The Mining Industry's Role in Enhanced Weathering and Mineralization for CO2 Removal.

Enhanced weathering and mineralization (EWM) aim to remove carbon dioxide (CO2) from the atmosphere by accelerating the reaction of this greenhouse gas with alkaline minerals. This suite of geochemical negative emissions technologies has the potential to achieve CO2 removal rates of >1 gigatonne per year, yet will require gigatonnes of suitable rock. As a supplier of rock powder, the mining industry will be at the epicenter of the global implementation of EWM. Certain alkaline mine wastes sequester CO2 under conventional mining conditions, which should be quantified across the industry. Furthermore, mines are ideal locations for testing acceleration strategies since tailings impoundments are contained and highly monitored. While some environmentally benign mine wastes may be repurposed for off-site use─reducing costs and risks associated with their storage─numerous new mines will be needed to supply rock powders to reach the gigatonne scale. Large-scale EWM pilots with mining companies are required to progress technology readiness, including carbon verification approaches. With its knowledge of geological formations and ore processing, the mining industry can play an essential role in extracting the most reactive rocks with the greatest CO2 removal capacities, creating supply chains, and participating in life-cycle assessments. The motivations for mining companies to develop EWM include reputational benefits and carbon offsets needed to achieve carbon neutrality.

Artificial intelligence-based prediction model for the elemental occurrence form of tailings and mine wastes.

With the advent of the second industrial revolution, mining and metallurgical processes generate large volumes of tailings and mine wastes (TMW), which worsens global environmental pollution. Studying the occurrence of metal and metalloid elements in TMW is an effective approach to evaluating pollution linked to TMW. However, traditional laboratory-based measurements are complicated and time-consuming; thus, an empirical method is urgently needed that can rapidly and accurately determine elemental occurrence forms. In this study, a model combining Bayesian optimization and random forest (RF) approaches was proposed to predict TMW occurrence forms. To build the RF model, a dataset of 2376 samples was obtained, with mineral composition, elemental properties, and total concentration composition used as inputs and the percentage of occurrence forms as the model output. The correlation coefficient (R), coefficient of determination, mean absolute error, root mean squared error, and root mean squared logarithmic error metrics were used for model evaluation. After Bayesian optimization, the optimal RF model achieved accurate predictive performance, with R values of 0.99 and 0.965 on the training and test sets, respectively. The feature significance was analyzed using feature importance and Shapley additive explanatory values, which revealed that the electronegativity and total concentration of the elements were the two features with the greatest influence on the model output. As the electronegativity of an element increases, its corresponding residual fraction content gradually decreases. This is because the solubility typically increases with the solvent's polarity and electronegativity. Overall, this study proposes an RF model based on the nature of TMW that can rapidly and accurately predict the percentage values of metal and metalloid element occurrence forms in TMW. This method can minimize testing time requirements and help to assess TMW pollution risks, as well as further promote safe TMW management and recycling.

Assessment of industrial by-products as amendments to stabilize antimony mine wastes.

The spread of antimony from mine wastes to the environment represents a matter of great concern due to its adverse effects on impacted ecosystems. There is an urgent need for developing and adopting sustainable and inexpensive measures to deal with this type of wastes. In this study the Sb leaching behavior of mine waste rocks and mine tailings derived from the exploitation of Sb ore deposits was characterized using standard batch leaching tests (TCLP and EN-12457-4) and column leaching essays. Accordingly, these mine wastes were characterized as toxic (>0.6 mg Sb L-1) and not acceptable at hazardous waste landfills (>5 mg Sb kg-1), showing also an ongoing Sb release under prolonged leaching conditions. Two industrial by-products were evaluated as amendments to stabilize them, namely deferrisation sludge (DFS) and a by-product derived from the treatment of aluminum salt slags (BP-Al). Mine wastes were amended with different doses (0-25%) of DFS or BP-Al and the performance of these treatments was evaluated employing also batch and column leaching procedures. The effectiveness of DFS to immobilize Sb was much higher than that exhibited by BP-Al. Thus, treatments with 25% BP-Al showed Sb immobilization levels of approximately 33-53%, whereas treatments with 5 and 25% DFS already attained Sb immobilization levels up to approximately 80-90 and 90-99%, respectively. Mine tailings amended with 5% DFS and mine waste rocks amended with 25% DFS decreased their leachable Sb contents below the limit for non-hazardous waste landfill acceptance (<0.7 mg Sb kg-1). Likewise, these DFS treatments were able to revert their toxic characterization. Moreover, the 25% DFS treatment showed to be a long-lasting stabilizing system, efficient at least during a leaching period equivalent to 10-year rainfall with a great Sb leaching reduction (close to 98%). After this long-term leaching process, DFS-treated mine wastes kept their non-hazardous and non-toxic characterization. The amorphous Fe (oxyhydr)oxides composing DFS were responsible for the important Sb removal capacity showed by this by-product. Thus, when DFS was applied to mine wastes mobile Sb was importantly fixed as non-desorbable Sb, showing also a considerable Sb removal capacity in presence of strong competing anions such as phosphate. The application of DFS as amendment presents a great potential to be used as a sustainable long-term stabilizing system of Sb mine wastes.

Effects on metal availability of the application of tree biochar and municipal waste biosolid in a metalliferous mine tailings substrate.

The phytostabilization of mine tailings requires a previous assessment of the effects of soil amendments on metal mobility. The goal of this work was to evaluate the response of metal availability (both labile and potentially available pools) to the addition of two organic amendments (a municipal waste biosolid and a tree biochar), separately and in combination, in a mine tailings substrate. For this purpose, a comprehensive comparison among several single extraction procedures and a sequential extraction procedure was performed. The effects on metals phytotoxicity were assessed through a germination test using seeds of Zygophyllum fabago. When evaluating the effect of the amendments in the labile metal pool, the biochar resulted effective in decreasing metal-extractable concentrations, especially for Cd, Mn and Zn. The treatment with biochar also showed better germination parameters (percentage of germinated seeds and sooner germination) than the rest of the unamended and amended treatments. The use of the municipal organic biosolid increased labile metal concentrations and potentially available metal pools assessed with EDTA and did not contribute to achieve better results of seed germination. Compared to the single biosolid treatment, the combination of biochar/biosolid modulated some labile metal concentrations and showed similar germination parameters to those obtained for the treatment amended only with biochar. This positive effect of biochar in modulating the soluble metal concentrations associated with certain urban/agricultural organic materials supported the suitability of using these combinations in field applications, although a higher rate of biochar application would be recommended to obtain a more beneficial effect.

A novel fast-vegetative propagation technique of the pioneer shrub Baccharis linearis on mine tailings by adding compost.

Interest in Baccharis linearis has increased as an alternative for assisted phytostabilization due to its spontaneous colonization of tailings dumps. The search for a novel fast-vegetative propagation technique to accelerate its coverage on mine tailings is a promising research area for sustainable mine closure plans. In this study, we determined the optimal proportion of compost and tailings as growing media to promote fast B. linearis propagation through a compound layering technique. The assessed growing substrates were: 100% tailings, 70% tailings + 30% compost, and 50% tailings + 50% compost. After 84 days of growth, the change in number and height of layering branches, root and shoot dry mass, percentage of ground coverage, and substrate chemical properties were assessed. The main results showed that compound layering of B. linearis is possible with compost addition. The growth of new roots and layering branches was significantly improved by either 30% or 50% compost addition into tailings, due to chemical improvements of substrate (higher nutrients and pH and decreased copper bioavailability). The study confirms that the compound layering of B. linearis may be an effective and novel technique for speeding the reclamation of post-operative mine tailings, which is improved by the incorporation of compost.

Biochar and urban solid refuse ameliorate the inhospitality of acidic mine tailings and foster effective spontaneous plant colonization under semiarid climate.

Phytomanagement is considered a suitable option in line with nature-based solutions to reduce environmental risks associated to metal(loid) mine tailings. We aimed at assessing the effectiveness of biochar from pruning trees combined with compost from urban solid refuse (USR) to ameliorate the conditions of barren acidic (pH ~5.5) metal(loid) mine tailing soils (total concentrations in mg kg-1: As ~220, Cd ~40, Mn ~1800, Pb ~5300 and Zn ~8600) from Mediterranean semiarid areas and promote spontaneous plant colonization. Two months after amendment addition were enough to observe improvements in chemical and physico-chemical tailing soil properties (reduced acidity, salinity and water-soluble metals and increased organic carbon and nutrients content), which resulted in lowered ecotoxicity for the soil invertebrate Enchytraeus crypticus. Recalcitrant organic carbon provided by biochar remained in soil whereas labile organic compounds provided by USR were consumed over time. These improvements were consistent for at least one year and led to lower bulk density, higher water retention capacity and higher scores for microbial/functional-related parameters in the amended tailing soil. Spontaneous growth of native vegetation was favored with amendment addition, but adult plants of remarkable size were only found after three years. This highlights the existence of a time-lag between the positive effects of the amendment on tailing soil properties being observed and these improvements being translated into effective spontaneous plant colonization.

Cyanotrophic and arsenic oxidizing activities of Pseudomonas mendocina P6115 isolated from mine tailings containing high cyanide concentration.

Mine tailings and wastewater generate man-made environments with several selective pressures, including the presence of heavy metals, arsenic and high cyanide concentrations, but severe nutritional limitations. Some oligotrophic and pioneer bacteria can colonise and grow in mine wastes containing a low concentration of organic matter and combined nitrogen sources. In this study, Pseudomonas mendocina P6115 was isolated from mine tailings in Durango, Mexico, and identified through a phylogenetic approach of 16S rRNA, gyrB, rpoB, and rpoD genes. Cell growth, cyanide consumption, and ammonia production kinetics in a medium with cyanide as sole nitrogen source showed that at the beginning, the strain grew assimilating cyanide, when cyanide was removed, ammonium was produced and accumulated in the culture medium. However, no clear stoichiometric relationship between both nitrogen sources was observed. Also, cyanide complexes were assimilated as nitrogen sources. Other phenotypic tasks that contribute to the strain's adaptation to a mine tailing environment included siderophores production in media with moderate amounts of heavy metals, arsenite and arsenate tolerance, and the capacity of oxidizing arsenite. P. mendocina P6115 harbours cioA/cioB and aoxB genes encoding for a cyanide-insensitive oxidase and an arsenite oxidase, respectively. This is the first report where P. mendocina is described as a cyanotrophic and arsenic oxidizing species. Genotypic and phenotypic tasks of P. mendocina P6115 autochthonous from mine wastes are potentially relevant for biological treatment of residues contaminated with cyanide and arsenic.

Phytotoxicity of polymetallic mine wastes from southern Tuscany and Saxony.

Restoration potential of mine wastes or approaches to improve soil conditions and to ameliorate phytotoxicity on these sites may be simulated in standardized greenhouse experiments. Plants can be cultivated side by side on materials from different origins in dilution series with defined admixtures of certain aggregates. Mine wastes used in the present study originated from Fenice Capanne (FC, Tuscany, Italy) and Altenberg (ALT, Saxony, Germany). Tailings of the Italian site contain high concentrations of lead, zinc, arsenic and sulphur while tin, wolfram, molybdenum and lithium are highly elevated in the German mine waste. We tested growth responses of five crop species and analyzed concentrations of various metals and nutrients in the shoot to evaluate the toxicity of the FC mine waste and found oilseed rape being the most and corn the least resistant crop. Interestingly, oilseed rape accumulated seven times higher levels of lead than corn without showing adverse effects on productivity. In a subsequent comparison of FC and ALT mine waste, we cultivated different species of buckwheat (Fagopyrum spec.), a fast growing genus that evolved in mountain areas and that has been shown to be tolerant to low pH and high concentrations of metals. We found that the FC mine waste was more toxic than the ALT substrate in F. tataricum and F. esculentum. However, lower admixtures of FC material (10%) resulted in stronger growth reductions than higher proportions (25%) of the mine waste which was primarily related to the slightly lower pH and higher availability of essential metals due to the admixture of sand. These results confirm the importance of managing the soil chemical and physical characteristics of wastelands and call for the development of assisted reclamation to prepare sites for regular biomass production.

Electrochemical effect on bioleaching of arsenic and manganese from tungsten mine wastes using Acidithiobacillus spp.

Mine wastes from tungsten mine which contain a high concentration of arsenic (As) may expose many environmental problems because As is very toxic. This study aimed to evaluate bioleaching efficiency of As and manganese (Mn) from tungsten mine wastes using the pure and mixed culture of Acidithiobacillus ferrooxidans and A. thiooxidans. The electrochemical effect of the electrode through externally applied voltage on bacterial growth and bioleaching efficiency was also clarified. The obtained results indicated that both the highest As extraction efficiency (96.7%) and the highest Mn extraction efficiency (100%) were obtained in the mixed culture. A. ferrooxidans played a more important role than A. thiooxidans in the extraction of As whereas A. thiooxidans was more significant than A. ferrooxidans in the extraction of Mn. Unexpectedly, the external voltage applied to the bioleaching did not enhance metal extraction rate but inhibited bacterial growth, resulting in a reverse effect on bioleaching efficiency. This could be due to the low electrical tolerance of bioleaching bacteria. However, this study asserted that As and Mn could be successfully removed from tungsten mine waste by the normal bioleaching using the mixed culture of A. ferrooxidans and A. thiooxidans.

Atmospheric dust deposition on soils around an abandoned fluorite mine (Hammam Zriba, NE Tunisia).

The present study focuses on the eolian dispersion and dust deposition, of major and trace elements in soils in a semi-arid climate, around an old fluorite (CaF2) and barite (BaSO4) mine, located in Hammam Zriba in Northern Tunisia. Ore deposits from this site contain a high amount of metal sulphides constituting heavy metal pollution in the surrounding environment. Samples of waste from the surface of mine tailings and agricultural topsoil samples in the vicinity of the mine were collected. The soil samples and a control sample from unpolluted area, were taken in the direction of prevailing northwest and west winds. Chemical analysis of these solids was performed using both X-ray fluorescence and X-ray diffraction. To determine the transfer from mine wastes to the soils, soluble fraction was performed by inductively coupled plasma and ionic chromatography. The fine grained size fraction of the un-restored tailings, still contained significant levels of barium, strontium, sulphur, fluorine, zinc and lead with mean percentages (wt%) of 30 (calculated as BaO), 13 (as SrO), 10 (as SO3), 4 (F), 2 (Zn) and 1.2 (Pb). Also, high concentrations of cadmium (Cd), arsenic (As) and mercury (Hg) were found with an averages of 36, 24 and 1.2mgkg-1, respectively. As a result of the eolian erosion of the tailings and their subsequent wind transport, the concentrations of Ba, Sr, S, F, Zn and Pb were extremely high in the soils near to the tailings dumps, with 5%, 4%, 7%, 1%, 0.8% and 0.2%, respectively. Concentration of major pollutants decreases with distance, but they were high even in the farthest samples. Same spatial distribution was observed for Cd, As and Hg. While, the other elements follow different spatial patterns. The leaching test revealed that most elements in the mining wastes, except for the anions, had a low solubility despite their high bulk concentrations. According the 2003/33/CE Decision Threshold, some of these tailings samples were considered as hazardous. Furthermore, other waste samples, considered non hazardous, were not inert. In contrast, the SO42-, Ba, Pb and Sb leachable contents measured in most of the soil samples were relatively high, exceeding the inert threshold for landfill disposal of wastes.

Ecotoxicological risks of the abandoned F-Ba-Pb-Zn mining area of Osor (Spain).

Due to its potential toxic properties, metal mobilization is of major concern in areas surrounding Pb-Zn mines. In the present study, metal contents and toxicity of soils, aqueous extracts from soils and mine drainage waters from an abandoned F-Ba-Pb-Zn mining area in Osor (Girona, NE Spain) were evaluated through chemical extractions and ecotoxicity bioassays. Toxicity assessment in the terrestrial compartment included lethal and sublethal endpoints on earthworms Eisenia fetida, arthropods Folsomia candida and several plant species, whereas aquatic tests involved bacteria Vibrio fischeri, microalgae Raphidocelis subcapitata and crustaceans Daphnia magna. Total concentrations of Ba (250-5110 mg kg-1), Pb (940 to >5000 mg kg-1) and Zn (2370-11,300 mg kg-1) in soils exceeded intervention values to protect human health. Risks for the aquatic compartment were identified in the release of drainage waters and in the potential leaching and runoff of metals from contaminated soils, with Cd (1.98-9.15 µg L-1), Pb (2.11-326 µg L-1) and Zn (280-2900 µg L-1) concentrations in filtered water samples surpassing US EPA Water Quality Criteria (2016a, b). Terrestrial ecotoxicity tests were in accordance with metal quantifications and identified the most polluted soil as the most toxic. Avoidance and reproduction tests with earthworms showed the highest sensitivity to metal contamination. Aquatic bioassays performed in aqueous extracts from soils confirmed the results from terrestrial tests and also detected toxic effects caused by the mine drainage waters. Algal growth inhibition was the most sensitive aquatic endpoint. In view of the results, the application of a containment or remediative procedure in the area is encouraged.

Prospective for remediation and purification of wastes from Xikuangshan mine by using Si-based substances.

Heavy metal mining includes several procedures producing water and solid wastes. These wastes may have high content of heavy metals and other pollutants. Usually, traditional technologies for purification of solid and liquid wastes are expensive and require a lot of special constructions. Recent investigations have shown that some Si-rich substances enable to regulate the mobility of pollutants in soil and water and enhance the plant resistance to its toxicity. Based on these findings, new way for purification of waste-waters and detoxification of pollutants can be elaborated. Laboratory test was conducted with contaminated solid and liquid wastes from Xikuangshan mine. In column and incubation tests, the contents and mobility of the following pollutants were evaluated in Si-treated and untreated samples: As, Cd, Co, Cr, Cu, Hg, Pb, Ni and Zn. The investigations have shown that the Si-rich substances can be used for filtration of contaminated waste-water. The concentrations of soluble pollutants were reduced by 5-10 times and more. The incubation tests with solid wastes and Si-rich compounds have demonstrated that some Si-based substances reduced the contaminant mobility by 2-4 times. The efficiency of tested substances depended on their solubility on Si. The data has demonstrated that some types of local materials including industrial wastes can be used for purification of waste-waters and detoxification of solid wastes.

Arsenic mineral and compound data as analyzed by X-ray absorption spectroscopy and X-ray diffraction.

Here, we present As K-edge X-ray absorption spectroscopy (XAS) data for 28 arsenic minerals and compounds. These minerals and compounds were obtained from mineral dealers, museum collections, and chemical suppliers, and were positively identified by synchrotron-based powder X-ray diffraction (XRD). All samples were analyzed for both XRD and XAS at the Canadian Light Source synchrotron (Saskatoon, Canada). The As K-edge XAS data were collected in both transmission and fluorescence modes and cover the extended X-ray absorption fine structure (EXAFS) region. Raw XAS data in both modes are provided to support XAS analysis obtained for geological or environmental research. Furthermore, As K-edge EXAFS spectra, the k3 weighted oscillatory χ(k) functions, and the Fourier-transforms in χ(R) of these K-edge data are processed and presented graphically. Corresponding XRD data was collected to confirm phase identity. Two-dimensional powder diffraction images were collected against an area detector and integrated to produce line scans. The XRD data were either collected at a wavelength of 0.68866 Å (18 keV) or 0.3497 Å (35.45 keV). Raw, tabulated asc files are available, while the patterns are also presented graphically over a 0-40 °2Θ range or 0-26.5 °2Θ range, respectively. The intent of this dataset is to provide reference XAS spectra to researchers conducting environmental or geological research on As.

Mercury Biogeochemical Cycle in Yanwuping Hg Mine and Source Apportionment by Hg Isotopes.

Although mercury (Hg) mining activities in the Wanshan area have ceased, mine wastes remain the primary source of Hg pollution in the local environment. To prevent and control Hg pollution, it is crucial to estimate the contribution of Hg contamination from mine wastes. This study aimed to investigate Hg pollution in the mine wastes, river water, air, and paddy fields around the Yanwuping Mine and to quantify the pollution sources using the Hg isotopes approach. The Hg contamination at the study site was still severe, and the total Hg concentrations in the mine wastes ranged from 1.60 to 358 mg/kg. The binary mixing model showed that, concerning the relative contributions of the mine wastes to the river water, dissolved Hg and particulate Hg were 48.6% and 90.5%, respectively. The mine wastes directly contributed 89.3% to the river water Hg contamination, which was the main Hg pollution source in the surface water. The ternary mixing model showed that the contribution was highest from the river water to paddy soil and that the mean contribution was 46.3%. In addition to mine wastes, paddy soil is also impacted by domestic sources, with a boundary of 5.5 km to the river source. This study demonstrated that Hg isotopes can be used as an effective tool for tracing environmental Hg contamination in typical Hg-polluted areas.

Microarthropod communities act as functional mediators of ecosystem recovery in abandoned metal(loid) mine tailings in semiarid areas.

Abandoned metal(loid) mine tailings show inhospitable conditions for the establishment of above- and below-ground communities (e.g., high metal(loid) levels, organic matter and nutrient deficiency). This worsens in semiarid areas due to the harsh climate conditions. Fertility islands (vegetation patches formed by plants that spontaneously colonize the tailings) can serve as potential nucleation spots fostering beneficial plant-microbial interactions. However, less attention has been paid to the soil invertebrates living beneath these patches and their functional role. Here, we studied whether the spontaneous plant colonization of abandoned metal(loid) mine tailings led to a greater presence of soil microarthropod communities and whether this could contribute to improving ecosystem functionality. Microarthropods were extracted, taxonomically identified and subsequently assigned to different functional groups (saphrophages, omnivores, predators) in bare soils and differently vegetated patches within metalliferous mine tailings and surrounding forests in southeast Spain. Microarthropod communities were significantly different in bare soils compared with vegetated patches in mine tailings and surrounding forests. Plant colonization led to an increase in microarthropod abundance in tailing soils, especially of mites and springtails. Moreover, saprophages and omnivores, but not predators, were favored in vegetated patches. The initial microarthropod colonization was mainly linked to higher organic matter accumulation and greater microbial activity in the vegetated patches within mine tailings. Moreover, soil formation processes already initiated in the tailings were beneficial for soil biota establishment. Thus, below-ground communities created an anchorage point for plant communities by primarily starting heterotrophic activities in the vegetated patches, thereby contributing to recover ecosystem functionality.

Using Mediterranean Native Plants for the Phytoremediation of Mining Sites: An Overview of the Past and Present, and Perspectives for the Future.

Mining exploitation in the Mediterranean Basin has left evident scars on the environment, and poses serious risks for human health and biodiversity, especially when mine wastes are left abandoned. This review analysed the main issues of metal(loid)s pollution related to mine exploitation in the Mediterranean Basin. Here, a list of Mediterranean native plant species studied for phytoremediation is given and, considering their biological forms, vegetational types, and ecology, we categorised them into halotolerant and hydro/hygrophilous vegetation, annual and perennial meadows, garrigues and maquis, and high maquis and woods. The main conclusions of the review are as follows: (1) plant communities established on mine environments are often rich in endemic taxa which ensure a high biodiversity and landscape value, and can help in the psychophysical health of local inhabitants; (2) political and land management should take greater account of the use of native plants for the remediation of contaminated soils; (3) a multidisciplinary approach that includes, among others, studies on biochemical response to metal(loid)s as well as the application of innovative soil amendments gives better results; (4) phytoextraction applications require a detailed recovery plan that takes into consideration several issues, including the negative influence on biodiversity due to extensive use of monotypic plantations, disposal of harvested hazardous plants, and the risk of phytoextracts entering the food chain; and (5) more studies are necessary to increase knowledge and to detect suitable species-especially halophytic ones-for phytoremediation purposes.

Antimony distribution and mobility in different types of waste derived from the exploitation of stibnite ore deposits.

Wastes derived from the exploitation of stibnite ore deposits were studied to determine their mineralogical, chemical, and environmental characteristics and establish the Sb distribution and the current and long-term risks of Sb mobilization. Representative samples of mine waste rocks, mine tailings, and smelting waste were studied by X-ray powder diffraction, polarized light microscopy, electron microprobe analysis, and digestion, leaching, and extraction procedures. The main Sb-bearing minerals and phases identified in the smelting waste were natrojarosite, iron (oxyhydr)oxides, mixtures of iron and antimony (oxyhydr)oxides, and tripuhyite; those in the mine tailings and mine waste rocks were iron (oxyhydr)oxides and/or mixtures of iron and antimony (oxyhydr)oxides. Iron (oxyhydr)oxides and natrojarosite had high Sb contents, with maximum values of 16.51 and 9.63 wt% Sb2O5, respectively. All three types of waste were characterized as toxic; the mine waste rocks and mine tailings would require pretreatment to decrease their leachable Sb content before they would be acceptable at hazardous waste landfills. Relatively little of the Sb was in desorbable forms, which accounted for <0.01 and <0.8% of the total Sb content in the smelting waste and mine waste rocks/mine tailings, respectively. Under reducing conditions, further Sb mobilization from mine waste rocks and mine tailings could occur (up to 4.6 and 3.3% of the total content, respectively), considerably increasing the risk that Sb will be introduced into the surroundings. Although the smelting waste had the highest total Sb content, it showed the lowest risk of Sb release under different environmental conditions. The significant Fe levels in the smelting waste facilitated the formation of various Fe compounds that greatly decreased the Sb mobilization from these wastes.

Spontaneous vegetation colonizing abandoned metal(loid) mine tailings consistently modulates climatic, chemical and biological soil conditions throughout seasons.

This study aimed to evaluate whether the improvement in soil conditions induced by the vegetation spontaneously colonizing abandoned metal(loid) mine tailings from semiarid areas is consistent throughout seasons and to identify if the temporal variability of that conditions is of similar magnitude of that of the surrounding forests. Soil climatic (temperature and moisture), chemical (pH, electrical conductivity and water-soluble salts and metal(loid)s) and biological (water-soluble organic carbon and ammonium, microbial biomass carbon, dehydrogenase and ß-glucosidase activity, organic matter decomposition and feeding activity of soil dwelling organisms) parameters were seasonally evaluated for one year in bare soils and different vegetated patches within metalliferous mine tailings and surrounding forests in southeast Spain. The results indicated that the improvement in soil conditions (as shown by softening of climatic conditions and lower scores for salinity and water-soluble metals and higher for biological parameters) induced by vegetation colonization was consistent throughout seasons. This amelioration was more evident in the more complex vegetation patches (trees with herbs and shrubs under the canopy), compared to bare soils and simpler soil-plant systems (only trees), and closer to forest soils outside the tailings. Bare soils and, to a lesser extent, vegetation patches solely composed by trees, showed stronger seasonal variability in temperature, moisture content, salinity, and water-soluble metals. In contrast, changes in biological and biological-related parameters were more pronounced in the more complex vegetation patches within mine tailings and surrounding forests due to its greater biological activity. In summary, the results demonstrated that vegetation patches formed by spontaneous colonization act as microsites that modulate seasonal variability in soil conditions and stimulate biological activity. This suggests that tailings vegetation patches might have higher resilience against climate change effects than bare soils. Therefore, they should be preserved as valuable spots in the phytomanagement of metal(loid)s mine tailings from semiarid areas.

Effect of the natural establishment of two plant species on microbial activity, on the composition of the fungal community, and on the mitigation of potentially toxic elements in an abandoned mine tailing.

In Mexico, millions of tons of mining wastes are deposited in the open pit. Their content in potentially toxic elements (PTE) represents an environmental risk. In the tailings, pioneer plant communities are established, associated with a determined diversity of fungi; plants, and fungi are fundamental in the natural rehabilitation of mining wastes. The objective was to evaluate the impact of the natural establishment of two plant species on the microbial activity, on the composition of the fungal community, and on the mitigation of the effect of PTE in a contaminated mine tailing. In a tailing, we selected three sites: one non-vegetated; one vegetated by Reseda luteola, and one vegetated by Asphodelus fistulosus. In the substrates, we conducted a physical and chemical characterization; we evaluated the enzymatic activity, the mineralization of the carbon, and the concentration of PTE. We also determined the fungal diversity in the substrates and in the interior of the roots, and estimated the accumulation of carbon, nitrogen, phosphorus and PTE in plant tissues. The tailings had a high percentage of sand; the non-vegetated site presented the highest electric conductivity, and the plant cover reduced the concentration of PTE in the substrates. Plants increased the carbon content in tailings. The enzymatic activities of ß-glucosidase and dehydrogenase, and the mineralization of carbon were highest at the site vegetated with A. fistulosus. Both plant species accumulated PTE in their tissues and exhibited potential in the phytoremediation of lead (Pb), cadmium (Cd), and copper (Cu). Fungal diversity was more elevated at the vegetated sites than in the bare substrate. Ascomycota prevailed in the substrates; the substrates and the plants shared some fungal taxa, but other taxa were specific. The plant coverage and the rhizosphere promoted the natural attenuation and a rehabilitation of the extreme conditions of the mining wastes, modulated by the plant species.

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.