Evaluation of mercury availability to pea parts (Pisum sativum L.) in urban soils: Comparison between diffusive gradients in thin films technique and plant model

Study of mercury bioavailability using isotope dilution and BCR sequential extraction in the sediment of Yellow Sea and East China Sea, China

Mercury (Hg) emitted from East Asian has increased the risk of Hg in China Marginal Seas for decades. However, the speciation of Hg (especially the bioavailable Hg) in these regions remains unclear. To address this problem, we analyzed total Hg (THg) and methylmercury (MeHg) in the sediment and porewater of Yellow sea (YS) and East China Sea (ECS) and determined the speciation of Hg using both improved BCR sequential extraction and isotope dilution (ID) techniques. Nearshore areas of YS and ECS exhibited higher THg levels in sediments and porewater, suggesting the significant contribution of terrestrial inputs. The spatial distribution of MeHg showed similar trends with THg, but the sites with higher MeHg concentrations did not align with those of THg. The improved BCR sequential extraction method showed the residual fraction dominated Hg content (∼44 %) in both systems, with a minor bioavailable carbonate fraction (1 %). The Spearman correlation analysis indicates that Eh and pH are the two factors significantly affected Hg bioavailability in the sediment. The bioavailability of Hg (estimated by the BCR method) showed a significant positive correlation with MeHg levels in the sediment (R²=0.47, P < 0.05), suggesting that BCR can be used to estimate the potential of Hg methylation in the sediment. However, the extent of bioavailable Hg in BCR and ID method were 1.15 ± 0.38 % and 29.5 ± 14.8 %, respectively, implying that Hg bioavailability may be underestimated by BCR techniques compared to ID methods (T-test, P < 0.01).

Use of diffusive gradients in thin-film technique to predict the mobility and transfer of nutrients and toxic elements from agricultural soil to crops-an overview of recent studies

The purpose of this review was to survey the recent applications of the diffusive gradients in thin films (DGT) technique in the assessment of mobility and bioavailability of nutrients and potentially toxic elements (PTEs) in agricultural soil. Many studies compared the capabilities of the DGT technique with those of classical soil chemical extractants used in single or sequential procedures to predict nutrients and PTE bioavailability to crops. In most of the published works, the DGT technique was reported to be superior to the conventional chemical extraction and fractionation methods in obtaining significant correlations with the metals and metalloids accumulated in crops. In the domain of nutrient bioavailability assessment, DGT-based studies focused mainly on phosphorous and selenium labile fraction measurement, but potassium, manganese, and nitrogen were also studied using the DGT tool. Different DGT configurations are reported, using binding and diffusive layers specific for certain analytes (Hg, P, and Se) or gels with wider applicability, such as Chelex-based binding gels for metal cations and ferrihydrite-based hydrogels for oxyanions. Overall, the literature demonstrates that the DGT technique is relevant for the evaluation of metal and nutrient bioavailability to crops, due to its capacity to mimic the plant root uptake process, which justifies future improvement efforts.

Assessment of mercury bioavailability in garden soils around a former nonferrous metal mining area using DGT, accumulation in vegetables, and implications for health risk

Mercury (Hg) is a toxic, non-essential element for living organisms, frequently present in high concentrations in soils from industrial areas. The total, dissolved, and labile Hg concentrations in garden soils and their accumulation in edible vegetables (onion, garlic, lettuce, and parsley) grown on contaminated soils in localities situated a former mining area were evaluated. The labile Hg fraction was estimated by diffusive gradient in thin films (DGT). The soil-to-vegetable transfer factors, as well as the health risk by exposure to Hg, were calculated based on the labile Hg concentration in soil. The total Hg concentration in soil varied widely (0.11-3.77 mg kg-1), Hg in soil solution ranged between 2.14 and 20.2 μg L-1 and labile Hg between 1.13 and 18.6 μg L-1. About 36-96% (84% on average) of the Hg concentration in soil solution was found in labile form. Multivariate analysis revealed significant correlations between the labile Hg concentration in soil and Hg accumulated in vegetables. The hazard indices showed that, although the study area is affected by legacy pollution, exposure to soil and consumption of locally grown vegetables do not pose health risks.

Mercury in air and soil on an urban-rural transect in East Africa

There are large knowledge gaps concerning concentrations, sources, emissions, and spatial trends of mercury (Hg) in the atmosphere in developing regions of the Southern Hemisphere, particularly in urban areas. Filling these gaps is a prerequisite for assessing the effectiveness of international regulation and for enabling a better understanding of the global transport of Hg in the environment. Here we use a passive sampling technique to study the spatial distribution of gaseous elemental Hg (Hg(0), GEM) and assess emission sources in and around Dar es Salaam, Tanzania's largest city. Included in the study were the city's main municipal waste dumpsite and an e-waste processing facility as potential sources of GEM. To complement the GEM data and for a better overview of the Hg contamination status of Dar es Salaam, soil samples were collected from the same locations where passive air samplers were deployed and analysed for total Hg. Overall, GEM concentrations ranged between <0.86 and 5.34 ng m^-3, indicating significant local sources within the urban area. The municipal waste dumpsite and e-waste site had GEM concentrations elevated above the background, at 2.41 and 1.77 ng m^-3, respectively. Hg concentrations in soil in the region (range 0.0067 to 0.098 mg kg^-1) were low compared to those of other urban areas and were not correlated with atmospheric GEM concentrations. This study demonstrates that GEM is a significant environmental issue in the urban region of Dar es Salaam. Further studies from urban areas in the Global South are needed to better identify sources of GEM.

Evaluation of mercury bioavailability to vegetables in the vicinity of cinnabar mine

Knowledge of the concentration of the bioavailable forms of mercury in the soil is necessary, especially, if these soils contain above-limit total mercury concentrations. The bioavailability of mercury in soil samples collected from the vicinity of abandoned cinnabar mines was evaluated using diffusive gradients in the thin films technique (DGT) and mercury phytoaccumulation by vegetables (lettuce, spinach, radish, beetroot, carrot, and green peas). Mercury was accumulated primarily in roots of vegetables. The phytoaccumulation of mercury into edible plant parts was site-specific as well as vegetable species-specific. The mercury concentration in edible parts decreased in the order: spinach leaf ≥ lettuce leaf ≥ carrot storage root ≥ beetroot storage root > radish storage root > pea legume. The translocation index as well as the target hazard quotient indicate the possible usability of soils from the vicinity of abandoned cinnabar mines for planting pod vegetables (peas). A strong positive correlation (r = 0.75 to 0.92, n > 30, p < 0.05) was observed between mercury concentration in secondary roots, the storage roots, leaves of vegetables and the flux of mercury from soil to the DGT units, and the effective concentration of mercury in soil solutions.

Applying the diffusive gradient in thin films method to assess soil mercury bioavailability to the earthworm Eisenia fetida

This study assessed the critical soil characteristics affecting mercury (Hg) bioavailability to the earthworm Eisenia fetida using the diffusive gradient in thin films (DGT) method. The soil samples were collected from a tributary of the Hyeongsan River contaminated with industrial waste and landfill leachates called Gumu Creek. The Hg concentration in the soil had a range of 0.33-170 μg g^-1 (average 33 ± 56 μg g^-1), and the Hg concentration of earthworms incubated in the soils was 0.83-11 μg g^-1 (average 2.9 ± 3.2 μg g^-1). When correlation analysis was used to detect the key variables among the soil properties related to Hg accumulation in the soils, earthworms, and resins, the water-holding capacity, which is covaried with the organic matter content, was determined to be a primary factor in increasing Hg accumulation in the soils, earthworms, and resins. However, the experimentally determined earthworm bioaccumulation factor and the DGT accumulation factor were negatively affected by the water-holding capacity. Therefore, the water-holding capacity played a dual role in the Gumu Creek deposits: increasing the soil Hg concentration and decreasing Hg bioavailability and leachability. Further, the DGT-Hg flux was positively correlated with the Hg concentration in earthworms (r = 0.93). Although the earthworm accumulation of Hg is not processed by passive diffusion, this study proves that the DGT method is promising for predicting soil Hg bioavailability to the earthworm E. fetida, and the water-holding capacity simultaneously regulates Hg availability to the DGT and the earthworms.

Fractionation Analysis of Mercury in Soils: A Comparison of Three Techniques for Bioavailable Mercury Fraction Determination

Knowledge of the fractionation of mercury in soils in the vicinity of abandoned cinnabar mines is essential for assessing the usability of soils for the cultivation of agriculturally important crops. Two different sequential extraction methods and the technique of diffusive gradients in thin films (DGT) were applied and compared for fractionation of mercury in soils from mercury-contaminated sites intended for farming purposes. The mercury found in these soils was primarily in the form of mercury sulfide (58.6-83.9%), followed by 6.7 to 15.4% of organically bound mercury and 2.9 to 23.2% of elemental mercury. Up to 10.3% of labile mercury species were determined by both sequential extraction methods in these soils. However, only 0.01 to 0.13% of mercury was determined as a bioavailable fraction using the DGT technique. Both sequential extraction methods tested for the fractionation analysis of mercury in contaminated soils were in excellent agreement. The content of the mobile (labile) mercury determined by the sequential extraction methods was statistically significantly higher (p < 0.0001) than the content of bioavailable mercury determined by the DGT technique. Environ Toxicol Chem 2020;39:1670-1677. © 2020 SETAC.

Determination of (Bio)-available mercury in soils: A review

Despite the mercury (Hg) control measures adopted by the international community, Hg still poses a significant risk to ecosystem and human health. This is primarily due to the ability of atmospheric Hg to travel intercontinentally and contaminating terrestrial and aquatic environments far from its natural and anthropogenic point sources. The issue of Hg pollution is further complicated by its unique physicochemical characteristics, most noticeably its multiple chemical forms that vary in their toxicity and environmental mobility. This meant that most of the risk evaluation protocols developed for other metal(loid)s are not suitable for Hg. Soil is a major reservoir of Hg and a key player in its global cycle. To fully assess the risks of soil Hg it is essential to estimate its bioavailability and/or availability which are closely linked to its toxicity. However, the accurate determination of the (bio)-available pools of Hg in soils is problematic, because the terms 'bioavailable' and 'available' are ill-defined. In particular, the term 'bioavailable pool', representing the fraction of Hg that is accessible to living organisms, has been consistently misused by interchanging with other intrinsically different terms e.g. mobile, labile, reactive and soluble pools. A wide array of physical, chemical, biological and isotopic exchange methods were developed to estimate the (bio)-available pools of Hg in soil in an attempt to offer a plausible assessment of its risks. Unfortunately, many of these methods do not mirror the (bio)-available pools of soil Hg and suffer from technical drawbacks. In this review, we discuss advantages and disadvantages of methods that are currently applied to quantify the (bio)-availability of Hg in soils. We recommended the most feasible methods and give suggestions how to improve the determination of (bio)-available Hg in soils.