Mobility Of Hg Research Articles

Mobilization of mercury by sediment transport after a prescribed fire in NE Portugal: Insight into size classes and temporal variation

Terrestrial ecosystems are important sinks for atmospheric mercury (Hg). It is well known that high severity wildfires can mobilize Hg in the surroundings of burned areas due to changes in ecosystem stability, but it is unclear whether this also occurs after lower severity fires, such as prescribed fires. The present study determined Hg concentrations and mobilization rates in different size fractions of sediments collected after a prescribed fire in a scrubland area. Sediments, collected from eight erosion plots on six occasions, were analysed for total Hg, C and N in several size classes (<0.2 mm, 0.2–0.5 mm, 0.5–2 mm and >2 mm) and Hg mobilization rates (HgST) were calculated for each size fraction. Average total Hg were 38, 57, 94 and 126 µg kg−1 for size fractions > 2, 0.5–2, 0.2–0.5 and < 0.2 mm, respectively. Total Hg was negatively correlated with C/N ratio, involving the humification degree of organic matter of sediments in Hg retention. In the last event (eight months after fire), sediments had 45–106 % more Hg, depending on size fraction, compared to the initial event. Mercury mobilization rates varied between 32 and 78 mg ha−1, with the fraction 0.5–2 mm accounting for 46 % of the mobilized Hg. The results revealed that prescribed fires can mobilize Hg, so their use to prevent wildfires must be done with caution.

The mercury and low molecular-weight antioxidants levels in ungulates of the Republic of Karelia

The high toxicity of mercury (Hg) poses a danger to the environment and humans, but studies of the concentration of this metal in organisms of terrestrial ecosystems are few. Ecotoxicologists also pay little attention to studying the role of antioxidant vitamins in protecting cells from toxic metals. The Republic of Karelia is one of the northwestern regions of Russia, the biogeochemical features of which can contribute to an increase in the mobility and bioavailability of Hg in food chains. The purpose of the work was to determine the concentration of Hg in the liver, kidneys, muscle and hair of ungulate mammals of the Republic of Karelia (wild boar Sus scrofa L. and moose Alces alces L.) and to analyze the relationship between the level of this toxic metal and the content of low molecular-weight antioxidants – reduced glutathione, retinol and α-tocopherol. Species and tissue-specific of the studied parameters in wild boars and moose are noted. The observations discovered by other researchers that omnivorous species accumulate more Hg in their tissues compared to herbivores, and also that this toxic metal is predominantly accumulated in the kidneys, while muscles contain a minimal amount, have been confirmed. Hg concentrations in most samples of liver and kidney of wild boars and in all samples of these same organs of moose were within the limits recorded for domestic pigs and deer, respectively. The levels of Hg we recorded in the tissues and hair of wild boars and moose were generally comparable to or lower than the levels of this metal noted in animals from other regions of Russia and other countries of the world. In wild boars and moose of Karelia, no statistically significant relationships were found between the Hg level and the content of the studied antioxidants in the internal organs. Moose were characterized by a higher content of α-tocopherol in the body than wild boars, which is a feature of this type of herbivorous ungulate mammal. The results of the study indicate a relatively low level of mercury pollution in terrestrial ecosystems in Karelia.

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 0-40cm (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.

Mobility of trace elements in a coastal contaminated site under groundwater salinization dynamics

Water pollution is a significant issue resulting from past long-term actions. The remediation projects carried out under law constraints for industrial plants, which have been the major contributors to environmental and water pollution, are currently providing a significant amount of data about contaminated soil, surface waters, and groundwater. Most of such plants worldwide are in coastal zones. Based on a significant amount of chemical and environmental data for a coastal contaminated site subject to variable groundwater salinization, this study aimed to understand the mobility of some trace elements because of coastal zone dynamics. Data concerned 688 groundwater samples, including As, Hg, Cd, Crtot, Cu, Ni, Pb, V, Se, Zn, pH, electrical conductivity, chlorides, total organic carbon and organic contaminants as quantitative variables, enhanced by additional qualitative variables such as groundwater salinity, season, water level, precipitation, and industrial activity type to make the dataset as representative as possible of the site under investigation. The study used robust multivariate statistical analyses to analyse the complex dataset and explain the relevant factors influencing contaminant behaviour under different environmental conditions. The Multivariate Statistical Analysis distinguished three clusters of trace elements with diverse reactivity to changes in groundwater salinization. The first includes Se, Cu, Crtot, V, and Ni, showing the highest correlation with electrical conductivity and chlorides because of their high affinity to form chloride or organic chloride complexes and for ion competition. Zn and Pb cluster in the second group: they are less reactive to groundwater salinization and more influenced by cation and anion competition and organic matter. The mobility of Hg and As (third cluster) significantly correlates with Fe and Mn, underlining the dominant role of reductive dissolution of trace elements-bearing minerals (Fe/Mn/Al-oxy-hydroxides) and metal-organic complexes. The correlation between the clustering of variables in groundwater and soils shows the influence of sediment structure, mineral composition, and physical and chemical soil conditions on the distribution in soils of trace elements and their transport to groundwater. The study proposes a valuable approach for assessing the effects of salinization in contaminated coastal aquifers. It supports planning multi-purpose characterization and monitoring campaigns of contaminated coastal sites and provides guidance on the correct associated remediation projects.

Precipitation patterns strongly affect vertical migration and methylation of mercury in legacy contaminated sites

Legacy-contaminated sites act as significant sources of mercury (Hg) to their surrounding surface and underground environments. Intensified extreme precipitation is posing great threats to the environment and human health by changing the fate of pollutants, yet little is known about its effect on the vertical migration and methylation of Hg in contaminated sites. Here, we applied a range of simulated extreme precipitation patterns (frequency and intensity) to column leaching assays with soils collected near a contaminated site. We observed that precipitation with high frequency but low intensity resulted in more vertical migration of Hg through the soil profile than that with low frequency but high intensity. The majority (> 90%) of leached Hg was prone to migrate vertically within the top 10 cm of the soil profile. Furthermore, rainfall stimulated microbial Hg methylation, as demonstrated by enhanced production of methylmercury (MeHg) in both simulated and field-contaminated soils. We identified specific microbial taxa including Geobacteraceae, Desulfuromonadaceae, Syntrophaceae, Oscillospiraceae, and Methanomicrobiaceae as key predictors of MeHg production, which differed from those typically observed in overlying water of croplands. Particularly, the relative abundance of these dominant Hg methylators significantly increased during rainfall-induced leaching compared to that of the control, suggesting the crucial yet previously overlooked impacts of increased precipitation events on the process of microbial Hg methylation in industry-contaminated sites. Given the rising incidences of extreme precipitation events worldwide due to climate change, this study highlights the significance of assessing Hg mobility and microbial transformation in legacy contaminated sites.

Mercury in river samples with artisanal and small-scale gold mining influence: The case of the upper Madeira River Basin (Brazil/Bolivia)

The issue of mercury (Hg) is becoming a growing concern in the Amazon basin. The Madeira River is known for intense artisanal and small-scale gold mining (ASGM), and Hg poses a significant issue for public health and ecosystems. In this study, we analyzed Hg riverine concentrations in water samples, suspended particulate matter (SPM) and sediments in the Upper Madeira River Basin. We also examined mineralogical and elemental compositions of the solid phase to enhance our understanding of Hg behavior in the environment. Dissolved Hg was not detected in water, and Hg concentrations in solids remained in the same order of magnitude across different rivers, despite variations in gold mining intensities along their courses. Generally, Hg concentrations were higher in compartments with a greater proportion of fine sediments (clay and silt), with average concentrations of 91.4 ± 13.0, 41.6 ± 11.0, and 22.2 ± 1.6 ng g−1 for SPM (n = 14), riverbank (n = 12), and riverbed (n = 4) sediments, respectively. Multivariate analysis shows Hg chemical associations related to clay minerals and organic matter (OM) for the riverbank and SPM compartments. However, in SPM, OM had a greater influence on Hg distribution. Conversely, one riverbed sample highlights with a high Hg concentration (547.78 ng g−1), large content of heavy minerals, and a distinct geochemical signature. This sample was collected in a river section upstream of Beni River's rapids where the river's hydrodynamics can induce the high concentration of heavy minerals in this riverbed sediment. This unexpectedly high Hg concentration encountered in sediment rich in heavy minerals is likely related to contamination by Hg-rich mine tailing discarded into the river. Overall, this study discusses the issue of Hg mobility in rivers suggesting its dispersion, i) associated with fine fraction rich in clay and OM, and ii) associated with heavy minerals.

Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry

Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments.

Investigating the role of poly-γ-glutamic acid in Pennisetum giganteum phytoextraction of mercury-contaminated soil

Farmland mercury (Hg) pollution poses a significant threat to human health, but there is a lack of highly efficient phytoextraction for its remediation at present. This study investigates the impact of poly-γ-glutamic acid (γ-PGA) on the phytoextraction capabilities of Pennisetum giganteum (P. giganteum) in Hg-contaminated soil. Our research indicates that amending γ-PGA to soil markedly enhances the assimilation of soil Hg by P. giganteum and transformation of Hg within itself, with observed increases in Hg concentrations in roots, stems, and leaves by 1.1, 4.3, and 18.9 times, respectively, compared to the control. This enhancement is attributed to that γ-PGA can facilitate the hydrophilic and bioavailable of soil Hg. Besides, γ-PGA can stimulate the abundance of Hg-resistance bacteria Proteobacteria in the rhizosphere of P. giganteum, thus increasing the mobility and uptake of soil Hg by P. giganteum roots. Moreover, the hydrophilic nature of Hg-γ-PGA complexes supports their transport via the apoplastic pathway, across the epidermis, and through the Casparian strip, eventually leading to immobilization in the mesophyll tissues. This study provides novel insights into the mechanisms of Hg phytoextraction, demonstrating that γ-PGA significantly enhances the effectiveness of P. giganteum in Hg uptake and translocation. The findings suggest a promising approach for the remediation of Hg-contaminated soil, offering a sustainable and efficient strategy for environmental management and health risk mitigation.

The binding effects and mechanisms of dissolved organic matter (DOM) on the fate of mercury in sludge anaerobic digestion combined with thermal hydrolysis

Dissolved organic matter (DOM) plays an important role in regulating the fate of mercury (Hg), e.g., mobility, bioavailability, and toxicity. Clarifying the role of DOM in binding Hg in the treatment processes of sewage sludge is important for relieving Hg contamination risks in land applications. However, the impacts of DOM on Hg binding in sewage sludge are still unclear. In this study, we investigated the evolution of Hg and its speciation in full-scale sludge anaerobic digestion (AD) with thermal hydrolysis. The role of DOM in binding Hg(II) was further analyzed. The results showed that AD with thermal hydrolysis led to an increase in the Hg content in the sludge (from 3.72 ± 0.47 mg/kg to 10.75 ± 0.16 mg/kg) but a decrease in Hg mobility (the mercury sulfide fraction increased from 60.56 % to 79.78 %). Further adsorption experiments revealed that at equivalent DOM concentrations, DOM with a low molecular weight (MW<1 kDa) in activated sludge, DOM with a medium molecular weight (1 kDa<MW<5 kDa) in thermally hydrolyzed sludge, and DOM with a high molecular weight (MW> 5 kDa) in both anaerobically digested sludge and conditioned sludge showed high binding amounts of Hg(II), with 1372.54, 535.28, 942.09 and 801.51 mg Hg/g DOM, respectively. Parallel factor analysis (PARAFAC) and fluorescence quotient (FQ) results showed that tryptophan-like and tyrosine-like substances had high binding affinities for Hg(II). Furthermore, X-ray photoelectron spectroscopy (XPS) indicated that the reduced organic sulfur contained in the DOM was potentially bound to Hg through the interactions of Hg-S and Hg-O. These results indicated that DOM may play special roles in regulating Hg speciation. The association between DOM and Hg(II), such as the significant positive correlation (p < 0.05) between the dissolution rate of Hg(II) and release of tryptophan-like substances during thermal hydrolysis, suggested the potential way for removing Hg from sludge.

Mercury Bioconcentration and Translocation in Rooted Macrophytes (Paspalum repens Berg.) from Floodplain Lakes in the Araguaia River Watershed, Brazilian Savanna

Macrophytes are fundamental photosynthetic organisms for functioning freshwater ecosystems, identified as potential bioindicators of mercury (Hg) in the environment. We quantified the concentrations of total Hg (THg) in water and macrophytes (Paspalum repens Berg.) from 17 lakes on the Araguaia River floodplain, aiming to compare the bioconcentration factor (BCF) in the aerial tissues and roots; evaluate the translocation factor (TF) between plant tissues; and assess the influence of environmental factors and land use on THg concentrations in water and macrophytes. The BCF was significantly higher in roots (1.29 ± 0.32) than in aerial tissues (0.41 ± 0.34), with low TF between plant tissues (0.14 ± 0.06). The highest concentrations of THg in water were determined in lakes with higher land use intensity and a pH close to neutral, indicating the transport of particulate-bound Hg and the immobilization in the water column. In contrast, wetlands were priority areas for the bioconcentration of THg in macrophytes, associated with sulfate, dissolved oxygen, and oxidation–reduction potential in the water. Thus, although P. repens is not a suitable bioindicator of Hg mobilization by anthropogenic land use in our study area, our results suggest the potential of macrophytes as bioindicators of sites that are favorable to Hg methylation.

Mercury supply limits methylmercury production in paddy soils

The neurotoxic methylmercury (MeHg) is a product of inorganic mercury (IHg) after microbial transformation. Yet it remains unclear whether microbial activity or IHg supply dominates Hg methylation in paddies, hotspots of MeHg formation. Here, we quantified the response of MeHg production to changes in microbial activity and Hg supply using 63 paddy soils under the common scenario of straw amendment, a globally prevalent agricultural practice. We demonstrate that the IHg supply is the limiting factor for Hg methylation in paddies. This is because IHg supply is generally low in soils and can largely be facilitated (by 336–747 %) by straw amendment. The generally high activities of sulfate-reducing bacteria (SRB) do not limit Hg methylation, even though SRB have been validated as the predominant microbial Hg methylators in paddies in this study. These findings caution against the mobilization of legacy Hg triggered by human activities and climate change, resulting in increased MeHg production and the subsequent flux of this potent neurotoxin to our dining tables.

Assessment of mobile mercury concentration in soils of an abandoned coalfield waste pile in Douro region: the Fojo waste pile (Portugal) study case

PurposePejão Mining Complex locates in Castelo de Paiva municipality and, until its closure in 1994, was one of the most important coal mines in the Douro Coalfield. This work aims to study the presence, quantify, and evaluate the dissemination of mercury (Hg), a potentially toxic element (PTE) of major public health concern by the World Health Organization (WHO), from a waste pile affected by coal fires.Materials and methodsSamples from areas affected and unaffected by the combustion and from surrounding soil were collected from Fojo waste pile region. First, the Hg pseudo-total concentration was estimated for all collected samples by soil microwave–assisted digestion with aqua regia (USEPA 3051A). Then, a sequential extraction procedure (SEP), the USEPA 3200, was applied for Hg fractionation and speciation aiming to evaluate Hg mobility and bioavailability to surrounding ecosystems.Results and discussionThe results obtained showed a Hg enrichment in soil samples when compared to Portuguese and international reference values for soils. Relatively to the Hg availability and mobility, although it predominates in the semi-mobile fraction, the waste pile materials exposed to combustion showed a concerning increase of Hg levels in the mobile fraction that contains the more labile Hg species, being a major source of environmental contamination by Hg.ConclusionsThis study allowed to conclude that combustion of mining residues increased Hg mobility, toxicity, and bioavailability, increasing the contamination potential of the coal waste pile. The methodology applied in this work can be replicated in other abandoned mines to monitor, control, and/or mitigate the Hg environmental impact in the surrounding soils and waters.

Mobilization of heavy metals from floodplain sediments of the Yellow River during redox fluctuations

The floodplain of the Yellow River is a typical area characterized by redox fluctuations and heavy metal pollution. However, the mobilization behavior of heavy metals in floodplain sediments during redox fluctuations remains poorly understood. In this study, reductive mobilization of Fe and Mn was observed under reducing environments through reduction and dissolution, leading to the subsequent release of adsorbed As. In contrast, the mobilization of U occurred under oxic conditions, as the oxidative state of U(VI) has higher solubility. Furthermore, insignificant effects on the mobilization of Cd, Cu, Pb, and Hg were noticed during redox fluctuations, indicating higher stability of these heavy metals. Additionally, we demonstrated that carbon sources can play a key role in the mobilization of heavy metals in floodplain sediments, amplifying the reductive mobilization of Fe, Mn, As and the oxidative mobilization of U. Our findings contribute to the understanding of the biogeochemical cycling of heavy metal in floodplain sediments of the Yellow River and the factors that control this cycling.

Mercury Biogeochemistry and Biomagnification in the Mediterranean Sea: Current Knowledge and Future Prospects in the Context of Climate Change

In the 1970s, the discovery of much higher mercury (Hg) concentrations in Mediterranean fish than in related species of the same size from the Atlantic Ocean raised serious concerns about the possible health effects of neurotoxic monomethylmercury (MMHg) on end consumers. After 50 years, the cycling and fluxes of the different chemical forms of the metal between air, land, and marine environments are still not well defined. However, current knowledge indicates that the anomalous Hg accumulation in Mediterranean organisms is mainly due to the re-mineralization of organic material, which favors the activity of methylating microorganisms and increases MMHg concentrations in low-oxygen waters. The compound is efficiently bio-concentrated by very small phytoplankton cells, which develop in Mediterranean oligotrophic and phosphorous-limited waters and are then transferred to grazing zooplankton. The enhanced bioavailability of MMHg together with the slow growth of organisms and more complex and longer Mediterranean food webs could be responsible for its anomalous accumulation in tuna and other long-lived predatory species. The Mediterranean Sea is a “hotspot” of climate change and has a rich biodiversity, and the increasing temperature, salinity, acidification, and stratification of seawater will likely reduce primary production and change the composition of plankton communities. These changes will likely affect the accumulation of MMHg at lower trophic levels and the biomagnification of its concentrations along the food web; however, changes are difficult to predict. The increased evasion of gaseous elemental mercury (Hg°) from warming surface waters and lower primary productivity could decrease the Hg availability for biotic (and possibly abiotic) methylation processes, but lower oxygen concentrations in deep waters, more complex food webs, and the reduced growth of top predators could increase their MMHg content. Despite uncertainties, in Mediterranean regions historically affected by Hg inputs from anthropogenic and geogenic sources, such as those in the northwestern Mediterranean and the northern Adriatic Sea, rising seawater levels, river flooding, and storms will likely favor the mobilization of Hg and organic matter and will likely maintain high Hg bioaccumulation rates for a long time. Long-term studies will, therefore, be necessary to evaluate the impact of climate change on continental Hg inputs in the Mediterranean basin, on air–sea exchanges, on possible changes in the composition of biotic communities, and on MMHg formation and its biomagnification along food webs. In this context, to safeguard the health of heavy consumers of local seafood, it appears necessary to develop information campaigns, promote initiatives for the consumption of marine organisms at lower trophic levels, and organize large-scale surveys of Hg accumulation in the hair or urine of the most exposed population groups.

Adsorption isotherm model of Hg2+ with biochar from young coconut waste

Biochar is a carbon-rich byproduct of biomass pyrolysis that may be used to restrict Hg mobility in soil by utilizing amelioration technology. This study examines the ability of biochar from young coconut waste to adsorb Hg in solution. Isothermal adsorption of Hg by batch equilibrium method. The basic principle of Hg adsorption behavior with biochar from young coconut waste (B-YCW) processed through the Kon-Tiki method at a temperature of 682 0C, moisture of 81.27%, and a yield ratio of 20.87% at a size of 0.5 mm. The adsorption of Hg2+ on B-YCW increased with increasing Hg concentration and decreasing pH. The capacity and adsorption coefficient of Hg2+ by biochar from young coconut waste was 312.88 mg g−1 and 69.64 L Kg−1 at a pH of 1.55 and a concentration of 100 mg L−1 Hg2+. The adsorption isotherm of Hg2+ occurs in the Freundlich and Langmuir models (Freundlich > Langmuir). The Freundlich model (y = 1.0375x - 1.2523; R² = 1) with a value of n of 0.96 and KF of 17.78 (L mg−1)1/n, and the Langmuir II model (y = 17.126x - 0.0244; R² = 1), with average Qm was 18.57 mg g−1; KL 68.198 L mg−1 and RL 0.0054 (favorable) from the Langmuir isotherm model.

Distribution and Homogenization of Multiple Mercury Species Inputs to Freshwater Wetland Mesocosms.

Mercury (Hg)-impaired aquatic ecosystems often receive multiple inputs of different Hg species with varying potentials for transformation and bioaccumulation. Over time, these distinct input pools of Hg homogenize in their relative distributions and bioaccumulation potentials as a result of biogeochemical processes and other aging processes within the ecosystem. This study sought to evaluate the relative time scale for homogenization of multiple Hg inputs to wetlands, information that is relevant for ecosystem management strategies that consider Hg source apportionment. We performed experiments in simulated freshwater wetland mesocosms that were dosed with four isotopically labeled mercury forms: two dissolved forms (Hg2+ and Hg-humic acid) and two particulate forms (nano-HgS and Hg adsorbed to FeS). Over the course of one year, we monitored the four Hg isotope endmembers for their relative distribution between surface water, sediment, and fish in the mesocosms, partitioning between soluble and particulate forms, and conversion to methylated mercury (MeHg). We also evaluated the reactivity and mobility of Hg through sequential selective extractions of sediment and the uptake flux of aqueous Hg in a diffusive gradient in thin-film (DGT) passive samplers. We observed that the four isotope spikes were relatively similar in surface water concentration (ca. 3000 ng/L) immediately after spike addition. At 1-3 months after dosing, Hg concentrations were 1-50 ng/L and were greater for the initially dissolved isotope endmembers than the initially particulate endmembers. In contrast, the Hg isotope endmembers in surface sediments were similar in relative concentration within 2 months after spike addition. However, the uptake fluxes of Hg in DGT samplers, deployed in both the water column and surface sediment, were generally greater for initially dissolved Hg endmembers and lower for initially particulate endmembers. At one year postdosing, the DGT-uptake fluxes were converging toward similar values between the Hg isotope endmembers. However, the relative distribution of isotope endmembers was still significantly different in both the water column and sediment (p < 0.01 according to one-way ANOVA analysis). In contrast, selective sequential extractions resulted in a homogeneous distribution, with >90% of each endmember extracted in the KOH fraction, suggesting that Hg species were associated with sediment organic matter. For MeHg concentrations in surface sediment and fish, the relative contributions from each endmember were significantly different at all sampling time points. Altogether, these results provide insights into the time scales of distribution for different Hg species that enter a wetland ecosystem. While these inputs attain homogeneity in concentration in primary storage compartments (i.e., sediments) within weeks after addition, these input pools remain differentiated for more than one year in terms of reactivity for passive samplers, MeHg concentration, and bioaccumulation.

New insights into Hg[sbnd]Se antagonism: Minor impact on inorganic Hg mobility while potential impacts on microorganisms

Selenium (Se) is a crucial antagonistic factor of mercury (Hg) methylation in soil, with the transformation of inorganic Hg (IHg) to inert mercury selenide (HgSe) being the key mechanism. However, little evidence has been provided of the reduced Hg mobility at environmentally relevant doses of Hg and Se, and the potential impacts of Se on the activities of microbial methylators have been largely ignored. This knowledge gap hinders effective mitigation for methylmercury (MeHg) risks, considering that Hg supply and microbial methylators serve as materials and workers for MeHg production in soils. By monitoring the mobility of IHg and microbial activities after Se spike, we reported that 1) active methylation might be the premise of HgSe antagonism, as higher decreases in MeHg net production were found in soils with higher constants of Hg methylation rate; 2) IHg mobility did not significantly change upon Se addition in soils with high DOC concentrations, challenging the long-held view of Hg immobilization by Se; and 3) the activities of iron-reducing bacteria (FeRB), an important group of microbial methylators, might be potentially regulated by Se addition at a dose of 4 mg/kg. These findings provide empirical evidence that IHg mobility may not be the limiting factor under Se amendment and suggest the potential impacts of Se on microbial activities.

Spatio-temporal trends of mercury levels in alluvial gold mining spoils areas monitored between rainy and dry seasons in the Peruvian Amazon

Artisanal and small-scale gold mining (ASGM) in the Amazon has degraded tropical forests and escalated mercury (Hg) pollution, affecting biodiversity, ecological processes and rural livelihoods. In the Peruvian Amazon, ASGM annually releases some 181 tons of Hg into the environment. Despite some recent advances in understanding the spatial distribution of Hg within gold mine spoils and the surrounding landscape, temporal dynamics in Hg movement are not well understood. We aimed to reveal spatio-temporal trends of soil Hg in areas degraded by ASGM.,. We analyzed soil and sediment samples during the dry and rainy seasons across 14 ha of potentially contaminated sites and natural forests, in the vicinities of the Native community of San Jacinto in Madre de Dios, Peru. Soil Hg levels of areas impacted by ASGM (0.02 ± 0.02 mg kg−1) were generally below soil environmental quality standards (6.60 mg kg−1). However, they showed high variability, mainly explained by the type of natural cover vegetation, soil organic matter (SOM), clay and sand particles. Temporal trends in Hg levels in soils between seasons differed between landscape units distinguished in the mine spoils. During the rainy season, Hg levels decreased up to 45.5% in uncovered soils, while in artificial pond sediments Hg increased by up to 961%. During the dry season, uncovered degraded soils were more prone to lose Hg than sites covered by vegetation, mainly due to higher soil temperatures and concomitantly increasing volatilization. Soils from natural forests and degraded soil covered by regenerating vegetation showed a high capacity to retain Hg mainly due to the higher plant biomass, higher SOM, and increasing concentrations of clay particles. Disturbingly, our findings suggest high Hg mobility from gold mine spoil to close by sedimentary materials, mainly in artificial ponds through alluvial deposition and pluvial lixiviation. Thus, further research is needed on monitoring, and remediation of sediments in artificial to design sustainable land use strategies.

Mercury distribution, mobilization and bioavailability in polluted sediments of Scheldt Estuary and Belgian Coastal Zone

In this study, the vertical distribution of mercury (Hg) in estuarine and marine sediment porewaters and solid phases was assessed by conventional and passive sampling techniques in the historically polluted Scheldt Estuary and Belgian Coastal Zone (BCZ). The Diffusive Gradients in Thin-films (DGT) measured labile Hg concentrations (HgLA) were mostly lower than the porewater Hg concentrations (HgPW), and they also presented different vertical distribution patterns. Still high Hg concentrations in the sediment solid phases, comparable to the historical ones, were observed. Even though pH, redox potential and dissolved sulfide concentration could influence the Hg biogeochemical behaviour, organic matter (OM) played a key role in governing Hg mobilization from sediment solid phase to porewater and in its partitioning between porewater and solid phase over depth. In the marine sediments, where OM had a marine signature, higher labile Hg concentrations in the porewater and faster resupply from the solid phase were observed. The DGT technique showed significant potential not only for the measurement of bioavailable Hg fractions in porewater, but also for the assessment of kinetic parameters governing the release of labile Hg species from the solid phase with the assistance of the DGT Induced Fluxes in Sediments (DIFS) model.

Mercury concentrations and export from small central Canadian boreal forest catchments before, during, and after forest harvest

Northern boreal forests are a strong sink for mercury (Hg), a global contaminant of significant concern to wildlife and human health. Mercury stored in forest soils can be mobilized via runoff and erosion, and under suitable conditions can be methylated to its much more bioaccumulative form, methylmercury. Forest harvesting can affect the mobilization and methylation of Hg, though the direction and magnitude of the impact is unclear or conflicting across previous studies. This study examined 5 harvested and 2 reference watersheds in northwestern Ontario, Canada, before, during, and after harvest to quantify changes in stream total and methylmercury concentration and loads and identified potential landscape and management factors that contribute to differences in stream response. In watersheds where streams were buffered by natural vegetation (≥30 m), no significant changes in total Hg or methylmercury concentrations or loads were observed. Significant increases in methylmercury concentrations and loads were observed downstream of a stream crossing in a watershed where the relatively small stream was unmapped and therefore only buffered by a 3 m machine exclusion zone. These results show that when current best management practices that minimize soil and water disturbance are followed, harvest can have a minimal impact on total and methylmercury loads, even in extensively harvested watersheds. However, there is a need for improved mapping of small streams to ensure best management practices are applied adequately across the landscape.