Characteristics, speciation, and bioavailability of mercury and methylmercury impacted by an abandoned coal gangue in southwestern China

Mercury supply limits methylmercury production in paddy soils

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

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.

Predictive modeling of methylmercury in rice (Oryza sativa L.) and species-sensitivity-distribution-based derivation of the threshold of soil mercury in karst mountain areas

The bioavailable mercury (Hg) in the soil is highly active and can affect the formulation of methyl-Hg (MeHg) in soil and its accumulation in rice. Herein, we predicted the concentration of MeHg in rice using bioavailable Hg extracted from soils; additionally, we determined the threshold value of soil Hg in karst mountain areas based on species sensitivity distribution. The bioavailable Hg was extracted using calcium chloride, hydrochloric acid (HCl), diethylenetriaminepentaacetic acid mixture, ammonium acetate, and thioglycolic acid. Results showed that HCl is the best extractant, and the prediction model demonstrated good predictability of the MeHg concentration in rice based on the HCl-extractable Hg, pH, and soil organic matter (SOM) data. Compared with the actual MeHg concentration in rice, approximately 99% of the predicted values (n = 103) were within the 95% prediction range, indicating the good performance of the rice MeHg prediction model based on soil pH, SOM, and bioavailable Hg in karst mountain areas. Based on this MeHg prediction model, the safety threshold of soil Hg was calculated to be 0.0936 mg/kg, which is much lower than the soil pollution risk screening value of agricultural land (0.5 mg/kg), suggesting that a stricter standard should be applied regarding soil Hg in karst mountain areas. This study presents the threshold of soil Hg pollution for rice safety in karst mountain areas, and future studies should target this threshold range.

Enrichment of Sulfate, Acidity and Mercury in Native outcrop coal, Southwest China

The coal found in the Longtan Formation of the Late Permian is widely distributed in Southwest China, including the northwestern Guizhou, southeastern Sichuan, and northern Yunnan regions. This coal typically has a high sulfur content. Eighty-two coal samples were collected from the coal strata in 11 counties spanning this area, including underground mine coal, native outcrop coal and man-made outcrop coal. The mercury, total fluorine, total sulfur, and sulfate contents and pH values were determined. The results showed that the average mercury content in native outcrop coal was 2233 ng/g, whereas that in underground mine coal was 306 ng/g, and the relative enrichment factor could reach 6.6. There was no significant difference in the total fluorine content among the three types of coals; furthermore, the total sulfur content in native outcrop coal and man-made outcrop coal was higher than that in underground mine coal because of the local policy, which strictly prohibits the mining of high-sulfur coal. Native outcrop coal is acidic, with a total average pH of 3.54 and an average sulfate content as high as 13,390 μg/g. In contrast, underground coal is almost neutral (average pH 7.33), with a low sulfate content (average 3221 μg/g). These characterizations indicate that native outcrop coal has been subjected to long-term weathering and the mercury enrichment is likely due to migration, oxidation, and precipitation of Hg from the underground coal seam and enriched in loose and pulverized coal particles. Further investigation is needed to determine whether other outcrop areas are affected by this phenomenon.

Biodegradation of organic compounds in the coal gangue by Bacillus sp. into humic acid

Coal gangue (CG), one of the world's largest industrial solid wastes produced during coal mining, is extremely difficult to be used owing to its combined contents of clay minerals and organic macromolecules. This study explored a novel process of degrading the harmful organic compounds in the CG into humic acid using a biological method characterized by scanning electron microscope-energy dispersive spectrometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and elemental analyzer. The results reveal that adding selected Bacillus sp. to the CG for 40 days can increase the humic acid content by ~ 17 times, reaching 17338.17 mg/kg, which is also the best level for promoting plant growth. FTIR and XPS spectra show that the organic compounds in the CG transforms primarily from C=C to C=O, COOH, and O-H groups, indicating that the organic compounds are gradually oxidized and activated, improving the humic acid concentration of soil. In addition, Bacillus sp. decreases pH and benzo[a]pyrene contents, and increases the content of available nutrients. After microbial degradation, coal gangue can be turned into ecological restoration materials.

Phytoremediation of Soils Contaminated with Heavy Metals from Gold Mining Activities Using Clidemia sericea D. Don

Soils contaminated by potentially toxic elements (PTEs) as a result of anthropogenic activities such as mining are a problem due to the adverse effects on human and environmental health, making it necessary to seek sustainable strategies to remediate contaminated areas. The objective of this study was to evaluate the species Clidemia sericea D. Don for the phytoremediation of soils contaminated with PTEs (Hg, Pb, and Cd) from gold mining activities. The study was conducted for three months, with soils from a gold mining area in northern Colombia, and seeds of C. sericea, under a completely randomized experimental design with one factor (concentration of PTEs in soil) and four levels (control (T0), low (T1), medium (T2), and high (T3)), each treatment in triplicate, for a total of twelve experimental units. Phytotoxic effects on plants, bioconcentration (BCF), and translocation (TF) factors were determined. The results obtained for the tissues differed in order of metal accumulation, with the root showing the highest concentration of metals. The highest values of bioconcentration (BCF > 1) were presented for Hg at T3 and Cd in the four treatments; and of translocation (TF > 1) for Hg and Pb at T0 and T1; however, for Pb, the TF indicates that it is transferable, but it is not considered for phytoextraction. Thus, C. sericea demonstrated its potential as a phytostabilizer of Hg and Cd in mining soils, strengthening as a wild species with results of resistance to the stress of the PTEs evaluated, presenting similar behavior and little phytotoxic affectation on the growth and development of each of the plants in the different treatments.

Improved holding and releasing capacities of coal gangue toward phosphate through alkali-activation

Activation is an effective method to improve adsorption capacity of coal gangue, however, most activated gangues reported have limited adsorption capacity for phosphate because of the electronegativity, affecting their use in agricultural production and environmental protection. In order to enhance the phosphate absorption capacity of coal gangue through alkali-activation, three alkali-activated silicate coal gangue according to fine-size (AS-FCG¹), medium-size (AS-MCG²), and large-size (AS-LCG³) were prepared through grinding, calcining (800 °C) and stimulating with Ca(OH)₂ solution (0.1 M), and raw coal gangue (RCG⁴) was used as control. The results revealed that AS-FCG has the best modified effects, its maximum phosphate adsorption capacity, according to Langmuir model, was 11.796 mg g^-1, which was 4.41, 8.16 and 73.73 times higher than that of AS-MCG, AS-LCG and RCG, respectively. The adsorption penetration time of AS-FCG packed column was over 30 h, which was longer than that of other samples. Besides, when the AS-FCG packed column loaded with phosphate was desorbed with distilled water, the phosphate concentration of filtrate was low and rapidly tended to 0 mg L^-1, while when it was desorbed with NaHCO₃ solution, the concentration remained high sustainably. The best adsorption performance of AS-FCG were associated with generation of calcium silicate hydrate gel and ettringite, which provided more Ca²⁺, Al³⁺, hydroxyl and larger specific surface area. It is suggested that alkali-activation is able to truly realize the recycling of solid waste, AS-FCG is an inexpensive, durable and eco-friendly material, which could not only be used to hold phosphate, but also release it slowly as fertilizer.

Preparation of a Gangue-Based X-type Zeolite Molecular Sieve as a Multiphase Fenton Catalyst and Its Catalytic Performance

In this study, a series of X-type zeolite molecular sieve catalysts, modified with copper (Cu-X), were prepared by an alkali fusion-hydrothermal synthesis using coal gangue from Inner Mongolia. These catalysts were used in the degradation of the methylene blue dye by a Fenton-like reaction. Characterization results showed that Cu is considered to be present in the surface structure of the zeolite in the form of doped Cu ions and metal oxide. It is believed that Cu2+ is the main active site involved in the Fenton reaction. The X-ray photoelectron spectroscopy (XPS) spectra indicated that Cu2+ and Cu+ coexist in the catalysts and participate together in the Fenton reaction. The degradation of methylene blue by the Cu-X catalysts was investigated to determine the optimal catalytic conditions in terms of six aspects: catalyst dosage, initial solution concentration, initial pH of the solution, H2O2 dosage, copper loading, and reaction temperature. The experimental results showed that CX-1.0 had excellent activity and stability for the degradation and decolorization of the methylene blue dye, which could completely degrade the dye within 90 min, and the total organic carbon removal rate reached as high as 97.8%. Electron spin resonance (ESR) and radical capture experiments showed that •OH played a dominant role in the Fenton-like reaction. Combined with XPS, ESR, and catalytic tests, the redox cycle of Cu+/Cu2+ was found to be accelerating the generation of reactive radicals in the Fenton system.