Acid Mine Drainage Environments Research Articles

The Application of Atomic Spectroscopy Techniques in the Recovery of Critical Raw Materials from Industrial Waste Streams, Part II

This month’s column is Part 2 of a contribution from my daughter Glenna, who recently completed her PhD studies in Environmental Science from the University of Copenhagen in Denmark. Her article explores the current landscape of global critical raw materials (CRM) trends in research and the applications of atomic spectroscopy (AS), including inductively coupled plasma–mass spectrometry (ICP-MS), inductively coupled plasma–optical emission spectrometry (ICP-OES), and X-ray analytical techniques in their identification of diverse industrial and environmental media, which have been essential in method validation and quantification of CRMs in complex matrices presenting high risks of interference. Some important examples to be presented include rare earth elements (REEs) in water leaching purification (WLP) residues that co-occur with radioactive materials; REEs and other metals in acid mine drainage (AMD) environments; REEs in coal combustion (fly ash) residues; arsenic (As) from groundwater treatment sediment; and platinum-group elements (PGEs) from sewage sludge. The article also classifies the different techniques in use at each stage of the CRM recovery train, investigates present challenges to each analytical method, and discusses the problem-solving tools used.

Interaction between acid-tolerant alga Graesiella sp. MA1 and schwertmannite under long-term acidic condition

Schwertmannite (Sch), a typical Fe(III)-oxyhydroxysulphate mineral, is the precipitation reservoir of toxic elements in acid mine drainage (AMD). Acid-tolerant microbes in AMD can participate in the microbe-mediated transformation of Sch, while Sch affects the physiological characteristics of these acid-tolerant microbes. Based on our discovery of algae and Sch enrichment in a contaminated acid mine pit lake, we predicted the interaction between algae and Sch when incubated together. The acid-tolerant alga Graesiella sp. MA1 was isolated from the pit-lake surface water of an acidic mine and incubated with different contents of Sch. Sch was detected as the main product at the end of 81 d; however, there was a weak transformation. The presence of dissolved Fe(II) could be largely attributed to the photoreduction dissolution of Sch, which was promoted by Graesiella sp. MA1. The adaptation and growth phases of Graesiella sp. MA1 differed under Sch stress. The photosynthetic and metabolic activities increased and decreased at the adaptation and growth phases, respectively. The MDA contents and antioxidant activity of SOD, APX, and GSH in algal cells gradually enhanced as the Sch treatment content increased, indicating a defense strategy of Graesiella sp. MA1. Metabolomic analysis revealed that Sch affected the expression of significant differential metabolites in Graesiella sp. MA1. Organic carboxylic acid substances were essentially up-regulated in response to Sch stress. They were abundant in the medium-Sch system with the highest Fe(III) reduction, capable of complexing Fe(III), and underwent photochemical reactions via photo-induced charge transfer. The significant up-regulation of reducing sugars revealed the high energy requirement of Graesiella sp. MA1 under Sch stress. And first enriched KEGG pathway demonstrated the importance of sugar metabolism in Graesiella sp. MA1. Data acquired in this study provide novel insights into extreme acid stress adaptation of acid-tolerant algae and Sch, contributing to furthering understanding of AMD environments.

Osmotic response in Leptospirillum ferriphilum isolated from an industrial copper bioleaching environment to sulfate

Iron and sulfur-oxidizing microorganisms play important roles in several natural and industrial processes. Leptospirillum (L.) ferriphilum, is an iron-oxidizing microorganism with a remarkable adaptability to thrive in extreme acidic environments, including heap bioleaching processes, acid mine drainage (AMD) and natural acidic water. A strain of L. ferriphilum (IESL25) was isolated from an industrial bioleaching process in northern Chile. This strain was challenged to grow at increasing concentrations of sulfate in order to assess changes in protein expression profiles, cells shape and to determine potential compatible solute molecules. The results unveiled changes in three proteins: succinyl CoA (SCoA) synthetase, isocitrate dehydrogenase (IDH) and aspartate semialdehyde dehydrogenase (ASD); which were notably overexpressed when the strain grew at elevated concentrations of sulfate. ASD plays a pivotal role in the synthesis of the compatible solute ectoine, which was identified along with hydroxyectoine by using matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF). The relationship between IDH, SCoA, and ectoine production could be due to the TCA cycle, in which both enzymes produce metabolites that can be utilized as precursors or intermediates in the biosynthesis of ectoine. In addition, distinct filamentous cellular morphology in L. ferriphilum IESL25 was observed when growing under sulfate stress conditions. This study highlights a new insight into the possible cellular responses of L. ferriphilum under the presence of high sulfate levels, commonly found in bioleaching of sulfide minerals or AMD environments.

The Application of Atomic Spectroscopy Techniques in the Recovery of Critical Raw Materials from Industrial Waste Streams, Part I

This month’s column is a contribution from my daughter Glenna, who recently completed her PhD studies in environmental science from the University of Copenhagen in Denmark. Her article explores the current landscape of global critical raw materials (CRM) trends in research and the applications of atomic spectroscopy (AS), including inductively coupled plasma–mass spectrometry (ICP–MS), inductively coupled plasma–optical emission spectrometry (ICP–OES), and X-ray analytical techniques in their identification of diverse industrial and environmental media, which have been essential in method validation and quantification of CRMs in complex matrices presenting high risks of interference. Some important examples that are presented include rare earth elements (REE) in water leaching purification (WLP) residues that co-occur with radio- active materials, REEs and other metals in acid mine drainage (AMD) environments, REEs in coal combustion (fly ash) residues, arsenic (As) from groundwater treatment sediment, and platinum-group elements (PGE) from sewage sludge. In addition, the article classifies the different techniques in use at each stage of the CRM recovery train, investigate present challenges to each analytical method, and discuss the problem-solving tools used.

The effect of acid mine drainage on the properties of an all-solid waste paste backfill body based on oil shale residue

The complex environment of mine water is closely linked to the effect of the backfill body. The goal of this study was to establish a method with which to reduce carbon emissions from cementitious materials while addressing the large amount of unutilized solid waste, such as oil shale residue, which lies on the surface of the planet, and pollutes the environment. Oil shale residue, fly ash, slag and coal gangue were used to produce an oil shale residue-based all-solid waste paste backfill (OSGPB) to study the changes in strength and environmental impact with corrosion time under acid mine drainage conditions. The development of cracks was monitored using digital image correlation (DIC) technology. The corrosion mechanism of OSGPB was analysed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). This study revealed that in an acid mine drainage environment, the unconfined compressive strength (UCS) decreased by 38.58% and 15.66% after 24 d of wetdry cycling and complete immersion corrosion, respectively. A significant quantity of corrosive gypsum was produced in the OSGPB, which expanded the internal micropores and gradually formed cracks. This led to more severe internal structural damage during wetdry cycles. Changes in the UCS of the OSGPB over 60 d were predicted using established grey models. The leached concentrations of heavy metal ions met the groundwater quality standard (class III) after the OSGPB was completely immersed in acid mine drainage and subjected to wetdry corrosion cycles. OSGPB showed great potential as a green backfilling material for mines.

Acetic acid stress response of the acidophilic sulfate reducer Acididesulfobacillus acetoxydans.

Acid mine drainage (AMD) waters are a severe environmental threat, due to their high metal content and low pH (pH <3). Current technologies treating AMD utilize neutrophilic sulfate-reducing microorganisms (SRMs), but acidophilic SRM could offer advantages. As AMDs are low in organics these processes require electron donor addition, which is often incompletely oxidized into organic acids (e.g., acetic acid). At low pH, acetic acid is undissociated and toxic to microorganisms. We investigated the stress response of the acetotrophic Acididesulfobacillus acetoxydans to acetic acid. A. acetoxydans was cultivated in bioreactors at pH 5.0 (optimum). For stress experiments, triplicate reactors were spiked until 7.5 mM of acetic acid and compared with (non-spiked) triplicate reactors for physiological, transcriptomic, and membrane lipid changes. After acetic acid spiking, the optical density initially dropped, followed by an adaptation phase during which growth resumed at a lower growth rate. Transcriptome analysis revealed a downregulation of genes involved in glutamate and aspartate synthesis following spiking. Membrane lipid analysis revealed a decrease in iso and anteiso fatty acid relative abundance; and an increase of acetyl-CoA as a fatty acid precursor. These adaptations allow A. acetoxydans to detoxify acetic acid, creating milder conditions for other microorganisms in AMD environments.

A strategy for source control of acid mine drainage by intelligent management of mine wastes

For mine site management, the release of acid and heavy metal ions from sulfur-containing waste rock or tailings (principally chalcopyrite and pyrite) under natural deposit conditions is a major environmental problem. In this study, the interaction between chalcopyrite and pyrite at different mass ratios under a simulated acid mine drainage environment was investigated. The dissolution experiments showed that a low proportion of pyrite promotes chalcopyrite dissolution, while a high proportion inhibits dissolution. However, this phenomenon is weakened by the presence of microorganisms. In addition, the combined characterization by X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy indicated that a high ratio of pyrite incorporation facilitated the formation of passivation layers on the chalcopyrite surface. Scanning electron microscopy results show that many oxidation etching pits are found on the surface of the chalcopyrite residues surface in a low ratio of pyrite addition, whereas numerous islands are observed and tightly wrapped around the surface of the chalcopyrite residues surface in a high ratio of pyrite addition. The current work presents a viable, practical approach to tailings stacking that reduces the release of the heavy metal ions copper and acid through the appropriate proportioning of ores.

Mechanism of the pore and molecular structure evolution of coal exposed to acid mine drainage (AMD)

In the post-extraction epoch, wastewater from mining activities, particularly acid mine drainage (AMD) residing in sulfur-laden coal terrains, assumes a pivotal role in the safety stewardship of decommissioned coal mines. This research aims to investigate the mechanism behind coal characteristic deterioration from prolonged exposure to AMD. Immersion assays were performed on coal samples across pH 2 to 5 to assess the impact of acid mine drainage. Subsequently, the pore and molecular architecture was appraised using microscopic methodologies. Computed Tomography (CT) findings elucidate that post-immersion, the porosity, and fissures proliferated longitudinally along the coal strata, engendering a marked amplification in surface porosity contiguous to pre-existing pores. This escalation in surface porosity was further accentuated in correlation with the intensification of AMD acidity. Nuclear Magnetic Resonance (NMR) data indicated a marginal augmentation in the content of both micropores and macropores within a tepid AMD milieu. However, in a more virulent AMD context, the proportion of micropores diminished, whereas that of macropores and pore throat size (PTS) experienced an upswing, thereby transmuting adsorptive pores into permeable conduits and consequently enhancing coal permeability. Scanning Electron Microscopy (SEM) corroborated the NMR outcomes; as AMD acidity transitioned from mild to severe, the coal matrix manifested many erosive pores, matrix layer disintegration, and an expansion in cleat width. Therefore, the microscopic pore evolution can be succinctly encapsulated as follows: in a mild AMD environment, dissolution of minerals predominates, generating erosive pores, whereas, in a more acidic AMD milieu, the matrix undergoes partial contraction, thereby augmenting pore volume, enhancing permeability, and inducing structural degradation. Additionally, Fourier Transform Infrared Spectroscopy (FTIR) analysis substantiated that AMD compromised the pore architecture and catalyzed the disintegration of coal macromolecules into lower molecular weight constituents. Therefore, AMD degrades coal macromolecules into smaller compounds, heightening matrix layer porosity and impairing coal characteristics. This research yields vital insights for the security and efficient management of abandoned mine excavations.

Different evolution of tensile and compressive strength in concrete affected by acid mine drainage

Although not many studies have analysed the effects of acid mine drainage (AMD) on the durability of concrete, it is clear that such an aggressive environment affects the different mechanical parameters of this material. AMD is an environmental problem that affects the five continents and is especially widespread in the south western part of the Iberian Peninsula, in an area known as the Iberian Pyrite Belt.In this work, the effect of AMD on tensile and compressive strength of concrete has been evaluated, analysing the evolution of these two fundamental parameters on this structural material. For this purpose, a series of concrete samples were subjected to a controlled AMD environment in the laboratory, using water with pH 2.8 collected from the Tharsis Mine (SW Spain). The concrete samples were prepared in accordance with the parameters required by various international standards.The samples were subjected to the aggressive environment by AMD for two periods of time (3 and 6 months), after which they were subjected to non-destructive testing and destructive testing until breaking the samples by means of compression and tensile (indirect tensile test). In addition, Emission Scanning Electron Microscopy studies were performed on samples before AMD influence and on samples affected by AMD for six months.The obtained results using the ultrasonic equipment showed a decrease in concrete quality, an increase in porosity and permeability, the reduction of modulus of elasticity that reaches 8.5% after six months, as well as the appearance of microcracks. The destructive the tests showed that tensile strength is much more affected than the compressive strength: the tensile strength went from 4.31 MPa to 3.27 MPa (24%) after six months, while the compressive strength went from 48.22 MPa to 43.83 MPa (9%) in the same period of time. This different evolution means that the formulas used in international standards that relating compressive strength to tensile strength need to be modified by means of a correction factor.

Coal-source acid mine drainage reduced the soil multidrug-dominated antibiotic resistome but increased the heavy metal(loid) resistome and energy production-related metabolism

A recent global scale study found that mining-impacted environments have multi-antibiotic resistance gene (ARG)-dominated resistomes with an abundance similar to urban sewage but much higher than freshwater sediment. These findings raised concern that mining may increase the risk of ARG environmental proliferation. The current study assessed how typical multimetal(loid)-enriched coal-source acid mine drainage (AMD) contamination affects soil resistomes by comparing with background soils unaffected by AMD. Both contaminated and background soils have multidrug-dominated antibiotic resistomes attributed to the acidic environment. AMD-contaminated soils had a lower relative abundance of ARGs (47.45 ± 23.34 ×/Gb) than background soils (85.47 ± 19.71 ×/Gb) but held high-level heavy metal(loid) resistance genes (MRGs, 133.29 ± 29.36 ×/Gb) and transposase- and insertion sequence-dominated mobile genetic elements (MGEs, 188.51 ± 21.81 ×/Gb), which was 56.26 % and 412.12 % higher than background soils, respectively.Procrustes analysis showed that the microbial community and MGEs exerted more influence on driving heavy metal(loid) resistome variation than antibiotic resistome. The microbial community increased energy production-related metabolism to fulfill the increasing energy needs required by acid and heavy metal(loid) resistance. Horizontal gene transfer (HGT) events primarily exchanged energy- and information-related genes to adapt to the harsh AMD environment. These findings provide new insight into the risk of ARG proliferation in mining environments.

Interaction of Organisms in Abandoned Tin Mining Pits: Perspective of Life in Acid Mine Drainage Environment

Acid Mine Drainage (AMD) occured after mining activity exposes metal sulphides to oxidizing conditions that impact acidic condition with low pH value in the waters and heavy metals contamination. These conditions are also occurred in abandoned tin mining pits, one of placed in Bangka Belitung Archipelago Province. This mini review aimed to elaborate information of micro- and macro-organism’ life in acid mine drainage to be associated with the possibility of life in abandoned tin mining pits. Acidophilic and acidotolerant organisms such as bacterial, phyto- and zooplankton, and some macroorganism included invertebrate or vertebrate like fishes and also water plants were found in these waters. Their presence developed a symbiosis interaction in aquatic environment. Phytoplankton is an autotroph organism, despite being considered autotroph organism, many phytoplankton require exogenous organic cofactors and nutrients for their life. These cofactors were often served by heterotroph bacterial to sustain the growth of phytoplankton. Instead, bacterial obtained dissolved organic matter derived from phytoplankton to survive in the aquatic environment. Furthermore, phytoplankton was consumed by zooplankton; zooplankton was consumed by small fish to big fish in the waters. In addition, water plants also support the interaction of organisms in the water by supplying dissolved oxygen, also anorganic and organic material for their life. The symbiosis and quorum sensing plays an important role in structuring the aquatic food web and creating a life in the acidic water-polluted heavy metal.

The Transformation of Hg2+ during Anaerobic S0 Reduction by an AMD Environmental Enrichment Culture.

Mercury (Hg) is a highly toxic and persistent heavy metal pollutant. The acid mine drainage (AMD) environment in sulfide-mining areas is a typical Hg pollution source. In this paper, the transformation of Hg2+ during anaerobic S0 reduction by an AMD environmental enrichment culture was studied by multiple spectroscopic and microscopic techniques. The experimental results showed that the microbial S0 reduction of the AMD enrichment culture was significantly inhibited in the presence of Hg2+. The results of cell surface morphology and composition analysis showed that there was obvious aggregation of flocculent particles on the cell surface in the presence of Hg2+, and the components of extracellular polymeric substances on the cell surface changed significantly. The results of surface morphology and C/S/Hg speciation transformation analyses of the solid particulate showed that Hg2+ gradually transformed to mercuric sulfide and Hg0 under anaerobic S0 reduction by the AMD enrichment culture. The microbial community structure results showed that Hg2+ significantly changed the enrichment community structure by decreasing their evenness. The dominant microorganisms with S0 reduction functions are closely related to mercury transformation and are the key driving force for the transformation of substrate solid particulate and cellular substances, as well as the fixation of Hg2+.

Glycerol amendment enhances biosulfidogenesis in acid mine drainage-affected areas: An incubation column experiment.

Microbial sulfate (SO4 2−) reduction in Acid Mine Drainage (AMD) environments can ameliorate the acidity and extreme metal concentrations by consumption of protons via the reduction of SO4 2− to hydrogen sulfide (H2S) and the concomitant precipitation of metals as metal sulfides. The activity of sulfate-reducing bacteria can be stimulated by the amendment of suitable organic carbon sources in these generally oligotrophic environments. Here, we used incubation columns (IC) as model systems to investigate the effect of glycerol amendment on the microbial community composition and its effect on the geochemistry of sediment and waters in AMD environments. The ICs were built with natural water and sediments from four distinct AMD-affected sites with different nutrient regimes: the oligotrophic Filón Centro and Guadiana acidic pit lakes, the Tintillo river (Huelva, Spain) and the eutrophic Brunita pit lake (Murcia, Spain). Physicochemical parameters were monitored during 18 months, and the microbial community composition was determined at the end of incubation through 16S rRNA gene amplicon sequencing. SEM-EDX analysis of sediments and suspended particulate matter was performed to investigate the microbially-induced mineral (neo)formation. Glycerol amendment strongly triggered biosulfidogenesis in all ICs, with pH increase and metal sulfide formation, but the effect was much more pronounced in the ICs from oligotrophic systems. Analysis of the microbial community composition at the end of the incubations showed that the SRB Desulfosporosinus was among the dominant taxa observed in all sulfidogenic columns, whereas the SRB Desulfurispora, Desulfovibrio and Acididesulfobacillus appeared to be more site-specific. Formation of Fe3+ and Al3+ (oxy)hydroxysulfates was observed during the initial phase of incubation together with increasing pH while formation of metal sulfides (predominantly, Zn, Fe and Cu sulfides) was observed after 1–5 months of incubation. Chemical analysis of the aqueous phase at the end of incubation showed almost complete removal of dissolved metals (Cu, Zn, Cd) in the amended ICs, while Fe and SO4 2− increased towards the water-sediment interface, likely as a result of the reductive dissolution of Fe(III) minerals enhanced by Fe-reducing bacteria. The combined geochemical and microbiological analyses further establish the link between biosulfidogenesis and natural attenuation through metal sulfide formation and proton consumption.

Machine learning-based prediction of toxic metals concentration in an acid mine drainage environment, northern Tunisia.

In northern Tunisia, Sidi Driss sulfide ore valorization had produced a large waste amount. The long tailings exposure period and in situ minerals interactions produced an acid mine drainage (AMD) which contributed to a strong increase in the mobility and migration of huge heavy metal (HM) quantities to the surrounding soils. In this work, the soil mineral proportions, grain sizes, physicochemical properties, SO42- and S contents, and Machine Learning (ML) algorithms such as the Random Forest (RF), Support Vector Machine (SVM), and Artificial Neural Network (ANN) models were used to predict the soil HM quantities transferred from Sidi-Driss mine drainage to surrounding soils. The results showed that the HM concentrations had significantly increased with the increase of decomposition and oxidation of galena, marcasite, pyrite, and sphalerite-marcasite and Fe-oxide-hydroxides quantities and the sulfate dissolution (marked with SO42- ions increase) that produced the decreased soil pH. Compared to SVM, and ANN models outputs, the RF model that revealed higher R2val, RPD, RPIQ, and lower error indices had satisfactorily predicted the soil HM accumulation coming from the AMD environment.

Biochar enhanced Cd and Pb immobilization by sulfate-reducing bacterium isolated from acid mine drainage environment

Sulfate reducing bacteria (SRB) is considered to be the most promising alternative biological treatment for immobilization of heavy metals due to its high efficiency and low cost. However, the mechanism underlying the biomineralization process has remained unclear. The strain used in this study was isolated from a nonferrous metal smelter, and it was immobilized on biochar to produce biochar inoculated bacterium(BIC); the Cd2+ and Pb2+ immobilization efficacy of BIC were investigated under differing reaction times, temperatures, pH, solid liquid ratios and heavy metal concentrations. Moreover, the BIC immobilization products were analyzed by a series of characterization techniques. The results indicated that the strain identified as Clostridium sulfidigenes HY-1 was found to exhibit high resistances to Cd2+ and Pb2+. The immobilization efficiency of BIC was much better than that of separate strain HY-1 and biochar. Altogether, the optimal immobilization conditions of BIC were found to be 3 d, pH 8.0, 30 °C, biochar addition of 0.6 g/L, and initial Cd2+ and Pb2+ concentrations of 5 and 30 mg/L, respectively. Under these conditions, the immobilization efficiencies of Cd2+ and Pb2+ were 67.9%, 95.7%, respectively. In addition, the BIC immobilization products were analyzed by Fourier transform infrared spectrometer, scanning electron microscope, energy dispersive spectroscopy and further confirmed to be CdS and PbS by X-ray diffraction and X-ray photoelectron spectroscopy. The results showed that biochar could enhance the bacterial immobilization effect; strain HY-1 produced S2− during sulfate reduction, which could accumulate and react with Cd2+ and Pb2+ to form metal sulfide precipitates, mainly in the form of CdS and PbS that were immobilized and removed. These results suggest that BIC shows promise as an efficient means of immobilizing Cd2+ and Pb2+ from acid mine drainage (AMD).

Fe(II)-mediated transformation of schwertmannite associated with calcium from acid mine drainage treatment

Schwertmannite is an important Fe(III)-oxyhydroxysulfate in acid mine drainage (AMD) polluted areas and its stability depends on surrounding environmental factors and previously bound elements. The treatment and neutralization of AMD normally involve the use of lime, which leads to the discharge of abundant Ca in the mining area. Such an environmental disturbance brings up an important and less considered problem of how the reductive transformation of schwertmannite associated with coexisting Ca occurred. Here, the Fe(II)-mediated transformation of Ca-adsorbed schwertmannite and subsequent Ca repartitioning behaviors were investigated. Results showed that adsorbed Ca had a weak inhibitory effect on Fe(II)-mediated schwertmannite transformation. Release of SO42− and SEM images both indicated that transformation rates of schwertmannite decreased under the influence of adsorbed Ca. XRD patterns indicated that adsorbed Ca altered schwertmannite transformation pathways and product compositions upon treatment with 0.4 mmol/L Fe(II). The end products of Sch notably contained both goethite and lepidocrocite; however, transformation products of SchCa only contained goethite all along. Approximately 33.5% of the surface adsorbed-Ca was released into solution within 6 hr after Fe(II) injection. Aqueous Ca behaved in a “first release and then im-mobilization” manner, which indicated dissolution and secondary mineralization drove Ca migration during the Fe(II)-mediated transformation of SchCa. Adsorbed Ca blocked the surface sites for subsequent Fe(II) adsorption, limited the Fe(II)-Fe(III) ETAE, and decreased the transformation rates. This work sheds light on the complex geochemical behavior of schwertmannite under the influences of environmental perturbations in AMD environments.

Exploring the microbial community inhabiting the phosphogypsum stacks of Huelva (SW SPAIN) by a high throughput 16S/18S rDNA sequencing approach.

Around 100 Mt of phosphogypsum (PG) have been deposited in large stacks on the salt marshes of the Tinto River estuary in Huelva (SW Spain), covering about 1000ha. These stacks contain extremely acidic water (pH < 2) with high concentrations of pollutants which can cause emissions into their surroundings, generating important environmental concerns. Despite many chemical, geological or hydrological studies have been conducted to characterize the PG stacks of Huelva, the microbial community inhabiting this extreme environment remains unexplored. Using a 16S/18S-rRNA-high throughput sequencing approach, we have uncovered the main taxonomic groups able to live in the acidic metal-contaminated water, which is in direct contact with the PG, demonstrating for the first time the existence of a huge diversity of microbial species in these extreme conditions. In addition, the physicochemical characteristics of the water sampled have been analyzed. These studies have revealed that the most abundant bacteria found in two different leachate samples of the PG stacks belong to the generaAcidiphilium, Pseudomonas, Leptosprillum, Acidithrix, or Acidithiobacillus, typically found in acid mine drainage (AMD) environments, which in total represent around 50% of the total bacterial community. Biodiversity of eukaryotes in PG water is lower than that of prokaryotes, especially in the water collected from the perimeter channel that surrounds the PG stacks, where the pH reaches a value of 1.5 and the activity concentrations exceed 300Bq L-1 for 238U or 20Bq L-1 for 210Po, values which are from four to five orders of magnitude higher than those usually found in unperturbed surface waters. Even so, an unexpected diversity of algae, fungi, and ciliates have been found in the PG stacks of Huelva, where chlorophyte microalgae and basidiomycetes fungi are the most abundant eukaryotes. Additional bioinformatics tools have been used to perform a functional analysis and predict the most common metabolic pathways in the PG microbiota. The obtained data indicate that the extreme conditions of these PG stacks hide an unexpected microbial diversity, which can play an important role in the dynamics of the contaminating compounds of the PG and provide new strains with unique biotechnological applications.

Acidithiobacillus ferrooxidans mediates morphology evolution of schwertmannite in the presence of Fe2+

Schwertmannite (Sch) possessing a typical “hedgehog” morphology is ubiquitously found in acid mine drainage (AMD) through biological oxidation of Fe2+ by Acidithiobacillus ferrooxidans (Acidithiobacillus ferrooxidans). However, little information is available on the effect of A. ferrooxidans on the morphology evolution of Sch during its formation. In this study, it was found that Fe2+ and A. ferrooxidans (with the presence of Fe2+) could promote the “hedgehog” structure formation of Sch in acid aerobic environment, respectively. Aerobic systems led to the increase in needle-like goethite on the surface of ellipsoidal aggregates with comparison to anaerobic systems. Hydroxyl radical (OH) produced only in the aerobic systems facilitated the formation of Fe3+. Meanwhile, A. ferrooxidans, containing mainly tryptophan, could significantly change the interfacial potential and favor the electron transfer from Fe2+ to Sch. Moreover, it also could coordinate with Fe (Fe2+, Fe3+). Thus, a possible mechanism for developing a typical “hedgehog” morphology of biogenic Sch was that Fe3+ from OH oxidization of Fe2+ and/or released from Sch would chelate with tryptophan to form Fe3+-tryptophan complex, and further induce the formation of needle-like goethite. However, during the crystal growth of goethite, tryptophan would be released to solution. These findings have important implications for comprehensively understanding the role of microorganism on mineral formation in AMD environment.

Sulfate-accelerated photochemical oxidation of arsenopyrite in acidic systems under oxic conditions: Formation and function of schwertmannite

The weathering of arsenopyrite is closely related to the generation of acid mine drainage (AMD) and arsenic (As) pollution. Solar radiation can accelerate arsenopyrite oxidation, but little is known about the further effect of SO42− on the photochemical process. Here, the photooxidation of arsenopyrite was investigated in the presence of SO42− in simulated AMD environments, and the effects of SO42− concentration, pH and dissolved oxygen on arsenopyrite oxidation were studied as well. SO42− could accelerate the photooxidation of arsenopyrite and As(III) through complexation between nascent schwertmannite and As(III). Fe(II) released from arsenopyrite was oxidized to form schwertmannite in the presence of SO42–, and the photooxidation of arsenopyrite occurred through the ligand-to-metal charge-transfer process in schwertmannite–As(III) complex along with the formation of reactive oxygen species in the presence of O2. The photooxidation rate of arsenopyrite first rose and then fell with increasing SO42− concentration. In the pH range of 2.0–4.0, the photooxidation rate of arsenopyrite progressively increased in the presence of SO42−. This study reveals how SO42− promotes the photooxidation of arsenopyrite and As release in the AMD environment, and improves the understanding of the transformation and migration of As in mining areas.

Integrative Assessments on Molecular Taxonomy of Acidiferrobacter thiooxydans ZJ and Its Environmental Adaptation Based on Mobile Genetic Elements.

Acidiferrobacter spp. are facultatively anaerobic acidophiles that belong to a distinctive Acidiferrobacteraceae family, which are similar to Ectothiorhodospiraceae phylogenetically, and are closely related to Acidithiobacillia class/subdivision physiologically. The limited genome information has kept them from being studied on molecular taxonomy and environmental adaptation in depth. Herein, Af. thiooxydans ZJ was isolated from acid mine drainage (AMD), and the complete genome sequence was reported to scan its genetic constitution for taxonomic and adaptative feature exploration. The genome has a single chromosome of 3,302,271 base pairs (bp), with a GC content of 63.61%. The phylogenetic tree based on OrthoANI highlighted the unique position of Af. thiooxydans ZJ, which harbored more unique genes among the strains from Ectothiorhodospiraceae and Acidithiobacillaceae by pan-genome analysis. The diverse mobile genetic elements (MGEs), such as insertion sequence (IS), clustered regularly interspaced short palindromic repeat (CRISPR), prophage, and genomic island (GI), have been identified and characterized in Af. thiooxydans ZJ. The results showed that Af. thiooxydans ZJ may effectively resist the infection of foreign viruses and gain functional gene fragments or clusters to shape its own genome advantageously. This study will offer more evidence of the genomic plasticity and improve our understanding of evolutionary adaptation mechanisms to extreme AMD environment, which could expand the potential utilization of Af. thiooxydans ZJ as an iron and sulfur oxidizer in industrial bioleaching.