Bioleaching Mechanism Research Articles

The Clay-SRB (sulfate-reducing bacteria) system: Dissolution and fractionation of REY

Rare earth elements (REYs) originate from the weathering of parent granite, whose clay-sized fractions are pivotal in the regolith-hosted rare earth elements (REEs) deposits. Regarding microbial action on REY mobilization and fractionation, their patterns remain unclear. Chemical extraction and bio-leaching experiments utilizing sulfate-reducing bacteria (SRB) were performed to exemplify the chemical and microbial effects on REY mobilization among the clay-sized phases. Our results indicate that the REYs occur primarily in the three fractions: i.e., amorphous FeMn phase, crystalline Fe phase, and carbonate in chemical reactors wherein the mineral phase was critical to the adsorption of REY. The 30-day SRB-leaching experiments led to an increase in the percentage of REY from 6% to 45% in the residue phase, implying that the residue phase, RAmor iron phase, and ROrg phase hosted the REYs. The disorder of iron-bearing minerals, formation of iron-organic matters (Fe-OM), and secondary iron-bearing minerals represented a significant bio-leaching mechanism. Compared to chemical extraction, relatively higher MREY and HREY release efficiencies were obtained via bio-leaching, with average LREY/HREY ratios of 1.34–5.91 and 0.2–2.24 in chemical and bio-reactors, respectively. Our findings exhibited high potential microbial effects on the mobilization and fractionation of REY among mineral phases, offering real insights into the biogeochemical processes between minerals and bacteria.

Investigating the acidophilic microbial community's adaptation for enhancement indium bioleaching from high pulp density shredded discarded LCD panels

As part of electronic waste (e-waste), the fastest growing solid waste stream in the world, discarded liquid crystal displays (LCDs) contain substantial amounts of both valuable and potentially harmful metal, offering valuable opportunities for resource extraction but posing environmental threats. The present comprehensive study is an investigation into the bioleaching of indium from discarded LCD panels, with a particular focus on high pulp density shredded (Sh-LCDs) and powdered (P-LCDs) materials. This study involved an acidophilic consortium, with two pathways, namely the mixed sulfur-iron pathways and sulfur pathways, being explored to understand the bioleaching mechanisms. Indium bioleaching efficiencies through the mixed sulfur-iron pathway were approximately 60% and 100% for Sh-LCDs and P-LCDs, respectively. Three mechanisms were involved in the extraction of indium from LCD samples: acidolysis, complexolysis, and redoxolysis. The microbial community adapted to a pulp density of 32.5 g/L was streak-plated and it was revealed that sulfur-oxizing bacteria dominated, resulting in the minimum indium extraction of 10% and 55% for both Sh-LCDs and P-LCDs samples, respectively. It was generally accepted that ferric ions as oxidants were effective for indium bioleaching from both the Sh-LCDs and P-LCDs. This implies that the cooperation or interaction within the microbial community used in the bioleaching process had a beneficial impact, enhancing the overall effectiveness of extracting indium from LCD panels. The adapted consortium utilizes a combination of microbial transformation, efflux systems, and chelation through extracellular substances to detoxify heavy metals. The adapted microbial community demonstrated better indium leaching efficiency (50%) compared to the non-adapted microbial community which achieved a maximum of 29% and 5% respectively from Sh-LCDs and P-LCDs at a pulp density of 32.5 g/L. The advantages of an adapted microbial community for indium leaching efficiency, attributing this advantage to factors such as high metabolic activity and improved tolerance to heavy metals. Additionally, the protective role of the biofilm formed by the adapted microbial community is particularly noteworthy, as it contributes to the community's resilience in the presence of inhibitory substances. This information is valuable for understanding and optimizing bioleaching processes for indium recovery, and by extension to possibly other metals.

Enhanced vanadium bioleaching from stone coal employing Aspergillus niger: Optimization of stone coal pretreatment method

Vanadium is a rare metal with excellent properties. The recovery of vanadium from low-grade stone coal and vanadium-containing solid wastes using microbial leaching, has become an increasingly popular technology. In this paper, Vanadium was obtained from stone coal using Aspergillus niger as the leaching strain, and the process of vanadium bioleaching was strengthened. Compared with plasma treatment and ultrasonic pretreatment of stone coal, the leaching effect of stone coal after roasting at 900 °C was the best, and the leaching rate of vanadium was as high as 94.52 %. The vanadium bioleaching mechanism was explored by SEM-EDS, XRD, XPS and FTIR analysis. The findings indicated that Aspergillus niger has the ability to adhere to the surface of stone coal, resulting in subsequent corrosion and degradation. The roasting treatment of stone coal and bioleaching of Aspergillus niger jointly lead to the decrease of crystallinity, the increase of interplanar spacing and the decrease of stability of stone coal. Meanwhile, the valence state and binding form of vanadium changed, and V (III) was oxidized to V (IV) and V (V). The mineral structure may be distorted as a result of the distinctive peak's vibratory stretching and the influence of functional categories that include oxygen, which in turn causes the change of the coordination form of the internal metal ions, which affects the symmetry of the mineral and facilitates the dissolution of vanadium from the stone coal. Research indicates that roasting and Aspergillus niger bioleaching have a great deal of promise to increase vanadium recovery from stone coal.

Removal of heavy metals from mine tailings by in-situ bioleaching coupled to electrokinetics

This work utilizes a combined biological-electrochemical technique for the in-situ removal of metals from polluted mine tailings. As the main novelty point it is proposed to use electrokinetics (EK) for the in-situ activation of a bioleaching mechanism into the tailings, in order to promote biological dissolution of metal sulphides (Step 1), and for the subsequent removal of leached metals by EK transport out of the tailings (Step 2). Mine tailings were collected from an abandoned Pb/Zn mine located in central-southern Spain. EK-bioleaching experiments were performed under batch mode using a lab scale EK cell. A mixed microbial culture of autochthonous acidophilic bacteria grown from the tailings was used. Direct current with polarity reversal vs alternate current was evaluated in Step 1. In turn, different biological strategies were used: biostimulation, bioaugmentation and the abiotic reference test (EK alone). It was observed that bioleaching activation was very low during Step 1, because it was difficult to maintain acidic pH in the whole soil, but then it worked correctly during Step 2. It was confirmed that microorganisms successfully contributed to the in-situ solubilization of the metal sulphides as final metal removal rates were improved compared to the conventional abiotic EK (best increases of around 40% for Cu, 162% for Pb, 18% for Zn, 13% for Mn, 40% for Ni and 15% for Cr). Alternate current seemed to be the best option. The tailings concentrations of Fe, Al, Cu, Mn, Ni and Pb after treatment comply with regulations, but Pb, Cd and Zn concentrations exceed the maximum values. From the data obtained in this work it has been observed that EK-bioleaching could be feasible, but some upgrades and future work must be done in order to optimize experimental conditions, especially the control of soil pH in acidic values.

Bioleaching rare earth elements from coal fly ash by Aspergillus niger

Rare earth elements (REEs) are abundant in certain mining coal, and further enriched in coal fly ash (CFA) after coal combustion. If the appropriate leaching technology is chosen, the CFA can be used as an unconventional source of REEs. In this paper, culture conditions and bioleaching process of Aspergillus niger (A. niger) were optimized, the mechanism of acid production and factors of the bioleaching process of A. niger were discussed, and the bioleaching mechanism of REEs from CFA by A. niger was expounded. The results showed that organic nitrogen sources affected the morphology and acid production capacity of A. niger, and the A. niger used in this study mainly secreted oxalic acid (4.05 mg/L). Secondly, the initial pH value, the most influential factor, further affected the bioleaching process by affecting the acid production of A. niger. Therefore, by optimizing the initial pH value to enhance the acid production capacity of A. niger and developing the best bioleaching system, the CFA can reach a leaching rate of 30.91 % for total REEs. Finally, based on the test results of organic acids and pH, combined with previous studies, we speculated the main mechanism of REES bioleaching from CFA by A. niger as proton exchange and organic ligand complexation. Microbial leaching (e.g., A. niger) integrates economic, environmental, and resource use values.

From understanding the rate limitations of bioleaching mechanisms to improved bioleach process design

While bioleaching has become established technology for the leaching of refractory gold concentrates and in the heap leaching of copper sulfide minerals, many other applications, although demonstrated at the laboratory scale, have not found significant uptake in industrial biomining. The reasons for this are manifold, but generally relate back to engineering constraints in operating processes at large scale while remaining economically viable.The paper unpacks acidophile sulfide bioleaching mechanisms within bioreactors into various sub-processes and analyses them in terms of their rate limitations, such as gas-liquid mass transfer of oxygen and CO2, diffusion constraints between and within mineral particles, competing side reactions etc. It becomes clear that microbial metabolic reactions are generally not what limits the kinetics of bioleaching reactions overall but rather the reaction environment. Improved process design to enhance the economic performance of bioleaching therefore needs to target the slowest sub-process in the bioleaching mechanism, which is rarely the biological one.Conventional tank and heap bioleaching processes are discussed in this context and routes taken towards their improvement discussed. Further, various promising novel concepts in biomining, such as leaching with biogenic lixiviants, phytomining and tailings leaching are evaluated within this framework, in each case identifying the potentially limiting sub-processes. It becomes clear that the relative ‘slowness’ of these sub-processes prohibits exploitation through designed active industrial processes; they can, however, still be facilitated through suitably designed ‘passive’ processes in which the time taken for slow processes is less of an economic constraint. While this is an obvious approach in bioremediation, it can also be employed for the economic recovery of value from otherwise sub-economic mineral deposits and waste materials through heap bioleaching. This way it can also play a role in the drive towards a circular economy.

Rapid and efficient extraction of Zn from wasted Zn-rich paint residue by indirect bioleaching and successive production of high-purity ZnCO3/ZnO by precipitation

Waste zinc-rich paint residue (WZPR) represents a typical hazardous waste containing both toxic organic substances and heavy metals. The extraction of Zn from WZPR by traditional direct bioleaching has been attracting attention owing to its eco-friendliness, energy conservation and low cost. However, a long bioleaching time and a low Zn release cast a shadow on the reputed bioleaching. To shorten the bioleaching time, the spent medium (SM) process was first used to free Zn from WZPR in this study. The results showed that the SM process had a much higher performance in Zn extraction. Zn removals of 100% and 44.2% (8.6 g/L and 15.2 g/L in the released concentration) were gained within 24 h under pulp densities of 2.0% and 8.0%, respectively, being over 1000 times of the release performance of Zn by previously reported direct bioleaching. On the one hand, the biogenic H+ in SM attacks ZnO to liberate Zn (Ⅱ) via quick acid dissolution. On the other hand, the biogenic Fe3+ not only highly oxidizes Zn0 in WZPR to generate and release Zn2+ but also intensely hydrolyzes to produce H+ to attack ZnO for further dissolution of Zn2+. Both biogenic H+ and Fe3+ contribute to over 90% of Zn extraction as the leading indirect bioleaching mechanism. Due to the high concentration of released Zn2+ and fewer impurity, the bioleachate was used to successfully produce high-purity ZnCO3/ZnO using a simple precipitation, thus achieving the high-value recycling of Zn in WZPR.

A Comparative Study on Bioleaching Properties of Various Sulfide Minerals Using Acidiphilium cryptum

Bioleaching has been regarded as a green alternative to chemical leaching in metal extraction processes. In this study, the bioleaching properties of indigenous acidophilic bacteria for various sulfide minerals were compared and evaluated in terms of pH reduction and metal leaching. The primary minerals in the samples were sphalerite (ZnS) (SP), galena (PbS) (GN1 and GN2), pyrite (FeS2) (PY), and chalcopyrite (CuFeS2) (CCP), and an indigenous acidophilic bacterium, Acidiphilium cryptum (99.56%), was applied for bioleaching experiments. The metal extraction in bioleaching differed according to the mineral content. The leached metal concentration of Zn was higher than that of Pb for the SP sample with a high ZnS content, whereas the concentration of Pb was higher than that of Zn for the GN1 and GN2 samples with a high PbS content. Meanwhile, the leaching rate of Zn was faster than that of Pb for all samples. Corrosion action was observed on the surface of bacterial residues in SP and GN1 samples. These results show that the bioleaching mechanism based on sulfide minerals proceeds through indirect biological oxidation, chemical oxidation, and direct bacterial oxidation. The results of this study can provide basic research data for process optimization when employing bioleaching to extract valuable metals.

Progress and Challenges of Biological Leaching of Heavy Metal in Coal Ash from a Power Plant

Bioleaching is a technique for reducing the heavy metal content of coal ash by using bacteria, fungi, or yeast. Previous studies in heavy metal bioleaching of coal ash discussed the factors affecting the process, but as yet there is little information on the challenges of using microorganisms. Therefore, this study aimed to obtain comprehensive information regarding the use of microorganisms in heavy metal bioleaching. Heavy metal concentrations in coal ash are low, and the metals are diverse. The components of coal ash are complexes that cannot leach certain heavy metals according to previous studies. These low concentrations and complex components make it difficult to investigate the bioleaching mechanism. The combination of biological and chemical interactions involves various components in this system. The high concentration of iron and heavy metal leached could be toxic for microorganisms. The process is influenced by several factors, such as particle size, pH, and pulp density. Most heavy metal bioleaching studies on coal ash have been conducted on a small scale to control conditions affecting the process. Bioleaching kinetics in coal is a liquid-solid reaction that can be represented by the shrinking core model, which was mainly used in this study.

A Critical Review on the Recovery of Base and Critical Elements from Electronic Waste-Contaminated Streams Using Microbial Biotechnology.

Pollution by end-of-life electronics is a rapid ever-increasing threat and is a universal concern with production of million metric tons of these wastes per annum. Electronic wastes (E-waste) are rejected electric or electronic equipment which have no other applications. The aggrandized unproper land filling of E-waste may generate hazardous effects on living organisms and ecosystem. At present, millions of tons of E-waste await the advancement of more efficient and worthwhile recycling techniques. Recovery of base and critical elements from electronic scraps will not only reduce the mining of these elements from natural resources but also reduces the contamination caused by the hazardous chemicals (mostly organic micropollutants) released from these wastes when unproperly disposed of. Bioleaching is reported to be the most eco-friendly process for metal recycling from spent electronic goods. A detailed investigation of microbial biodiversity and a molecular understanding of the metabolic pathways of bioleaching microorganisms will play a vital function in extraction of valuable minerals from the end-of-life scraps. Bioleaching technique as an economic and green technology costs around 7 USD per kg for effective reusing of E-waste as compared to other physical and chemical techniques. This review provides a summary of worldwide scenario of electronic pollutants; generation, composition and hazardous components of electronic waste; recycling of valuable elements through bioleaching; mechanism of bioleaching; microorganisms involved in base and critical element recovery from E-waste; commercial bioleaching operations; and upcoming aspects of this eco-friendly technique.

Understanding the mechanism of microcrack-enhanced bioleaching of copper

Investigations on the microcrack distribution characteristics and bioleaching behaviors of particles were conducted to obtain better bioleaching efficiency and deepen the mechanism understanding. Results indicate that comminution by high-pressure grinding roll (HPGR) is the desirable method for obtaining particles with more abundant and wider microcracks. Fixing the HPGR pressure at 4 MPa results in the highest bioleaching efficiency for copper extraction, namely 86% for the shaking flask bioleaching (particle size: <1.7 mm) and 52% for the column bioleaching of particles (particle size: 3.35–6.7 mm). The suggested model for reaction kinetics of microcrack-containing particles implies that more abundant and wider microcracks can promote the exposure of both Cu-containing phases and ferrous iron as well as solution infiltration, as evidenced by the COMSOL simulation results and bacteria proliferation analysis. The results highlight the importance of microcracks for enhancing the bioleaching process and provide insights for the mechanism understanding.

Bioleaching of critical metals using microalgae

&lt;abstract&gt; &lt;p&gt;Critical metals, which mainly include the platinum group of metals, and the rare earth elements, have gained much importance because these elements are essential for economic development. A matter of concern is their availability, which is scarce, and so a constant supply is at risk. Bioleaching is one of the commonly used methods to extract these critical metals from various sources, such as industrial wastewater and mining water.&lt;/p&gt; &lt;p&gt;In this study, we have discussed the mechanisms of bioleaching, the factors that affect bioleaching, and a correlation between the extraction of the critical metals using microalgae which has many positive aspects. The review also suggests the future prospects for the use of microalgae in the extraction of critical metals.&lt;/p&gt; &lt;/abstract&gt;

In situ detection of Cu2+, Fe3+ and Fe2+ ions at the microbe-mineral interface during bioleaching of chalcopyrite by moderate thermophiles

Bioleaching of metal sulfides represents an interfacial process in which the spatial relationship of metal ions with microbial cells and extracellular polymeric substances (EPS) is complex. The dissolution of chalcopyrite, cell attachment, and biofilm formation patterns were assessed during bioleaching with mixed cultures at 45 °C. The analysis of confocal laser scanning microscopy (CLSM) indicated that EPS were laid flat on the surface of chalcopyrite coupons, forming monolayer biofilms, with significant amounts of Fe3+, Fe2+ and Cu2+ enriched at the cell-chalcopyrite interface. In addition, the metal ions from mineral surfaces were extracted by ultrasonication. The results showed that the mineral surface is enriched with large amounts of Fe3+ (12.07 mg/g) and Cu2+ (10.82 mg/g) during the final stage of bioleaching, and more Cu2+ ions and iron maters from bio-oxidizing maybe were enclosed in the EPS space. It was inferred that the EPS with jarosite on the surface of chalcopyrite gradually acted as a weak diffusion barrier for Cu2+, and Fe3+ ions transference during bioleaching. These results can provide important evidence to reveal the hypothesis of ''metal ions enrichment on the mineral surface'' and benefit a further understanding of the bioleaching mechanism.

Simulated bioleaching of ion-adsorption rare earth ore using metabolites of biosynthetic citrate: An alternative to cation exchange leaching

Ion-adsorption rare earth ore (IRE-ore) is a strategic resource that provides almost all the market demand for heavy rare earth elements (HREEs). The commonly used ammonium sulfate technology in industrial production is gradually being restricted for use due to environmental damage, thus resulting in the huge challenge of the IRE-ore industry. Bioleaching has potential prospects in the clean and efficient utilization of IRE-ore, mainly due to its advantages of low-cost and eco-friendliness. Since IRE-ore is mainly extracted by in-situ leaching in industrial production, contact/direct bioleaching seems infeasible. Hence, noncontact/indirect bioleaching of IRE-ore by microbial metabolites would be the best choice. In this work, Aspergillus niger and Yarrowia lipolytica were found to be effective in IRE-ore bioleaching, and citrate was reported to be the main microbial metabolite. To further investigate the bioleaching mechanism and process intensification, simulated bioleaching of IRE-ore by biosynthetic citrate/((NH4)3Cit) was conducted. Erlenmeyer flask leaching results showed that more than 90% of the total REE leaching yield can be obtained at extremely low citrate concentrations (approximately 3.3 mmol/L), which was only 10% of the commonly used ammonium sulfate concentration. Analytical results showed that Cit species directly transferred REEs from the IRE-ore surface to solution via a complexation reaction, which was synchronized and synergistic with the ion-exchange reaction of NH4+. This work provides a new idea for the clean and efficient utilization of IRE-ore and is expected to overcome the common disadvantages and bottlenecks of traditional technology of inorganic salt leaching and the principle of cation exchange.

Enhanced metal bioleaching mechanisms of extracellular polymeric substance for obsolete LiNixCoyMn1-x-yO2 at high pulp density.

Harmful chemicals present in electric vehicle Li-ion batteries (EV LIBs) can limit the pulp density of bioleaching processes using Acidithiobacillus sp. to 1.0% (w/v) or lower. The strong enhancing mechanisms of extracellular polymeric substances (EPS) on the bioleaching of metals from spent EV LIBs at high pulp density (4% w/v) were studied using bio-chemical, spectroscopic, surface structure imaging and bioleaching kinetic methods. Results demonstrated that the added EPS significantly improved bioleaching efficiency of Ni, Co and Mn improved by 42%, 40% and 44%, respectively. EPS addition boosted the growth of cells under adverse conditions to produce more biogenic H+ while Fe3+ and Fe2+ were adsorbed by the biopolymer. This increased Li extraction by acid dissolution and concentrated the Fe3+/Fe2+ cycle via non-contact mechanisms for the subsequent contact bioleaching of Ni, CO and Mn at the EV LIB-bacteria interface. During the leaching process, added EPS improved adhesion of the bacterial cells to the EV LIBs, and the resultant strong interfacial reactions promoted bioleaching of the target metals. Hence, a combination of non-contact and contact mechanisms initiated by the addition of EPS enhanced the bioleaching of spent EV LIBs at high pulp density.

Importance of laccase enzyme and triiodide for gold leaching from silicate ore by marine bacterium Acinetobacter sp.

The effect of enzymes and other substances on gold bioleaching was investigated. Acinetobacter sp. had a higher gold leaching performance than two strains of Vibrio sp. because it grew well in nutrient broth (NB) medium containing potassium iodide (KI) and produced high levels of both laccase and triiodide (I3-). Under two-step condition of Acinetobacter sp. in NB medium containing KI 10.9 g·L−1 and 1% (w/v) alkali lignin had the highest efficiency in gold leaching. Plausible mechanisms of gold bioleaching involved in biogenic iodide-iodine lixiviant and laccase reaction mechanisms. Laccase produced by Acinetobacter sp. could directly oxidize insoluble gold to soluble gold and also increase iodide oxidation. Afterward, the gold can be oxidized completely in the biogenic iodide-iodine lixiviant to form gold (I) diiodide [AuI2]- or gold (III) tetraiodide [AuI4]-. Interestingly, it was found that silicate ore could act as an inducer of laccase production. Furthermore, alkali lignin addition could increase gold bioleaching due to it can be used as a substrate for laccase production. Therefore, the two-step condition of Acinetobacter sp. could be applied in gold leaching under mild alkaline pH conditions. The advantage of this technology was safer and more environmentally friendly than cyanidation as a conventional extraction.

Catalytic mechanism of manganese ions and visible light on chalcopyrite bioleaching in the presence of Acidithiobacillus ferrooxidans

The bioleaching of chalcopyrite is low cost and environmentally friendly, but the leaching rate is low. To explore the mechanism of chalcopyrite bioleaching and improve its leaching rate, the effect and mechanism of manganese ions (Mn 2+ ) and visible light on chalcopyrite mediated by Acidithiobacillus ferrooxidans ( A. ferrooxidans ) were discussed. Bioleaching experiments showed that when both Mn 2+ and visible light were present, the copper extraction was 14.38% higher than that of the control system (without Mn 2+ and visible light). Moreover, visible light and Mn 2+ promoted the growth of A. ferrooxidans . Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis revealed that Mn 2+ promoted the formation of extracellular polymeric substance (EPS) on the surface of chalcopyrite, changed the morphology of A. ferrooxidans , enhanced the adsorption of bacteria on chalcopyrite surface with light illumination, and thus promoted the bioleaching of chalcopyrite. UV–vis absorbance spectra indicated that Mn 2+ promoted the response of chalcopyrite to visible light and enhanced the catalytic effect of visible light on chalcopyrite bioleaching. Based on X-ray photoelectron spectroscopy (XPS), the relevant sulfur speciation of chalcopyrite before and after bioleaching were analyzed and the results revealed that visible light and Mn 2+ promoted chalcopyrite bioleaching by reducing the formation of passivation layer (S n 2- /S 0 ). Investigation into electrochemical results further indicated that Mn 2+ and visible light improved the electrochemical activity of chalcopyrite, thus increasing the bioleaching rate.

Bioleaching of Copper from Electronic Waste Using Aspergillus niger

The recycling of printed circuit boards (PCBs) causes environmental problems by releasing dioxins and furans. The objective of this study is the recovery of copper from PCBs through the bioleaching process using Aspergillus niger which was isolated from the surface of Abu Tartur phosphate. In the bioleaching process, the optimum conditions were used to modify ammonium medium with inoculum spore size are 2Χ106 SFU/50 ml for 5 days at 30ͦ C in a pulp of 0.5% solid 150 mesh particle size and aeration at 200 rpm. Different carbon and nitrogen sources were used. Glucose (1.5%) and ammonium chloride (0.2%) were the best source of carbon and nitrogen, respectively. Also, the optimum initial medium’s pH was 7. At these conditions, about 100% of Cu was extracted. The mechanism of bioleaching was studied by detecting the production of organic acid through using brome cresol green as an indicator and HPLC analysis. The color change of agar medium of the incubated plate with A. niger from blue to yellow, and HPLC analysis showed detection malic and citric acids in the sample in presence e-waste higher than the sample without e-waste. E-waste before and after the bioleaching process was investigated using SEM. The surface of e-waste becomes more smooth and porous due to the bioleaching process.

Applications of Microbial Communities for the Remediation of Industrial and Mining Toxic Metal Waste: A Review

The increased amount of heavy metal pollution, due to rapid urbanization is one of the main reasons for ecosystem degradation. The sewer systems of industries, as well as the mining grounds, are the potential sources of toxic heavy metals. Broad application of mutagenic metals in different industrial processes results in their bioaccumulation in the soil ecosystem. The primary metal wastes are mercury, arsenic, lead, cadmium, cyanide, chromium, etc. Long-term exposure to even the low concentration of metals induces multiple health hazards and organ failure. The safe removal or treatment of wastes is crucial as many approaches might lead to the generation of harmful by-products or incomplete transformation of toxic metals. Therefore, eco-friendly biodegradation of toxic metals administrating efficient bioleachers is generally preferred. However, the activity of the microbes depends on their ability to tolerate large content of metals, and the use of metal-tolerant microbes found in nature is the simplest and biogenic way of degrading the metal contaminants of industries such as mining, manufacturing, textile, petroleum, plastics, etc. Microbial remediation includes the following mechanisms; surface absorption, bioleaching, transformation to less toxic products, biomineralization, and bioaccumulation. The objective of this review is to describe the industrial production and environmental occurrence of carcinogenic metals, highlight the role of the bioleachers in the decontamination process, harmful effects of toxic metals, and microbial bioleaching mechanism.

Bioleaching metals from waste electrical and electronic equipment (WEEE) by Aspergillus niger: a review.

In the twenty-first century, the increasing demand for electrical and electronic equipment (EEE) has caused its quick update and the shortening of its service life span. As a consequence, a large number of waste electrical and electronic equipment (WEEE) needs to be processed and recycled. As an environmentally friendly method, biometallurgy has received extensive attention in the disposal of WEEE in recent years. Aspergillus niger is an acid-producing fungus with a potential applicability to improve metals' recycling efficiency. This review article describes the latest statistical status of WEEE and presents the latest progress of various metallurgical methods involved in WEEE recycling for metal recovery. Moreover, based on the summary and comparison towards studies have been reported for bioleaching metals from WEEE by A. niger, the bioleaching mechanisms and the bioleaching methods are explained, as well as the effects of process parameters on the performance of the bioleaching process are also discussed. Some insights and perspectives are provided for A. niger to be applied to industrial processing scale.