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Acidophiles are a group of microorganisms that thrive in acidic environments where pH level is far below the neutral value 7.0. They belong to a larger family called extremophiles, which is a group that thrives in various extreme environmental conditions which are normally inhospitable to other organisms. Several human activities such as mining, construction and other industrial processes release highly acidic effluents and wastes into the environment. Those acidic wastes and wastewaters contain different types of pollutants such as heavy metals, radioactive, and organic, whose have adverse effects on human being as well as on other living organisms. To protect the whole ecosystem, those pollutants containing effluents or wastes must be clean properly before releasing into environment. Physicochemical cleanup processes under extremely acidic conditions are not always successful due to high cost and release of toxic byproducts. While in case of biological methods, except acidophiles, no other microorganisms cannot survive in highly acidic conditions. Therefore, acidophiles can be a good choice for remediation of different types of contaminants present in acidic conditions. In this review article, various roles of acidophilic microorganisms responsible for removing heavy metals and radioactive pollutants from acidic environments were discussed. Bioremediation of various acidic organic pollutants by using acidophiles was also studied. Overall, this review could be helpful to extend our knowledge as well as to do further relevant novel studies in the field of acidic pollutants remediation by applying acidophilic microorganisms.

The discharge of strongly acidic industrial wastewater containing high concentration of chloride ions (Cl−) has become one of the major environmental challenges faced globally. For the removal of extremely stable Cl− in acidic aqueous conditions, precipitation method possesses major advantages of strong adaptability and simple operation. This study proposed a novel cuprous oxide (Cu2O) method assisted by ascorbic acid (VC) for the removal of Cl− from strongly acidic wastewater. First, liquid-phase reduction was chosen as the optimal Cu2O synthesis method based on product purity and composition. Then, parameter optimization results show that increased reagent dosage and acidity significantly enhanced Cl− removal efficiency, while other factors had negligible impacts. After treatment with the sole addition of Cu2O, the dosed Cu2O existed in four forms, including cuprous chloride (CuCl), copper ion (Cu2+), elemental copper (Cu0), and Cu2O, among which the generation of Cu2+ and Cu0, through the oxidation and disproportionation of cuprous ion (Cu+), served as the main reason for the unsatisfactory efficiency in the removal of Cl−. Fortunately, VC is precisely capable of inhibiting the side reactions of Cu+, and under the assistance of 0.10 g VC, the removal of Cl− by Cu2O was greatly improved with the multiple of theoretical reagent dosage decreasing from 12 to 3, the residual concentration of Cu2+ decreasing from 1197 to 18.4 mg/L and the residual concentration of Cl− decreasing from 88.4 to 53.8 mg/L, thus validating the feasibility of this method.

Lime treatment is an important technique for softening of drinking water, treating the municipal-based wastewaters, and treating wastewater discharged from industries and inorganic-based hazardous wastes liquid in nature. Lime treatment is also considered a most applicable technology in terms of economics and efficiency for treating effluents of acidic mine drainage compared to different advanced physical and biological treatment methods. These acidic waste effluents are rich in toxic metals, metalloids, and hazardous/toxic wastes and contaminants (Cu, Cd, Pb, Mn, Zn, Fe, As, Si, SO 4 2− , etc.) due to oxidation and dissolution of pyrite, pyrrhotite, and other sulphidic materials and must undergo suitable eco-friendly treatment(s) before disposing in a landfill or into the impoundment along with tailings. Neutralization is the perquisite for the elimination of these toxic metals and contaminants from wastewater of acidic nature followed by the precipitation and co-precipitation reactions. Lime treatment of acidic wastewaters brings a change in pH (alkalization) resulting in the insolubility of these toxic metals that leads to the formation of minuscule particles due to precipitation reactions. These precipitates are then separated to achieve clear effluents by forming a low-density sludge (LDS) or a high-density sludge (HDS) depending upon the separation methods. The HDS had an advantage over LDS in terms of handling and disposability due to high density, less volume, and better particulate characteristics. The produced sludge as a result of solid/liquid separation should be disposed of by eco-friendly approaches/strategies or recycled or reused for other industrial or manufacturing processes. Nevertheless, diverse lime materials like calcite-enriched powdered oyster shell might have the potential for reducing concentrations of toxicants within permissible limits; however, these are expensive and less efficient for defluoridation from hemihydrated gypsum (CaSO 4 ·0.5H 2 O) produced from recyclable dihydrated gypsum (CaSO 4 ·2H 2 O) by heating at 130–180 °C. Therefore, more cost-effective strategies such as deployment of resultant sludge need to be tested for the defluoridation and stabilization of CaSO 4 ·0.5H 2 O. The present manuscript reviews the science of lime treatment of acidic waste water effluents and defluoridation techniques.

Waste-iron-filling (WIF) served as a precursor to synthesize α- Fe 2 O 3 through the co-precipitation process. The α- Fe 2 O 3 was converted to solid acid catalysts of RBC500, RBC700, and RBC900 by calcination with temperatures of 500, 700 and 900 °C respectively and afterwards sulfonated. Among the various techniques employed to characterize the catalysts is Fourier transforms infrared spectrometer (FT-IR), X-ray diffraction (XRD and Scanning electron microscopy (SEM). Performance of the catalysts was also investigated for biodiesel production using waste cooking oil (WCO) of 6.1% free fatty acid. The XRD reveals that each of the catalysts composed of Al– Fe 2 O 3 / SO 4 . While the FT-IR confirmed acid loading by the presence of SO 4 2 - groups. The RBC500, RBC700, and RBC900 possessed suitable morphology with an average particle size of 259.6, 169.5 and 95.62 nm respectively. The RBC500, RBC700, and RBC900 achieved biodiesel yield of 87, 90 and 92% respectively, at the process conditions of 3 h reaction time, 12:1 MeOH: WCO molar ratio, 6 wt% catalyst loading and 80 °C temperature. The catalysts showed the effectiveness and relative stability for WCO trans-esterification over 3 cycles. The novelty, therefore, is the synthesis of nano-solid acid catalyst from WIF, which is cheaper and could serve as an alternative source for the ferric compound.

Eggshells offer many advantages as adsorbents, such as affordability without special preparations other than pulverization and calcination. However, the manufacturing industry generally has a severe problem with high concentrations of heavy metals in wastewater. The purpose of this study was to use eggshell byproducts and calcined eggshell treatment for the adsorption of copper in an aqueous solution. The reaction time, metal concentration, adsorbent dose, temperature, and pH were evaluated using primary factors followed by the response surface method (RSM) to investigate the optimum conditions for eggshell byproducts and calcined eggshell adsorption treatment. The results of the one-factor-at-a-time experiment showed that the optimal adsorption rate was obtained from treatment at 24 h, 25 mg/L, 10 mg, and 25 °C. In addition, the effect of pH on the adsorption rates of eggshells and eggshells with membrane were detected at pH values of 5 and 5.9 and found to be 95.2, 90.5, and 73.3%. The reaction surface experiment showed that the best adsorption rate reached 99.3% after calcination at 900 °C for 2 h and a 20 min reaction. The results showed that eggshells, eggshell membranes, eggshells with membrane, and calcined eggshells could be applied to remove copper ions from industrial wastewater. The adsorption capacity of the calcined eggshell is better than that of the non-calcined eggshell and has good neutrality in acidic industrial wastewater. Therefore, it is convenient and practical for practical production and application. Likewise, this study conveys promising findings in the context of improving wastewater treatment based on a circular economy approach to waste reuse in the food industry and represents a valuable direction for future research.

Microbial life is not restricted to any particular setting. Over the past several decades, it has been evident that microbial populations can exist in a wide range of environments, including those with extremes in temperature, pressure, salinity, and pH. Bacteria and Archaea are the two most reported types of microbes that can sustain in extreme environments, such as hot springs, ice caves, acid drainage, and salt marshes. Some can even grow in toxic waste, organic solvents, and heavy metals. These microbes are called extremophiles. There exist certain microorganisms that are found capable of thriving in two or more extreme physiological conditions simultaneously, and are regarded as polyextremophiles. Extremophiles possess several physiological and molecular adaptations including production of extremolytes, ice nucleating proteins, pigments, extremozymes and exopolysaccharides. These metabolites are used in many biotechnological industries for making biofuels, developing new medicines, food additives, cryoprotective agents etc. Further, the study of extremophiles holds great significance in astrobiology. The current review summarizes the diversity of microorganisms inhabiting challenging environments and the biotechnological and therapeutic applications of the active metabolites obtained as a response to stress conditions. Bioprospection of extremophiles provides a progressive direction with significant enhancement in economy. Moreover, the introduction to omics approach including whole genome sequencing, single cell genomics, proteomics, metagenomics etc., has made it possible to find many unique microbial communities that could be otherwise difficult to cultivate using traditional methods. These findings might be capable enough to state that discovery of extremophiles can bring evolution to biotechnology.

Phosphogypsum is an acidic solid waste mainly composed of CaSO₄-2H₂O by-products of the wet process phosphoric acid industry, which has the characteristics of high impurity content, poor stability of stockpiling, but can be utilized in a resourceful way. Phosphogypsum waste utilization can reduce environmental pollution, save resources and create economic value. In order to investigate the fatigue characteristics and the mechanism of dynamic strength change of cement-phosphogypsum-red clay under wet and dry cycles, the cumulative deformation characteristics and the rule of change of critical dynamic stress of the mixed materials were investigated by dynamic triaxial fatigue test, SEM and XRD test, and the mechanism of dynamic strength change was analyzed according to the microstructure and the chemical mineral composition of the mixed materials. The test results show that: the cumulative deformation curve of the mix under the action of dry and wet cycles is divided into three types: stable, destructive and critical, and the critical dynamic stress is positively related to the peripheral pressure and consolidation ratio, and negatively related to the number of dry and wet cycles; the mechanism of dynamic strength change: the hydration of cement generates hydrated calcium silicate gel, which coalesces fine particles of the mix to form agglomeration, and the calcium sulfate dihydrate in phosphogypsum reacts with it to generate calcium alumina, and at the same time, the particles form clusters through electrostatic adsorption, and the particles of calcium sulfate dihydrate react with it to form calcium alumina. Through electrostatic adsorption to form clusters, a variety of agglomerates intertwined to form a stable structure with a certain strength. Relative to other proportions of phosphogypsum and red clay, the mix has better dynamic strength at a 1:1 mass ratio of phosphogypsum to red clay; and too much phosphogypsum will make the mix acidic increase, resulting in some of the calcium alumina dissolved, while the dry and wet cycle will increase the internal pores of the mix, reducing the strength of the mixture.

Microbial communities inhabiting the acid mine drainage (AMD) have been extensively studied, but the microbial communities in the coal mining waste dump that may generate the AMD are still relatively under-explored. In this study, we characterized the microbial communities within these under-explored extreme habitats and compared with those in the downstream AMD creek. In addition, the interplay between the microbiota and the environmental parameters was statistically investigated. A Random Forest ensemble model indicated that pH was the most important environmental parameter influencing microbial community and diversity. Parameters associated with nitrogen cycling were also critical factors, with positive effects on microbial diversity, while S-related parameters had negative effects. The microbial community analysis also indicated that the microbial assemblage was driven by pH. Various taxa were enriched in different pH ranges: Sulfobacillus was the indicator genus in samples with pH < 3 while Acidobacteriaceae-affiliated bacteria prevailed in samples with 3 < pH < 3.5. The detection of some lineages that are seldom reported in mining areas suggested the coal mining dumps may be a reservoir of phylogenetic novelty. For example, potential nitrogen fixers, autotrophs, and heterotrophs may form diverse communities that actively self-perpetuate pyrite dissolution and acidic waste generation, suggesting unique ecological strategies adopted by these innate microorganisms. In addition, co-occurrence network analyses suggest that members of Acidimicrobiales play important roles in interactions with other taxa, especially Fe- and S-oxidizing bacteria such as Sulfobacillus spp.

This work has been supported by grants from the MINECO-Spain (www.mineco.gob.es) (CGL2011-22540 and CGL2009-11059), IGME 478 Grant (www.igme.es) and an ERC project (http://erc.europa.eu) (250-350-IPBSL).

Copper mine road dust is the major source of dust in mine operations. The dust produced on the road surface is a great hazard to the workers. Aiming at the road dust of an open-pit mine, this paper conducts a physical and chemical analysis of a new type of chemical dust suppressant. It is prepared by using sodium polyacrylate as a binder, sodium carbonate as a moisture absorbent, polyethylene glycol as a water-retaining agent, and alkyl glycoside as a surfactant. Physical and chemical characteristics and dust suppression performance of dust suppressant were tested. The results show that the dust suppressant has a pH of 11.03, a viscosity of 18.5 mPa·s, and a surface tension of 28.1 mN/m. The content of heavy metal ions contained is less than the maximum concentration defined by “The norms for the integrated treatment of copper mine acidic waste water.” Under the same temperature condition, the greater the humidity, the stronger the hygroscopicity. Especially when the humidity is 30%, the hygroscopic effect is contrary to water. The dust suppressant also has good anti-evaporation properties, and it could maintain a moisture content of 4% to 5% after being placed at room temperature for 10 days. Compared with water, the dust suppressant has better performance of wind erosion, water erosion, and compression resistance. Under the same conditions, the loss rate of water is 2 times that of the dust suppressant, and the pressure of the dust suppressant sample is about 3 times that of water. The dust suppressant has a much higher dust removal efficiency for all dust and respirable dust than water under the same conditions. Finally, the test results and mechanism of the dust suppression mechanism of the dust suppressant are described and analyzed, which shows that the dust suppressant studied in this paper has good performance and is suitable for road dust prevention.