Selective removal of transition metals from acidic mine waters by novel consortia of acidophilic sulfidogenic bacteria

Sulphidogenic Bioprocesses for Acid Mine Water Treatment and Selective Recovery of Arsenic and Metals

Human communities need water and mineral resources, the supply of which requires the implementation of recycling and saving strategies. Both closed and active mining sites could beneficiate of the implementation of nature-based solutions, including bioreactors involving sulphate-reducing prokaryotes (SRP), in order to separate and recover arsenic (As) and metals from aqueous stream while producing clean water. Selective precipitation strategies can be designed based on the selection of microbial communities adapted to the pH conditions, generally acidic, and to available low-cost electron donors. Laboratory batch and continuous experiments must be implemented for each type of mine water in order to determine the optimal flow-sheet in which As could be precipitated as sulphides (orpiment or realgar), inside the bioreactor or offline, through stripping of biologically produced hydrogen sulphides (H2S). The respective concentrations and proportions of As and metals and the initial acid mine drainage pH are key parameters that will influence the feasibility of efficient selective precipitation. SRP-based bioreactors could be combined with complementary treatment steps in optimised mine water management solutions that will minimise the production of As-contaminated end-solid waste.

Nutrient structure dynamics and microbial communities at the water-sediment interface in an extremely acidic lake in northern Patagonia

Lake Caviahue (37° 50 'S and 71° 06' W; Patagonia, Argentina) is an extreme case of a glacial, naturally acidic, aquatic environment (pH ~ 3). Knowledge of the bacterial communities in the water column of this lake, is incipient, with a basal quantification of the bacterioplankton abundance distribution in the North and South Basins of Lake Caviahue, and the described the presence of sulfur and iron oxidizing bacteria in the lake sediments. The role that bacterioplankton plays in nutrient utilization and recycling in this environment, especially in the phosphorus cycle, has not been studied. In this work, we explore this aspect in further depth by assessing the diversity of pelagic, littoral and sediment bacteria, using state of the art molecular methods and identifying the differences and commonalties in the composition of the cognate communities. Also, we investigate the interactions between the sediments of Lake Caviahue and the microbial communities present in both sediments, pore water and the water column, to comprehend the ecological relationships driving nutrient structure and fluxes, with a special focus on carbon, nitrogen, and phosphorus. Two major environmental patterns were observed: (a) one distinguishing the surface water samples due to temperature, Fe2+, and electrical conductivity, and (b) another distinguishing winter and summer samples due to the high pH and increasing concentrations of N-NH4+, DOC and SO42-, from autumn and spring samples with high soluble reactive phosphorus (SRP) and iron concentrations. The largest bacterial abundance was found in autumn, alongside higher levels of dissolved phosphorus, iron forms, and increased conductivity. The highest values of bacterial biomass were found in the bottom strata of the lake, which is also where the greatest diversity in microbial communities was found. The experiments using continuous flow column microcosms showed that microbial growth over time, in both the test and control columns, was accompanied by a decrease in the concentration of dissolved nutrients (SRP and N-NH4+), providing proof that sediment microorganisms are active and contribute significantly to nutrient utilization/mobilization.

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.

Bio-recovery of CuS nanoparticles from the treatment of acid mine drainage with potential photocatalytic and antibacterial applications

In the present work, CuS nanoparticles were biorecovered from a real acid mine drainage (AMD) and its photocatalytic and antibacterial activities were studied. CuS were formed by delivering biogenic H2S produced by a continuous sulfidogenic bioreactor to an off-line vessel containing the AMD. The main physico-chemical properties of CuS nanoparticles were analyzed by UV-vis spectroscopy, TEM, FE-SEM, XRD and XPS. Moreover, its photocatalytic activity on the photodegradation of organic dyes in water and its antibacterial activity against several bacterial strains were studied and compared with CuS nanoparticles synthetized from a CuSO4 aqueous solution based on the same synthesis method. CuS nanoparticles from the real AMD showed similar physico-chemical properties and photocatalytic and antibacterial activities in comparison to CuS nanoparticles formed with the copper solutions. These results open the way to recover valorous CuS nanoparticles from AMD with potential industrial applications using a metal bioremediation process based on sulfidogenic bioreactors.

Selective separation of metals from wastewater using sulfide precipitation: A critical review in agents, operational factors and particle aggregation

Extensive research has been conducted on the separation and recovery of heavy metals from wastewater through the targeted precipitation of metal sulfides. It is necessary to integrate various factors to establish the internal correlation between sulfide precipitation and selective separation. This study provides a comprehensive review of the selective precipitation of metal sulfides, considering sulfur source types, operating factors, and particle aggregation. The controllable release of H2S from insoluble metal sulfides has garnered research interest due to its potential for development. The pH value and sulfide ion supersaturation are identified as key operational factors influencing selectivity precipitation. Effective adjustment of sulfide concentration and feeding rate can reduce local supersaturation and improve separation accuracy. The particle surface potential and hydrophilic/hydrophobic properties are crucial factors affecting particle aggregation, and methods to enhance particle settling and filtration performance are summarized. The regulation of pH and sulfur ion saturation also controls the zeta potential and hydrophilic/hydrophobic properties on the particles surface, thereby affecting particle aggregation. Insoluble sulfides can decrease sulfur ion supersaturation and improve separation accuracy, but they can also promote particle nucleation and growth by acting as growth platforms and reducing energy barriers. The combined influence of sulfur source and regulation factors is vital for achieving precise separation of metal ions and particle aggregation. Finally, suggestions and prospects are proposed for the development of agents, kinetic optimization, and product utilization to promote the industrial application of selective precipitation of metal sulfides in a better, safer, and more efficient way.

From Genes to Bioleaching: Unraveling Sulfur Metabolism in Acidithiobacillus Genus

Sulfur oxidation stands as a pivotal process within the Earth's sulfur cycle, in which Acidithiobacillus species emerge as skillful sulfur-oxidizing bacteria. They are able to efficiently oxidize several reduced inorganic sulfur compounds (RISCs) under extreme conditions for their autotrophic growth. This unique characteristic has made these bacteria a useful tool in bioleaching and biological desulfurization applications. Extensive research has unraveled diverse sulfur metabolism pathways and their corresponding regulatory systems. The metabolic arsenal of the Acidithiobacillus genus includes oxidative enzymes such as: (i) elemental sulfur oxidation enzymes, like sulfur dioxygenase (SDO), sulfur oxygenase reductase (SOR), and heterodisulfide reductase (HDR-like system); (ii) enzymes involved in thiosulfate oxidation pathways, including the sulfur oxidation (Sox) system, tetrathionate hydrolase (TetH), and thiosulfate quinone oxidoreductase (TQO); (iii) sulfide oxidation enzymes, like sulfide:quinone oxidoreductase (SQR); and (iv) sulfite oxidation pathways, such as sulfite oxidase (SOX). This review summarizes the current state of the art of sulfur metabolic processes in Acidithiobacillus species, which are key players of industrial biomining processes. Furthermore, this manuscript highlights the existing challenges and barriers to further exploring the sulfur metabolism of this peculiar extremophilic genus.

Microbial functionalities and immobilization of environmental lead: Biogeochemical and molecular mechanisms and implications for bioremediation

The increasing environmental and human health concerns about lead in the environment have stimulated scientists to search for microbial processes as innovative bioremediation strategies for a suite of different contaminated media. In this paper, we provide a compressive synthesis of existing research on microbial mediated biogeochemical processes that transform lead into recalcitrant precipitates of phosphate, sulfide, and carbonate, in a genetic, metabolic, and systematics context as they relate to application in both laboratory and field immobilization of environmental lead. Specifically, we focus on microbial functionalities of phosphate solubilization, sulfate reduction, and carbonate synthesis related to their respective mechanisms that immobilize lead through biomineralization and biosorption. The contributions of specific microbes, both single isolates or consortia, to actual or potential applications in environmental remediation are discussed. While many of the approaches are successful under carefully controlled laboratory conditions, field application requires optimization for a host of variables, including microbial competitiveness, soil physical and chemical parameters, metal concentrations, and co-contaminants. This review challenges the reader to consider bioremediation approaches that maximize microbial competitiveness, metabolism, and the associated molecular mechanisms for future engineering applications. Ultimately, we outline important research directions to bridge future scientific research activities with practical applications for bioremediation of lead and other toxic metals in environmental systems.

Treatment of acid rock drainage using a sulphate-reducing bioreactor with a limestone precolumn

Sulphate reducing bacteria (SRB) offer promise for the treatment of mine waste due to their effectiveness removing toxic heavy metals as highly insoluble metal sulphides and their ability to generate alkalinity. The main objective of this study was to develop a treatment composed of a sulphate-reducing bioreactor with a limestone precolumn for the removal of Cu(II) from a synthetic ARD. The purpose of the limestone column was to increase the pH values and decrease the level of Cu in the effluent to prevent SRB inhibition. The system was fed with a pH-2.7 synthetic ARD containing Cu(II) (10-40 mg/L), sulphate (2000 mg/L) and acetate (2.5 g COD/L) for 150 days. Copper removal efficiencies in the two-stage system were very high (95-99%), with a final concentration of 0.53 mg/L Cu, and almost complete removal occurred in the limestone precolumn. In the same manner, the acidity of the synthetic ARD was effectively reduced in the limestone precolumn to 7.3 and the pH was raised in the bioreactor (7.3-8.0). COD consumption by methanogens was predominant from day 0-118, but SRB dominated at the end of the experiment (day 150) when the average COD removal and sulphide production were 74.8% and 61.7%, respectively. Study of the microbial taxonomic composition in the bioreactor revealed that Methanosarcina and Methanosaeta were the most prevalent methanogens while the genera Desulfotomaculum and Syntrophobacter were the dominant SRB. Among the SRB identified Desulfotomaculum intricatum (99% identity) and Desulfotomaculum acetoxidans (96%) were the most abundant sequences of bacteria capable of using acetate.

Comparison of two acidophilic sulfidogenic consortia for the treatment of acidic mine water

Sulfate-reducing bioreactors are a biotechnological alternative for the treatment of acid mine drainage (AMD). In this study, two separate bioreactors with pH and temperature-controlled (Bio I and II) were operated with two different acidophilic microbial consortia to determine their efficiencies in sulfate removal from a synthetic acidic mine water. The bioreactors were operated for 302 days in continuous flow mode under the same parameters: fed with a sulfate solution of ∼30 mM with a pH of 2.5, the temperature at 30°C, stirred gently at 40 rpm and using a continuous stream of nitrogen to help remove the H2S produced in the bioreactor. The glycerol consumption, acetate production, and sulfate removal were monitored throughout the course of the experiment. The community composition and potential metabolic functional groups were analyzed via 16S rRNA partial gene sequencing. Bio I consortium reduced the sulfate, achieving a range of sulfate concentration from 4.7 to 19 mM in the effluent liquor. The removal of sulfate in Bio II was between 5.6 and 18 mM. Both bioreactors’ communities showed the presence of the genus Desulfosporosinus as the main sulfate-reducing bacteria (SRB). Despite differences in microbial composition, both bioreactors have similar potential metabolism, with a higher percentage of microorganisms that can use sulfate in respiration. Overall, both bioreactors showed similar performance in treating acidic mine water containing mostly sulfate using two different acidophilic sulfidogenic consortia obtained from different global locations.

Comparison of different small molecular weight alcohols for sustaining sulfidogenic bioreactors maintained at moderately low pH

Sulfate-reducing bacteria (SRB) catalyse the dissimilatory reduction of sulfate to hydrogen sulfide using a wide range of small molecular weight organic compounds, and hydrogen, as electron donors. Here we report the effects of different combinations of small molecular weight alcohols on the performance and bacterial composition of a moderately low pH sulfidogenic bioreactor (pH 4.0–5.5) operated at 35°C in continuous flow mode. Ethanol alone and methanol or ethanol used in combination with glycerol were evaluated based on their equivalent amounts of carbon. Although evidenced that methanol was utilised as electron donor to fuel sulfidogenesis at pH 5.5, rates of sulfate reduction/sulfide production were negatively impacted when this alcohol was first introduced to the system, though these rates increased in subsequent phases as a result of adaptation of the microbial community. Further increased dosage of methanol again caused rates of sulfidogenesis to decrease. Methanol addition resulted in perturbations of the bioreactor microbial community, and species not previously detected were present in relatively large abundance, including the sulfate-reducer Desulfovibrio desulfuricans. Ethanol utilization was evidenced by the increase in rates of sulfidogenesis as the dosage of ethanol increased, with rates being highest when the bioreactor was fed with ethanol alone. Concentrations of acetate in the effluent liquor also increased (up to 8 mM) as a result of incomplete oxidation of ethanol. This alcohol continued to be used as the electron donor for sulfate reduction when the bioreactor pH was decreased incrementally (to pH 4.0), but rates of sulfidogenesis decreased. The relative abundance of Dv. desulfuricans diminished as the bioreactor pH was lowered, while that of the acidophilic Firmicute Desulfosporosinus acididurans increased. This study has shown that all three alcohols can be used to fuel microbial sulfidogenesis in moderately acidic liquors, though the cost-effectiveness, availability and toxicity to the microbial community will dictate the choice of substrate.

Sulfate-reducing bioreactors subjected to high sulfate loading rate or acidity: variations in microbial consortia

Sulfate-reducing bioreactors are used in e.g. the mining industry to remove sulfate and harmful metals from process waters. These bioreactors are expected to be run for extended periods of time and may experience variations in the influent quality, such as increasing sulfate loading rate and decrease in pH, while being expected to function optimally. In this study we followed the sulfate removal rate and variation in microbial communities over a period of up to 333 days in three different up-flow anaerobic sludge blanket (UASB) bioreactors being submitted to increasing sulfate loading rate or decreasing pH. Sodium lactate was used as the sole carbon source and electron donor. All three bioreactors contained highly diverse microbial communities containing archaea, fungi and bacteria. Sulfurospirillum and Desulfovibrio were the most prominent bacterial genera detected in the bioreactors receiving the highest sulfate loading rates, and the greatest relative abundance of methanogenic archaea and the fungal genus Cadophora coincided with the highest sulfate reduction rates. In contrast, Sulfuricurvum was dominant in the bioreactor receiving influent with alternating pH, but its relative abundance receded in response to low pH of the influent. All bioreactors showed excellent sulfate removal even under extreme conditions in addition to unique responses in the microbial communities under changing operational conditions. This shows that a high diversity in the microbial consortia in the bioreactors could make the sulfate removal process less sensitive to changing operational conditions, such as variations in influent sulfate loading rate and pH.

Influents with high sulfate loading rate or low pH were successfully treated.

Microbial consortia showed versatility and adaption to changing operation.

Unknown interactions among microbial groups makes interpretation challenging.

Novel Microorganisms Contribute to Biosulfidogenesis in the Deep Layer of an Acidic Pit Lake

Cueva de la Mora is a permanently stratified acidic pit lake with extremely high concentrations of heavy metals at depth. In order to evaluate the potential for in situ sulfide production, we characterized the microbial community in the deep layer using metagenomics and metatranscriptomics. We retrieved 18 high quality metagenome-assembled genomes (MAGs) representing the most abundant populations. None of the MAGs were closely related to either cultured or non-cultured organisms from the Genome Taxonomy or NCBI databases (none with average nucleotide identity >95%). Despite oxygen concentrations that are consistently below detection in the deep layer, some archaeal and bacterial MAGs mapped transcripts of genes for sulfide oxidation coupled with oxygen reduction. Among these microaerophilic sulfide oxidizers, mixotrophic Thermoplasmatales archaea were the most numerous and represented 24% of the total community. Populations associated with the highest predicted in situ activity for sulfate reduction were affiliated with Actinobacteria, Chloroflexi, and Nitrospirae phyla, and together represented about 9% of the total community. These MAGs, in addition to a less abundant Proteobacteria MAG in the genus Desulfomonile, contained transcripts of genes in the Wood-Ljungdahl pathway. All MAGs had significant genetic potential for organic carbon oxidation. Our results indicate that novel acidophiles are contributing to biosulfidogenesis in the deep layer of Cueva de la Mora, and that in situ sulfide production is limited by organic carbon availability and sulfur oxidation.

Acetate Degradation at Low pH by the Moderately Acidophilic Sulfate Reducer Acididesulfobacillus acetoxydans gen. nov. sp. nov

In acid drainage environments, biosulfidogenesis by sulfate-reducing bacteria (SRB) attenuates the extreme conditions by enabling the precipitation of metals as their sulfides, and the neutralization of acidity through proton consumption. So far, only a handful of moderately acidophilic SRB species have been described, most of which are merely acidotolerant. Here, a novel species within a novel genus of moderately acidophilic SRB is described, Acididesulfobacillus acetoxydans gen. nov. sp. nov. strain INE, able to grow at pH 3.8. Bioreactor studies with strain INE at optimum (5.0) and low (3.9) pH for growth showed that strain INE alkalinized its environment, and that this was more pronounced at lower pH. These studies also showed the capacity of strain INE to completely oxidize organic acids to CO₂, which is uncommon among acidophilic SRB. Since organic acids are mainly in their protonated form at low pH, which increases their toxicity, their complete oxidation may be an acid stress resistance mechanism. Comparative proteogenomic and membrane lipid analysis further indicated that the presence of saturated ether-bound lipids in the membrane, and their relative increase at lower pH, was a protection mechanism against acid stress. Interestingly, other canonical acid stress resistance mechanisms, such as a Donnan potential and increased active charge transport, did not appear to be active.

Acetotrophic sulfate-reducing consortia develop active biofilms on zeolite and glass beads in batch cultures at initial pH 3

Sulfate-reducing microbial communities remain a suitable option for the remediation of acid mine drainage using several types of carrier materials and appropriate reactor configurations. However, acetate prevails as a product derived from the incomplete oxidation of most organic substrates by sulfate reducers, limiting the efficiency of the whole process. An established sulfate-reducing consortium, able to degrade acetate at initial acidic pH (3.0), was used to develop biofilms over granular activated carbon (GAC), glass beads, and zeolite as carrier materials. In batch assays using glycerol, biofilms successfully formed on zeolite, glass beads, and GAC with sulfide production rates of 0.32, 0.26, and 0.14 mmol H₂S/L·d, respectively, but only with glass beads and zeolite, acetate was degraded completely. The planktonic and biofilm communities were determined by the 16S rRNA gene analysis to evaluate the microbial selectivity of the carrier materials. In total, 46 OTUs (family level) composed the microbial communities. Ruminococcaceae and Clostridiaceae families were present in zeolite and glass beads, whereas Peptococcaceae was mostly enriched on zeolite and Desulfovibrionaceae on glass beads. The most abundant sulfate reducer in the biofilm of zeolite was Desulfotomaculum sp., while Desulfatirhabdium sp. abounded in the planktonic community. With glass beads, Desulfovibrio sp. dominated the biofilm and the planktonic communities. Our results indicate that both materials (glass beads and zeolite) selected different key sulfate-reducing microorganisms able to oxidize glycerol completely at initial acidic pH, which is relevant for a future application of the consortium in continuous bioreactors to treat acidic streams. KEY POINTS: • Complete consumption of glycerol and acetate at acidic pH by sulfate reduction. • Glass beads and zeolite are suitable materials to form sulfate-reducing biofilms. • Acetotrophic sulfate-reducing bacteria attached to zeolite preferably.

Sulfate removal rate and metal recovery as settling precipitates in bioreactors: Influence of electron donors

Four down-flow structured bed bioreactors were operated targeting biological sulfate-reduction and metal recovery. Three different electron donors were tested: glycerol (R1), lactate (R2), sucrose (R3), and a blend of the previous three (R4) with an increasing copper influent load (5, 15, and 30 mg Cu²⁺.L^-1). Copper inhibited sulfate-reduction in R1 (15 mg Cu²⁺.L^-1) and R3 (5 mg Cu²⁺.L^-1), but the fermentative activity was not affected. R2 and R4 were not inhibited by the copper influent concentration. R2 provided the highest sulfate reduction rate (1767.3 ± 240.1 mg SO₄^2-.L.day^-1). Nonetheless, the accumulation of settling precipitates was 22 % higher in R4 than in R2, indicating the former yielded the highest metal recovery as settling precipitates (24.8 g FSS.L^-1, 25 % Fe²⁺, 5% Cu²⁺). 16S rRNA sequencing showed highest diversity of sulfate-reducing bacteria in R2. A predominance of sulfate-reducing and fermentative bacteria with more similarity was observed between microbial populations in R1 and R4, despite the difference in toxicity thresholds. Hence, the electron donor influenced not only the biological sulfate reduction, but also metal toxicity thresholds and metal recovery as settling precipitates.

Recent advances in heavy metal recovery from wastewater by biogenic sulfide precipitation

Biological sulfide precipitation by sulfate reducing bacteria (SRB) is an emerging technique for the recovery of heavy metals from metal contaminated wastewater. Advantages of this technique include low capital cost, ability to form highly insoluble salts, and capability to remove and recover heavy metals even at very low concentrations. Therefore, sulfate reduction under anaerobic conditions has become a suitable alternative for the treatment of wastewaters that contain metals. However, bioreactor configurations for recovery of metals from sulfate rich metallic wastewater have not been explored widely. Moreover, the recovered metal sulfide nanoparticles could be applied in various fields such as solar cells, dye degradation, electroplating, etc. Hence, metal recovery in the form of nanoparticles from wastewater could serve as an incentive for industries. The simultaneous metal removal and recovery can be achieved in either a single-stage or multistage systems. This paper aims to present an overview of the different bioreactor configurations for the treatment of wastewater containing sulfate and metal along with their advantages and drawbacks for metal recovery. Currently followed biological strategies to mitigate sulfate and metal rich wastewater are evaluated in detail in this review.

Sulfur Reduction at Hyperthermoacidophilic Conditions with Mesophilic Anaerobic Sludge as the Inoculum

Sulfur reduction at hyperthermoacidophilic conditions represents a promising opportunity for metal sulfide precipitation from hot acidic metallurgical streams, avoiding costly cooling down. The suitability of mesophilic anaerobic sludges as the inoculum for sulfur-reducing bioreactors operated at high temperature and low pH was explored. We examined sludges from full-scale anaerobic reactors for sulfur-reducing activity at pH 2.0-3.5 and 70 or 80 °C, with H₂ as an electron donor. At pH 3.5 in batch experiments, sulfidogenesis started within 4 days, reaching up to 100-200 mg·L^-1 of dissolved sulfide produced after 19-24 days, depending on the origin of the sludge. Sulfidogenesis resumed after removing H₂S by flushing with nitrogen gas, indicating that sulfide was limiting the conversion. The best performing sludge was used to inoculate a 4 L gas-lift reactor fed with H₂ as the electron donor, CO₂ as the carbon source, and elemental sulfur as the electron acceptor. The reactor was operated in semibatch mode at a pH 3.5 and 80 °C, and stable sulfide production rates of 60-80 mg·L^-1·d^-1 were achieved for a period of 24 days, without formation of methane or acetate. Our results reveal the potential of mesophilic anaerobic sludges as seed material for sulfur-reducing bioprocesses operated at hyperthermoacidophilic conditions. The process needs further optimization of the volumetric sulfide production rate to gain relevance for practice.

Evaluating the Effect of pH, Temperature, and Hydraulic Retention Time on Biological Sulphate Reduction Using Response Surface Methodology

Biological sulphate reduction (BSR) has been identified as a promising alternative for treating acid mine drainage. In this study, the effect of pH, temperature, and hydraulic retention time (HRT) on BSR was investigated. The Box–Behnken design was used to matrix independent variables, namely pH (4–6), temperature (10–30 °C), and HRT (2–7 days) with the sulphate reduction efficiency and sulphate reduction rate as response variables. Experiments were conducted in packed bed reactors operating in a downflow mode. Response surface methodology was used to statistically analyse the data and to develop statistical models that can be used to fully understand the individual effects and the interactions between the independent variables. The analysis of variance results showed that the data fitted the quadratic models well as confirmed by a non-significant lack of fit. The temperature and HRT effect were significant (p < 0.0001), and these two variables had a strong interaction. However, the influence of pH was insignificant (p > 0.05).

Characteristics of an Iron-Reducing, Moderately Acidophilic Actinobacterium Isolated from Pyritic Mine Waste, and Its Potential Role in Mitigating Mineral Dissolution in Mineral Tailings Deposits

Reactive pyritic mine tailings can be populated by chemolithotrophic prokaryotes that enhance the solubilities of many metals, though iron-reducing heterotrophic microorganisms can inhibit the environmental risk posed by tailings by promoting processes that are the reverse of those carried out by pyrite-oxidising autotrophic bacteria. A strain (IT2) of Curtobacterium ammoniigenes, a bacterium not previously identified as being associated with acidic mine wastes, was isolated from pyritic mine tailings and partially characterized. Strain IT2 was able to reduce ferric iron under anaerobic conditions, but was not found to catalyse the oxidation of ferrous iron or elemental (zero-valent) sulfur, and was an obligate heterotrophic. It metabolized monosaccharides and required small amounts of yeast extract for growth. Isolate IT2 is a mesophilic bacterium, with a temperature growth optimum of 30 °C and is moderately acidophilic, growing optimally at pH 4.0 and between pH 2.7 and 5.0. The isolate tolerated elevated concentrations of many transition metals, and was able to grow in the cell-free spent medium of the acidophilic autotroph Acidithiobacillus ferrooxidans, supporting the hypothesis that it can proliferate in acidic mine tailings. Its potential role in mitigating the production of acidic, metal-rich drainage waters from mine wastes is discussed.

Enrichment and isolation of acid-tolerant sulfate-reducing microorganisms in the anoxic, acidic hot spring sediments from Copahue volcano, Argentina

The geothermal Copahue-Caviahue (GCC) system (Argentina) is an extreme acidic environment, dominated by the activity of Copahue volcano. Environments characterised by low pH values, such as volcanic areas, are of particular interest for the search of acidophilic microorganisms with application in biotechnological processes. In this work, sulfate-reducing microorganisms were investigated in geothermal acidic, anaerobic zones from GCC system. Sediment samples from Agua del Limón (AL1), Las Máquinas (LMa2), Las Maquinitas (LMi) and Baño 9 (B9-2, B9-3) were found to be acidic (pH values 2.1-3.0) to moderate acidic (5.1-5.2), containing small total organic carbon values, and ferric iron precipitates. The organic electron donor added to the enrichment was completely oxidised to CO2. Bacteria related to 'Desulfobacillus acidavidus' strain CL4 were found to be dominant (67-83% of the total number of clones) in the enrichment cultures, and their presence was confirmed by their isolation on overlay plates. Other bacteria were also detected with lower abundance (6-20% of the total number of clones), with representatives of the genera Acidithiobacillus, Sulfobacillus, Alicyclobacillus and Athalassotoga/Mesoaciditoga. These enrichment and isolates found at low pH confirm the presence of anaerobic activities in the acidic sediments from the geothermal Copahue-Caviahue system.

Characterization and application of an anaerobic, iron and sulfate reducing bacterial culture in enhanced bioremediation of acid mine drainage impacted soil

Development of an appropriate bioremediation strategy for acid mine drainage (AMD) impacted environment is imperative for sustainable mining but remained critically challenged due to the paucity of knowledge on desired microbiological factors and their nutrient requirements. The present study was conducted to utilize the potential of an anaerobic, acid-tolerant, Fe³⁺ and SO₄^2- reducing microbial consortium for in situ remediation of highly acidic (pH 3.21), SO₄^2- rich (6285 mg/L) mine drainage impacted soil (AIS). A microbial consortium enriched from AMD system and composed of Clostridiales and Bacillales members was characterized and tested for in situ application through microcosms. A combination of bioaugmentation (enriched consortium) and biostimulation (cellulose) allowed 97% reduction in dissolved sulfate and rise in pH up to 7.5. 16S rRNA gene-based amplicon sequencing confirmed that although the bioaugmented community could survive in AIS, availability of carbon source was necessary for superior iron- and sulfate- reduction. Quantitative PCR of dsrB gene confirmed the role of carbon source in boosting the SO₄^2- reduction activities of sulfate reducers. This study demonstrated that native AIS harbored limited catabolic activities required for the remediation but addition of catabolically active microbial populations along with necessary carbon and energy source facilitate the bioremediation of AIS.

Dissimilatory reduction of sulfate and zero-valent sulfur at low pH and its significance for bioremediation and metal recovery

Redox transformations of sulfur, involving dissimilatory and assimilatory oxidation and reduction reactions, occurs in water bodies and terrestrial environments worldwide, leading to dynamic cycling of this element throughout the biosphere. In cases where zero-valent (elemental) sulfur, sulfate and other oxidized forms are used as electron acceptor in (primarily) anaerobic microbial metabolisms, the end product is hydrogen sulfide (HS^- or H₂S, dependent on pH). While neutrophilic and alkalophilic sulfidogenic prokaryotes have been known for many decades, acid-tolerant and acidophilic strains and species have been isolated and characterized only in the past twenty or so years, even though evidence for sulfide generation on these environments was previously well documented. This review outlines the background and current status of the biodiversity and metabolisms of sulfate- and sulfur-reducing prokaryotes that are metabolically active in low pH environments, and describes the developing technologies in which they are being used to remediate acidic waste waters (which are often metal-contaminated) and to recover metal resources.

Chitin as a substrate for the biostimulation of sulfate-reducing bacteria in the treatment of mine-impacted water (MIW)

This study aims to know the basis of sulfate-reducing bacteria (SRB) and chitin source relationship for the development of a biotreatment system for mine-impacted water (MIW). The MIW consists of river water impacted by coal acid mine drainage (AMD), an extremely acid effluent, rich in sulfate and dissolved metal ions, with a high pollutant potential. Chitin was used as metal ion sorbent and biostimulant of SRB, whose anaerobic dissimilatory metabolism reduces sulfate to sulfide. Microcosms were built in an oxygen-free atmosphere using chitin from two different sources: commercial chitin and shrimp shell waste, which contains calcium carbonate, an acidity removal agent, in addition to chitin. The results indicate that the shrimp shell performs best in removing sulfate (99.75%), iron (99.04%), aluminum (98.47%), and manganese (100%) ions. The iron ion sorption kinetics of the sediments were also studied; pseudo-second order behavior was observed. High-throughput sequencing analysis revealed the present bacterial community and its abundance in the microcosms after 11 and 30 treatment days: SRB were detected but were not the majority. Thus, this research aims to contribute to the sustainable treatment MIW through the employment of an abundant and low-cost biomaterial.

Upconversion of Cellulosic Waste Into a Potential "Drop in Fuel" via Novel Catalyst Generated Using Desulfovibrio desulfuricans and a Consortium of Acidophilic Sulfidogens

Biogas-energy is marginally profitable against the "parasitic" energy demands of processing biomass. Biogas involves microbial fermentation of feedstock hydrolyzate generated enzymatically or thermochemically. The latter also produces 5-hydroxymethyl furfural (5-HMF) which can be catalytically upgraded to 2, 5-dimethyl furan (DMF), a "drop in fuel." An integrated process is proposed with side-stream upgrading into DMF to mitigate the "parasitic" energy demand. 5-HMF was upgraded using bacterially-supported Pd/Ru catalysts. Purpose-growth of bacteria adds additional process costs; Pd/Ru catalysts biofabricated using the sulfate-reducing bacterium (SRB) Desulfovibrio desulfuricans were compared to those generated from a waste consortium of acidophilic sulfidogens (CAS). Methyl tetrahydrofuran (MTHF) was used as the extraction-reaction solvent to compare the use of bio-metallic Pd/Ru catalysts to upgrade 5-HMF to DMF from starch and cellulose hydrolyzates. MTHF extracted up to 65% of the 5-HMF, delivering solutions, respectively, containing 8.8 and 2.2 g 5-HMF/L MTHF. Commercial 5% (wt/wt) Ru-carbon catalyst upgraded 5-HMF from pure solution but it was ineffective against the hydrolyzates. Both types of bacterial catalyst (5wt%Pd/3-5wt% Ru) achieved this, bio-Pd/Ru on the CAS delivering the highest conversion yields. The yield of 5-HMF from starch-cellulose thermal treatment to 2,5 DMF was 224 and 127 g DMF/kg extracted 5-HMF, respectively, for CAS and D. desulfuricans catalysts, which would provide additional energy of 2.1 and 1.2 kWh/kg extracted 5-HMF. The CAS comprised a mixed population with three patterns of metallic nanoparticle (NP) deposition. Types I and II showed cell surface-localization of the Pd/Ru while type III localized NPs throughout the cell surface and cytoplasm. No metallic patterning in the NPs was shown via elemental mapping using energy dispersive X-ray microanalysis but co-localization with sulfur was observed. Analysis of the cell surfaces of the bulk populations by X-ray photoelectron spectroscopy confirmed the higher S content of the CAS bacteria as compared to D. desulfuricans and also the presence of Pd-S as well as Ru-S compounds and hence a mixed deposit of PdS, Pd(0), and Ru in the form of various +3, +4, and +6 oxidation states. The results are discussed in the context of recently-reported controlled palladium sulfide ensembles for an improved hydrogenation catalyst.

Recent advances in dissimilatory sulfate reduction: From metabolic study to application

Sulfate-reducing bacteria (SRB) are a group of diverse anaerobic microorganisms omnipresent in natural habitats and engineered environments that use sulfur compounds as the electron acceptor for energy metabolism. Dissimilatory sulfate reduction (DSR)-based techniques mediated by SRB have been utilized in many sulfate-containing wastewater treatment systems worldwide, particularly for acid mine drainage, groundwater, sewage and industrial wastewater remediation. However, DSR processes are often operated suboptimally and disturbances are common in practical application. To improve the efficiency and robustness of SRB-based processes, it is necessary to study SRB metabolism and operational conditions. In this review, the mechanisms of DSR processes are reviewed and discussed focusing on intracellular and extracellular electron transfer with different electron donors (hydrogen, organics, methane and electrodes). Based on the understanding of the metabolism of SRB, responses of SRB to environmental stress (pH-, temperature-, and salinity-related stress) are summarized at the species and community levels. Application in these stressed conditions is discussed and future research is proposed. The feasibility of recovering energy and resources such as biohydrogen, hydrocarbons, polyhydroxyalkanoates, magnetite and metal sulfides through the use of SRB were investigated but some long-standing questions remain unanswered. Linking the existing scientific understanding and observations to practical application is the challenge as always for promotion of SRB-based techniques.

Bioreductive Dissolution as a Pretreatment for Recalcitrant Rare-Earth Phosphate Minerals Associated with Lateritic Ores

Recent research has demonstrated the applicability of a biotechnological approach for extracting base metals using acidophilic bacteria that catalyze the reductive dissolution of ferric iron oxides from oxidized ores, using elemental sulfur as an electron donor. In Brazil, lateritic deposits are frequently associated with phosphate minerals such as monazite, which is one of the most abundant rare-earth phosphate minerals. Given the fact that monazite is highly refractory, rare earth elements (REE) extraction is very difficult to achieve and conventionally involves digesting with concentrated sodium hydroxide and/or sulfuric acid at high temperatures; therefore, it has not been considered as a potential resource. This study aimed to determine the effect of the bioreductive dissolution of ferric iron minerals associated with monazite using Acidithiobacillus (A.) species in pH- and temperature-controlled stirred reactors. Under aerobic conditions, using A. thiooxidans at extremely low pH greatly enhanced the solubilization of iron from ferric iron minerals, as well that of phosphate (about 35%), which can be used as an indicator of the dissolution of monazite. The results from this study have demonstrated the potential of using bioreductive mineral dissolution, which can be applied as pretreatment to remove coverings of ferric iron minerals in a process analogous to the bio-oxidation of refractory golds and expand the range of minerals that could be processed using this approach.

Co-culture of a Novel Fermentative Bacterium, Lucifera butyrica gen. nov. sp. nov., With the Sulfur Reducer Desulfurella amilsii for Enhanced Sulfidogenesis

Biosulfidogenesis can be used to remediate low pH and high metal content waters such as acid mine drainage and recover the present metals. The selection of a cheap electron donor for the process is important for the economic viability. In this work we isolated a novel versatile acidotolerant fermentative bacterium (strain ALE^T) that is able to use a great variety of substrates including glycerol. Strain ALE^T is an obligate anaerobe, and cells are motile, rod-shaped, spore-forming, and stain Gram-positive. Growth occurred in a pH range from 3.5 to 7 (optimum 5.5), and temperature range from 25 to 40°C (optimum 37°C). It grows by fermentation of sugars, organic acids and glycerol. It has the ability to use thiosulfate, iron and DMSO as electron acceptors. Its genome is 4.7 Mb with 5122 protein-coding sequences, and a G+C content of 46.9 mol%. Based on 16S rRNA gene sequence analysis, the closest cultured species is Propionispora hippei (91.4% 16S rRNA gene identity) from the Sporomusaceae family (Selenomonadales order, Negativicutes class, Firmicutes phylum). Based on the distinctive physiological and phylogenetic characteristics of strain ALE^T, a new genus and species Lucifera butyrica gen. nov., sp. nov., is proposed. The type strain is ALE^T (=JCM 19373^T = DSM 27520^T). Strain ALE^T is an incomplete oxidizer and acetate, among other products, accumulates during glycerol conversion. Strain ALE^T was used to extend the substrate range for sulfur reduction by constructing co-cultures with the acetate oxidizer and sulfur reducer Desulfurella amilsii. The co-culture was tested with glycerol as substrate in batch and chemostat experiments. Acetate formed by fermentation of glycerol by strain ALE^T resulted in sulfur reduction by D. amilsii. The co-culture strategy offers good perspectives to use a wide range of cost-efficient substrates, including glycerol, to produce sulfide by specialized sulfur reducers. The recovery of heavy metals from metalliferous streams may become economically feasible by this approach.

Note: The locus tag for the genes encoded in Lucifera butyrica is LUCI_^∗. To avoid repetition of the prefix along the text, the locus tags are represented by the specific identifier.

How to tackle the stringent sulfate removal requirements in mine water treatment-A review of potential methods

Sulfate (SO₄^2-) is a ubiquitous anion in natural waters. It is not considered toxic, but it may be detrimental to freshwater species at elevated concentrations. Mining activities are one significant source of anthropogenic sulfate into natural waters, mainly due to the exposure of sulfide mineral ores to weathering. There are several strategies for mitigating sulfate release, starting from preventing sulfate formation in the first place and ending at several end-of-pipe treatment options. Currently, the most widely used sulfate-removal process is precipitation as gypsum (CaSO₄·2H₂O). However, the lowest reachable concentration is theoretically 1500 mg L^-1 SO₄^2- due to gypsum's solubility. At the same time, several mines worldwide have significantly more stringent sulfate discharge limits. The purpose of this review is to examine the process options to reach low sulfate levels (< 1500 mg L^-1) in mine effluents. Examples of such processes include alternative chemical precipitation methods, membrane technology, biological treatment, ion exchange, and adsorption. In addition, aqueous chemistry and current effluent standards concerning sulfate together with concentrate treatment and sulfur recovery are discussed.

Design and Application of a Low pH Upflow Biofilm Sulfidogenic Bioreactor for Recovering Transition Metals From Synthetic Waste Water at a Brazilian Copper Mine

A sulfidogenic bioreactor, operated at low pH (4–5), was set up and used to remove transition metals (copper, nickel, cobalt, and zinc) from a synthetic mine water, based on the chemistry of a moderately acidic (pH 5) drainage stream at an operating copper mine in Brazil. The module was constructed as an upflow biofilm reactor, with microorganisms immobilized on porous glass beads, and was operated continuously for 462 days, during which time the 2 L bioreactor processed >2,000 L of synthetic mine water. The initial treatment involved removing copper (the most abundant metal present) off-line in a stream of H₂S-containing gas generated by the bioreactor, which caused the synthetic mine water pH to fall to 2.1. The copper-free water was then amended with glycerol (the principal electron donor), yeast extract and basal salts, and pumped directly into the bioreactor where the other three transition metals were precipitated (also as sulfides), concurrent with increased solution pH. Although some acetate was generated, most of the glycerol fed to the bioreactor was oxidized to carbon dioxide, and was coupled to the reduction of sulfate to hydrogen sulfide. No archaea or eukaryotes were detected in the bioreactor microbial community, which was dominated by acidophilic sulfate-reducing Firmicutes (Peptococcaceae strain CEB3 and Desulfosporosinus acididurans); facultatively anaerobic non-sulfidogens (Acidithiobacillus ferrooxidans and Actinobacterium strain AR3) were also found in small relative abundance. This work demonstrated how a single low pH sulfidogenic bioreactor can be used to remediate a metal-rich mine water, and to facilitate the recovery (and therefore recycling) of target metals. The system was robust, and was operated empirically by means of pH control. Comparison of costs of the main consumables (glycerol and yeast extract) and the values of the metals recovered showed a major excess of the latter, supporting the view that sulfidogenic biotechnology can have significant economic as well as environmental advantages over current approaches used to remediate mine waters which produce secondary toxic wastes and fail to recover valuable metals.

The Evolution, Current Status, and Future Prospects of Using Biotechnologies in the Mineral Extraction and Metal Recovery Sectors

The current global demand in terms of both the amounts and range of metals for industrial and domestic use greatly exceeds that at any previous time in human history. Recycling is inadequate to meet these needs and therefore mining primary metal ores will continue to be a major industry in the foreseeable future. The question of how metal mining can develop in a manner which is less demanding of energy and less damaging of the environment in a world whose population is increasingly aware of, and concerned about, the environment, requires urgent redress. Increased application of biotechnologies in the mining sector could go some way in solving this conundrum, yet, biomining (harnessing microorganisms to enhance the recovery of base and precious metals) has remained a niche application since it was first knowingly used in the 1960s. This manuscript reviews the development and current status of biomining applications and highlights their limitations as well as their strengths. New areas of biotechnology that could be applied in the mining sector, and their potential impact in terms of both their potential environmental and economic benefits, are also discussed.

Recent Developments for Remediating Acidic Mine Waters Using Sulfidogenic Bacteria

Acidic mine drainage (AMD) is regarded as a pollutant and considered as potential source of valuable metals. With diminishing metal resources and ever-increasing demand on industry, recovering AMD metals is a sustainable initiative, despite facing major challenges. AMD refers to effluents draining from abandoned mines and mine wastes usually highly acidic that contain a variety of dissolved metals (Fe, Mn, Cu, Ni, and Zn) in much greater concentration than what is found in natural water bodies. There are numerous remediation treatments including chemical (lime treatment) or biological methods (aerobic wetlands and compost bioreactors) used for metal precipitation and removal from AMD. However, controlled biomineralization and selective recovering of metals using sulfidogenic bacteria are advantageous, reducing costs and environmental risks of sludge disposal. The increased understanding of the microbiology of acid-tolerant sulfidogenic bacteria will lead to the development of novel approaches to AMD treatment. We present and discuss several important recent approaches using low sulfidogenic bioreactors to both remediate and selectively recover metal sulfides from AMD. This work also highlights the efficiency and drawbacks of these types of treatments for metal recovery and points to future research for enhancing the use of novel acidophilic and acid-tolerant sulfidogenic microorganisms in AMD treatment.

Metallic bionanocatalysts: potential applications as green catalysts and energy materials

Microbially generated or supported nanocatalysts have potential applications in green chemistry and environmental application. However, precious (and base) metals biorefined from wastes may be useful for making cheap, low-grade catalysts for clean energy production. The concept of bionanomaterials for energy applications is reviewed with respect to potential fuel cell applications, bio-catalytic upgrading of oils and manufacturing 'drop-in fuel' precursors. Cheap, effective biomaterials would facilitate progress towards dual development goals of sustainable consumption and production patterns and help to ensure access to affordable, reliable, sustainable and modern energy.

Mechanisms and effectivity of sulfate reducing bioreactors using a chitinous substrate in treating mining influenced water

Mining-influenced water (MIW) is one of the main environmental challenges associated with the mining industry. Passive MIW remediation can be achieved through microbial activity in sulfate-reducing bioreactors (SRBRs), but their actual removal rates depend on different factors, one of which is the substrate composition. Chitinous materials have demonstrated high metal removal rates, particularly for the two recalcitrant MIW contaminants Zn and Mn, but their removal mechanisms need further study. We studied Cd, Fe, Zn, and Mn removal in bioactive and abiotic SRBRs to elucidate the metal removal mechanisms and the differences in metal and sulfate removal rates using a chitinous material as substrate. We found that sulfate-reducing bacteria are effective in increasing metal and sulfate removal rates and the duration of operation in SRBRs, and that the main mechanism involved was metal precipitation as sulfides. The solid residues provided evidence of the presence of sulfides in the bioactive column, more specifically ZnS, according to XPS analysis. The feasibility of passive treatments with a chitinous substrate could be an important option for MIW remediation.

Microbial Diversity and Community Assembly across Environmental Gradients in Acid Mine Drainage

Microorganisms play an important role in weathering sulfide minerals worldwide and thrive in metal-rich and extremely acidic environments in acid mine drainage (AMD). Advanced molecular methods provide in-depth information on the microbial diversity and community dynamics in the AMD-generating environment. Although the diversity is relatively low and in general inversely correlated with the acidity, a considerable number of microbial species have been detected and described in AMD ecosystems. The acidophilic microbial communities dominated by iron/sulfur-oxidizing microbes vary widely in their composition and structure across diverse environmental gradients. Environmental conditions affect the microbial community assembly via direct and indirect interactions with microbes, resulting in an environmentally dependent biogeographic pattern. This article summarizes the latest studies to provide a better understanding of the microbial biodiversity and community assembly in AMD environments.

Indirect Redox Transformations of Iron, Copper, and Chromium Catalyzed by Extremely Acidophilic Bacteria

Experiments were carried out to examine redox transformations of copper and chromium by acidophilic bacteria (Acidithiobacillus, Leptospirillum, and Acidiphilium), and also of iron (III) reduction by Acidithiobacillus spp. under aerobic conditions. Reduction of iron (III) was found with all five species of Acidithiobacillus tested, grown aerobically on elemental sulfur. Cultures maintained at pH 1.0 for protracted periods displayed increasing propensity for aerobic iron (III) reduction, which was observed with cell-free culture liquors as well as those containing bacteria. At. caldus grown on hydrogen also reduced iron (III) under aerobic conditions, confirming that the unknown metabolite(s) responsible for iron (III) reduction were not (exclusively) sulfur intermediates. Reduction of copper (II) by aerobic cultures of sulfur-grown Acidithiobacillus spp. showed similar trends to iron (III) reduction in being more pronounced as culture pH declined, and occurring in both the presence and absence of cells. Cultures of Acidithiobacillus grown anaerobically on hydrogen only reduced copper (II) when iron (III) (which was also reduced) was also included; identical results were found with Acidiphilium cryptum grown micro-aerobically on glucose. Harvested biomass of hydrogen-grown At. ferridurans oxidized iron (II) but not copper (I), and copper (I) was only oxidized by growing cultures of Acidithiobacillus spp. when iron (II) was also included. The data confirmed that oxidation and reduction of copper were both mediated by acidophilic bacteria indirectly, via iron (II) and iron (III). No oxidation of chromium (III) by acidophilic bacteria was observed even when, in the case of Leptospirillum spp., the redox potential of oxidized cultures exceeded +900 mV. Cultures of At. ferridurans and A. cryptum reduced chromium (VI), though only when iron (III) was also present, confirming an indirect mechanism and contradicting an earlier report of direct chromium reduction by A. cryptum. Measurements of redox potentials of iron, copper and chromium couples in acidic, sulfate-containing liquors showed that these differed from situations where metals are not complexed by inorganic ligands, and supported the current observations of indirect copper oxido-reduction and chromium reduction mediated by acidophilic bacteria. The implications of these results for both industrial applications of acidophiles and for exobiology are discussed.

Long-term effects of increasing acidity on low-pH sulfate-reducing bioprocess and bacterial community

An ethanol-fed, sulfate-reducing anaerobic baffled reactor was operated over a period of 260 days to assess the effects of sequentially more acidic conditions (pH 4.5-2.5) on sulfate reduction and bacterial community. Results showed that the reactor could reduce sulfate and generate alkalinity at progressively lower pH values of 4.5, 3.5, and 2.5 in a synthetic wastewater containing 2500 mg/L sulfate. About 93.9% of the influent sulfate was removed at a rate of 4691 mg/L/day, and the effluent pH was increased to 6.8 even when challenged with influent pH as low as 2.5. Illumina MiSeq sequencing revealed that a step decrease in influent pH from 4.5 to 2.5 resulted in noticeable decrease in the biodiversity inside the sulfidogenic reactor. Additionally, complete and incomplete organic oxidizers Desulfobacter and Desulfovibrio were observed to be the most dominant sulfate reducers at pH 2.5, sustaining the low-pH, high-rate sulfate removal and alkalinity generation.

Biosynthesis of zinc sulfide quantum dots using waste off-gas from a metal bioremediation process

Waste H₂S biogas from a mine-water remediation bioprocess is used to make zinc sulfide quantum dots which are identical to ZnS QDs made by chemical methods.

Electricity generation from an inorganic sulfur compound containing mining wastewater by acidophilic microorganisms

Sulfide mineral processing often produces large quantities of wastewaters containing acid-generating inorganic sulfur compounds. If released untreated, these wastewaters can cause catastrophic environmental damage. In this study, microbial fuel cells were inoculated with acidophilic microorganisms to investigate whether inorganic sulfur compound oxidation can generate an electrical current. Cyclic voltammetry suggested that acidophilic microorganisms mediated electron transfer to the anode, and that electricity generation was catalyzed by microorganisms. A cation exchange membrane microbial fuel cell, fed with artificial wastewater containing tetrathionate as electron donor, reached a maximum whole cell voltage of 72 ± 9 mV. Stepwise replacement of the artificial anolyte with real mining process wastewater had no adverse effect on bioelectrochemical performance and generated a maximum voltage of 105 ± 42 mV. 16S rRNA gene sequencing of the microbial consortia resulted in sequences that aligned within the genera Thermoplasma, Ferroplasma, Leptospirillum, Sulfobacillus and Acidithiobacillus. This study opens up possibilities to bioremediate mining wastewater using microbial fuel cell technology.

Solid and liquid media for isolating and cultivating acidophilic and acid-tolerant sulfate-reducing bacteria

Growth media have been developed to facilitate the enrichment and isolation of acidophilic and acid-tolerant sulfate-reducing bacteria (aSRB) from environmental and industrial samples, and to allow their cultivation in vitro The main features of the 'standard' solid and liquid devised media are as follows: (i) use of glycerol rather than an aliphatic acid as electron donor; (ii) inclusion of stoichiometric concentrations of zinc ions to both buffer pH and to convert potentially harmful hydrogen sulphide produced by the aSRB to insoluble zinc sulphide; (iii) inclusion of Acidocella aromatica (an heterotrophic acidophile that does not metabolize glycerol or yeast extract) in the gel underlayer of double layered (overlay) solid media, to remove acetic acid produced by aSRB that incompletely oxidize glycerol and also aliphatic acids (mostly pyruvic) released by acid hydrolysis of the gelling agent used (agarose). Colonies of aSRB are readily distinguished from those of other anaerobes due to their deposition and accumulation of metal sulphide precipitates. Data presented illustrate the effectiveness of the overlay solid media described for isolating aSRB from acidic anaerobic sediments and low pH sulfidogenic bioreactors.

Temporal evolution of bacterial communities associated with the in situ wetland-based remediation of a marine shore porphyry copper tailings deposit

Mine tailings are a serious threat to the environment and public health. Remediation of these residues can be carried out effectively by the activation of specific microbial processes. This article presents detailed information about temporal changes in bacterial community composition during the remediation of a section of porphyry copper tailings deposited on the Bahía de Ite shoreline (Peru). An experimental remediation cell was flooded and transformed into a wetland in order to prevent oxidation processes, immobilizing metals. Initially, the top oxidation zone of the tailings deposit displayed a low pH (3.1) and high concentrations of metals, sulfate, and chloride, in a sandy grain size geological matrix. This habitat was dominated by sulfur- and iron-oxidizing bacteria, such as Leptospirillum spp., Acidithiobacillus spp., and Sulfobacillus spp., in a microbial community which structure resembled acid mine drainage environments. After wetland implementation, the cell was water-saturated, the acidity was consumed and metals dropped to a fraction of their initial respective concentrations. Bacterial communities analyzed by massive sequencing showed time-dependent changes both in composition and cell numbers. The final remediation stage was characterized by the highest bacterial diversity and evenness. Aside from classical sulfate reducers from the phyla δ-Proteobacteria and Firmicutes, community structure comprised taxa derived from very diverse habitats. The community was also characterized by an elevated proportion of rare phyla and unaffiliated sequences. Numerical ecology analysis confirmed that the temporal population evolution was driven by pH, redox, and K. Results of this study demonstrated the usefulness of a detailed follow-up of the remediation process, not only for the elucidation of the communities gradually switching from autotrophic, oxidizing to heterotrophic and reducing living conditions, but also for the long term management of the remediation wetlands.

Sulfur Reduction in Acid Rock Drainage Environments

Microbiological suitability of acidophilic sulfur reduction for metal recovery was explored by enriching sulfur reducers from acidic sediments at low pH (from 2 to 5) with hydrogen, glycerol, methanol and acetate as electron donors at 30 °C. The highest levels of sulfide in the enrichments were detected at pH 3 with hydrogen and pH 4 with acetate. Cloning and sequencing of the 16S rRNA gene showed dominance of the deltaproteobacterial sulfur-reducing genus Desulfurella in all the enrichments and subsequently an acidophilic strain (TR1) was isolated. Strain TR1 grew at a broad range of pH (3-7) and temperature (20-50 °C) and showed good metal tolerance (Pb(2+), Zn(2+), Cu(2+), Ni(2+)), especially for Ni(2+) and Pb(2+), with maximal tolerated concentrations of 0.09 and 0.03 mM, respectively. Different sources of sulfur were tested in the enrichments, from which biosulfur showed fastest growth (doubling time of 1.9 days), followed by colloidal, chemical and sublimated sulfur (doubling times of 2.2, 2.5, and 3.6 days, respectively). Strain TR1's physiological traits make it a good candidate to cope with low pH and high metal concentration in biotechnological processes for treatment of metal-laden acidic streams at low and moderately high temperature.