission source shifts of atmospheric Hg⁰. However, the radial translocation would result in uncertainties at the higher resolution because of the mixing of odd-MIF signatures with active rings. Caution should be taken and additional supporting evidence collected from independent methods should be used for verifying the tree-ring records.

Role of H+, HF, SO42- and kaolin in fixing Hg of coal fire sponge

Coal fire sponges (CFS) are common in coal-fire areas. Due to the enrichment of Hg in CFS, large amounts of Hg are released by CFS into the atmosphere via natural weathering or solar radiation. Therefore, CFS should be of concern in Hg pollution management and control globally. In addition, CFS changes the Hg cycle path by capturing Hg from coal fires that would have entered the atmosphere. In this study, the concentration, distribution, species, and enrichment mechanism of CFS Hg were investigated. The results showed that the Hg concentration in CFS ranged from 1008 to 35,310 ng/g, with an average of 8932 ng/g (CFS number, n = 153). The Hg concentration of CFS in different types of land was found to be significantly inhomogeneous. To determine the status of subterranean spontaneous combustion, the Hg concentration was added, which can improve the effect of coal-fire monitoring. Compared to the background area topsoil, CFS was enriched in Hg, acid, SO₄^2-, and total fluoride. The Hg species in CFS was primarily HgSO₄, followed by HgO. However, the primary Hg species in the surrounding topsoil were HgCl₂ and HgO. By the simulation experiment, it was determined that hydrofluoric acid (HF) was beneficial to activate the stable species in the coal-fire areas. HgCl₂, HgO, or Hg⁰ were ionized by acid liquor or HF, which can promote Hg migration and increase the adsorbed ratio; in the presence of SO₄^2-, the primary Hg species was HgSO₄. Ultimately, Hg was absorbed by clay minerals and organic matter. The high-efficiency activation of steady Hg species by the coal-fire HF should be studied further.