Overview and challenges of mercury fractionation and speciation in soils

Thermal desorption technique to speciate mercury in carbonate, silicate, and organic-rich soils

Thermal desorption is a well-assessed technique to speciate mercury (Hg) in soils and sediments. However, the effects related to the different matrices are still not properly assessed. In this study, thermal desorption was applied to Hg-free calcite mixed with Hg standard and soils rich in carbonate and silicate minerals, as well as organic matter. Hg0, HgCl2, HgO, α-HgS, β-HgS and organo-mercuric compounds were recognized, pointing out that the soil matrix operates notable differences in terms of breakdown temperatures of the Hg-compounds and suggesting that the mineralogical composition of soil has to be investigated before applying the thermal desorption technique. Furthermore, the presence of Hg0 was carefully evaluated since, as already observed, it forms Hg2+, which increases mercury mobility in the pedological cover with important consequences for those soils contaminated and located close to decommissioned or active mining areas and/or industrial sites (e.g. chloro-alkali industries). Experimental runs were thus carried out by using carbonate-, silicate- and organic-rich soils doped with liquid Hg. It was observed that Hg0 tends to be oxidized to form Hg+ and then Hg2+ as a function of soil matrix and reaction time. Surprisingly, the oxidation rate is rather fast, since after 42 days the initial content of Hg0 is halved, thus following an exponential decay. This implies that in Hg0-polluted areas, the fate of the resulting Hg2+ can be that to: i) be adsorbed by organic matter and/or Fe-Mn-Al oxides and/or ii) feed shallow aquifers. This study is a further step ahead to understand the behavior of Hg in contaminated soils from industrial and mining areas where liquid Hg is occurring in different soil matrices and may provide useful indications for remediation operations.

Comparative study of the sequential extraction methodologies for fractionation analysis of mercury in coal of Thar coalfield

The primary objective of this study was to evaluate the bound fractions of mercury (Hg), physicochemical parameters, and mineral composition of coal. Coal samples were collected from various depths within Block-VII of the Thar coalfield in Pakistan. The Hg associated with different chemical fractions of coal was extracted using a sequential extraction scheme as per the community bureau of reference (BCR) protocol. This study utilized both the BCR-sequential extraction method (BCR-SEM) and a single-step sequential extraction based on an ultrasonic-assisted method (SSE-UAM) for the fractionation analysis of Hg in coal. The extraction methodologies, BCR-SEM and SSE-UAM, were specifically designed for analyzing Hg fractionation in coal samples. The SSE-UAM offers an operational advantage, requiring only 2 h compared to the 51 h needed for BCR-SEM. The analyses were validated using standard reference material (SRM-1635a) and the spiking addition method, achieving a recovery percentage of 97.1% for total Hg concentrations using the pseudo-extraction method in SRM-1635A. Total Hg content in the coal samples ranged from 0.60 to 2.34 µg g-1 across four different coal seams from Block-VII of the Thar coalfield. Additionally, Hg concentration was observed to decrease with increasing depth, attributed to changes in mineralogical composition. The highest concentration of Hg was detected at a depth of 200-203 m, while the lowest concentration was at a depth of 152-154 m. The concentration of Hg in various fractions was 32-60% in the acid-soluble fraction, 1.72-4.92% in the reducible fraction, and 9.58-50.8% in the oxidizable fractions. The coal sample characteristics were analyzed using an elemental analyzer and scanning electron microscopy with energy-dispersive spectroscopy. Cold vapor atomic absorption spectrometry (CV-AAS) was used to measure the extracted fractional concentration of Hg in coal.

Discarded substrates from soilless hydroponic horticulture as potential amendments for metal-contaminated soils

Soil contamination with metals is a major threat for the environment and public health since most metals are toxic to humans and to non-human biota, even at low concentrations. Thus, new sustainable remediation approaches are currently needed to immobilize metals in soils to decrease their mobility and bioavailability. In this work, we explore the application of discarded substrates from hydroponic cultivation, namely coconut shell and a mixture of coconut shell and pine bark, for immobilization of metals (Cd, Cr, Ni, Cu, Pb, Hg, Sb and As) in a naturally contaminated soil from a mining region in Portugal. The immobilization capacity of substrates (added to the soil at 5% mass ratio) was assessed both individually and also combined with other traditional agriculture soil additives (limestone and gypsum, at 2% mass ratio) and nanoparticles of zero-valent iron (nZVI) at 1-3% mass ratio. The overall results obtained after a 30-d incubation showed that the discarded substrates are a viable, economic, and environmental-friendly solution for metal remediation in soils, with the capacity of immobilization ranging from 20 to 91% for the metals and metalloids studied. Furthermore, they showed the capacity to reduce the soil toxicity (EC50 ∼ 6000 mg/L) to non-toxic levels (EC50 > 10000 mg/L) to the bacteria Aliivrio fischeri.

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).

Adapted Sequential Extraction Protocol to Study Mercury Speciation in Gold Mining Tailings: Implications for Environmental Contamination in the Amazon

Artisanal small-scale gold mining (ASGM), an increasingly prevalent activity in South America, generates mercury-contaminated tailings that are often disposed of in the environment, leading to the introduction of mercury into ecosystems and the food web, where it bioaccumulates. Therefore, studying the geochemical processes involved in the desorption and dissolution of mercury in these tailings is essential for critical risk evaluations in the short and long term. For this purpose, sequential extraction procedures (SEPs) can be useful because they help to identify the phases to which Hg is associated, although they also have limitations such as a lack of selectivity and specificity. In this work, we propose a modified four-step SEP: exchangeable mercury (F1), oxidizable mercury (F2), mercury bound to Fe oxides (F3), and strongly bound mercury (F4). To test this adapted sequential extraction method, we evaluated the Hg contamination in mercury-contaminated tailings of the Amazon basin. The results revealed a total mercury concentration of 103 ± 16 mg·kg-1 in the tailings, with a significant portion in F1 (28% of the total), where Hg was bioavailable. The large Hg concentration in F3 (36%) suggested that Fe oxides likely contribute to mercury retention. Together, the SEP results emphasize the urgent need for improved surveillance of gold mining activities and responsible tailings management practices to mitigate environmental contamination and safeguard the health of the Amazon ecosystem.

The transformation of soil Hg oxidation states controls elemental Hg release in the greenhouse with applying organic fertilizer

The foliage vegetables cultivated in greenhouse of Hg-contaminated regions suffer from severe Hg contamination issues because of soil elemental Hg (Hg(0)) release. Application of organic fertilizer (OF) is the indispensable part of farming, but its influences on soil Hg(0) release are unclear. A new method of thermal desorption coupled with cold vapor atomic fluorescence spectrometry was developed to measure transformations of Hg oxidation states to elucidate the impact mechanism of OF on Hg(0) release process. Our results showed that the soil Hg(0) concentrations can directly determine its release fluxes. The application of OF causes that oxidizing reactions of Hg(0)/Hg(I) and Hg(I)/Hg(II) are excited; then soil Hg(0) concentrations decreases. Besides, the elevated soil organic matter by amending OF can complex with Hg(II), resulting in that the reductions of Hg(II) to Hg(I) and Hg(0) are inhibited. Additionally, the OF can directly adsorb soil Hg(0), decreasing the removability of Hg(0). Subsequently, the application of OF can significantly inhibit soil Hg(0) release, resulting in a pronounced decrease in interior atmospheric Hg(0) concentrations. Our results provide a novel perspective for enriching the fate of soil Hg that transformation of soil Hg oxidation states plays a crucial role in affecting soil Hg(0) release process.

Mechanisms and biological effects of organic amendments on mercury speciation in soil-rice systems: A review

Mercury (Hg) pollution is a well-recognized global environmental and health issue and exhibits distinctive persistence, neurotoxicity, bioaccumulation, and biomagnification effects. As the largest global Hg reservoir, the Hg cumulatively stored in soils has reached as high as 250-1000 Gg. Even more concerning is that global soil-rice systems distributed in many countries have become central to the global Hg cycle because they are both a major food source for more than 3 billion people worldwide and the central bridge linking atmospheric and soil Hg circulation. In this review, we discuss the form distribution, transformation, and bioavailability of Hg in soil-rice systems by focusing on the Hg methylation and demethylation pathways and distribution, uptake, and accumulation in rice plants and the effects of Hg on the community structure and ecological functions of microorganisms in soil-rice systems. In addition, we clarify the mechanisms through which commonly used humus and biochar organic amendments influence Hg and its environmental effects in soil-rice systems. The review also elaborates on the advantages of sulfur-modified biochars and their critical role in controlling Hg migration and bioavailability in soils. Finally, we provide key information about Hg pollution in soil-rice systems, which is of great significance for developing appropriate strategies and mitigation planning to limit Hg bioconcentration in rice crops and achieving key global sustainable development goals, such as the guarantee of food security and the promotion of sustainable agriculture.

Identification of mercury species in coal combustion by-products from power plants using thermal desorption-atomic fluorescence spectrometry on-line coupling system

Along with the environmental protection policies becoming strict in China, the air pollution control devices (especially selective catalytic reduction (SCR)) are widely equipped in coal-fired power plants. The installation and run of these devices will inevitably affect mercury (Hg) species distribution in coal fired by-products such like fly ash (FA) and gypsum. In this work, a new on-line coupling system based on atomic fluorescence spectrometry (AFS) with a home-made chromatographic workstation was successfully developed to identify Hg species through thermal programmed desorption (TPD). The influences of matrix, furnace temperature, and carrier gas flow on analytical performance were investigated and the parameters were optimized. The FA and gypsum samples from coal-fired power plants equipped with SCR were collected and the mercury species were analyzed by the developed coupling system. HgCl₂ and HgO were the main species in FA, while Hg₂Cl₂ and HgO were the main species in gypsum. All of Hg species in the studied FA and gypsum samples were released below 400 °C. A sequential extraction procedure was applied to further verify the operational Hg species including mobile and non-mobile fractions in FA and gypsum samples. This study demonstrated that AFS coupled with TPD procedure was an effective method to analyze Hg species in coal combustion by-products from power plants.

Mercury fractionation, bioavailability, and the major factors predicting its transfer and accumulation in soil-wheat systems

Soil mercury (Hg) and its bioaccumulation in food crops have attracted widespread concerns globally due to its harmful effects on biota. However, soil mercury fractionation, bioavailability, and the major factors predicting its transfer and accumulation in soil-wheat-systems have not been thoroughly explored. Twenty-one (21) soil samples collected throughout China with a wide spectrum of physico-chemical characteristics were contaminated with HgCl₂ and winter wheat (Triticum aestivum L.) was grown on the soils in a greenhouse pot-culture experiment for 180 days. A four-step sequential extraction was used segregating soil Hg into water-soluble (F1, 0.21 %), exchangeable (F2, 0.07 %), organically bound (F3, 16.40 %), and residual fractions (F4, 83.32 %). Step-wise multiple linear regression (SMLR) and path analysis (PA) were used to develop a prediction model and identify the major controlling factors of soil-wheat Hg transference. The SMLR results revealed that wheat Hg in leaves, husk, and grain was positively correlated with soil total and available Hg, and crystalline manganese (Cryst-Mn), while negatively correlated with soil pH, amorphous manganese (Amor-Mn) and crystalline aluminium (Cryst-Al). Bioconcentration factor (BCF) values were significantly higher in acidic soils (highest 0.05), with phytotoxic effects in some soils, as compared to alkaline soils (lowest 0.006). Furthermore, wheat grain Hg was significantly correlated with total (R² = 0.25), water-soluble (R² = 0.54) and NH₄Ac-extractable Hg (R² = 0.43) while also had a good correlation with soil pH (R² = -0.20). In conclusion, the soil total and available Hg (water-soluble + exchangeable fraction), pH, organic matter, and Amor-Mn are the most important soil variables that support Hg uptake in the wheat plants, which benefit managing Hg-enriched agricultural soils for safe wheat production.

Demystifying mercury geochemistry in contaminated soil-groundwater systems with complementary mercury stable isotope, concentration, and speciation analyses

Interpretation of mercury (Hg) geochemistry in environmental systems remains a challenge. This is largely associated with the inability to identify specific Hg transformation processes and species using established analytical methods in Hg geochemistry (total Hg and Hg speciation). In this study, we demonstrate the improved Hg geochemical interpretation, particularly related to process tracing, that can be achieved when Hg stable isotope analyses are complemented by a suite of more established methods and applied to both solid- (soil) and liquid-phases (groundwater) across two Hg2+-chloride (HgCl2) contaminated sites with distinct geological and physicochemical properties. This novel approach allowed us to identify processes such as Hg2+ (i.e., HgCl2) sorption to the solid-phase, Hg2+ speciation changes associated with changes in groundwater level and redox conditions (particularly in the upper aquifer and capillary fringe), Hg2+ reduction to Hg0, and dark abiotic redox equilibration between Hg0 and Hg(II). Hg stable isotope analyses play a critical role in our ability to distinguish, or trace, these in situ processes. While we caution against the non-critical use of Hg isotope data for source tracing in environmental systems, due to potentially variable source signatures and overprinting by transformation processes, our study demonstrates the benefits of combining multiple analytical approaches, including Hg isotope ratios as a process tracer, to obtain an improved picture of the enigmatic geochemical behavior and fate of Hg at contaminated legacy sites.

Immobilization of mercury in tailings originating from the historical artisanal and small-scale gold mining using sodium polysulfide

A series of sodium polysulfides (SPSs) with different sulfur indexes was prepared as stabilizers to amend elemental mercury-contaminated artisanal small-scale gold mine (ASGM) tailings in Hubei, China, by controlling the molar ratio of sulfur and sodium sulfides as 1:1, 2:1, 3:1, and 4:1 during the synthesis. XRD, XPS, and laser Raman spectroscopy all suggested that the synthesized SPSs were a mixture of multiple polysulfides, sulfur, sodium sulfides, and sodium thiosulfate. Based on toxicity characteristic leaching procedure test (TCLP), mercury stabilization efficiency of SPSs was evaluated and proved to be more superior than sulfur, sodium sulfide, and also calcium polysulfide, with an optimal stabilization efficiency of 97.16% at SPS/THg = 1:2, SPSs pH = initial pH, and liquid-to-solid ratio = 20:7. A pseudo-second-order kinetic model was able to interpret the stabilization kinetics and demonstrated that mercury stabilization rate increased with the sulfur index in the SPSs, but excess SPSs were potentially to inhibit the precipitation of mercury. Speciation analysis results determined with sequential extraction indicated that the unstable mercury, elemental mercury, and organic-bound mercury fractions decreased respectively by up to 88.6%, 53.5%, and 26.3%. Pearson correlation analysis showed that the mercury stabilization in the mine tailings amended with SPSs mainly occurs from the precipitation of the elemental mercury, and the organic mercury fraction reduction was correlated with the decrease of the unstable mercury.

The Legacy of Mercury Contamination from a Past Leather Manufacturer and Health Risk Assessment in an Urban Area (Pisa Municipality, Italy)

An abandoned open green space in the urban setting of the Municipality of Pisa (Tuscany, Italy) has been designed for renewal to foster the development of recreational activities and improve the lives of the surrounding communities. However, the geochemical site characterization revealed Pb, Cu, Zn and Hg concentrations in the soil exceeding the thresholds imposed by Italian regulations for residential use. Pb, Cu and Zn contents likely reflect the effects of urban vehicle traffic, while Hg contamination represents the legacy of a past artisanal tannery that used Hg(II)-chloride in leather processing in the mid-1900s. Mercury is widely distributed in the area, with the highest concentration in the uppermost soil layer, and reaching about 170 mg/kg in the common dandelion rhizosphere. Chemical extractions and thermal desorption experiments have indicated that most Hg is in the elemental free and matrix-bound fraction, with a possible minor amount (less than 4 wt%) of HgS and negligible methylated forms (0.1 wt%). The data suggest that soil processes could reduce Hg2+ to volatile Hg0. Mercury in groundwater, hosted in a shallow aquitard in the area, was below 0.2 µg/L. However, the presence of chloride in groundwater might result in the formation of Hg stable aqueous complexes, increasing Hg release from solids. Future water quality monitoring is hence recommended. The risk assessment highlighted that mercury in soil carries a risk of non-cancerous effects, in particular for children, posing the basis for management planning.

Distribution, alkylation, and migration of mercury in soil discharged from the Itomuka mercury mine

The purpose of this study was to investigate the behavior of previously discharged mercury (Hg) released from the Itomuka Hg mine into the surrounding environment, especially into soil. Total-Hg (T-Hg), methylmercury (MeHg), and ethylmercury (EtHg) concentrations in the surface soil at eight sample sites around the mine were 3.8-64.2 mg/kg, 6.0-54.7 μg/kg, and undetected to 4.5 μg/kg, respectively. Core samples collected from seven of the eight sample sites showed that the vertical distribution of T-Hg was the highest in the surface soil layer and decreased rapidly in the lower layers. A strong positive correlation was observed between T-Hg and MeHg concentrations in the core samples; however, the slope of the regression line varied considerably for each core. This suggests that Hg and MeHg were not supplied from the atmosphere simultaneously, but rather that MeHg was produced on-site. Further, the formation of MeHg and EtHg in soil was considered in terms of the total organic carbon/total nitrogen ratio, which is a decomposition index of soil organic matter. The strong positive correlation between T-Hg and MeHg can be attributed to the migration of organic matter containing Hg species to the lower layers. There was no relationship between T-Hg and MeHg at the riverbed sample site because of the high T-Hg in the lower soil layers, suggesting that Hg was supplied by ore at this sample site. These assumptions of the formation change and migration of Hg in soil were supported by the results of the fractionation experiment and the elution test. To understand the current conditions in this area, measurements of Hg in the water, sediment, atmosphere, and plants were also conducted.

Effect of inorganic carbonate and organic matter in thermal treatment of mercury-contaminated soil

Thermal treatment of mercury (Hg)-contaminated soil was studied to investigate the desorption behavior of Hg at different temperatures. The soil samples were collected from two locations with different land uses around the mine and industrial site. The effect of soil properties such as inorganic carbonate minerals and organic matter content on Hg desorption was investigated to understand the thermal desorption process. The effect of soil composition on Hg desorption showed that behavior at 100 °C was similar, but a different behavior could be found at 300 °C. The thermal desorption efficiency at 300 °C is affected by the thermal properties of soils and the Hg desorption capacity of the soils. The Hg from both soil types was removed above 300 °C, and Hg was effectively removed from mine soil due to the partial decomposition of carbonate in the soil composition, while industrial soil showed that desorption would be restrained by Hg organic matter complexes due to organic matter content. Despite a relatively higher concentration of Hg in the mine soil, Hg removal efficiency was greater than that in the industrial soil. Sequential extraction results showed that only the Hg fractions (residual fractions, step 6) in mine soil changed, while the industrial soil was affected by changes in Hg fractions (step 3 to step 6) at 300 °C. Changes in soil pH during thermal desorption are also influenced by heating time and temperature. Therefore, the mechanisms of Hg desorption during thermal treatment were observed by soil properties. The volatilization of Hg in the soil is induced by organic carbon, while soil Hg release is controlled by organic matter complexes.

Distribution of mercury isotope signatures in Yundang Lagoon, Xiamen, China, after long-term interventions

Isotope signatures of mercury (Hg) were determined for Hg fractions in seawater, sediments, porewaters, core sediments and fish from the Yundang Lagoon, Xiamen, China. Sequential extraction was used to extract Hg fractions in sediments and the purge-trap method was used to preconcentrate Hg in seawater. A large variation in mass dependent fractionation (δ²⁰²Hg: -2.50‰ to -0.36‰) was observed in the lagoon. Seawater and fish samples showed positive mass-independent fractionation (Δ¹⁹⁹Hg: -0.06‰-0.45‰), while most of sediment and porewater samples displayed insignificant mass-independent fractionation (Δ¹⁹⁹Hg: -0.10‰-0.07‰). Ancillary parameters (total organic carbon, sulfide, pH, Eh, water content and grain size) were also measured in the sediments to investigate correlations with Hg isotopes. Three sources (domestic sewage, sediments and atmospheric deposition) were identified as the main sources of Hg in the lagoon seawater. Photochemical reaction was the main process causing isotope fractionation in seawater. Through Hg partitioning and deposition, light isotopes were enriched from dissolved Hg to particulate Hg, then to sediments, and then to porewaters. Finally, Hg isotope signatures were used to identify the Hg sources and fractionation processes in core sediments from different depths. Our results demonstrate that Hg isotopes are powerful tools for tracing Hg sources and arriving at a better understanding of Hg biogeochemical cycling in the lagoon after long-term interventions.

Mercury speciation in mine tailings amended with biochar: Effects on mercury bioavailability, methylation potential and mobility

Biochar is a low-cost and environmentally friendly amendment with strong ability for adsorption of mercury (Hg) from aqueous solutions, contaminated soils, and sediments. In the present study, six biochars were prepared from the pyrolysis of cocoa pod husk, sugarcane bagasse and banana pseudostem at 400 and 600 °C in order to use them as an organic amendment and to evaluate their capacities to reduce the bioavailability, methylation potential, and mobility of Hg present in mine tailings without environmental treatment. To quantify the effects of each variety of biochar, incubation experiments of soil were established by mixing mine tailings with 5% by weight of biochar for 90 days. Once the incubation time concluded, sequential extraction procedures were carried out to determine the fractionation of the Hg species. Speciation analysis results indicated that the remedial effects of biochar depended on the source of organic matter and pyrolysis temperature. The bioavailable and organic Hg fractions decreased respectively by up to 75 and 79%, indicating a methylation potential reduction. Immobile Hg fraction increased to 76% with respect to the control. Adsorption and stabilization to HgS from the soluble forms of Hg reduce the percentage of bioavailable Hg. The organic Hg fraction reduction was correlated with the decrease of the bioavailable Hg fraction and with direct adsorption processes in the biochar structure. Highly porous biochars developed at high temperature, with large contents of superficial polar functional groups (H/C), and high pH, electrical conductivity, ash percentage and cation exchange capacity values favor the stabilization and adsorption of Hg in mine tailings. In summary, the application of biochar could be an effective method for the remediation of Hg-contaminated mine tailings, transforming the Hg species into less toxic, soluble, reactive, and bioavailable forms.

Assessment of bioavailability and mobility of major and trace elements in agricultural soils collected in Port St Johns, Eastern Cape, South Africa using single extraction procedures and pseudo-total digestion

PURPOSE

The aim of this study was to evaluate the applicability of ultrasound assisted single extraction and pseudo-total digestion procedures to investigate the bioavailability and mobility of major and trace elements collected from agricultural soil collected in Mzimvubu farmstead area located in Port St Johns, Eastern Cape Province (South Africa).

MATERIALS AND METHODS

The potential metal availability was assessed using complexing agent (ethylenediaminetetraacetic acid (EDTA)), mild (cacium chloride (CaCl2) and ammonium nitrate (NH4NO3)), deionized water and acidic (CH3COOH) extractants with the assistance of ultrasound to evaluate mobility and bioavailability of metals. The ultrasound radiation was used to assist the single extraction of major and trace metals from agricultural soils as well shortening the extraction time. The pseudo-total metal content in agricultural soils was obtained using pseudo-total digestion employing aqua-regia.

RESULTS AND DISCUSSION

The results obtained using different solvents for single extraction procedures, revealed that higher extraction of Al (256-681 mg kg-1), Fe (172-430 mg kg-1) and Mn (35-136 mg kg-1), was observed compared to other metals. Among the investigated solvents, deionized water, acetic acid and EDTA proved to be the most aggressive extractants. The geo-accumulation index (2-6) and contamination factor (3-6) demonstrated that Fe, Cr, Mn, Co, Cu, Ni, and Zn could pose significant environmental contamination risk.

CONCLUSIONS

Based on the results obtained, it can be concluded that the high levels of some studied metals was due to anthropogenic activities.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40201-020-00581-x.

Mercury fractionation in tropical soils: A critical point of view

Problems related to specificity and re-precipitation of metals in sequential chemical extractions can impair their routine use. In order to test the efficiency of a sequential chemical procedure, model compounds composed by soil components commonly found in tropical soils such as goethite, Al-goethite, ferrihydrite, hematite, bauxite, and humic acid were incubated with either Hg(NO₃)₂ or HgSO₄ and submitted to chemical extraction. The procedure aims to assess: (i) water soluble Hg; (ii) bioaccessible Hg at pH near human stomach; (iii) Hg associated with organic matter; (iv) reduced Hg; (v) Hg associated with Fe, Al, and Mn oxides; and, (vi) residual Hg. This procedure was also tested via single and sequential extractions using the surface and subsurface samples of two tropical soils, i.e., a Rhodic Acrudox and a Typic Hapludox, with and without lime application. Soil samples were submitted to an adsorption experiment with HgCl₂ and a high adsorption percentage was observed. The majority of Hg at both single and sequential procedure was extracted by an acetic acid solution (pH = 2). Liming, soil depth, and soil type were not determinative on Hg extractability. The sequential extraction applied showed a lack specificity of Hg fractions, confirmed by the model components.

Mercury-bearing wastes: Sources, policies and treatment technologies for mercury recovery and safe disposal

Due to the lenient environmental policies in developing economies, mercury-containing wastes are partly produced as a result of the employment of mercury in manufacturing and consumer products. Worldwide, the presence of mercury as an impurity in several industrial processes leads to significant amounts of contaminated waste. The Minamata Convention on Mercury dictates that mercury-containing wastes should be handled in an environmentally sound way according to the Basel Convention Technical Guidelines. Nevertheless, the management policies differ a great deal from one country to another because only a few deploy or can afford to deploy the required technology and facilities. In general, elemental mercury and mercury-bearing wastes should be stabilized and solidified before they are disposed of or permanently stored in specially engineered landfills and facilities, respectively. Prior to physicochemical treatment and depending on mercury's concentration, the contaminated waste may be thermally or chemically processed to reduce mercury's content to an acceptable level. The suitability of the treated waste for final disposal is then assessed by the application of standard leaching tests whose capacity to evaluate its long-term behavior is rather questionable. This review critically discusses the main methods employed for the recovery of mercury and the treatment of contaminated waste by analyzing representative examples from the industry. Furthermore, it gives a complete overview of all relevant issues by presenting the sources of mercury-bearing wastes, explaining the problems associated with the operation of conventional discharging facilities and providing an insight of the disposal policies adopted in selected geographical regions.

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.

Evolution of the Speciation and Mobility of Pb, Zn and Cd in Relation to Transport Processes in a Mining Environment

Elements in mining extracts can be potentially toxic if they are incorporated into soils, sediments or biota. Numerous approaches have been used to assess this problem, and these include sequential extractions and selective extractions. These two methods have limitations and advantages, and their combined use usually provides a rough estimate of the availability or (bio)availability of potentially toxic elements and, therefore, of their real potential as toxicants in food chains. These indirect speciation data are interesting in absolute terms, but in the work described here, this aspect was developed further by assessing the evolution of availability-related speciation in relation to the transport processes from the emission source, which are mainly fluvial- and wind-driven. This objective was achieved by characterizing tailings samples as the source of elements in soils and sediments at increasing distances to investigate the evolution of certain elements. The standard procedures employed included a sequential five-step extraction and a selective extraction with ammonium acetate. The results show that the highest percentages of Zn and Pb in tailings, soils and sediment samples are associated with oxyhydroxides, along with a significant presence of resistant mineralogical forms. In the case of Cd, its association with organic matter is the second-most important trapping mechanism in the area. The physicochemical mechanisms of transport did not transform the main mineralogical associations (oxyhydroxides and resistant mineralogical forms) along the transects, but they produced a chaotic evolution pattern for the other minor matrix associations for Zn and a decrease in exchangeable and carbonate-bound forms for Pb in soils. Interestingly, in sediments, these mobile forms showed a decrease in Zn and a chaotic evolution for Pb. The most probable reason for these observations is that Zn²⁺ can form smithsonite (ZnCO₃) or hydrozincite (Zn₅(CO₃)₂(OH)₆), which explains the retention of a carbonate-bound form for Zn in the soil transect. In contrast, Pb and Cd can appear as different mineral phases. The order of (bio)availability was Pb > Zn > Cd in tailings but Cd > Pb > Zn in soils. The physicochemical processes involved in transport from tailings to soils produce an increase in Cd (bio)availability. The trend is a decrease in bioavailability on moving away from the source (tailings), with maximum values obtained for Cd near to the source area (200-400 m).

Functionalized Mesoporous Silicon Nanomaterials in Inorganic Soil Pollution Research: Opportunities for Soil Protection and Advanced Chemical Imaging

"Innovative actions towards a pollution free-planet" is a goal of the United Nations Environment Assembly (UNEA). Aided by both the Food and Agricultural Organisation (FAO) and its Global Soil Partnership under the 3rd UNEA resolution, a consensus from > 170 countries have agreed a need for accelerated action and collaboration to combat soil pollution. This initiative has been tasked to find new and improved solutions to prevent and reduce soil pollution, and it is in this context that this review provides an updated perspective on an emerging technology platform that has already provided demonstrable utility for measurement, mapping, and monitoring of toxic trace elements (TTEs) in soils, in addition to the entrapment, removal, and remediation of pollutant sources. In this article, the development and characteristics of functionalized mesoporous silica nanomaterials (FMSN) will be discussed and compared with other common metal scavenging materials. The chemistries of the common functionalizations will be reviewed, in addition to providing an outlook on some of the future directions/applications of FMSN. The use of FMSN in soil will be considered with some specific case studies focusing on Hg and As. Finally, the advantages and developments of FMSN in the widely used diffusive gradients-in-thin films (DGT) technique will be discussed, in particular, its advantages as a DGT substrate for integration with oxygen planar optodes in multilayer systems that provide 2D mapping of metal pollutant fluxes at submillimeter resolution, which can be used to measure detailed sediment-water fluxes as well as soil-root interactions, to predict plant uptake and bioavailability.

Detailed investigation of methylmercury accumulation in rice grain from Hg2+-spiked non-contaminated paddy field soils

Total-Hg (T-Hg) and methylmercury (MeHg) concentrations in rice grains were measured to understand the MeHg accumulation process. Rice plants were cultivated in Hg²⁺-spiked non-contaminated soils in experimental pots at three different places. Although soil MeHg concentrations in the pots changed significantly and individually during the rice-growing season, T-Hg concentration of brown rice grain was high at high soil MeHg concentration. In addition, there was no significant variation in T-Hg concentration in brown rice grains from individual panicles or among panicles obtained from the same pot, although the period of growth for each panicle was different. The highest T-Hg concentration of brown rice grains recorded for a panicle was 1.4 ± 0.1 mg kg^-1 (n = 8), and the corresponding MeHg ratio was 76%. In addition, the T-Hg and MeHg concentrations in various parts of the brown rice grain-white rice (endosperm), bran, and embryo-were measured. Among the parts of the brown rice grain, the embryo had the highest Hg concentration. Furthermore, Hg concentration in the grain was constant during grain filling. These findings suggest that MeHg formed in soil accumulates in the rice plant during growth and is supplied to the rice grains continuously for the entire duration of the grain development period.

Influence of ferrous-metal production on mercury contamination and fractionation in farmland soil around five typical iron and steel enterprises of Tangshan, China

Iron and steel production is one of the main anthropogenic sources of mercury (Hg) emission and release. Oxidized and particulate Hg discharged from iron and steel enterprises deposit into the surrounding soil, which accumulate and introduce environmental risks. Therefore, it is important to assess Hg pollution in the soil surrounding iron and steel enterprises. In this study, the Hg pollution, Hg distribution from steel plants and Hg fractionation in farmland soil around five typical steel plants were analysed in Tangshan of China. The Hg pollution indexes (Pi) of more than 90% soil samples were greater than 3 by the single factor pollution index method, which showed that most soil samples around the five steel plants were strongly contaminated by Hg. The Hg contents in soil increased first and then decreased, and the maximum content presented at 250-300 m away from the boundary of the steel plants. The order of Hg fraction proportion in the soil samples was extractable (35%-43%) > volatile (24%-36%) > residual (10%-26%) > reducible (0-15%) > oxidizable (0-12%). The distribution of Hg fraction in farmland soil had no regular trend with the distance from the steel plants. The volatile Hg and extractable Hg were dominant in farmland soil, and their combined proportion was greater than 60%. These two fractions of Hg are at risk of re-volatilization into the atmosphere or potential absorption by plants.

Bioavailability and sequential extraction of mercury in soils and organisms of a mangrove contaminated by a chlor-alkali plant

Botafogo river estuary poses the highest Hg concentration reported for mangrove soils in Brazil. Such high contamination took place owing to the improper waste disposal for 24 years from a chlor-alkali plant nearby the estuary. Here we determined Hg concentrations in soils, mangrove plants (Rhizophora mangle and Laguncularia racemosa), and an aquatic organism (the oyster species Crassostrea rhizophorae) to assess Hg bioavailability. Besides, a sequential extraction procedure was used to separate soil Hg into five fractions: water-soluble; 'human stomach acid' soluble; organically bound; elemental Hg; mercuric sulfide. Results showed that environmentally available concentration of Hg in the mangrove soils were up to 150-fold higher than a pristine mangrove under the same geological context used as a reference. Additionally, Hg concentration in soils was also above sediment quality guidelines and Hg adverse effects towards sensitive estuarine organisms are likely. Mercury concentrations in oysters were the highest reported in Brazil, but within safety standards according to the country food security agency. It seems that Hg stocks in the studied soils are governed by organic matter and redox conditions, but changing on environmental conditions and land use can alter this balance and convert these mangrove areas from sink to source of Hg to the environment.

The Mercury Behavior and Contamination in Soil Profiles in Mun River Basin, Northeast Thailand

To determine the geochemical characteristics and contamination of soil mercury in the Mun River basin, northeast Thailand, the vertical mercury distribution patterns and mercury contamination levels in six soil profiles under different land uses are studied. A total of 240 soil samples collected from agricultural land, abandoned agricultural land, and woodland were analyzed by an RA-915M mercury analyzer to determine the total mercury (THg) content, which ranged from 0.13 to 69.40 μg∙kg^-1 in the study area. In the soil cultivation layer (0-30 cm), the average content of THg in the woodland (15.89 μg∙kg^-1) and the agricultural land (13.48 μg∙kg^-1) were higher than that in the abandoned agricultural land (4.08 μg∙kg^-1), indicating that the plants or crops could increase the content of mercury in the surface soil layer. The total organic carbon (TOC) and iron content with high positive correlations with the THg content significantly contributed to the adsorption of soil mercury. Moreover, a higher pH value in the soil and a finer grain size in soil texture can be beneficial for the enrichment of mercury. A geoaccumulation index was used to evaluate the contamination of mercury, showing that this area had a slight contamination, and a few soil sites were moderate contamination.

Mercury Isotope Fractionation in the Subsurface of a Hg(II) Chloride-Contaminated Industrial Legacy Site

To understand the transformations of mercury (Hg) species in the subsurface of a HgCl₂-contaminated former industrial site in southwest Germany, Hg isotope analysis was combined with an investigation of Hg forms by a four-step sequential extraction protocol (SEP) and pyrolytic thermodesorption. Data from two soil cores revealed that the initial HgCl₂ was partly reduced to metallic Hg(0) and that Hg forms of different mobility and oxidation state coexist in the subsurface. The most contaminated sample (K2-8, 802 mg kg^-1 Hg) had a bulk δ²⁰²Hg value of around -0.43 ± 0.06‰ (2SD), similar to published average values for industrial Hg sources. Other sample signatures varied significantly with depth and between SEP pools. The most Hg-rich samples contained mixtures of Hg(0) and Hg(II) phases, and the water-extractable, mobile Hg pool exhibited heavy δ²⁰²Hg values of up to +0.18‰. Sequential water extracts revealed slow dissolution kinetics of mobile Hg pools, continuously releasing isotopically heavy Hg into solution. This was further corroborated by heavy δ²⁰²Hg values of groundwater samples. Our results demonstrate that the Hg isotope signature of an industrial contamination source can be significantly altered during the transformations of Hg species in the subsurface, which complicates source tracing applications but offers the possibility of using Hg isotopes as process tracers in contaminated subsurface systems.

Mercury speciation, transformation, and transportation in soils, atmospheric flux, and implications for risk management: A critical review

Mercury (Hg) is a potentially harmful trace element in the environment and one of the World Health Organization's foremost chemicals of concern. The threat posed by Hg contaminated soils to humans is pervasive, with an estimated 86 Gg of anthropogenic Hg pollution accumulated in surface soils worldwide. This review critically examines both recent advances and remaining knowledge gaps with respect to cycling of mercury in the soil environment, to aid the assessment and management of risks caused by Hg contamination. Included in this review are factors affecting Hg release from soil to the atmosphere, including how rainfall events drive gaseous elemental mercury (GEM) flux from soils of low Hg content, and how ambient conditions such as atmospheric O₃ concentration play a significant role. Mercury contaminated soils constitute complex systems where many interdependent factors, including the amount and composition of soil organic matter and clays, oxidized minerals (e.g. Fe oxides), reduced elements (e.g. S^2-), as well as soil pH and redox conditions affect Hg forms and transformation. Speciation influences the extent and rate of Hg subsurface transportation, which has often been assumed insignificant. Nano-sized Hg particles as well as soluble Hg complexes play important roles in soil Hg mobility, availability, and methylation. Finally, implications for human health and suggested research directions are put forward, where there is significant potential to improve remedial actions by accounting for Hg speciation and transportation factors.

Environmental impacts of the life cycle of alluvial gold mining in the Peruvian Amazon rainforest

Alluvial gold mining activities in the Peruvian Amazon rainforest are responsible for mercury emissions and deforestation. To understand related environmental impacts, specifically toxicity and climate change, this study uses Life Cycle Assessment methodology. Four predominant extraction systems were selected and modelled and three scenarios that reflect currently available gold recovery systems were modelled: amalgamation, amalgamation with mercury recovery through retort system and gravimetric tables. The USEtox and IPCC life cycle impact assessment methods were used to assess the environmental impacts in term of human toxicity, freshwater ecotoxicity and climate change. Results show that for all systems, human toxicity values are governed by mercury emissions in gold recovery activities (ca. 80%). However, the use of retort significantly lowers these impacts (ca. 90%). Machines and diesel use for ore extraction and freighting activities drive freshwater ecotoxicity. Moreover, deforestation has a major contribution on the environmental impacts related to climate change. However, these impacts are dependent on the type of extraction system. Although human toxicity, freshwater ecotoxicity and climate change are frequently studied separately, a direct relationship between them has been identified in this system. Finally, beyond the environmental burdens related to alluvial gold mining, there are impacts affecting the social, cultural, and economic dimensions that will need to be analyzed to ensure a comprehensive understanding of the system.

Comparison of different sequential extraction procedures for mercury fractionation in polluted soils

Three sequential extraction procedures (SEPs), modified Tessier, modified BCR, and CIEMAT, were compared for mercury fractionation in polluted soils. With satisfactory total mercury recovery, the modified Tessier and modified BCR SEPs were comparable with each other in terms of extraction efficiency in equivalent mercury fractions, whereas both SEPs were not as efficient as the CIEMAT SEP. However, the CIEMAT SEP might underestimate the oxidizable mercury fractions due to the humic and fulvic complexes instead of the organic matter of the other two SEPs. For mercury bioavailability identification, based on Pearson correlation analysis, all fractions in each SEP were significantly correlated with mercury uptake in Ipomoea aquatica, causing difficulty in comparison. Partial correlation analysis indicated that the mobile mercury fractions extracted by the first step in all three SEPs had a positive correlation with mercury uptake by plant, while mercury bound to organic matter extracted by both modified Tessier and modified BCR SEPs presented negative correlation with mercury uptake by plant which was in contrast to CIEMAT SEP. Meanwhile, clearly positive correlations between mercury fractions extracted by the former three steps of CIEMAT SEP and mercury uptake in Ipomoea aquatica were observed, demonstrating that CIEMAT SEP provided more accurate results related to Hg bioavailability than did the other two SEPs.

Mercury isotope signatures of digests and sequential extracts from industrially contaminated soils and sediments

Environmental mercury (Hg) pollution is a matter of global concern. Mercury speciation controls its environmental behaviour, and stable isotope ratios can potentially trace Hg movement through environmental compartments. Here we investigated Hg in industrially contaminated soils and sediments (Visp, Valais, Switzerland) using concentration and stable isotope analysis (CV-MC-ICP-MS) of total digests, and a four-step sequential extraction procedure. The sequential extraction employed (1) water (labile Hg species), (2) NaOH or Na₄P₂O₇ (organically-bound Hg), (3) hydroxylamine-HCl (Hg bound to Mn and Fe (oxyhydr)oxides), and (4) aqua regia (residual Hg pools). The majority of Hg was extracted in step 4 and up to 36% in step 2. Mercury bound to organic matter was the dominant source of Hg in water, NaOH and Na₄P₂O₇ extracts. Sulfides and colloidal oxide minerals were possible additional sources of Hg in some samples. The inconsistent comparative performance of NaOH and Na₄P₂O₇ extractions showed that these classical extractants may not extract Hg exclusively from the organically-bound pool. Samples taken at the industrial facility displayed the greatest isotopic variation (δ²⁰²Hg: -0.80‰ ± 0.14‰ to 0.25‰ ± 0.13‰, Δ¹⁹⁹Hg: -0.10‰ ± 0.03‰ to 0.02‰ ± 0.03‰; all 2SD) whereas downstream of the facility there was much less variation around average values of δ²⁰²Hg = -0.47‰ ± 0.11‰ and Δ¹⁹⁹Hg = -0.05‰ ± 0.03‰ (1SD, n = 19). We interpret the difference as the result of homogenisation by mixing of canal sediments containing Hg from the various sources at the industrial facility with preservation of the mixed industrial Hg signature downstream. In contrast to previous findings, Hg isotopes in the sequential extracts were largely similar to one another (2SD < 0.14‰), likely demonstrating that the Hg speciation was similar among the extracts. Our results reveal that Hg resides in relatively stable soil pools which record an averaged isotope signature of the industrial sources, potentially facilitating source tracing studies with Hg isotope signatures at larger spatial scales further downstream.

Spatial distribution characteristics of mercury in the soils and native earthworms (Bimastos parvus) of the leachate-contaminated zone around a traditional landfill

The contents and spatial distribution of mercury (Hg), including soil-Hg fractionation and Hg-containing native earthworm Bimastos parvus (B. parvus) species, were investigated in the leachate-contaminated zone of a large traditional landfill, Japan. Soil-Hg was fractionated into 5 categories: F1/water soluble Hg (Hg-w), F2/human stomach acid soluble Hg (Hg-h), F3/organic-chelated (Hg-o), F4/elemental Hg (Hg-e), and F5/mercuric sulfide (Hg-s). The total mercury (T-Hg) and methylmercury (MeHg) of native B. parvus, and the geochemical properties of soils were examined in this study. Soil T-Hg concentration ranged between 0.227 and 2.919 mg kg^-1 dry weight (dw). The T-Hg and MeHg concentrations of B. parvus species ranged from 1.242 to 6.775 mg kg^-1 dw and from 0.031 to 0.218 mg kg^-1 dw, respectively. Percentages of soil-Hg fractions were in the order of F3/Hg-o > F4/ Hg-e > F5/Hg-s > F1/Hg-w > F2/Hg-h, and the fractions of Hg-o and Hg-e were 55.50% and 35.31%, respectively. Similar distributions and close correlations between the levels of B. parvus Hg and soil Hg-o, Hg-e, and Hg-s were observed in this study. The distribution of Hg in B. parvus was associated with soil organic matter (SOM) content and particle size (sand, clay); however, it was not correlated with Hg-w or Hg-h. The results indicated that easily bioavailable and soluble Hg fractions (Hg-w, Hg-h) of the soil were not appropriate to illustrate the distribution of Hg in native B. parvus. Instead, the stable soil-Hg fractions (Hg-o, Hg-e, and Hg-s) demonstrated good relationships of spatial distribution with B. parvus Hg in leachate-contaminated soil. It is advisable to preclude the evaluation of Hg biological distribution using soluble Hg fractions only. Stable Hg fractions in leachate-contaminated soil should also be included for assessing the biological distribution of Hg in leachate-contaminated soils.

Speciation and Mobility of Mercury in Soils Contaminated by Legacy Emissions from a Chemical Factory in the Rhône Valley in Canton of Valais, Switzerland

Legacy contamination of soils and sediments with mercury (Hg) can pose serious threats to the environment and to human health. Assessing risks and possible remediation strategies must consider the chemical forms of Hg, as different Hg species exhibit vastly different environmental behaviors and toxicities. Here, we present a study on Hg speciation and potential mobility in sediments from a chemical factory site, and soils from nearby settlement areas in the canton of Valais, Switzerland. Total Hg ranged from 0.5 to 28.4 mg/kg in the soils, and 3.5 to 174.7 mg/kg in the sediments, respectively. Elemental Hg(0) was not detectable in the soils by thermal desorption analysis. Methylmercury, the most toxic form of Hg, was present at low levels in all soils (<0.010 mg/kg; <0.8% of total Hg). Sequential extractions and thermal desorption analyses suggested that most of the Hg in the soils was present as “matrix-bound Hg(II)”, most likely associated with soil organic matter. For factory sediments, which contained less organic matter, the results suggested a higher fraction of sulfide-bound Hg. Batch extractions in different CaCl2 solutions revealed that Hg solubility was low overall, and there was no Hg-mobilizing effects of Ca2+ or Cl− in solution. Only in some of the factory sediments did high CaCl2 concentrations result in increased extractability of Hg, due to the formation of Hg-chloride complexes. Additional experiments with soil redox reactors showed that even mildly reducing conditions led to a sharp release of Hg into solution, which may be highly relevant in soils that are prone to periodic water saturation of flooding.

Influence of ore processing activity on Hg, As and Sb contamination and fractionation in soils in a former mining site of Monte Amiata ore district (Italy)

A geochemical study was carried out at the former Abbadia San Salvatore (ASS) mining site of the Monte Amiata ore district (Italy). Hg, As and Sb total contents and fractionation using a sequential extraction procedure were determined in soil and mining waste samples. Ore processing activities provided a different contribution to Hg contamination and concentration in soil fractions, influencing its behaviour as volatility and availability. Soils of roasting zone showed the highest Hg contamination levels mainly due to the deposition of Hg released as Hg⁰ by furnaces during cinnabar roasting. High Hg contents were also measured in waste from the lower part of mining dump due to the presence of cinnabar. The fractionation pattern suggested that Hg was largely as volatile species in both uncontaminated and contaminated soils and mining waste, and concentrations of these Hg species increased as contamination increased. These findings were in agreement with the fact that the ASS mining site is characterized by high Hg concentrations in the air and the presence of Hg⁰ liquid droplets in soil. Volatile Hg species were also prevalent in uncontaminated soils likely because the Monte Amiata region is an area characterized by anomalous fluxes of gaseous Hg from natural and anthropogenic inputs. At the ASS mining site soils were also contaminated by Sb, while As contents were comparable with its local background in soil. In all soil and waste samples Sb and As were preferentially in residual fraction.

Mercury behaviour and C, N, and P biogeochemical cycles during ecological restoration processes of old mining sites in French Guiana

Several decades of gold mining extraction activities in the Amazonian rainforest have caused deforestation and pollution. While ecological rehabilitation is essential for restoring biodiversity and decreasing erosion on deforested lands, few studies note the behaviour or toxicity of trace elements during the rehabilitation process. Our original study focused on the potential use of microbial activity and Hg speciation and compared them with As, Cu, Zn and Cr speciation in assessing the chemical and biological quality of ecological restoration efforts. We sampled two sites in French Guyana 17 years after rehabilitation efforts began. The former site was actively regenerated (R) with the leguminous species Clitoria racemosa and Acacia mangium, and the second site was passively regenerated with spontaneous vegetation (Sv). We also sampled soil from a control site without a history of gold mining (F). We performed microcosm soil experiments for 30 days, where trace element speciation and enzyme activities (i.e., FDA, dehydrogenase, β-glucosidase, urease, alkaline and acid phosphatase) were estimated to characterise the behaviour of trace elements and the soil microbial activity. As bioindicators, the use of soil microbial carbon biomass and soil enzyme activities related to the carbon and phosphorus cycles seems to be relevant for assessing soil quality in rehabilitated and regenerated old mining sites. Our results showed that restoration with leguminous species had a positive effect on soil chemical quality and on soil microbial bioindicators, with activities that tended toward natural non-degraded soil (F). Active restoration processes also had a positive effect on Hg speciation by reducing its mobility. While in Sv we found more exchangeable and soluble mercury, in regenerated sites, Hg was mostly bound to organic matter. These results also suggested that enzyme activities and mercury cycles are sensitive to land restoration and must be considered when evaluating the efficiency of restoration processes.

Comparing the influence of selenite (Se4+) and selenate (Se6+) on the inhibition of the mercury (Hg) phytotoxicity to pak choi

Selenite (Se (IV)) and selenate (Se (IV)) have recently been demonstrated to be equally effective in inhibiting mercury (Hg) phytotoxicity to plants. This assertion is still unclear. In this study, we aimed to explore the potential effects of Se species (Se⁴⁺ and Se⁶⁺) on the inhibition of the mercury (Hg) bioavailability to pak choi in dry land. Pot experiments with exposure to different dosages of mercuric chloride (HgCl₂) and selenite (Na₂SeO₃) or selenate (Na₂SeO₄) were treated. To compare the influence of Se (IV) and Se (VI) on the bioaccumulation and bioavailability of Hg, the levels of total Hg in different pak choi (Brassica chinensis L.) tissues (roots and shoots) and the distribution changes of Hg fractions in soil before planting and after harvest were determined as well as the Hg I_R values in soils (relative binding intensity) were analyzed. Results showed that application Se (IV) reduced the concentrations of Hg in pak choi roots more than Se (VI). Hg concentrations were also decreased in pak choi shoots in Se (IV) treatments, while which notably increased in Se (VI) treatments. Thus, Se (IV) plays a more important role than Se (VI) in limiting the absorption and bioaccumulation of Hg in pak choi. Moreover, this inhibition may only significantly occur when Se (IV) is at an appropriate level (2.5mg/kg). In addition, the good correlations between the proportions of mobile Hg fractions (soluble and exchangeable fractions), I_R values with the Hg concentrations in plants were observed. This affirmed the importance of the Hg fractions transformation and the I_R indicator of Hg in the assessment of their bioavailability. Our findings regarding the importance of Se (IV) influence in reducing Hg bioaccumulation not only provided the correct appraisal about the effect of Se species on the inhibition of the Hg phytotoxicity to pak choi in dry land, but also be a good reference for selecting Se fertilizer forms (Se⁴⁺ or Se⁶⁺).

Comparison of extractants used for the assessment of mercury availability in a soil from the Almadén mining district (Spain)

Single extraction methods have been extensively used to assess the availability of metals in polluted soils. This work focused on checking the feasibility of several chemicals, i.e. CaCl₂, EDTA, diethylenetriaminepentaacetic acid (DTPA) and a low-molecular-weight organic acid mixture (rhizosphere-based method), to be used as extractants for mercury (Hg) in a soil from the Almadén mining district (Spain). Moreover, the effect of several experimental parameters, i.e. extraction time (0.5, 1, 2, 5, 16 and 24 h), concentration of extractant (0.01, 0.05, 0.1 and 1 M) and soil/extractant ratio (1:2, 1:5 and 1:10), on the amount of Hg extracted was investigated. The Hg extraction ability followed the descending order EDTA > rhizosphere-based method > DTPA ≈ CaCl₂. This ranking was attributed to the higher complexation power of EDTA and organic acids. It was also found that extraction times between 2 and 5 h are required to avoid underestimation of mobile Hg and re-adsorption of the Hg previously extracted. Although some exceptions were found, Hg extraction efficiency was generally enhanced with higher extractant concentrations. Finally, the amount of Hg extracted by the four extractants increased with decreasing soil/extractant ratios.