Increases of Total Mercury and Methylmercury Releases from Municipal Sewage into Environment in China and Implications

Quantifying the impact of anthropogenic emissions and aquatic environmental impacts on sedimentary mercury variations in a typical urban river

In urban and industrial regions, sedimentary mercury (Hg) serves as the crucial indicator for Hg pollution, posing potential risks to ecology and human health. The physicochemical processes of Hg in aquatic environments are influenced by various factors such as anthropogenic emissions and aquatic environmental impacts, making it challenging to quantify the drivers of total mercury (THg) variations. Here, we analyzed the spatiotemporal variations, quantified driving factors, and assessed accumulation risks of sedimentary THg from the mainstream of a typical urban river (Haihe River). THg in the urban region (37-3237 ng g-1) was significantly higher (t-test, p < 0.01) than in suburban (71-2317 ng g-1) and developing regions (156-916 ng g-1). The sedimentary THg in suburban and developing regions increased from 2003 to 2018, indicating the elevated atmospheric deposition of Hg. Together with the temperature, grain size of sediments, total organic carbon (TOC), the pH and salinity of water, 40 components of parent and substituted polycyclic aromatic hydrocarbons (PAHs) were first introduced to quantify the driver of sedimentary THg based on generalized additive model. Results showed that anthropogenic emissions, including three PAHs components (31%) and TOC (63%), accounted for 94% of sedimentary THg variations. The aquatic environmental impacts accounted for 5% of sedimentary THg variations. The geo-accumulation index of THg indicated moderate to heavy accumulation in the urban region. This study demonstrates that homologous pollutants such as PAHs can be used to trace sources and variations of Hg pollution, supporting their co-regulation as international conventions regulate pollutants.

Temporal variation of mercury and methyl mercury in water and accumulation by phytoplankton in the eutrophic estuary, northern Taiwan

Three surveys were carried out to study the phytoplankton role in influencing the Hg distribution in a poorly eutrophic estuary by measuring the total Hg (THg) and methylHg (MeHg) concentrations in waters and four-size fractions of phytoplankton. The THg and MeHg concentrations in waters and phytoplankton varied markedly temporal during the three surveys. The total concentrations of THg and MeHg in the four-size fractions of phytoplankton ranged between 0.62 and 28.15 mg/kg and 0.022-4.411 mg/kg, respectively. The dominance of THg and MeHg phytoplankton concentrations differed from different size fractions and varied with the various surveys. The huge uptake of Hg by abundant phytoplankton decreased both Hg concentrations in waters and phytoplankton, which was attributed to the biomass dilution effect during the July survey. The Hg partition between water and phytoplankton provided substantial evidence to illustrate the huge uptake of Hg by the abundant phytoplankton.

Degradation of organic mercury in high salt environments by a marine aerobic bacterium Alteromonas macleodii KD01

Mercury (Hg), particularly organic mercury, poses a global concern due to its pronounced toxicity and bioaccumulation. Bioremediation of organic mercury in high-salt wastewater faces challenges due to the growth limitations imposed by elevated Cl- and Na+ concentrations on microorganisms. In this study, an isolated marine bacterium Alteromonas macleodii KD01 was demonstrated to degrade methylmercury (MeHg) efficiently in seawater and then was applied to degrade organic mercury (MeHg, ethylmercury, and thimerosal) in simulated high-salt wastewater. Results showed that A. macleodii KD01 can rapidly degrade organic mercury (within 20 min) even at high concentrations (>10 ng/mL), volatilizing a portion of Hg from the wastewater. Further analysis revealed an increased transcription of organomercury lyase (merB) with rising organic mercury concentrations during the exposure process, suggesting the involvement of mer operon (merA and merB). These findings highlight A. macleodii KD01 as a promising candidate for addressing organic mercury pollution in high-salt wastewater.

Multimedia fate simulation of mercury in a coastal urban area based on the fugacity/aquivalence method

Due to intensive industrial production and living activities, urban areas are the main anthropogenic mercury (Hg) emission sources. After entering the environment through exhaust gases, wastewater or waste residues, Hg can migrate and transform among different environmental compartments in various species, such as elemental mercury (Hg0), divalent mercury (Hg2+) and methylmercury (MeHg). Studies have yet to report on the multimedia behaviors of Hg in urban areas due to the complexity of the processes involved. In this study, the atmospheric Hg emission in Dalian, a coastal city in Northeast China, was estimated by an anthropogenic emission inventory, and a Level III multimedia model was constructed based on the fugacity/aquivalence method to simulate the fate of Hg in air, water, soil, sediment, vegetation and film. The total annual atmospheric emission was 9.91 t, of which coal combustion and non-coal sources accounted for 70.1 % and 29.9 %, respectively. Atmospheric emission and advection were dominated by Hg0, and aquatic emission and advection were dominated by Hg2+. The migration of air-vegetation, vegetation-soil and soil-air were three important pathways of Hg in urban areas. The model was validated by collecting local soil and vegetation samples and regional air, seawater and sediment monitoring data. The scenario simulation indicated that the local load would decrease to different extents with a 21.0 % reduction in atmospheric Hg emission by implementing the "coal-to-gas" measures. Our developed model can characterize the fate of Hg in coastal urban areas and provide a reference for control strategies.

Elimination of methylmercury production potential in excessive sludge in wastewater treatment plants by sulfur addition

Mercury ions (Hg(II)) in wastewater can accumulate and transform into the highly neurotoxic methylmercury (MeHg) in activated sludge. The release of MeHg can have severe environmental consequences, making the treatment of MeHg-contaminated sludge a pressing concern. In this study, we found that all the collected activated sludge samples, from different wastewater treatment plants in four cities, had the potential for Hg methylation. The Hg-methylating capacity reached a maximum level of 0.70-0.92 μg/g volatile suspended solids after 48 h of exposure to 5 μg/L Hg(II) and showed an average MeHg production rate of 4.8±0.5%. Accordingly, a sludge treatment method involving the addition of elemental sulfur (S0) for a short-term or long-term duration (3 or 180 days, respectively) was proposed. The results demonstrated that this treatment approach effectively mitigated and potentially eliminated MeHg formation by simultaneously reducing Hg bioavailability and Hg-methylating bioactivity. We found that bioavailable Hg(II) ions were converted to a secondary phase similar to insoluble HgS after S0 addition treatment, leading to a decrease in Hg bioavailability in sludge. The enhancement of Hg and extracellular polymeric substances (EPS) complexation via the increasing amount of thiol groups in EPS also reduced the Hg bioavailability after the long-term treatment. Furthermore, the long-term S0 addition significantly reduced the abundance of Hg-methylators with hgcA gene and promoted the growth of Hg-reducers with merA gene, which ensured the complete elimination of MeHg production potential of the excessive activated sludge. Our findings demonstrated that the proposed S0-addition sludge treatment is a promising and safe biotechnology for treating Hg-contaminated sludge. This approach has the potential to contribute significantly to the mitigation of MeHg pollution within environmental contexts.

Microbial diversity and abundance of Hg related genes from water, sediment and soil the Colombian amazon ecosystems impacted by artisanal and small-scale gold mining

The Amazon region abounds in precious mineral resources including gold, copper, iron, and coltan. Artisanal and small-scale gold mining (ASGM) poses a severe risk in this area due to considerable mercury release into the surrounding ecosystems. Nonetheless, the impact of mercury on both the overall microbiota and the microbial populations involved in mercury transformation is not well understood. In this study we evaluated microbial diversity in samples of soil, sediment and water potentially associated with mercury contamination in two localities (Taraira and Tarapacá) in the Colombian Amazon Forest. To this end, we characterized the bacterial community structure and mercury-related functions in samples from sites with a chronic history of mercury contamination which today have different levels of total mercury content. We also determined mercury bioavailability and mobility in the samples with the highest THg and MeHg levels (up to 43.34 and 0.049 mg kg-1, respectively, in Taraira). Our analysis of mercury speciation showed that the immobile form of mercury predominated in soils and sediments, probably rendering it unavailable to microorganisms. Despite its long-term presence, mercury did not appear to alter the microbial community structure or composition, which was primarily shaped by environmental and physicochemical factors. However, an increase in the relative abundance of merA genes was detected in polluted sediments from Taraira. Several Hg-responsive taxa in soil and sediments were detected in sites with high levels of THg, including members of the Proteobacteria, Acidobacteria, Actinobacteria, Firmicutes and Chloroflexi phyla. The results suggest that mercury contamination at the two locations sampled may select mercury-adapted bacteria carrying the merA gene that could be used in bioremediation processes for the region.

Abiotic and biotic roles of metals in the anaerobic digestion of sewage sludge: A review

Anaerobic digestion (AD) is a promising technique for sludge treatment and resource recovery. Metals are very important components of sludge and can have substantial effects on its complex nature and microbial activity. However, systematic reviews have not addressed how metals in sludge affect AD and how they can be regulated to improve AD. This paper comprehensively reviews the effects of metals on the AD of sludge from both abiotic and biotic perspectives. First, we introduce the contents and basic characteristics (e.g., chemical forms) of intrinsic metals in sewage sludge. Then, we summarise the main mechanism by which metals influence sludge properties and the methods for removing metals and thus improving AD. Next, we analyze the effects of both intrinsic and exogenous metals on the enzymes and microbial communities involved in anaerobic bioconversion, focusing on the types, critical concentrations and valence states of the metals. Finally, we propose ideas for future research on the roles of metals in the AD of sludge. In summary, this review systematically clarifies the roles of metals in the AD of sludge and provides a reference for improving AD by regulating these metals.

Revealing varying relationships between wastewater mercury emissions and economic growth in Chinese cities

Mercury emission from industrial wastewater has a great impact on the aquatic environment but is not well studied. Inventory analysis, decoupling and decomposition methods have been conducted based on the China Pollution Source Census dataset, which combines industry removal efficiencies to calculate mercury emissions from industrial wastewater in 340 cities in China during 2000-2010. The results show that over these 11 years, total mercury emissions and per capita mercury emissions increased by approximately 5 times, while the emission intensity increased by only about 3%. From 2000 to 2010, only 0.59% of cities showed strong decoupling between economic growth and mercury emissions, and 37.65% of cities showed weak decoupling, whereas 38.82% of cities showed negative decoupling. We attribute the decoupling of economic development and emissions in individual cities to several socioeconomic factors and find that a decline in emission intensity is the main driver. The Gini coefficient indicates a significant imbalance between cities' emissions, but this situation improved during 2000-2010. The objective of this article is to provide a historical perspective on the situation of mercury emissions from wastewater in China, thereby contributing' to the broader understanding of industrial pollution.

Mercury biomagnification at higher rates than the global average in aquatic ecosystems of the Qinghai-Tibet Plateau

Mercury biomagnification in aquatic ecosystems is a global issue. Biomagnification patterns and drivers in alpine regions remain poorly understood. Hg biomagnification in the aquatic food web of the Qinghai-Tibetan Plateau (Q-T Plateau) was investigated. A total of 302 fish and macroinvertebrate tissue samples were analysed for total mercury (THg) and nitrogen (δ¹⁵N) stable isotope ratios. Overall, 26.75% of fish individuals exceeded the USFWS consumption guidelines. A total of 52.17% of the sampling sites covering different habitats exhibited a significantly positive THg-δ¹⁵N relationship, which confirmed the Hg biomagnification potential of Q-T Plateau aquatic ecosystems. The Q-T Plateau Hg biomagnification rates were generally far higher than global averages regardless of the habitat type. Hg in sediments, elevation and population density were positively related to the Hg biomagnification magnitude on the Q-T Plateau, which could be attributed to the disproportionate response of Hg concentrations in macroinvertebrates and fishes along environmental gradients. Our findings offer empirical evidence that fish consumption on the Q-T Plateau poses a substantial Hg exposure risk to people living along river and lake shores. Higher biomagnification rates could further disproportionately accelerate Hg pollution in Q-T Plateau aquatic ecosystems under future anthropogenic activities and climate warming trajectories.

Modeling transport and fate of heavy metals at the watershed scale: State-of-the-art and future directions

A predictive understanding of the source-specific (e.g., point and diffuse sources) land-to-river heavy metal (HM) loads and HM dynamics in rivers is essential for mitigating river pollution and developing effective river basin management strategies. Developing such strategies requires adequate monitoring and comprehensive models based on a solid scientific understanding of the watershed system. However, a comprehensive review of existing studies on the watershed-scale HM fate and transport modeling is lacking. In this review, we synthesize the recent developments in the current generation of watershed-scale HM models, which cover a wide range of functionalities, capabilities, and spatial and temporal scales (resolutions). Existing models, constructed at various levels of complexity, have their strengths and weaknesses in supporting diverse intended uses. Additionally, current challenges in the application of watershed HM modeling are covered, including the representation of in-stream processes, organic matter/carbon dynamics and mitigation practices, the issues of model calibration and uncertainty analysis, and the balance between model complexity and available data. Finally, we outline future research requirements regarding modeling, strategic monitoring, and their combined use to enhance model capabilities. In particular, we envisage a flexible framework for future watershed-scale HM models with varying degrees of complexity to accommodate the available data and specific applications.

Critical review on biogeochemical dynamics of mercury (Hg) and its abatement strategies

Mercury (Hg) is among the naturally occurring heavy metal with elemental, organic, and inorganic distributions in the environment. Being considered a global pollutant, high pools of Hg-emissions ranging from >6000 to 8000 Mg Hg/year get accumulated by the natural and anthropogenic activities in the atmosphere. These toxicants have high persistence, toxicity, and widespread contamination in the soil, water, and air resources. Hg accumulation inside the plant parts amplifies the traces of toxic elements in the linking food chains, leads to Hg exposure to humans, and acts as a potential genotoxic, neurotoxic and carcinogenic entity. However, excessive Hg levels are equally toxic to the plant system and severely disrupt the physiological and metabolic processes in plants. Thus, a plausible link between Hg-concentration and its biogeochemical behavior is highly imperative to analyze the plant-soil interactions. Therefore, it is requisite to bring these toxic contaminants in between the acceptable limits to safeguard the environment. Plants efficiently incorporate or absorb the bioavailable Hg from the soil thus a constructive understanding of Hg uptake, translocation/sequestration involving specific heavy metal transporters, and detoxification mechanisms are drawn. Whereas recent investigations in biological remediation of Hg provide insights into the potential associations between the plants and microbes. Furthermore, intense research on Hg-induced antioxidants, protein networks, metabolic mechanisms, and signaling pathways is required to understand these bioremediations techniques. This review sheds light on the mercury (Hg) sources, pollution, biogeochemical cycles, its uptake, translocation, and detoxification methods with respect to its molecular approaches in plants.

Understanding of mercury and methylmercury transformation in sludge composting by metagenomic analysis

Municipal sewage especially the produced sewage sludge is a significant source releasing mercury (Hg) to the environment. However, the Hg speciation especially methylmercury (MeHg) transformation in sewage sludge treatment process remains poorly understood. This study investigated the transformation of Hg speciation especially MeHg in sludge composting. The distribution of Hg transformation related gene pairs hgcAB and merAB, and their putative microbial hosts were comprehensively analyzed. Both Hg (from 3.16±0.22 mg/kg to 3.20±0.19 mg/kg) and MeHg content (from 4.77±0.64 ng/g to 4.36±0.37 ng/g) were not obviously changed before and after composting, but about 19.69% of Hg and 27.36% of MeHg were lost according to mass balance calculation. The metagenomic analysis further revealed that anaerobes (Desulfobacterota and Euryarchaeota) were the mainly putative Hg methylators especially carrying high abundance of hgcA gene in the initial periods of composting. Among the 151 reconstructed metagenome-assembled genomes (MAGs), only 4 hgcA gene carriers (Myxococcota, Firmicutes, Cyclobacteriaceae, and Methanothermobacter) and 16 merB gene carriers were identified. But almost all of the MAGs carried hgcB gene and merA gene. The merA gene was widely distributed in genomes, which indicated the widespread functionality of microbes for reducing Hg(II) to Hg(0). The hgcA carrying microbes tends to present the similar metabolic pathways including methanogenesis and sulfur metabolism. Besides, both the irregular distribution of hgcA in various species (including Actinobacteria, Archaea, Bacteroidetes, Desulfobacterota, Euryarchaeota, and Nitrospirae, etc.) and opposite evolution trends between hgcA gene abundance and its host genome abundance can be an indication of horizontal gene transfer or gene deletions of hgcA during composting. Our findings thus revealed that sludge composting is not only a hotspot for Hg speciation transformation, but also a potential hotspot for MeHg transformation.

Variation of mercury fractionation and speciation in municipal sewage treatment plant: effects of mercury on the atmosphere

The speciation of mercury in various processing units of sewage treatment in autumn and winter were studied to understand the conversion and fate of mercury. The results show that the average concentrations of total mercury (THg) in the influent were 130.5 ± 69.8 ng/L and 231.3 ± 107.2 ng/L in autumn and winter, respectively, and the particulate mercury was the main speciation (accounting for 59.3% and 86.9%, respectively). The proportion of dissolved mercury increased after treatment, and the total removal efficiencies of THg were 78.9% and 90.8%, respectively. The release of mercury into the atmosphere during wastewater treatment was studied for the first time. The dissolved gaseous mercury levels in the influent in autumn and winter were 0.60 ± 0.40 ng/L and 0.34 ± 0.21 ng/L, respectively. The average gaseous element mercury concentration in aerobic tank air was 6.34 ± 0.49 ng/m³. The estimated mercury from aeration was 6.4 kg per year in China. The dissolved gaseous mercury will be released to the air if the sewage treatment plant is open-air. Closed sewage treatment and collection of waste gas treatment can reduce the influence of released mercury. Mercury releases into the atmosphere in the process of sewage treatment are one of the fates of mercury in sewage. Closed sewage treatment and collection of waste gas treatment are necessary to reduce the influence of released mercury. The reactive mercury levels in the influents of autumn and winter were 1.28 ± 0.49 ng/L and 1.96 ± 0.43 ng/L, and these levels account for a small proportion of THg, only 1.7% and 0.8%. Hg²⁺ were released by the degradation of organic matter in the secondary biological treatment. The THg levels in dehydrated sludge were higher than those in biochemical sludge but lower than the maximum limit of THg in agricultural sludge.

Removal of methylmercury and its potential relationship to microbiota in sludge anaerobic digestion under thermal hydrolysis

Reducing health risk of mercury (Hg)/methylmercury (MeHg) in sewage sludge is vital to its land application. This study revealed that thermal hydrolysis reduced MeHg content both during pretreatment process and subsequent anaerobic digestion (AD), which resulted in decrease of MeHg content from 4.24 ng/g to 0.95 ng/g after thermal hydrolysis (150 ℃) and further decreased to 0.39 ng/g after AD. Notably, thermal hydrolysis at high temperature (120 ℃ and 150 ℃) promoted both Hg methylation and MeHg demethylation rather than the control or at low temperature (100 ℃). Hg methylation dominated in hydrolysis and acidogenesis stage, whereas MeHg demethylation dominated in methanogenesis stage. Though abundance of related genes (HgcA and merA) was dramatically reduced, Ruminococcaceae, Peptococcaceae, and Lachnospiraceae were potentially Hg methylators in hydrolysis and acidogenesis stage. Whereas, MeHg demethylation dominated in the late period of AD due to the improved syntrophic methanogenesis and possibly reduced Hg²⁺ biodegradability by precipitation.

Nanocellulose as green material for remediation of hazardous heavy metal contaminants

Heavy metal pollution generated by urban and industrial activities has become a major global concern due to its high toxicity, minimal biodegradability, and persistence in the food chain. These are the severe pollutants that have the potential to harm humans and the environment as a whole. Mercury, chromium, copper, zinc, cadmium, lead, and nickel are the most often discharged hazardous heavy metals. Nanocellulose, reminiscent of many other sustainable nanostructured materials, is gaining popularity for application in bioremediation technologies owing to its many unique features and potentials. The adsorption of heavy metals from wastewaters is greatly improved when cellulose dimension is reduced to nanometric levels. For instance, the adsorption efficiency of Cr³⁺ and Cr⁶⁺ is found to be 42.02% and 5.79% respectively using microcellulose, while nanocellulose adsorbed 62.40% of Cr³⁺ ions and 5.98% of Cr⁶⁺ ions from contaminated water. These nanomaterials are promising in terms of their ease and low cost of regeneration. This review addresses the relevance of nanocellulose as biosorbent, scaffold, and membrane in various heavy metal bioremediation, as well as provides insights into the challenges, future prospects, and updates. The methods of designing better nanocellulose biosorbents to improve adsorption efficiency according to contaminant types are focused.

Mercury emission profile for the torrefaction of sewage sludge at a full-scale plant and application of polymer sorbent

We evaluated mercury (Hg) behavior in a full-scale sewage sludge torrefaction plant with a capacity of 150 wet tons/day, which operates under a nitrogen atmosphere at a temperature range of 250-350 °C. Thermodynamic calculations and monitoring results show that elemental Hg (Hg⁰) was the dominant species in both the pyrolysis gas during the torrefaction stage and in the flue gas from downstream air pollution control devices. A wet scrubber (WS) effectively removed oxidized Hg from the flue gas and moved Hg to wastewater, and an electrostatic precipitator (ESP) removed significant particulate-bound Hg but showed a limited capacity for overall Hg removal. Hg bound to total suspended solids had a much higher concentration than that of dissolved Hg in wastewater. Total suspended solid removal from wastewater is therefore recommended to reduce Hg discharge. Existing air pollution control devices, which consist of a cyclone, WS, and ESP, are not sufficient for Hg removal due to the poor Hg⁰ removal performance of the WS and ESP; a further Hg⁰ removal unit is necessary. A commercial packed tower with sorbent polymer catalyst composite material was effective in removing Hg (83.3%) during sludge torrefaction.

Tracing the Transboundary Transport of Mercury to the Tibetan Plateau Using Atmospheric Mercury Isotopes

Deposition of atmospheric mercury (Hg) is the most important Hg source on the high-altitude Himalayas and Tibetan Plateau. Herein, total gaseous Hg (TGM) at an urban and a forest site on the Tibetan Plateau was collected respectively from May 2017 to October 2018, and isotopic compositions were measured to clarify the influences of landforms and monsoons on the transboundary transport of atmospheric Hg to the Tibetan Plateau. The transboundary transported anthropogenic emissions mainly originated over Indo-Gangetic Plain and carried over the Himalayas by convective storms and mid-tropospheric circulation, contributing over 50% to the TGM at the Lhasa urban site, based on the binary mixing model of isotopes. In contrast, during the transport of TGM from South Asia with low altitude, the uptake by evergreen forest in Yarlung Zangbo Grand Canyon largely decreased the TGM level and shifted isotopic compositions in TGM at the Nyingchi forest site, which are located at the high-altitude end of the canyon. Our results provided direct evidence from Hg isotopes to reveal the distinct patterns of transboundary transport to the Tibetan Plateau shaped by landforms and climates, which is critical to fully understand the biogeochemical cycling of Hg in the high-altitude regions.

Observation-Based Mercury Export from Rivers to Coastal Oceans in East Asia

Globally, the consumption of coastal fish is the predominant source of human exposure to methylmercury, a potent neurotoxicant that poses health risks to humans. However, the relative importance of riverine inputs and atmospheric deposition of mercury into coastal oceans remains uncertain owing to a lack of riverine mercury observations. Here, we present comprehensive seasonal observations of riverine mercury and methylmercury loads, including dissolved and particulate phases, to East Asia's coastal oceans, which supply nearly half of the world's seafood products. We found that East Asia's rivers annually exported 95 ± 29 megagrams of mercury to adjacent seas, 3-fold greater than the corresponding atmospheric deposition. Three rivers alone accounted for 71% of East Asia's riverine mercury exports, namely: Yangtze, Yellow, and Pearl rivers. We further conducted a metadata analysis to discuss the mercury burden on seawater and found that riverine export, combined with atmospheric deposition and terrestrial nutrients, quantitatively elevated the levels of total, methylated, and dissolved gaseous mercury in seawater by an order of magnitude. Our observations support that massive amounts of riverine mercury are exported to coastal oceans on a continental scale, intensifying their spread from coastal seawater to the atmosphere, marine sediments, and open oceans. We suggest that the impact of mercury transport along the land-ocean aquatic continuum should be considered in human exposure risk assessments.

Highly Resolved Inventory of Mercury Release to Water from Anthropogenic Sources in China

This study developed an up-to-date and point-source-based inventory of mercury (Hg) releases to water in China by applying probabilistic release factors that combined industry removal efficiencies, reuse of reclaimed water, and receiving water types. In 2017, the national mercury release to water was estimated to be 50 (35-66) tons, in which 47%, 8%, 7%, and 25% were from nonferrous metal smelting, vinyl chloride monomer (VCM) production, coal-fired boilers, and domestic sewage, respectively. Approximately 95% of mercury was released to inland rivers, and the rest was discharged to lakes or coastal water. The significant sources were identified based on their mercury releases to water. The control of mercury release to water in China shall focus on zinc smelting plants, municipal sewage treatment plants, and the VCM production process. For zinc smelting plants, China can tighten the limit of mercury concentration in discharged wastewater and combine Hg-catcher device in traditional integrated treatment. For municipal sewage treatment plants and the VCM production process, promoting processes of Hg-free production can reduce mercury inputs at the source. Our study provides insights for other parties to identify the relevant sources of mercury release to water and to conduct control measures, so as to promote the global convention implementation.

Different Mercury Species Partitioning and Distribution in the Water and Sediment of a Eutrophic Estuary in Northern Taiwan

The total Hg and methyl Hg in waters and sediments, as well as particulate total Hg (PTHg), were analyzed to study their distribution and partitioning in the Danshuei River Estuary (DRE), northern Taiwan. TOC and grain size were also determined in the sediment samples. The dissolved total Hg (DTHg) in waters ranged from 24.0 to 45.8 ng/L. The dissolved methyl Hg (DMeHg) concentrations contributed 0.6–30.4% of the DTHg pool, with the higher percentage appearing in the upper estuary. The DMeHg concentration positively correlated with the Chl.a within the estuary, suggesting that phytoplankton plays an important role in influencing the DMeHg concentration. The partitioning results indicated that DTHg chiefly dominates the THg (DTH + PTHg) pool, especially at a salinity of >15 psu region. The value of partition coefficient, log(KD), was within a range of 3.54 to 4.68, and the value linearly decreased with increasing salinity. The sediment total Hg (STHg) concentrations ranged from 80 to 379 ng/g, and most data exceeded the NOAA guidelines value (ERL < 150 ng/g), indicating that the DRE is contaminated with Hg. The STHg concentrations inversely and positively correlated with the grain size and TOC content, respectively, suggesting that sediment Hg distributions are strongly influenced by the both parameters.

Human Methylmercury Exposure and Potential Impacts in Central Tibet: Food and Traditional Tibetan Medicine

Methylmercury presents potent neurotoxicity to humans. Fish consumption is the leading source of human exposure to methylmercury worldwide. However, the exposure source in Tibet remains poorly understood because of the scarcity of observational data on most Tibetan foods, although high mercury levels were recently detected in some traditional Tibetan medicines. Here, the results of field investigations show that the joint consumption of traditional Tibetan medicines (TTMs), fish, and rice constitutes a primary exposure pathway to methylmercury in Tibetans and that the probable daily intake of methylmercury is close to that for many coastal regions. People who are young and high-income may have higher methylmercury exposure levels mainly because of economic development and cultural exchanges among regions. Our analysis indicates that a large proportion of the Tibetan population are likely to face a high methylmercury exposure risk and that mercury-susceptible populations in Tibet should be attentive to consuming TTMs with fish.

Heavy metal(loid)s in sewage sludge in China: concentrations and spatial-temporal variations

Sewage sludge (SS) production in China has increased rapidly, accompanying the fast expansion of its sewage treatment capacity. Heavy metals (HMs) in SS have been a great concern, hampering the utilization and disposal of SS. In this study, heavy metal (HM) contents in SS from throughout China were determined. The median values of HMs in SS decreased in the order Zn > Cu > Cr > Pb > Ni > As > Hg > Cd. The general attainment rates of HMs in SS are satisfying (> 90%). Combining the present data with those obtained from references, spatial distributions and temporal trends of HMs in SS were analyzed. Depending on the specific HM element, the spatial variation trend might be decreasing trends from south to north and from east to west of China. The element-specific hot spots of SS with relatively high HM contents were identified. Analysis of the historical data in different time intervals reviewed obvious decreasing trends in HM contents of SS in China, indicating the well implementation of more and more stringent environmental regulations.

Mercury and methylmercury in China's lake sediments and first estimation of mercury burial fluxes

Lake sediments are key materials for mercury deposition and methylation. To understand the mercury concentrations in China's lakes, 100 lake surface sediment samples were collected from 35 lakes in 2014. Total mercury (THg), methylmercury (MeHg) concentrations and the annual Hg burial rates in lake sediments were measured. THg and MeHg concentrations in the sediment ranged from 13.6 to 1488 ng‧g^-1 and 0.05 to 1.70 ng‧g^-1, respectively, and urban lakes reported most high values, indicating direct anthropogenic inputs. The Inner Mongolia-Xinjiang Region (MX) and Qinghai-Tibet Plateau Region (QT) reported relatively lower mercury burial rates, while the Eastern Plain Region (EP), Northeast Mountain and Plain Region (NE), and Yunnan-Guizhou Plateau Region (YG) reported higher mercury burial rates. Regional variances of THg burial fluxes were dominated by atmospheric deposition, terrestrial input, and sediment accumulation rates in different lakes. In 2014, the estimated average THg burial rate in China's lakes was 139 μg‧m^-2‧yr^-1, comparable to the average in mid-latitude North America in recent years; however, due to China's much smaller lake area relative to NA, the annual THg burial flux in China was much lower than that in North America. EP and NE, where most freshwater aquatic products in China are harvested, accounted for 58.2% and 22.9%, respectively, of the THg burial flux. High sedimentary MeHg concentrations and MeHg:THg ratios were reported in most of the NE but low MeHg concentrations and MeHg:THg ratios were reported in EP. MeHg concentrations and MeHg:THg ratios were positively correlated with water COD levels and negatively correlated with average temperature. The results of this study indicate that in addition to the adjacent seas, lake sediments are an important mercury sink in China's aquatic environment, which could cause health risks due to MeHg intake, especially in NE.

Machine learning-assisted calibration of Hg2+ sensors based on carbon nanotube field-effect transistors

Nanomaterial-based electronic sensors have demonstrated ultra-low detection limits, down to parts-per-billion (ppb) or parts-per-trillion (ppt) concentrations. However, these extreme sensitivities also make them susceptible to signal saturation at higher concentrations and restrict their usage primarily to low concentrations. Here, we report machine learning techniques to create a calibration method for carbon nanotube-based field-effect transistor (FET) devices. We started with linear regression, followed by regression splines to capture the non-linearity in the data. Further improvements in model performance were obtained with regression trees. Finally we lowered the model variance and further boosted the model performance by introducing random forest. The resulting performance as measured by R² was estimated to be 0.8260 using out-of-bag error. The methodology avoids saturation and extends the dynamic range of the nanosensors up to 12 orders of magnitude in analyte concentrations. Further investigations of the sensing mechanism include analysis of feature importance in each of the model we tested. Functionalized nanosensors demonstrate selective detection of Hg²⁺ ions with detection limits 10^-14.36±0.78 M, and maintain calibration to concentrations as high as 1 mM. Application of machine learning techniques to investigate which features in the FET signal maximally correlate with concentration changes provide valuable insight into the carbon nanotube sensing mechanism and assist in the rational design of future nanosensors.

Fate of mercury and methylmercury in full-scale sludge anaerobic digestion combined with thermal hydrolysis

Methylmercury (MeHg) is one of the highly toxic and bio-accumulated forms of mercury. Its presence in wastewater treatment processes has been evidenced in recent studies. Considering its enrichment in sewage sludge and the ecological risk associated with its land application, this study investigated the fate of mercury and MeHg in full-scale anaerobic digestion combined with Cambi thermal hydrolysis based on one-year sampling. Results showed that the advanced anaerobic digestion could increase the total mercury (THg) content from 4.35 ± 0.43 mg/kg in raw sludge to 6.37 ± 1.05 mg/kg in digested sludge, and the MeHg content decreased from 1.61 to 8.94 ng/g in raw sludge to 0.21-2.03 ng/g after anaerobic digestion. The demethylation of MeHg was dominant in both thermal hydrolysis and anaerobic digestion; it was mostly derived from the physico-chemical impacts such as chemical decomposition in thermal hydrolysis and precipitation in anaerobic digestion. Although the reported microbial methylators, such as Methanosarcina and Clostridia, were dominant in anaerobic digestion, the relative abundances of hgcA and merA were relatively low and did not correlate with the MeHg profiles. Thus, microbial methylation or demethylation seems negligible in terms of MeHg transformation.

The impact of the Three Gorges Dam on the fate of metal contaminants across the river-ocean continuum

The Three Gorges Dam (TGD) is the world's largest hydropower construction. It can significantly impact contaminant transport in the Yangtze River-East China Sea Continuum (YR-ECSC). In addition to evaluating the impact of the TGD on the deposition of contaminants in the reservoir, we also address their cycles in the river below the dam and in the coastal East China Sea. A comprehensive study of metal contaminant transport along the YR-ECSC has not been previously attempted. We quantified the fates of mercury (Hg), arsenic (As), lead (Pb), cadmium (Cd) and chromium (Cr) within the YR-ECSC, and the impacts of the TGD, by sampling water and suspended particles along the Yangtze River during spring, summer, fall, and winter and by modeling. We found that the Yangtze River transports substantial amounts of heavy metals into the coastal ocean. In 2016, riverine transport amounted to 48, 5900, 11,000, 230, and 15,000 megagrams (Mg) for Hg, As, Pb, Cd, and Cr, respectively, while other terrestrial contributions were negligible. Metal flux into the coastal ocean was primarily derived from the downstream portion of the river (84-97%), while metals transported from upstream were largely trapped in the Three Gorges Reservoir (TGR, 72%-96%). For example, 34 Mg of Hg accumulated in the TGR owing to river damming, large-scale soil erosion, and anthropogenic point source releases, while 21 Mg of Hg was depleted from the riverbed downstream owing to the altered river hydrology caused by the TGD. Overall the construction of TGD resulted in a 6.9% net decrease in the Hg burden of the East China Sea, compared to the pre-TGD period.

Dissolved Black Carbon Facilitates Photoreduction of Hg(II) to Hg(0) and Reduces Mercury Uptake by Lettuce (Lactuca sativa L.)

Here, we investigated the photoreduction of Hg(II) (Hg(NO3)2) mediated by dissolved black carbon (DBC, <0.45 μm size fraction) collected from water extracts of biochar derived by pyrolyzing crop residues (rice, soybean, and peanut). Under simulated sunlight conditions, the presence of 5 mg C/L DBC significantly facilitated the production of Hg(0) from Hg(II) (initially at 10 nmol/L) with a reduction ratio of 73 ± 4% in 5.3 h. Inhibition of photolysis-induced reactive oxygen species by a quencher or removal of dissolved oxygen indicated that Hg(II) was mainly reduced by superoxide anion (O2•-). Reduction by electrons transferred from photoexcited DBC components or by organic free radicals produced from photo-Fenton-like reactions was also proposed to play a role. Contrary to dissolved humic substances, the DBC-mediated photoreduction of Hg(II) led to unique positive mass-independent isotopic fractionation (MIF) of Hg(0) (Δ199Hg = 1.8 ± 0.3‰), which was attributed to the dominance of secondary Hg(II) reduction by O2•-. The leachate from soil amended with rice biochar at 1-5% mass ratios exhibited significantly higher photocatalytic efficiency than that from unamended soil (wherein the reduced Hg(0) increased from 27 ± 1 to 63 ± 2% in maximum), and the efficiency positively correlated with the percentage of amended biochar. Under natural illumination conditions, the total mercury and/or methylmercury uptake by roots, shoots, and leaves of lettuce (Lactuca sativa L.) grown in water extracts of rice biochar-amended soil was consistently lower (up to 70 ± 20%) than that without the biochar amendment. The findings highlight that DBC might play an important and previously unrecognized role in the biogeochemical cycle and the environmental impact of mercury.

A critical review on the occurrence of resistomes in the environment and their removal from wastewater using apposite treatment technologies: Limitations, successes and future improvement

Recent reports are pointing towards the potential increasing risks of resistomes in human host. With no permissible limit in sight, resistomes are continually multiplying at an alarming rate in the ecosystem, with a disturbing level in drinking water source. The morphology and chemical constituent of resistomes afford them to resist degradation, elude membrane and counter ionic charge, thereby, rendering both conventional and advanced water and wastewater treatment inefficient. Water and wastewater matrix may govern the propagation of individual resistomes sub-type, co-selection and specific interaction towards precise condition may have enhanced the current challenge. This review covers recent reports (2011-2019) on the occurrence of ARB/ARGs and ease of spread of resistance genes in the aquatic ecosystem. The contributions of water matrix to the spread and mitigation, treatment options, via bulk removal or capture, and intracellular and extracellular DNA lysis were discussed. A complete summary of recent occurrences of ARB/ARGs, fate after disinfection and optimum conditions of individual treatment technology or in tandem, including process limitations, with a brief assessment of removal or degradation mechanism were highlighted.

Drastic reduction of sludge in wastewater treatment plants: co-digestion of sewage sludge and aqueous waste in a thermophilic membrane reactor

Sewage sludge and aqueous wastes are usually treated in separate facilities. Both may pose specific issues, mainly related to the uncertainty of the recovery/disposal route and costs, for the sludge, and to the extremely variable quantitative and qualitative properties, for the aqueous waste. In the present work, the co-digestion of thickened sludge and aqueous wastes in a Thermophilic Aerobic Membrane Reactor (TAMR) was studied in order to allow the almost complete reduction of sludge directly in wastewater treatment plants (WWTPs). Different conditions (aerobic and alternate aeration) were tested in a pilot plant, at the semi-industrial scale. The TAMR plant was operated at 48°C with constant organic load rate (5 kgCOD m^-3 d^-1) and hydraulic retention time (5 days). The main results obtained are the following: (I) high overall COD (78-97%) and total phosphorus (>60%) removal rate under both the studied aeration conditions; (II) increase of ammonia concentration due to the effective ammonification of organic nitrogen; (III) low specific sludge production (0.04[Formula: see text]) in the thermophilic reactor.

The upgrading of conventional activated sludge processes with thermophilic aerobic membrane reactor: Alternative solutions for sludge reduction

Sludge recovery/disposal represents one of the most crucial aspects related to the management of wastewater treatment plants. The most widely diffused technology for the treatment of industrial and municipal wastewaters is the conventional activated sludge (CAS) process, which is characterized by a relatively high excess sludge production. Different technical solutions are proposed in the literature for sludge minimization and they can be applied either on wastewater line (WL) or sludge line (SL). This work is focused on different approaches based on the use of Thermophilic Aerobic Membrane Reactor (TAMR): this can be added to a CAS plant, and integrated to WL or SL, yielding a significant sludge reduction. The process performance was analysed in terms of volatile solids (VS) reduction and specific sludge production. The TAMR was tested both at full-scale and pilot-scale with different feeding substrates: industrial wastewater for the full-scale plant; industrial wastewater, sludge and a mix of these for the pilot-scale plants. The results obtained are: (i) good solids removal (38-90% and 40-50% in terms of VS for sludge and mix of industrial wastewater and sludge, respectively), (ii) low specific sludge production (0.01-0.09 kg_{VSS produced} kg_{COD removed}^-1 for industrial wastewater and 0.014-0.069 kg_{VSS produced} kg_{COD removed}^-1 for mix of industrial wastewater and sludge) and (iii) a significant reduction of sludge when CAS is improved with the TAMR technology.

Mercury loads and fluxes from wastewater: A nationwide survey in Switzerland

Mercury (Hg) pollution threatens ecosystems and human health. Wastewater treatment plants (WWTPs) play a key role in limiting Hg discharges from wastewaters to rivers and lakes, but large-scale studies to estimate Hg loads and discharge at national levels are scarce. We assessed the concentration, flux, speciation, and removal of Hg in municipal wastewater throughout Switzerland by investigating 64 WWTPs in a pre-study and a subset of 28 WWTPs in the main study. We also studied the behavior and pathways of Hg along the various treatment steps in a state-of-the-art WWTP. The resulting dataset, representative of industrialized countries, provides an overview of (i) current Hg concentration ranges, (ii) average per capita loads, and (iii) wastewater Hg inputs into surface waters. The results allowed estimation of a total Hg (THg) load in Swiss wastewater of 130 ± 30 kg THg/year (15.7 mg/capita/y), of which 96 ± 4% is retained in sewage sludge. About 4.7 ± 0.5 kg THg/year (0.57 mg/capita/y) is discharged with the treated wastewater into surface waters. This corresponds to only 1.5-3% of the THg load carried by the major Swiss rivers, indicating that >95% of riverine Hg originates from other sources. Extrapolation to the population of Europe would yield a total amount of 11,700 kg THg/year in raw wastewater, with some 480 kg THg/year discharged to surface waters. Monomethyl mercury on average accounted for 0.23% of THg, and its fraction remained constant along the different treatment steps.

Mercury isotope compositions in large anthropogenically impacted Pearl River, South China

Rivers integrate natural and anthropogenic mercury (Hg), and are important vectors of terrestrial Hg to the oceans. Here, we report the total Hg concentration and Hg isotope compositions of dissolved load in the Pearl River, the second largest river in China, in order to understand the processes and sources affecting Hg systematics in large anthropogenically-impacted river water. The dissolved Hg showed a concentration varying from 0.45 to 2.44 ng/L, within the range reported for natural background lake and river waters. All river water samples showed significantly negative δ²⁰²Hg (-2.89‰ to -0.57‰), slightly positive Δ²⁰⁰Hg (-0.05‰ to 0.52‰), and mostly positive Δ¹⁹⁹Hg (0.10‰ to 0.57‰), except for three extremely negative values (-2.25‰ to -0.76‰). Combined with other geochemical parameters, we suggest that the influence of in-river processes, such as sorption and reduction, on the Hg isotope compositions is very limited, and the dissolved Hg in the Pearl River mainly comes from atmospheric precipitation and surface soil weathering. Although the whole river basin is largely affected by urban, industrial and mining activities, unlike other heavy metals, their direct contributions to dissolved Hg seem limited. It is worth noting that the three samples with very negative Δ¹⁹⁹Hg values (down to -2.25‰) are derived from special source which attribute to the input of Hg released from the local incineration of electronic wastes. This study demonstrates that isotope approach is a powerful tool for tracing sources and pathways of Hg in large complex river systems.

Occurrence and Fate of Heavy Metals in Municipal Wastewater in Heilongjiang Province, China: A Monthly Reconnaissance from 2015 to 2017

As one of the major sources of pollutions in the environments, effluents from municipal wastewater recently became a hot topic. This study quantified monthly county-level releases of five heavy metals, i.e., lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), and mercury (Hg), from municipal wastewater into the environment in the Heilongjiang Province of China, based on sampling, measurement, and modeling tools. Wastewater samples were collected from 27 municipal wastewater treatment plants (MWTPs) in 15 county-level cities of Heilongjiang every month from 2015 to 2017. The concentrations of five heavy metals were analyzed in both influents (Pb: 160 ± 100 μg/L; Cd: 15 ± 9.0 μg/L; Cr: 170 ± 64 μg/L; Hg: 0.67 ± 1.5 μg/L; As: 6.2 ± 4.8 μg/L) and effluents (Pb: 45 ± 15 μg/L; Cd: 5.2 ± 5.1 μg/L; Cr: 57 ± 13 μg/L; Hg: 0.28 ± 0.12 μg/L; As: 2.6 ± 1.4 μg/L). The removal ratios of the five heavy metals ranged from 50% to 67%. Inflow fluxes of Pb, Cr, and Cd displayed increasing trends first then decreased after reaching a maximum value, whereas those of Hg and Pb remained stable. Material flow analysis reveals that constructions of MWTPs are conducive to significantly reduce the releases of heavy metals from urban areas into the aquatic environment in the study area. Additionally, municipal wastewater sludge (used as fertilizer or spread on the land) could be a significant source of heavy metals in the land.

Significant elevation of human methylmercury exposure induced by the food trade in Beijing, a developing megacity

Methylmercury (MeHg) poses health risks to humans worldwide. The investigation of a longer chain of biogeochemical MeHg transport from production to consumption than that addressed in previous studies could provide additional scientific foundation for the reduction of risks. The main objective of this study is to identify the impacts of the interregional food trade along with the age, gender and socioeconomic status of people on human MeHg exposure in a developing megacity. Based on a field investigation, sampling and measurements, we provide experimental evidence regarding the substantial displacement of human MeHg exposure from production areas to consumption areas induced by the food trade. In 2018, 20% and 64% of the exposure in Beijing originated from the international and interprovincial food trade, respectively. Meanwhile, the ingestion of fish contributed 79% to the total exposure, followed by rice (4.4%), crab (3.8%) and shrimp (2.7%), and the exposure risk in urban districts was higher than that in rural areas by a factor of 2.2. A significantly higher contribution of imported deep-sea species to exposure among young people than among older people was observed (P < 0.01**), and a larger contribution of the international food trade to the MeHg exposure risk for women of childbearing age (average: 27%) than that among other groups (average: 10%) was found. Overall, our efforts demonstrate the dramatic impact of the food trade on MeHg exposure in a developing megacity, and we suggest that MeHg-susceptible populations in China should choose indigenous fish species (e.g., hairtail, yellow croaker and carp species) rather than imported deep-sea species as their dietary protein source.

Occurrence, speciation and fate of mercury in the sewage sludge of China

Municipal sewage has been identified to be an important source of mercury (Hg) to the environment. However, as the major sink of sewage-borne Hg, sewage sludge (SS) remains unresolved in terms of the occurrence status of Hg species. We presented here, a nation-wide survey on the speciation of Hg in SS of China. Total Hg (THg) and methylmercury (MeHg) were detected in all SS samples, within ranges of 0.4-12.4 mg/kg and 0.1-27.0 μg/kg, respectively. Sludge-borne Hg mainly occurred in the mercury sulfide and organo-chelated phases, with only tiny portions occurring as soluble Hg. The mass loadings of sludge-borne THg and MeHg in China for year 2016 were estimated to be 12.2 metric tons and 19.9 kg, respectively. Landfill was the most important sink of sludge-borne Hg, followed by incineration, land application, and building materials.

Sources and transport of methylmercury in the Yangtze River and the impact of the Three Gorges Dam

The magnitude of environmental change due to anthropogenic impacts might greatly exceed that of natural disturbances. In this work, we quantitatively examine the impacts of river damming, soil erosion, and point-source release on the transport of methylmercury (MeHg) throughout the Yangtze River, the third longest river in the world. Based on seasonal observations and the subsequent material flow analysis, we found that in 2016, the Yangtze River discharged 470 ± 200 kg MeHg to the coastal and shelf areas, a value at least ten-fold larger than existing observations in other large rivers around the world. The construction of the Three Gorges Dam (TGD), the world's largest hydropower dam, induced a substantial amount of MeHg (at least 250 ± 220 kg) accumulation in the reservoir and a relatively small amount of MeHg (150 ± 37 kg) discharge to the downstream region in 2016. The reservoir itself is not expected to be more contaminated by MeHg than the downstream areas of the river after the TGD, and the TGD has an additive effect on downstream MeHg transport. The riverine MeHg flux in the river mouth was 3-fold that discharged from the TGD mainly due to TGD-induced resuspension of MeHg from the downstream riverbed, as well as MeHg imports to the downstream area from tributaries, soil erosion, municipal wastewater, and in situ production. Our analysis offers new evidence that in future decades, the increase in estuarine MeHg contamination resulting from the increasing construction of large dams might pose a challenge for global coastal fisheries.

Adsorption of low-concentration mercury in water by 3D cyclodextrin/graphene composites: Synergistic effect and enhancement mechanism

The efficient removal of mercury from aqueous media remains a severe challenge in ensuring environmental safety, especially for low-concentration mercury, which requires adsorbents with high mercury affinity. In this work, we reported a nanocomposite of β-cyclodextrin and three-dimensional graphene (3D CD@RGO) to enhance the adsorption affinity and capacity for mercury with low concentrations. Characterization of the nanocomposite revealed that cyclodextrin was well dispersed on the 3D graphene support structure to provide highly exposed hydroxyl groups. Adsorption experiments showed that CD@RGO exhibited different adsorption behaviors for mercury within different concentration ranges of 0.2-4.0 mg/L and 4.0-10.0 mg/L, and the adsorption affinity for the former range (K_L = 10.05 L/mg) was 1.5 times higher than that for the latter range (K_L = 6.69 L/mg). Moreover, CD@RGO had a high adsorption efficiency of 96.6% with a superb adsorption affinity (172.09 L/g) at C_e = 0.01 mg/L, which is 6.70 and 41.25 times higher than that of RGO and RCD (physical mixture of RGO and cyclodextrin), respectively, indicating a synergistic effect of CD@RGO for mercury adsorption. This enhancement can be attributed to the transformation of the adsorption mechanism from the outer-sphere force of electrostatic interaction in RGO to the inner-sphere surface complexation in CD@RGO.

An experimental study of the impacts of solar radiation and temperature on mercury emission from different natural soils across China

Mercury (Hg) emission from natural soil is one of the most important contributors to global Hg cycles. Research on Hg emission from soil to air has been carried out in China. Currently, most of the research focuses on contaminated sites in China, while research in other regions is rare. To provide more accurate information on Hg emissions from soil to air in China and obtain additional laboratory data to verify the role of solar radiation and temperature in this process, we sampled and measured Hg emission fluxes from various natural soils (range, 48-240 ng/g) across mainland China under different solar radiation (0-900 W·m^-2) and temperature (15-45 °C) conditions in a laboratory. We found that in different places in China, Hg emissions from natural soils occurred more easily when the soil Hg concentration, temperature, and solar radiation were high, but the impacts were different among the regions due to different soil types. Hg emissions from natural soils (0.071-24 ng·m²·h^-1) were typically lower than those from contaminated sites, suggesting that additional measurements in natural soils are desirable. The results of this study could provide more accurate information on Hg emission from natural soil to air and help establish a nationwide natural soil Hg emission inventory in China.

Mercury in Municipal Sewage and Sewage Sludge

Wide occurrence of mercury species, including the highly toxic and readily bioaccumulative methylmercury (MeHg), in municipal sewage (MS) and sewage sludge (SS) has been evidenced in recent studies. Considering that vast amounts of MS and SS are produced globally each year and the majority of MS is discharged into aqueous environments, i.e., the main sites for MeHg bioaccumulation, special attention should be paid on the source and environmental behaviors of sewage-borne and sludge-borne mercury species. This review aims to summarize the findings on the occurrence of mercury species in MS and SS, their transport and transformation in MS treatment plants, as well as their fate and environmental implications.

Rapid Increase in the Lateral Transport of Trace Elements Induced by Soil Erosion in Major Karst Regions in China

Soil erosion, which has been recently shown to significantly perturb carbon cycling, occurs naturally but can be either enhanced or reduced by human activities. However, the impacts of soil erosion on terrestrial contaminant cycles remain unclear. Here, we select eight trace elements, i.e., arsenic, cadmium, chromium, copper, nickel, lead, zinc, and mercury, to examine the erosional impacts of the elements' fate and transport across China. By synthesizing the detailed distribution of soil erosion fluxes, soil element inventories, and diverse modeling methods, we reveal that while human activities have reduced the lateral transport of these elements in the Loess Plateau (Central North China, a 56% decline in the past two decades with a range of 46% to 110%) due to soil conservation projects, they have increased these transport fluxes in China's major karst regions (Southwest China, a 84% increase with a range of 55% to 150%) because of severe rocky desertification. These fluxes have completely overwhelmed the soil conservation efforts in the Loess Plateau. Fluxes of these elements into aquatic environments from Southwest China reached 46% of the total input in China in 2010. These fluxes were higher than the inputs from point sources in the region by a factor of 50 because of impacts of excessive agricultural cultivation and geographical and climatic factors. These findings indicate the enormous perturbation of terrestrial contaminant cycles caused by soil erosion in karst regions and demonstrate the need for long-term sustainable management of soil erosion and contaminant discharge to protect fragile terrestrial ecosystems.

Mercury/silver resistance genes and their association with antibiotic resistance genes and microbial community in a municipal wastewater treatment plant

Municipal wastewater treatment plants (WWTPs) are an important reservoir for heavy metal (e.g., Hg and Ag) resistance genes and antibiotic resistance genes (ARGs). However, current knowledge on Hg/Ag resistance genes and their association with ARGs in WWTPs remains largely unknown. In this study, the fates of five Hg/Ag resistance genes (merB, merD, merR, silE, and silR), five ARGs (sulI, sulII, tetO, tetQ, tetW), and class 1 integrase (intI1) in a WWTP were investigated. Results show that the absolute abundances of all target genes were greatly reduced through the treatment systems. The dynamics of merB, merD and silE were significantly correlated with tetW and sulII. Based on network analysis, Hg/Ag resistance genes might share the same microbial hosts with tetQ and tetW, implying the potential importance of Hg/Ag in ARGs evolution and spread. These findings advanced our understanding of the occurrence of Hg/Ag resistance genes and ARGs in WWTPs.

Comparative effects of mercury chloride and methylmercury exposure on early neurodevelopment in zebrafish larvae

Mercury (Hg) is a ubiquitous environmental toxicant with important public health implications. Hg causes neurotoxicity through astrocytes, Ca²⁺, neurotransmitters, mitochondrial damage, elevations of reactive oxygen species and post-translational modifications. However, the similarities and differences between the neurotoxic mechanisms caused by different chemical forms of Hg remain unclear. Zebrafish embryos were exposed to methylmercury (MeHgCl) or mercury chloride (HgCl₂) (0, 4, 40, 400 nM) up for 96 h. HgCl₂ exposure could significantly decrease survival rate, body length and eye size, delay the hatching period, induce tail bending and reduce the locomotor activity, and these effects were aggravated in the MeHgCl group. The compounds could increase the number of apoptotic cells in the brain and downregulate the expression of Shha, Ngn1 and Nrd, which contribute to early nervous development. The underlying mechanisms were investigated by metabolomics data. Galactose metabolism, tyrosine metabolism and starch and sucrose metabolism pathways were disturbed after HgCl₂ or MeHgCl exposure. In addition, the levels of three neurotransmitters including tyrosine, dopamine and tryptophan were reduced after HgCl₂ or MeHgCl exposure. Oxidative stress is related to metabolite changes, such as changes in the putrescine, niacinamide and uric acid contents in the HgCl₂ group, and squalene in the MeHgCl group. These data indicated that downregulation of these genes and abnormal metabolic profile and pathways contribute to the neurotoxicity of HgCl₂ and MeHgCl.

The metabolomics and neurodevelopmental endpoints were integrated to reveal that abnormal metabolic pathway and expression of Shha, Ngn1 and Nrd may contribute to neurotoxicity induced by MeHg, which was more toxic than HgCl₂ in zebrafish larvae.

Impacts of farmed fish consumption and food trade on methylmercury exposure in China

The global pollutant mercury (Hg), especially as methylmercury (MeHg), threatens human and ecosystem health. But major contributors of MeHg exposure to people in China remain highly debated. We developed the China Mercury Exposure Assessment (CMEA) model, which incorporates human exposure pathways for MeHg and total Hg (THg), the interregional, including international and interprovincial, food trading as well as human physiology to provide a comprehensive system that can evaluate the pathway of Hg forms to human consumers in China. Based on the CMEA model that employed the most comprehensive and recent data, we have found that the Probable Daily Intake (PDI) of MeHg for the Chinese population was 0.057 (range: 0.036-0.091 as 60% confidence interval) μg·kg^-1·day^-1, while that of THg was 0.35 (range: 0.22-0.55) μg·kg^-1·day^-1. MeHg exposure was dominated by fish intake, especially by farm-raised freshwater fish due to higher consumption of these fish. In 2011, fish intake contributed to 56% to the total MeHg exposure, followed by rice (26%). Consumption of farm-raised fish reduced human exposure to MeHg by 33%. On the other hand, interregional food trading increased MeHg exposure of the Chinese population, as a whole, by 7.6%. The international and interprovincial food trades contributed to 5.1% and 22% of MeHg intake, respectively. For the whole China, fish intake related exposure to MeHg was highest for the Eastern and Northeastern populations, while Tibetans were chronically exposed to the highest MeHg from other sources. Our findings highlight the importance of farmed fish and food trade for MeHg exposure.

Enrichments of Metals, Including Methylmercury, in Sewage Spills in South Carolina, USA

Exposure to microbial pathogens is the primary concern of sanitary sewer overflows; however, sewage spills may also be a significant source of toxic metals, including methylmercury (MeHg). Between November 2015 and January 2017, after Hurricane Joaquin, surface water samples were collected routinely from three creeks in Columbia, SC. Routine sampling coincided with six sewage spills. Total mercury (THg) and MeHg (unfiltered and filtered) and 32 other metals (filtered) were measured. Compared with surface water samples, THg (unfiltered and filtered), MeHg (unfiltered), and 19 other metals were significantly higher in sewage spills (all log-transformed) (two-tailed test, < 0.05 for all, = 38-42). Toxic weighting factors were applied to 18 metals, including THg and MeHg, in samples collected directly from sewage spills ( = 3-4) and a wastewater outfall ( = 5). On average, sewage was 18.2 and 12.0 times more toxic for THg and MeHg, respectively, and 1.75 times more toxic for all 18 metals, compared to treated effluent from the wastewater outfall. Results suggest sewage spills were a source of inorganic Hg, MeHg, and other metals to the receiving waters and may potentially contribute to water quality impairments.

Traditional Tibetan Medicine Induced High Methylmercury Exposure Level and Environmental Mercury Burden in Tibet, China

Highly elevated concentrations of total mercury (THg) and methylmercury (MeHg) were found in the municipal sewage in Tibet. Material flow analysis supports the hypothesis that these elevated concentrations are related to regular ingestion of Hg-containing Traditional Tibetan Medicine (TTM). In Tibet in 2015, a total of 3600 kg of THg was released from human body into the terrestrial environment as a result of TTM ingestion, amounting to 45% of the total THg release into the terrestrial environment in Tibet, hence substantially enhancing the environmental Hg burden. Regular ingestion of TTM leads to chronic exposure of Tibetans to inorganic Hg (IHg) and MeHg, which is 34 to 3000-fold and 0-12-fold higher than from any other known dietary sources, respectively. Application of a human physiology model demonstrated that ingestion of TTM can induce high blood IHg and MeHg levels in the human body. Moreover, 180 days would be required for the MeHg to be cleared out of the human body and return to the initial concentration i.e. prior to the ingestion of 1 TTM pill. Our analysis suggests that high Hg level contained in TTM could be harmful to human health and elevate the environmental Hg burden in Tibet.

A case study on the occurrence, transport, and fate of mercury species in a sewage treatment plant in Jiaozuo, China

Occurrence and fate of sewage- and sludge-borne total mercury (THg) and methylmercury (MeHg) have received little attention, although they might be an important component of the global Hg inventory. To address the knowledge gap, sewage and sewage sludge (SS) samples were collected from a secondary municipal sewage treatment plant (MSTP) located in Jiaozuo, China. Total Hg in the raw sewage varied between 95.3 ng/L and 1.2 × 10³ ng/L, while MeHg occurred in the sewage within ranges of 2.7-21.3 ng/L. On average, 93.4% of THg and 89.1% of MeHg were removed from the raw sewage by the MSTP. Around 80% of net THg input into the MSTP was transported to the SS, while 80% of net MeHg input was decomposed during the treatment processes. Thus, MSTPs would be regarded as an important sink of sewage-borne THg and MeHg. On the other hand, MeHg widely occurred in the SS, with concentrations between 7.2 and 10.2 ng/g, accounting for 0.2% of the corresponding THg contents in the SS. Occurrence of THg and MeHg in the SS indicated that SS would be a significant source of MeHg and THg to the environment.

Impact of Water-Induced Soil Erosion on the Terrestrial Transport and Atmospheric Emission of Mercury in China

Terrestrial mercury (Hg) transport, induced by water erosion and exacerbated by human activities, constitutes a major disturbance of the natural Hg cycle, but the processes are still not well understood. In this study, we modeled these processes using detailed information on erosion and Hg in soils and found that vast quantities of total Hg (THg) are being removed from land surfaces in China as a result of water erosion, which were estimated at 420 Mg/yr around 2010. This was significantly higher than the 240 Mg/yr mobilized around 1990. The erosion mechanism excavated substantial soil THg, which contributed to enhanced Hg(0) emissions to the atmosphere (4.9 Mg/yr around 2010) and its transport horizontally into streams (310 Mg/yr). Erosion-induced THg transport was driven by the extent of precipitation but was further enhanced or reduced by vegetation cover and land use changes in some regions. Surface air temperature may exacerbate the horizontal THg release into water. Our analyses quantified the processes of erosion-induced THg transport in terrestrial ecosystems, demonstrated its importance, and discussed how this transport is impacted by anthropogenic inputs and legacy THg in soils. We suggest that policy makers should pay more attention to legacy anthropogenic THg sources buried in soil.