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

The duality of sulfate-reducing bacteria: Reducing methylmercury production in rhizosphere but enhancing accumulation in rice plants

Sulfate-reducing bacteria (SRB) are known to alter methylmercury (MeHg) production in paddy soil, but the effect of SRB on MeHg dynamics in rhizosphere and rice plants remains to be fully elucidated. The present study investigated the impact of SRB on MeHg levels in unsterilized and γ-sterilized mercury-polluted paddy soils, with the aim to close this knowledge gap. Results showed that the presence of SRB reduced MeHg production by ∼22 % and ∼17 % in the two soils, but elevated MeHg contents by approximately 55 % and 99 % in rice grains, respectively. Similar trend at smaller scales were seen in roots and shoots. SRB inoculation exerted the most profound impact on amino acid metabolism in roots, with the relative response of L-arginine positively linking to MeHg concentrations in rhizosphere. The SRB-induced enrichment of MeHg in rice plants may be interpreted by the stronger presence of endophytic nitrogen-related microbes (e.g. Methylocaldum, Hyphomicrobium and Methylocystis) and TGA transcription factors interacting with glutathione metabolism and calmodulin. Our study provides valuable insights into the complex effects of SRB inoculation on MeHg dynamics in rice ecosystems, and may help to develop strategies to effectively control MeHg accumulation in rice grains.

Utilizing biochars to stabilize mercury in contaminated floodplain sediment: Implications on mercury remediation

Major weather events contribute to the mobility and remobilization of legacy mercury (Hg) contamination and sequestration within sediments. Remediation using biochar as a soil amendment is a useful technique to immobilize and decrease Hg toxicity. This study explored whether biochar application is effective at stabilizing labile mercury (LaHg) from floodplain sediment. Controlled mesocosms simulating contamination events and flooding conditions were conducted. Floodplain sediment, which experiences annual periodic flooding, was collected. Sediment was spiked with inorganic Hg, applied with different types of biochar, and experienced simulated flooding events. Four types of biochar, pure rice husk (RH), pure peanut hull (PH), sulfur-modified rice husk (SMRH), and sulfur-modified peanut hull (SMPH), were applied at 10 and 40 g/kg rates (i.e., RH 10, RH 40; PH 10, PH 40, SMRH 10, SMRH 40, SMPH 10, SMPH 40). Total Hg, methylmercury, and LaHg concentrations were analyzed by coupling with redox potential measurements. Results indicate that SMRH 10, PH 10, PH 40, SMPH 10, and SMPH 40 successfully remediate Hg by stabilizing and reducing LaHg species from floodplain sediment. However, a high Hg methylation potential was observed with unsulfated and sulfated peanut hulls (PH 10, PH 40, SMPH 10, and SMPH 40), as they tend to create a reducing microenvironment that favors sulfate reduction reactions. Additionally, sulfur-modified biochar tends to promote Hg methylation potential at high application rates (i.e., 40 g/kg). We thus recommend using SMRH at a relatively low application rate (SMRH 10) for the remediation of Hg from floodplain sediment.

Metal organic framework-derived CuO/Cu2O polyhedron-CdS quantum dots double Z-scheme heterostructure for cathodic photoelectrochemical detection of Hg2+ in food and environment

This study employed an innovative copper oxide/cuprous oxide (CuO/Cu2O) polyhedron‑cadmium sulphide quantum dots (CdS QDs) double Z-scheme heterostructure as a matrix for the cathodic PEC determination of mercury ions (Hg2+). First, the CuO/Cu2O polyhedral composite was prepared by calcining a copper-based metal organic framework (Cu-MOF). Subsequently, the amino-modified CuO/Cu2O was integrated with mercaptopropionic acid (MPA)-capped CdS QDs to form a CuO/Cu2O polyhedron-CdS QDs double Z-scheme heterostructure, producing a strong cathodic photocurrent. Importantly, this heterostructure exhibited a specifically reduced photocurrent for Hg2+ when using CdS QDs as Hg2+-recognition probe. This was attributed to the extreme destruction of the double Z-scheme heterostructure and the in situ formation of the CuO/Cu2O-CdS/HgS heterostructure. Besides, p-type HgS competed with the matrix for electron acceptors, further decreasing the photocurrent. Consequently, Hg2+ was sensitively assayed, with a low detection limit (0.11 pM). The as-prepared PEC sensor was also used to analyse Hg2+ in food and the environment.

Iodide- and electrochemical assisted removal of mercury by Cirsium arvense from gold tailings in the Amansie West District, Ghana

Mercury (Hg) pollution in Ghana through mining has become a serious environmental challenge. This study investigates the potential of Cirsium arvense to photostabilize Hg using electrokinetic current with or without an iodide solution in gold mine tailings heavily contaminated through mining activities in southern Ghana. An initial Hg concentration of 9.60 mg/kg using cold vapor atomic absorption spectrometry (CVAAS) was determined. The biological absorption coefficient, bioconcentration factor, and translocation factor of Hg have been presented. Cirsium arvense therefore had a higher bioconcentration factor (BCF) of 2.6-5.15 mg/kg, and a transfer factor (TF) of 0.24-0.36 indicating a higher efficiency for phytostabilization. Both the rate and time of extractions of Hg from the tailings by Cirsium arvense are efficiently improved in the combined electric current and iodide treatment. Plant and electric current combined treatment and plant and iodide combined treatment had only 60 and 50% phytostabilization rates, respectively. The combined plant, iodide, and electric current treatment has proven to be superior with about >90% Hg removal rate. Therefore, the combined plant, iodide, and electric current treatment resulted in a higher Hg removal efficiency by Cirsium arvense in a shorter period due to higher solubilization rate and electromigration effects on Hg species.

Insights into the biogeochemical transformation, environmental impacts and biochar-based soil decontamination of antimony

Every year, a significant amount of antimony (Sb) enters the environment from natural and anthropogenic sources like mining, smelting, industrial operations, ore processing, vehicle emissions, shooting activities, and coal power plants. Humans, plants, animals, and aquatic life are heavily exposed to hazardous Sb or antimonide by either direct consumption or indirect exposure to Sb in the environment. This review summarizes the current knowledge about Sb global occurrence, its fate, distribution, speciation, associated health hazards, and advanced biochar composites studies used for the remediation of soil contaminated with Sb to lessen Sb bioavailability and toxicity in soil. Anionic metal(loid) like Sb in the soil is significantly immobilized by pristine biochar and its composites, reducing their bioavailability. However, a comprehensive review of the impacts of biochar-based composites on soil Sb remediation is needed. Therefore, the current review focuses on (1) the fundamental aspects of Sb global occurrence, global soil Sb contamination, its transformation in soil, and associated health hazards, (2) the role of different biochar-based composites in the immobilization of Sb from soil to increase biochar applicability toward Sb decontamination. The review aids in developing advanced, efficient, and effective engineered biochar composites for Sb remediation by evaluating novel materials and techniques and through sustainable management of Sb-contaminated soil, ultimately reducing its environmental and health risks.

Selenium- and chitosan-modified biochars reduce methylmercury contents in rice seeds with recruiting Bacillus to inhibit methylmercury production

Biochar could reshape microbial communities, thereby altering methylmercury (MeHg) concentrations in rice rhizosphere and seeds. However, it remains unclear whether and how biochar amendment perturbs microbe-mediated MeHg production in mercury (Hg) contaminated paddy soil. Here, we used pinecone-derived biochar and its six modified biochars to reveal the disturbance. Results showed that selenium- and chitosan-modified biochar significantly reduced MeHg concentrations in the rhizosphere by 85.83% and 63.90%, thereby decreasing MeHg contents in seeds by 86.37% and 75.50%. The two modified bicohars increased the abundance of putative Hg-resistant microorganisms Bacillus, the dominant microbe in rhizosphere. These reductions about MeHg could be facilitated by biochar sensitive microbes such as Oxalobacteraceae and Subgroup_7. Pinecone-derived biochar increased MeHg concentration in rhizosphere but unimpacted MeHg content in seeds was observed. This biochar decreased the abundance in Bacillus but enhanced in putative Hg methylator Desulfovibrio. The increasing MeHg concentration in rhizosphere could be improved by biochar sensitive microbes such as Saccharimonadales and Clostridia. Network analysis showed that Saccharimonadales and Clostridia were the most prominent keystone taxa in rhizosphere, and the three biochars manipulated abundances of the microbes related to MeHg production in rhizosphere by those biochar sensitive microbes. Therefore, selenium- and chitosan-modified biochar could reduce soil MeHg production by these microorganisms, and is helpful in controlling MeHg contamination in rice.

Evaluation of novel biomass-derived materials as binding layers for determining labile mercury in water by diffusive gradient in thin-films technique

In this work, several binding gels were successfully prepared in Diffusive Gradient in Thin-film (DGT) that targeted the inclusion of novel biomass-derived materials for the determination of the labile fraction of mercury (Hg) in water. First, five biomass-derived materials were tested and the descending order as a function of the average percentage of Hg removal in solution was feathers > biochar > cork > canola meal > rice husk. The best two materials were treated and pulverized into powder to be embedded in a hydrogel; and so, feathers were pyrolyzed preserving the sulfur contained in their keratin structure (FBC), and biochar (BC) was modified and pyrolyzed with sublimated sulfur (SBC) to increase the Hg sorption sites in its structure. Analysis by Energy Dispersive X-ray fluorescence (EDXRF) spectrometry confirmed that the different pyrolysis procedures increased sulfur absorption successfully. The efficiency of the new gels (BC, SBC and FBC) in agarose was evaluated by comparative Hg uptake tests, showing a larger efficacy in the following order: SBC > BC > FBC. To assess the suitability of their application in freshwater environments, novel DGT devices were also evaluated to determine their diffusion coefficients (D). This test was conducted under controlled laboratory conditions, with particular focus on the potential competence of trace elements (Mn, Cu, Zn, Ni, Pb, Cd and As), which are commonly present in natural waters affected by mining. A stronger linear relationship between the Hg uptake by binding layers and the deployment time were obtained for the DGT devices with SBC (R2 = 0.948) vs. BC (R2 = 0.885). Therefore, the D obtained for Hg were 8.94 × 10-6 cm2 s-1 for DGT-SBC and 5.12 × 10-6 cm2 s-1 for DGT-BC devices at 25 °C, both within the same order of magnitude reported by previous studies. The good performance obtained by DGT-SBC devices is a promising result and indicates the potential for valorization of waste materials in the DGT technique.

Bacterial assemblages imply methylmercury production at the rice-soil system

The plant microbiota can affect plant health and fitness by promoting methylmercury (MeHg) production in paddy soil. Although most well-known mercury (Hg) methylators are observed in the soil, it remains unclear how rice rhizosphere assemblages alter MeHg production. Here, we used network analyses of microbial diversity to identify bulk soil (BS), rhizosphere (RS) and root bacterial networks during rice development at Hg gradients. Hg gradients greatly impacted the niche-sharing of taxa significantly relating to MeHg/THg, while plant development had little effect. In RS networks, Hg gradients increased the proportion of MeHg-related nodes in total nodes from 37.88% to 45.76%, but plant development enhanced from 48.59% to 50.41%. The module hub and connector in RS networks included taxa positively (Nitrososphaeracea, Vicinamibacteraceae and Oxalobacteraceae) and negatively (Gracilibacteraceae) correlating with MeHg/THg at the blooming stage. In BS networks, Deinococcaceae and Paludibacteraceae were positively related to MeHg/THg, and constituted the connector at the reviving stage and the module hub at the blooming stage. Soil with an Hg concentration of 30 mg kg-1 increased the complexity and connectivity of root microbial networks, although microbial community structure in roots was less affected by Hg gradients and plant development. As most frequent connector in root microbial networks, Desulfovibrionaceae did not significantly correlate with MeHg/THg, but was likely to play an important role in the response to Hg stress.

Pyrolysis temperature influences the capacity of biochar to immobilize copper and arsenic in mining soil remediation

Biochar is a promising material used for multiple remediation approaches, mainly in polluted soils. Its properties can differ depending on feedstock and pyrolysis temperature. In this context, we tested the capacity of three biochar products made from corncob, pyrolyzed at different temperatures (350, 500, and 650 °C), to remediate a mining soil affected by high levels of Cu and As. We performed an exhaustive characterization of the biochar. We found that biochar showed a higher surface area with increasing pyrolysis temperature, whereas high molecular weight PAHs were detected in biochar produced at the maximum temperature, thus indicating potential ecotoxicological risks. After the application of biochar to the soil, Cu was partially immobilized, especially when using that obtained at 500 °C. This effect is attributed to the structure of this material and an increase in soil pH and organic matter content. Conversely, As was increased in the soluble fraction for all three types of biochar but in a proportion that lacks relevance. On the whole, given its lower PAH content, higher Cu immobilization ratio, and an almost negligible increase in As availability, biochar obtained at 500 °C outperformed the other two products with respect to soil recovery. Of note, data on Cu and As availability were doubled-checked using two extraction methodologies. We propose that this operational approach for determining the most suitable pyrolysis temperature will find application in other soil remediation actions.

Investigating spatial-temporal contamination for two environments of the Amazon estuary: A multivariate approach

An assessment of environmental quality in Amazonian estuaries utilizing histological and immunohistochemical biomarkers concomitantly with analyses of trace metals in the tissues of Sciades herzbergii, also considering physical chemical analyzes of the water. 352 animals were captured from two sites and during two periods (dry and rainy). Site 1: São Marcos Bay - heavy anthropic influence and Site 2: Caeté estuary-preserved estuary. In the laboratory, the fish were weighed (g) and measured (cm). Fragments of gills and liver were analyzed using histology and immunohistochemistry (Caspase 3). The specimens from Site 1 presented a low-value condition factor, with the highest concentrations of Al, Cd, and Hg appearing in the muscle, and most severe damages to gills and liver. In contrast, individuals from Site 2 presented a high-value condition factor and showed low metal concentrations in the muscle with only slight tissue lesions. Furthermore, our results demonstrated that seasonal changes affect metal modulation and pathologies in fish at Site 1. The sentinel species chosen in this study is considered a strong bioindicator of pollution and the combination of different biomarkers was efficient in providing a clear view of the signs of exposure to pollutants, and the risks posed to fish health by the presence of metals in the environment, especially in Site 1.

Unveiling the Role of Dissolved Organic Matter on the Hg Phytoavailability in Biochar-Amended Soils

Biochar can effectively reduce the phytoavailability of mercury (Hg) in soil, but the mechanisms are not fully understood. In this study, the dynamic changes in Hg content adsorbed by the biochar (BC-Hg), Hg phytoavailability in the soil (P-Hg), and soil dissolved organic matter (DOM) characteristics were determined over a 60-day treatment period. Biochar obtained at 300 °C, 500 °C and 700 °C reduced the P-Hg concentration assessed by MgCl2 extraction by 9.4%, 23.5% and 32.7%, respectively. However, biochar showed a very limited adsorption on Hg, with the maximum BC-Hg content only accounting for 1.1% of the total amount. High-resolution scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) results showed that the proportion of Hg atoms in biochar after 60 d was barely detectable. Biochar treatment can shift soil DOM toward higher aromatic content and molecular weight. Additionally, the addition of high-temperature biochar increased more humus-like components, but low-temperature biochar increased more protein-like components. Correlation analysis and partial least squares path modeling (PLS-PM) showed that biochar promoted humus-like fractions formation to reduce the Hg phytoavailability. This research has deepened the understanding of the mechanisms by which biochar stabilizes Hg in agricultural soils.

The Size Screening Could Greatly Degrade the Health Risk of Fish Consuming Associated to Metals Pollution-An Investigation of Angling Fish in Guangzhou, China

Fish size can heavily impact the bioaccumulation of metals, but it was rarely applied to screen out the fish with low health risk for consuming. Given the widespread metals contamination of angling fish, the angling fish collected from Guangzhou, China, were taken as an example in this study. The screening length and weight were detailed in accordance with the investigation of metals contamination among angling fish. Importantly, the feasibility of size screening on mitigating the health risk of angling fish was evaluated. The results revealed that the concentration of Cr and As were relatively high and beyond the maximum residue limit (MRL) in some fish. The mean pollution index (Pi) of As, Cr, and Pb were beyond 0.2, suggesting the widespread minor contamination. The total metal pollution index (MPI) manifested Oreochroms mossambcus was the most contaminated. The target hazard quotient (THQ) of Cr, As, and Hg were relatively higher, but the higher probability of THQ > 1 indicated the health risk should be dominantly from As. The highest TTHQ suggested the highest risk of Oreochroms mossambcus. Regression analysis determined the fish of THQ < 1 should be more likely centralized in the size that is beyond 13.7 cm and 45.0 g for adults and 19.8 cm and 127.9 g for children. Significantly reducing THQ among these screened fish confirmed their effect on the degrading health risk of metals; particularly, the children’s THQ returned below 1. The commonly contaminated Oreochroms mossambcus was further excluded to remove the screened fish with THQ > 1; the further decrease in THQ confirmed that the exclusion of a contaminated species could improve the effect of size screening.

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.

Biochar as a potential strategy for remediation of contaminated mining soils: Mechanisms, applications, and future perspectives

Soil heavy metal contamination caused by mining activities is a global issue. These heavy metals can be enriched in plants and animals through the food chain, and eventually transferred to the human system and threatening public health. Biochar, as an environmentally friendly soil remediation agent, can effectively immobilize heavy metals in soil. However, most researchers concern more about the remediation effect and mechanism of biochar for industrial and agricultural contaminated soil, while related reviews focusing on mining soil remediation are limited. Furthermore, the remediation effect of soil in mining areas is affected by many factors, such as physicochemical properties of biochar, pyrolysis conditions, soil conditions, mining environment and application method, which can lead to great differences in the remediation effect of biochar in diverse mining areas. Therefore, it is necessary to systematically unravel the relevant knowledge of biochar remediation, which can also provide a guide for future studies on biochar remediation of contaminated soils in mining areas. The present paper first reviews the negative effects of mining activities on soil and the advantages of biochar relative to other remediation methods, followed by the mechanism and influencing factors of biochar on reducing heavy metal migration and bioavailability in mining soil were systematically summarized. Finally, the main research directions and development trends in the future are pointed out, and suggestions for future development are proposed.

The legacy of artisanal gold mining and its impact on fish health from Tapajós Amazonian region: A multi-biomarker approach

Tapajós Region, is an area with intense historical artisanal and small-scale gold mining. Therefore, the core objective of this study was to evaluate the environmental status of different rivers located in this region, using biomarker endpoints in Serrasalmus rhombeus as a tool. Fish and sediment were collected from two rivers, Tropas and Crepori, affluent of Tapajós River, located inside a Federal Protection Area and in a Reference site. Mercury concentration in sediment and fish were traced, and biomarkers in gills and liver were analyzed. Results showed a clear difference between these two rivers compared to the Reference site. Fish tissues presented biomarker responses according to the site of collection. Catalase (CAT) activity was statistically higher in fish gills from Crepori, confirming the capacity of mercury interference with redox equilibrium. High levels of lipid peroxidation were also noted to contribute greatly in incidence of morphological changes in the liver and gills, suggesting that mercury bioaccumulation during continuous exposure promote biological responses in a cumulative manner, from molecules to tissues. This study also indicates adaptation in fish defense mechanisms given the conditions in the Tropas River, as well as a variation in biomarker responses to that of the Crepori river. In summary, Tapajós affluents presented high mercury levels in fish tissues leading to biomarker responses, demonstrating a hazardous signal of a long history of mercury pollution.

Responses of bacterial taxonomic attributes to mercury species in rhizosphere paddy soil under natural sulphur-rich biochar amendment

Biochar and sulphur (S) are important factors regulating the level, speciation and transformation of mercury (Hg), leading to alterations in the assemblage of the soil microbial community. However, variations in the taxonomic attributes of the rhizosphere soil bacterial community arising from the Hg speciation in paddy soil, amended with natural S-rich biochar (NSBC) derived from the pyrolysis of S-rich oilseed rape straw, remain unclear. Herein, a rice pot experiment was conducted. Hg-polluted paddy soils were amended with NSBC and low-S biochar (LSBC) to evaluate the role of Hg chemical form affected by NSBC in regulating the taxonomic attributes of rhizosphere soil, including microbial abundance, composition, and ecological clusters within the co-occurrence network of microbial communities. Results showed that microbial abundance was higher in soils with lower Hg levels, and mean increases of 149 observed operational taxonomic units (OTUs) and 238 predicted OTUs (Chao 1) were observed, with a 1 mg kg^-1 decrease in the total Hg (T-Hg) content. Among the 13 predictor variables, the T-Hg content was the strongest and most consistent predictor of the bacterial taxonomic attributes. This finding may be attributed to the fact that the drastic reduction in T-Hg and Hg bioavailability induced by NSBC results in the decrease of Hg stress on the soil microbiome. Moreover, NSBC amendment shifted the ecological clusters toward the amelioration of Hg pollution.

The Variation of Heavy Metals Bioavailability in Sediments of Liujiang River Basin, SW China Associated to Their Speciations and Environmental Fluctuations, a Field Study in Typical Karstic River

The bioavailability of heavy metals (HMs) in sediments is closely related to the security of the aquatic environment, but their impacts are poorly researched, particularly in karstic rivers. Therefore, Liujiang River Basin was taken as an example in this study. Seven HMs were analyzed to determine the bioavailability and speciations of HMs in sediments. Moreover, the impacts of environmental factors on HMs were identified. The obtained results suggested that HMs in the sediments are all within their permissible exposure limit (PEL), but Cd and Zn are significantly higher than the soil baseline. Most HMs were found to be in a residual fraction, while their exchangeable fraction was found to be in an extremely low ratio. HMs in bioavailable parts are significantly higher than in the exchangeable and carbonate-bound phases but lower than in the non-residual phase, which demonstrated that HM bioavailability is not confined to the exchangeable and carbonate-bound phases. The correlation coefficients commonly decreased with decreasing speciation ratios, which suggested that the overall bioavailability of metals should be determined by speciation ratios instead of speciations themselves. Noteworthily, most HMs in the residual form were found to be significantly correlated with their overall bioavailability, which highlighted the potential bioavailability of residual form. The non-correlations between pH, electrical conductivity (EC), total dissolved solids (TDS), and HM bioavailability suggested that HMs in the carbonate-bound phase are stable and unsusceptible to environmental variations, while the significant correlations between redox potential (Eh), turbidity, organic matter (OM), main grain size (Mz), and HM bioavailability suggested that HMs in the reducible and oxidizable forms are susceptible to environmental fluctuations. Therefore, the variation of HM bioavailability in karstic rivers is largely regulated by their reducible and oxidizable forms instead of their carbonate-bound form.