Distinct Roles of Acidophiles in Complete Oxidation of High-Sulfur Ferric Leach Product of Zinc Sulfide Concentrate

A Case of Predominance of Alicyclobacillus tolerans in Microbial Community during Bioleaching of Pentlandite-Chalcopyrite Concentrate

Bacterial isolates assigned to the species Alicyclobacillus tolerans, which occupies an intermediate position between an organotrophic genus Alicyclobacillus and mixotrophic genus Sulfobacillus, were revealed as members of the acidophilic chemolithotrophic community during stirred-tank bioleaching of violarite–pentlandite–chalcopyrite concentrate at 40 °C. Surprisingly, this species succeeded more common iron-oxidizing community members after a series of bioleaching processes in bioreactors. The possibility of mixotrophic and organoheterotrophic growth of Al. tolerans, tolerance to low pH values (1.0–1.15), as well as preservation of cells via sporulation under unfavorable conditions, may explain its key role in the bioleaching of the copper–nickel bulk concentrate. Isolation of two other sulfur-oxidizing pure cultures dominating the microbial community, together with their phylogenetic characterization, allowed the assignment of these isolates to the species Acidithiobacillus caldus. This and other studies of acidophilic microbial communities are important for the development and intensification of the bioleaching processes, including a biobeneficiation approach previously proposed by us.

Effect of diurnal temperature range on bioleaching of sulfide ore by an artificial microbial consortium

Temperature is considered to be one of the main factors affecting bioleaching, but few studies have assessed the effects of diurnal temperature range (DTR) on the bioleaching process. This study investigates the effects of different bioleaching temperatures (30 and 40 °C) and DTR on the bioleaching of metal sulfide ores by microbial communities. The results showed that DTR had an obvious inhibitory effect on the bioleaching efficiency of the artificial microbial community, although this effect was mainly concentrated in the early and middle stages (0-18 days) of exposure, gradually decreasing until almost disappearing in the late stage (18-24 days). Extracellular polymeric substance (EPS) analysis showed that DTR did not change the composition of the EPS matrix (humic acid-like substances, polysaccharides and protein-like substances), but had a significant effect on the generative behavior of EPS, inhibiting the secretion of EPS during the early and middle stages of the bioleaching process. However, the continual increase in EPS secretion in the bioleaching system gradually reduced the adverse effects of DTR on mineral dissolution. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Scanning electron microscopy- energy dispersive spectrometry (SEM-EDS) analysis of the bioleached residue showed that DTR had no obvious effect on the mineralogical characteristics of sulfide ore. Therefore, in industrial sulfide ore bioleaching applications, in order to accelerate the artificial microbial community start-up process, temperature control measures should be increased in the bioleaching process to reduce the adverse effects of DTR on mineral dissolution.

Effect of Temperature on Biobeneficiation of Bulk Copper-Nickel Concentrate with Thermoacidophilic Microbial Communities

Bioleaching of the bulk copper–nickel sulfide concentrate was proposed as a method to remove nickel from it and to obtain a concentrate containing copper as chalcopyrite. This approach is based on the different refractoriness of sulfide minerals in ferric sulfate solutions and oxidation by acidophilic microorganisms. The bulk concentrate contained 10.8% copper in the form of chalcopyrite (CuFeS2) and 7.2% nickel that occurred in pentlandite ((Ni,Fe)9S8) and violarite (FeNi2S4). Three microbial communities grown at 35, 40, and 50 °C were used for bioleaching. The microbial community at 40 °C was the most diverse in the genus and species composition. At all temperatures of the process, the key roles in bioleaching belonged to mixotrophic and heterotrophic acidophiles. The highest levels of nickel leaching of 97.2 and 96.3% were observed in the case of communities growing at 40 and 50 °C, respectively. At the same time, the bioleach residue, which could be characterized as a marketable high-grade copper (chalcopyrite) concentrate, was obtained only at 40 °C. This solid contained 15.6% copper and 0.54% nickel. Thus, the biobeneficiation of bulk sulfide concentrates can be a promising field of biohydrometallurgy.

Sequential Bioleaching of Pyritic Tailings and Ferric Leaching of Nonferrous Slags as a Method for Metal Recovery from Mining and Metallurgical Wastes

In this work, we proposed a method for biohydrometallurgical processing of mining (old pyritic flotation tailings) and metallurgical (slag) wastes to recover gold and other nonferrous metals. Since this processing allows the removal of toxic metals or at least decreases their content in the solids, this approach may reduce the negative environmental impacts of such waste. The proposed process was based on pyritic tailings’ bioleaching to recover metals and produce leach liquor containing a strong oxidizing agent (ferric sulfate) to dissolve nonferrous metal from slag. This approach also allows us to increase concentrations of nonferrous metals in the pregnant leach solution after pyritic waste bioleaching to allow efficient extraction. The old pyritic tailings were previously leached with 0.25% sulfuric acid for 10 min to remove soluble metal sulfates. As a result, 36% of copper and 35% of zinc were extracted. After 12 days of bioleaching with a microbial consortium containing Leptospirillum spp., Sulfobacillus spp., Ferroplasma spp., and Acidithiobacillus spp. at 35 °C, the total recovery of metals from pyritic tailings reached 68% for copper and 77% for zinc; and subsequent cyanidation allowed 92% recovery of gold. Ferric leaching of two types of slag at 70 °C with the leachate obtained during bioleaching of the tailings and containing 15 g/L of Fe3+ allowed 88.9 and 43.4% recovery of copper and zinc, respectively, from copper slag within 150 min. Meanwhile, 91.5% of copper, 84.1% of nickel, and 70.2% of cobalt were extracted from copper–nickel slag within 120 min under the same conditions.

Insights into the Metabolism and Evolution of the Genus Acidiphilium, a Typical Acidophile in Acid Mine Drainage

Here, we report three new Acidiphilium genomes, reclassified existing Acidiphilium species, and performed the first comparative genomic analysis on Acidiphilium in an attempt to address the metabolic potential, ecological functions, and evolutionary history of the genus Acidiphilium In the genomes of Acidiphilium, we found an abundant repertoire of horizontally transferred genes (HTGs) contributing to environmental adaption and metabolic expansion, including genes conferring photosynthesis (puf, puh), CO2 assimilation (rbc), capacity for methane metabolism (mmo, mdh, frm), nitrogen source utilization (nar, cyn, hmp), sulfur compound utilization (sox, psr, sqr), and multiple metal and osmotic stress resistance capacities (czc, cop, ect). Additionally, the predicted donors of horizontal gene transfer were present in a cooccurrence network of Acidiphilium Genome-scale positive selection analysis revealed that 15 genes contained adaptive mutations, most of which were multifunctional and played critical roles in the survival of extreme conditions. We proposed that Acidiphilium originated in mild conditions and adapted to extreme environments such as acidic mineral sites after the acquisition of many essential functions.IMPORTANCE Extremophiles, organisms that thrive in extreme environments, are key models for research on biological adaption. They can provide hints for the origin and evolution of life, as well as improve the understanding of biogeochemical cycling of elements. Extremely acidophilic bacteria such as Acidiphilium are widespread in acid mine drainage (AMD) systems, but the metabolic potential, ecological functions, and evolutionary history of this genus are still ambiguous. Here, we sequenced the genomes of three new Acidiphilium strains and performed comparative genomic analysis on this extremely acidophilic bacterial genus. We found in the genomes of Acidiphilium an abundant repertoire of horizontally transferred genes (HTGs) contributing to environmental adaption and metabolic ability expansion, as indicated by phylogenetic reconstruction and gene context comparison. This study has advanced our understanding of microbial evolution and biogeochemical cycling in extreme niches.

Understanding Stress Response to High-Arsenic Gold-Bearing Sulfide Concentrate in Extremely Metal-Resistant Acidophile Sulfobacillus thermotolerans

Biooxidation of gold-bearing arsenopyrite concentrates, using acidophilic microbial communities, is among the largest commercial biohydrometallurgical processes. However, molecular mechanisms of microbial responses to sulfide raw materials have not been widely studied. The goal of this research was to gain insight into the defense strategies of the acidophilic bacterium Sulfobacillus thermotolerans, which dominates microbial communities functioning in industrial biooxidation processes at >35 °C, against the toxic effect of the high-arsenic gold-bearing sulfide concentrate. In addition to extreme metal resistance, this acidophile proved to be one of the most As-tolerant microorganisms. Comparative proteomic analysis indicated that 30 out of 33 differentially expressed proteins were upregulated in response to the ore concentrate, while the synthesis level of the functional proteins required for cell survival was not negatively affected. Despite a high level of cellular metal(loid) accumulation, no specific metal(loid)-resistant systems were regulated. Instead, several proteins involved in the metabolic pathways and stress response, including MBL fold metallo-hydrolase, sulfide:quinone oxidoreductase, and GroEL chaperonin, may play crucial roles in resistance to the sulfide ore concentrate and arsenic, in particular. This study provides the first data on the microbial responses to sulfide ore concentrates and advances our understanding of defense mechanisms against toxic compounds in acidophiles.