1. INTRODUCTION:

1.1. The Significance of bioleaching in gold extraction

Standard gold extraction techniques result in high expenses while creating substantial environmental disturbances so alternative sustainable techniques must be developed. Traditional cyanidation methods remain effective however they face technical, economic along with environmental constraints. The demand for modern hydrometallurgical extraction approaches requires inexpensive harmless effective reagents such as biogenic lixiviants.

Conventional mining operations bring serious ecological damage to surface waters and groundwater supply alongside soil contamination thus demonstrating the immediate need for clean extraction methods. The biological method called bioleaching utilizes microorganisms to extract gold and proves itself as an effective protocol for resolving these extraction issues¹.

The process of bioleaching enables sustainable and economical extraction of gold using microorganisms as highlighted by references. The bioleaching process utilizes biological operations instead of chemical procedures and energy-consuming methods to extract gold through its sources. The bioleaching process decreases environmental impacts while retaining the possibility of extracting valuable gold from low-grade mineral sources and e-waste although traditional extraction methods may not make them cost-effective. Bioleaching technology gains interest because it provides a solution to financial restrictions of traditional methods as well as anticipated legislation regarding environmental impact^2,3.

The method is becoming more important because it enables processing of low-grade ores and reduces environmental effects. Extracting gold from declining high-grade deposits requires new methods for obtaining gold from lower-grade minerals because the access to premium deposits is critically declining. Bioleaching technology establishes an extraction solution to extract metal from difficult-to-process low-grade ores. Bioleaching supports sustainable development through its waste reduction and chemical consumption diminution which brings valuable advantages to a circular economy framework^3,4.

1.2. Fundamentals of Bioleaching

Bioleaching uses microorganisms to extract gold content from materials which include ores and other forms of mining rocks. Certain microorganisms use their natural abilities to work on metal compounds which permits extraction from solid materials. Through biochemical processes bacteria together with fungi cause the dissolving of gold by producing mechanisms that expose the metal for recovery. Specific metabolites together with enzymatic reactions create the driving force needed for mineral solubilization that targets gold-bearing chemicals⁵.

Through the production of cyanide and organic acids microbes drive the interaction with gold particles to transform them into soluble compounds. The secondary metabolite cyanide production by Chromobacterium violaceum results in the formation of gold-cyanide complexes thus rendering the gold accessible within solution. Aspergillus niger among other fungus species generates citric and oxalic acids that serve as chelator agents to form gold ion complexes and enhance gold solubility. The particular chemical processes that microorganisms use determine the effectiveness of gold dissolution from its mineral state^6,7.

Gold in solution can be recovered from the solution through different extraction procedures. A sufficient method must be established to extract and concentrate the solubilized gold from its leaching solution. Cementation and mobilization methods that humans use for metal recovery require extensive chemical substances and energy expenditure. Biosorption along with bioreduction and bioprecipitation represent biotechnological solutions that present both economic potential and environmental advantages for precious metal extraction from solutions. These techniques utilize microorganism and biological material abilities to choose specific gold substances from solutions for better recovery methods⁴.

1.3. Types of Microorganisms Used in Gold Bioleaching:

The bioleaching process utilizes several microorganisms consisting of bacteria and fungi according to⁶. The selection procedure for microbes needs to consider multiple aspects including precious material type and environmental conditions alongside desired gold extraction efficiency. A bioleaching process needs careful design since different microorganisms’ present different levels of gold solubilizing power along with diverse resistance capabilities to environmental factors. The advantages of bacteria and fungi match each other thereby allowing users to optimize gold extraction through individual use or coordinated application^2,6.

Three primary bacterial organisms used for bioleaching operations are C. violaceum, P. aeruginosa and Bacillus megaterium. Research demonstrates that these bacteria succeed in dissolving gold through cyanide production and organic acid secretion and redox reactions. The bacteria C. violaceum produces cyanide that creates gold dissolving complexes. P. aeruginosa effectively participates in bioremediation activities thanks to its resistance against many heavy metals which allows it to work efficiently in complicated setting for extracting gold. Bioleaching operations are supported by B. megaterium and other Bacillus species through pathways of biosorption and bioaccumulation⁷.

Bioleaching processes use the fungus Aspergillus niger together with other microorganisms such as Aspergillus niger. The bioleaching properties of A. niger are optimal because its production of organic acids functions as chelating agents that improve gold solubility. Laboratory tests demonstrated that this fungus effectively removes gold from electronic waste and low-grade ores thereby proving its wide functionality and operational excellence. Fungal bioleaching operates as an alternative extraction method which substitutes harsh chemicals thus decreasing environmental effect^2,8.

2. Bacterial species in gold bioleaching:

2.1. Chromobacterium violaceum:

The facultative anaerobic gram-negative bacterium C. violaceum generates cyanide along with other researchers in their study. The bacterium demonstrates its flexibility by surviving under both aerobic and anaerobic conditions which enables it to adapt throughout bioleaching process environments. The bacterium organizes into the gram-negative category because of its particular cell wall composition which affects its contact with surroundings and resistance to chemical agents. C. violaceum generates cyanide the potent complexing agent for gold which proves essential for its usefulness in gold bioleaching processes^7,9.

Cyanide enables the dissolution of gold particles extracted from crushed ores so that gold recovery procedures become possible. C. violaceum produces cyanide that reacts with gold particles and generates the soluble complex named Au (CN)₂-. The complex shows stability in water-based solutions which enables extraction of gold from solid rock ore material. The efficiency of cyanide leaching depends on three main factors which include cyanide concentration together with solution pH levels and metal interference that binds available cyanide⁹.

Research indicates that C. violaceum demonstrates a gold extraction yield higher than 70% according to studies. Studies on C. violaceum bioleaching for electronic waste extraction show its effectiveness in recovering gold extraction and its performance matches or outperforms standard chemical practices. Adjustments to the bacterial culture environment through optimization of nutrient sources alongside pH control and aeration management will lead to improved effectiveness of gold extraction using this bacterium. The cyanogenic potential of C. violaceum has been improved through metabolic engineering approaches which yielded superior gold extraction rates^9,10.

2.2. Pseudomonas aeruginosa:

P. aeruginosa functions as a bacteria species that helps in heavy metal bioremediation particularly for gold extraction along with other metal bioremediation tasks. This environmental microorganism exists throughout the world because it shows versatility in its metabolism and can adapt to numerous environmental conditions. Heavy metal bioremediation represents an important sphere for P. aeruginosa where it helps remove multiple heavy metals from polluted soils and water sources. The heavy metal tolerance abilities of P. aeruginosa establish it as a key biotechnological agent which performs effectively in environmental remediation operations and recycling programs^6,
11.

Participating in heavy metal processing are multiple microorganisms among which Pseudomonas spp. stands as one example. All Pseudomonas species display different metabolic properties which includes heavy metal transformation alongside heavy metal accumulation and heavy metal solubilization. Several mechanisms like biosorption or bioaccumulation or biotransformation help microorganisms to interact with heavy metals. These microorganisms play essential roles in heavy metal processing because they find use in bioremediation and bioleaching applications¹¹.

When P. aeruginosa is combined with iodide the production of cyanide improves which leads to greater efficiency in gold leaching processes. Laboratory studies show that P. aeruginosa cultures will produce more cyanide when supplied with iodide which leads to improved gold mineral dissolution rates. The oxidizing activity of iodide allows it to accept electrons while simultaneously accelerating the oxidation of cyanide precursors which produces elevated cyanide yields. The integration of P. aeruginosa with iodide shows potential as a superior method to boost gold bioleaching from different mining materials⁹.

2.3. Bacillus spp.:

Bacillus spp. belong to the well-studied bioremediation microorganisms among them are B. subtilis, B. cereus and B. thuringiensis. This group of bacteria consists of gram-positive microorganisms which produce endospores while they reside across soil and aquatic environments. Through endospore formation Bacillus spp. can endure extreme conditions leading to their high resistance which enables bioleaching together with bioremediation operations under diverse circumstances. Bacillus spp. utilize diverse metabolic pathways which provide these microorganisms with high bioremediation potential that includes the deletion of heavy metals and organic pollutants and other environmental toxic compounds¹².

The bacteria utilize biosorption as well as EPS-mediated bioaccumulation and bioprecipitation mechanisms to remove heavy metals according to. The surface cell binding mechanism of Bacillus spp. is categorized as biosorption whereas bioaccumulation describes metallic substance accumulation inside bacterial cells. The heavy metal removal process through Bacillus spp. is boosted by extracellular polymeric substances (EPS) which function to bind and precipitate heavy metals within contaminated areas. Clustering mechanisms work in unison to make Bacillus spp. effective within bioremediation and bioleaching operations¹².

Bacillus spp. helps with phytoremediation by fostering plant development in addition to making contaminated soil areas friendlier to plant accumulation. The utilization of plants for environmental remediation through phytoremediation benefits from Bacillus spp. strains present in soil environments. The bacteria enhance plant growth through hormone production and better nutrient access and stronger metal resistance in plants. Bacillus spp. helps plants accumulate heavy metals in their tissues which supports soil cleaning activities of remediation¹².

3. Iron and sulfur-oxidizing bacteria:

3.1. Role in Biooxidation:

Bioleaching operations in the industrial sector rely on highly acid-tolerant microbes for extracting gold from pyrite and arsenopyrite through biooxidation according to references. Refractory gold ores containing gold inside sulfide minerals become accessible through these processes due to their effectiveness. Acidophilic microorganisms succeed in breaking down sulfide minerals through highly acidic environments and this process releases trapped gold. During biooxidation multiple complicated biochemical reactions dissolve both the mineral framework and lead to the liberation of gold throughout the process^4,13.

The bacteria function best at high acid levels while handling heavy metal concentrations. Extreme conditions become possible for these microorganisms because of their unique adaptations across both physiological and genetic levels. Bacteria have developed specific strategies which help them control their pH levels and shield them from toxic metal exposure as well as extract beneficial elements from their challenging environment. The organisms make essential adaptations for survival and activity within bioleaching environments because such high acidic conditions and metal concentrations would be toxic to common life forms¹⁴.

Iron and sulfur-oxidizing bacteria transform insoluble sulfides into soluble sulfates helping the recovery of metals. The sulfide minerals undergo bacterial oxidation during which process soluble sulfates form which are subsequently removed from the ore material. The iron ions that form during this process have the ability to initiate additional sulfide mineral oxidation thus creating an automatically repeating cycle of chemical decomposition. The breakdown of the mineral framework through this procedure allows extraction of valuable metals such as gold along with other precious materials for recovery processes^5,15.

3.2. Key species:

Bioleaching depends on Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans which use their capabilities to oxidize iron and sulfur compounds. While capable of using ferrous iron and sulfur compounds as its energy source A. ferrooxidans functions as a chemolithoautotrophic bacterium. A. thiooxidans specializes in sulfur compound oxidation as its main biological process. The acidophilic species exist under extremely low pH ranges from 1.5 to 2.0 while actively playing a vital role in sulfide mineral bioleaching^4,5.

The metal dissolution process of sulfide minerals occurs through the assistance of these chemolithotrophic bacteria. A. ferrooxidans together with A. thiooxidans break down sulfide minerals through their metabolic processes which releases copper, zinc and gold among other valuable metals. These minerals become oxidized when bacteria consume iron and sulfur compounds to maintain a sustainable energy-based process that bioleaching experts rely on. The dissolution process remains critical because it enables metal extraction from substandard mineral ores as well as ore concentrates¹⁶.

A. ferrooxidans and A. thiooxidans play an essential role during the bioleaching of refractory gold ores as per¹⁷. Getaway gold ores with gold embedded inside sulfide minerals of arsenopyrite and pyrite prove resistant to typical cyanide leaching processes. Biooxidation pretreatment with A. ferrooxidans and A. thiooxidans plays a vital role in sulfide mineral breakdown through their activity to make gold available for cyanide extraction. Bioleaching operations as a pretreatment step enhance the complete recovery of gold material from refractory ores to a significant extent¹⁷.

3.3. Mechanisms of action:

The bacterial process produces ferric and hydrogen ions through the oxidation of metal sulfides according to research findings. acidic environmental conditions induced by A. ferrooxidans and A. thiooxidans convert pyrite (FeS₂) metal sulfides into ferric ions (Fe³⁺) and hydrogen ions (H⁺). The ions function as essential elements that facilitate the chemical dissolution process of minerals that contain gold. An overall chemical process involves FeS2 reacting with 14Fe³⁺ and 8H₂O to form 15Fe²⁺ together with 2SO₄^2- and 16H^{+ 13, 16}.

The chemical reaction of ferric iron works as an oxidizing agent to dissolve the gold-bearing minerals. The active ferric ions from bacterial fermentation break down and dissolve gold-bearing minerals in solution. The mineral matrix breakdown through chemical attack allows the gold to be extracted because it becomes available as a result. The bioleaching process needs continuous ferric ion regeneration by bacteria to maintain continuous operation¹⁶.

The bacteria's biofilm creation of these microenvironments supports bioleaching process advancement. Several bacterial species form biofilms on sulfide minerals which create an ideal area for the bioleaching operation to run smoothly. An area with concentrated bacterial density and metabolic activity within the biofilm increases mineral oxidation while releasing gold. Biofilms protect the bacteria from environmental stresses because they create a protective barrier around them while simultaneously protecting them from high acidity and metal concentrations which enables better bacterial activity⁴.

4. Fungal species in gold bioleaching:

4.1. Aspergillus niger:

A tremendous number of studies support the bioleaching uses of A. niger due to its metal solubilization capabilities. A filamentous fungus named A. niger exists prevalently in soil together with plant debris and within numerous industrial facilities. A. niger has become a widely-used bioleaching organism because it grows vigorously while tolerating environmental stress and producing multiple organic acids. The combination of properties found in A. niger makes it suitable for releasing metals from multiple resources including metallic ores together with electronic waste^2,8,18.

The microorganism generates citric acid together with oxalic acid to dissolve gold in electronic waste and ores but also in other substrates. Organic acid formation by A. niger consists of citric acid together with oxalic acid and gluconic acid along with various other acids that rely on culture conditions and nutrient delivery. Organic acids serve as chelating agents because they create metal ion complexes which make the ions more soluble in aqueous solutions. A. niger uses acid production as its principal method to perform bioleaching operations when dissolving gold and other metals^8,18.

Research shows that A. niger achieves outstanding efficiency in extracting gold from PCBs combined with computer components. Scientists have proven that A. niger extracts gold effectively from electronic waste materials that contain printed circuit boards (PCBs) and computer parts. A. niger proves to be a sustainable option for metal recycling when it achieves gold extraction levels of up to 87%. A. niger provides a beneficial tool for extracting valuable metals from complex matrices because of its ability to dissolve precious metals in these systems⁸.

4.2. Other fungal strains:

The bioleaching process can be conducted by several fungal strains which include both Aspergillus niger alongside Penicillium simplicissimum and Paecilomyces spp. P. simplicissimum represents another filamentous fungus that produces organic acids and dissolves metals in the environment. The rare earth elements recovery process benefits from bioleaching through the use of P. lilacinus and other members of the Paecilomyces spp. taxonomic grouping. By employing different fungal microorganisms for bioleaching labs can broaden their operational possibilities while finding organisms that match particular mining requirements or environmental settings¹⁸.

The fungi achieve bioleaching results through metabolic acid production together with other metabolites which dissolve metal compounds. The fungi achieve metal solubilization through their production of organic acids with three main examples being citric acid, oxalic acid and gluconic acid. The acids function as chelating agents that develop metallic ion complexes which boost their dissolution capacity in water-based solutions. The fungi can produce two major classes of chemical compounds which facilitate mineral matrix breakdown and metal release¹⁸.

Research demonstrates that combining different fungal microorganisms improves the efficiency of gold retrieval. When different fungal strains function together in bioleaching operations these organisms create beneficial effects that improve the entire process of gold extraction. The combination of different fungal strains allows the production of diverse organic acids and enzymes together with the possible synergistic functioning which leads to efficient gold solubilization. Bioleaching becomes more resistant to environmental stress because mixed cultures enhance process stability together with robustness².

4.3. Mechanisms of fungal bioleaching:

Gold bioleaching occurs through fungi use of organic acids that function as chelating agents according to¹⁸. Bioleaching depends heavily on the organic acids fungi generate specifically including citric acid together with oxalic acid and gluconic acid. The organic acids function as chelating agents to generate stable complexes with gold ions which improves their ability to dissolve in water-based solutions. The organic acid molecules bind to gold ions through coordinate bonds which neutralizes the positive gold charge therefore inhibiting its precipitation from solution¹⁸.

The formation of organic acid gold ion complexes makes extraction processes more efficient by improving their aqueous solubility rate. The compound formation between organic acids and gold ions provides the necessary condition to dissolve gold from solid materials. These organic acid-metal complexes behave as anionic particles that create electrostatic stability because of their negative charge making them dissolve in water-based solutions. The stability and solution properties of these complexes relate to both the specific organic acid type and solution pH together with additional metal ion composition¹⁸.

Modern studies demonstrate that fungal biological processes serve as essential agents to improve metal extraction from various solid waste materials. Fungi execute two fundamental metabolic processes through organic acid and enzyme production to enhance metal extraction from solid waste materials of electronic waste and low-grade ores. Through these processes both metal solubilization happens while the solid matrix breaks down which renders metals available for extraction. The performance of metal recovery can be improved by optimizing fungal metabolic activities through proper control of culture operating conditions along with selecting optimal nutrient supplies⁸.

Figure 1: Comparative process efficiency of gold recovery methods

The chart shows efficiency ratings and operational parameters for four microorganisms selected for gold extraction methods. It also includes their processing duration. The optimal conditions for A. ferrooxidans result in 85-92% recovery at pH 1.5-2.5 and 30-35°C during a 15-25-day process. The mixed culture provides optimal recovery (88-95%) under pH 1.8-3.0 and temperature ranges of 28-32°C for a processing time of 12-18 days thus establishing itself as the most suitable method for slightly acidic to neutral environments with moderate temperatures and short production times according to Table (1) and Figure (1).

5. Alternative bacterial Species:

5.1. Delftia acidovorans:

D. acidovorans uses nanoparticles to precipitate gold into a new method for extracting gold. D. acidovorans represents a bacterial species which exhibits gold ion binding properties that lead to the formation of gold nanoparticles. Delftibactin which the bacterium produces act as the primary agent to reduce gold ions into forming nanoparticles. The nanoparticle creation method stands apart from ordinary bioleaching processes because it delivers an exclusive framework for gold extraction⁷.

The bacterium transforms bioleached gold material into stable gold nanoparticles. During gold mineralization with D. acidovorans bacteria create stable gold nanoparticles that can be simply obtained from solution samples. The nanoparticle materials form spherical shapes that measure between several nanometers and multiple tens of nanometers. The solution stabilization of these nanoparticles results from Delftibactin peptide coverage which stops particle aggregation while retaining their spread throughout the solution⁷.

The use of D. acidovorans with similar bioleaching bacteria leads to improved total gold extraction rates ⁷. Bioleaching bacteria from the species C. violaceum and A. ferrooxidans display synergetic biomining behavior together with D. acidovorans for gold extraction. The gold extraction process involves other bacteria dissolving minerals from ores then D. acidovorans forms gold nanoparticles for easily reclaiming the metal. Gold extraction operations become more effective and environmentally friendly when implementing this combined bacterial method.

5.2. Stenotrophomonas sp.:

The microorganism Stenotrophomonas sp. possesses the ability to eliminate gold contamination in wastewater by carrying out the Au (III) reduction into metallic gold Au (0). Stenotrophomonas sp. functions as a bacterial agent that transforms Au (III) gold ions into Au ⁽⁰⁾elemental gold and thus separates them from wastewater pollutants. The bacterium transforms atomic gold through its metabolic actions by accepting electrons from various organic or inorganic material to transform gold ions into gold (0). The formation of elemental gold occurs in the wastewater solution so it becomes possible to extract it¹.

The recovery of gold nanocrystals occurs following the deposition stage through which these nanocrystals are obtained. During gold ion reduction through Stenotrophomonas sp. activity gold nanocrystals develop and they either settle on bacterial cell surfaces or accumulate in the environmental surroundings. The recovery process for nanocrystals contains several choices including filtration and centrifugation and adsorption. The recovery of gold nanocrystals through the utilization of Stenotrophomonas sp. provides an environment-friendly method to manage wastewater with gold contamination¹.

Research shows that Stenotrophomonas sp. possesses capabilities to clean up gold contaminants in specific locations. Stenotrophomonas sp.'s capability to extract gold from wastewater makes it suitable for applying bioremediation methods toward gold-contaminated sites particularly mining areas and industrial facilities. The bacterium receives introduction to contaminated sites to reduce gold ions into elemental gold and arrest further gold dispersion. The strategy provides both an affordable means and a sustainable treatment solution when compared to conventional remediation techniques¹.

5.3. Other Heavy Metal-Resistant Bacteria:

Priestia aryabhattai along with Enterobacter cloacae show resistance against heavy metals when isolated from gold mining locations. Bacteria extracted from gold mining sites displayed the ability to survive heavy metals during their characterization process. Bacteria which reside in environments rich with heavy metals have developed two distinct survival strategies: harmony with the metals and successful usage of metals as part of their basic processes. The bacteria Enterobacter cloacae and P. aryabhattai represent microorganisms that successfully inhabit gold mining sites¹⁹.

The bacterial DNA includes merA, cnrA and pocC genes which allow them to resist heavy metals. The resistance genes present in these bacteria allow them to detoxify and sequester heavy metals through specific genetic instructions. Mercuric reductase enzyme originates from the merA gene which reduces mercury ions until they become safer elemental mercury. Cobalt and nickel resistance originates from the cnrA gene whereas pocC provides copper resistance. Identification of these genes in P. aryabhattai and Enterobacter cloacae explains their survival capability in heavy metal-tainted locations¹⁹.

Expert scientists view these bacteria as potential agents for bioremediation studies and further research in gold mine environments. Bacteria obtained from gold mining sites show particular potential for bioremediation operations. Heavy metals do not cause harm to these bacteria because they have natural detoxification mechanisms which make them suitable for cleaning up mining site contamination. Studies of these microorganisms' metabolic capabilities together with their genetic mechanisms would enable researchers to create new bioremediation approaches for gold mining areas¹⁹.

6. Mechanisms of Microbial Gold Recovery:

6.1. Biooxidation:

Biooxidation stands as the first-line processing approach for treating refractory gold ores which include sulfide minerals according to literature sources. The extraction process of gold through cyanide leaching is limited when sulfide minerals like FeS₂ and FeAsS contain gold inside their structure. Biooxidation functions as a preprocessing operation which decomposes sulfide minerals thereby freeing gold particles so they become ready for extraction. Biooxidation plays an important role in the process of improving gold extraction rates from refractory ores^13,17.

A. ferrooxidans (Fe²⁺ → Fe³⁺ + e⁻)

4FeS₂ + 15O₂ + 2H₂O → 2Fe₂(SO₄) ₃ + 2H₂SO₄

Gold that is trapped inside sulfide matrices becomes liberated when iron and sulfur-oxidizing microorganisms perform sulfide mineral oxidation. The biooxidation process uses A. ferrooxidans along with A. thiooxidans as well as other iron and sulfur-oxidizing microorganisms to play this crucial role. An oxidation reaction occurring between microbial bacteria enables them to transform sulfide minerals by transforming them into soluble sulfates which frees up the gold found in the mineral structure. Biochemical reactions form a complex sequence that the bacterial enzymatic systems catalyze through the oxidation process^4,13.

Through ferric ions and hydrogen ion production the dissolution of gold occurs by oxidant action. The chemical activity of iron and sulfur-oxidizing bacteria during sulfide mineral oxidation generates both hydrogen (H⁺) and ferric (Fe³⁺) ions. The extracted minerals dissolve due to gold oxidation enabled by these ions acting as oxidants. The ferric ions develop exceptional ability to transform elemental gold into gold ions (Au³⁺) that build soluble complexes with cyanide or additional ligands¹⁶.

6.2. Biocyanidation:

The production of cyanide as a secondary metabolite falls under the capacity of bacteria including C. violaceum. The secondary metabolite cyanide production of certain bacteria functions as the base of the biocyanidation process for dissolving gold. Cyanide produces C. violaceum among bacteria which hold cyanide production capabilities. The cyanide that bacteria of this kind create interacts with gold minerals to develop extractable soluble gold-cyanide complexes^7,9.

The chemical reaction between cyanide and gold results in soluble gold-cyanide complexes according to research in. The microbes produce cyanide to create Au (CN)^2- which dissolves gold atoms. The aqueous solution stability of this complex enables extraction of gold from the solid mining ore material. The efficiency of this extraction relies strongly on three major system components which include cyanide concentration and pH value along with other metals which take part in cyanide complexation^{7, 9}.

Au⁰ + 2CN⁻ → [Au (CN)₂] ⁻

The application of cyanide-producing bacteria for extraction is restricted by two main limitations which include low cyanide toxicity along with difficult cyanide production optimization. The application of cyanide-producing bacteria for gold extraction encounters multiple restrictions despite their extraction potential. Higher cyanide concentrations become necessary for effective gold dissolution because its acquired toxicity levels remain lower than those from other lixiviants including thiourea. Cyanide production optimization by these bacteria faces difficulties because environmental conditions together with nutritional resources affect its generation¹⁰.

6.3. Organic acid Production:

A. niger among other fungi secretes organic acids which help dissolve gold material. The fungal organism A. niger and other fungal species naturally generate two types of organic acids including citric acid together with oxalic acid. The acids act as metal ion chelating agents to form solution complexes which enhance the extractability of metals in water. The bioleaching operation that A. niger performs greatly depends on its ability to produce these organic acids that dissolve metals and gold^{8, 18}.

The solubility of gold ions rises through their complex formation with organic acid chelating agents. The formation of gold ion complexes through organic acids enhances their solution rate until extraction becomes possible. Gold solubilization requires solid matrixes to undergo complex formation through organic acids uniting with gold ions. Such metal complexes possess an anionic charge system that facilitates their dissolving in aqueous solutions¹⁸.

Such acid production depends on both solution pH levels and nutrient sources in the environment. The process through which fungi produce organic acids depends on three elements such as pH, nutrients and temperature conditions. A. niger produces its most efficient organic acids at a pH level ranging from 2.0 to 3.0. Organic acid production undergoes significant stimulation through the availability of necessary nutrients and especially through the presence of carbon molecules in the growth medium¹⁸.

7. Applications of Bioleaching:

7.1. Processing Refractory Gold Ores:

The wide adoption of bioleaching occurs to process refractory gold ores because the conventional methods fail to extract gold from sulfide minerals. The ores need prior treatment to break free the gold which creates conditions for cyanidation extraction and additional processing techniques. Bioleaching functions as a successful pretreatment method which is friendly to the environment^13,17.

Refractory ores capture gold inside sulfide minerals such as pyrite and arsenopyrite hence they resist regular leaching solutions. Sulfide minerals seal off the gold particles so leaching agents cannot reach the gold particles. A pretreatment method needs to be applied for breaking down sulfide minerals¹⁷.

Biooxidation enables gold extraction by cyanide through oxidation of sulfide minerals which liberates gold particles. Iron and sulfur-oxidizing bacteria serve during biooxidation to speed up sulfide mineral oxidation through their catalytic activity. After biooxidation the recovered gold becomes accessible through cyanide extraction together with alternative solutions¹⁸.

7.2. Gold recovery from electronic waste:

Bioleaching processes target electronic waste because it contains abundant gold and other precious metals to make it a lucrative candidate according to research. The worldwide surge of e-waste requires new sustainable and effective recycling techniques to handle the increasing amounts of waste. The bioleaching approach presents itself as a superior methodology compared to conventional methods in e-waste recycling^7,20,21.

The growing worldwide e-waste generation requires the establishment of environmentally-friendly and efficient recycling technologies. Traditional e-waste recycling operations use high quantities of energy together with harsh chemicals which produce environmental harms while generating substantial expenses. Bioleaching presents both ecological advantages and economic benefits to the process²⁰.

Bioleaching represents a sustainable method for e-waste gold extraction as it substitutes traditional methods which use harsh chemicals and consume large amounts of energy. The method minimizes environmental destruction during e-waste treatment while it helps gather essential resource materials from the waste stream. Through bioleaching technology, the recovery of copper alongside palladium and silver metals is possible from e-waste materials²¹.

The process minimizes environmental harm throughout e-waste disposal because it cuts down on toxic chemical requirements and energy requirements. Bioleaching outperforms conventional e-waste procedures because it produces fewer wastes thus minimizing its environmental effect. Bioleaching that uses microorganism’s functions as an advanced method of e-waste recycling because it provides environmental sustainability through its operations²¹.

7.3. Bioremediation of mining sites:

Through bioleaching techniques heavy metals along with pollutants are removed from contaminated mining sites thus benefiting environmental cleanup efforts. During mining operations heavy metals get released into local soils and water thus endangering human health together with environmental systems. Bio-remediation provides an economical as well as environmentally sustainable technique for addressing contamination issues^22,23.

The capability of microorganisms to clean heavy metals and pollutants from contaminated sites exists through transforming these contaminants into less dangerous products. The removal process uses different methods starting with biosorption followed by bioaccumulation and ending with biotransformation. Specified microbial methods depend on two factors: the microorganism type as well as the polluting substance type²³.

The reduction of environmental footprints through mining operations becomes possible because heavy metal contamination effects are minimized. Mining sites regain their ecological wellness while human health protection happens with the implementation of bioremediation methods. Nor does it degrade the available natural resources²².

8. Advantages and Disadvantages of Bioleaching:

8.1. Advantages:

Bioleaching serves as an environmentally beneficial approach for mining since it replaces traditional mining methods that require strong chemicals and create extensive manufacturing wastes. The environmentally friendly extraction process provides a sustainable solution for gold recovery in accordance with rising environmental rules. The implementation of minimal chemicals helps to protect water sources from pollution together with soil pollution^1,21.

The process becomes cheaper for low-grade ores and electronic waste operations because it uses less power along with fewer chemicals. The technique creates financial feasibility to extract previously marginal resources in the industry. The reduced expenses provide greater financial gain to gold extraction operations³.

Bioleaching demonstrates powerful effectiveness in treating different sources of gold extraction including refractory ores and electronic waste materials. The tool shows value in resource recovery because it efficiently processes multiple material types. The high levels of extraction provide big economic opportunities⁸.

8.2. Disadvantages:

The extraction speed of bioleaching through microbes operates at a prolonged rate extending from weeks to months because this creates challenges in commercial-scale gold mining. The slowing reaction speeds force operators to utilize big leaching volumes since process times extend for long durations. Bioleaching methods tend to increase both capital expenditure and operational costs for industry applications²⁴.

The bioprocessing depends sensitively on environmental factors including pH levels and temperature as well as the availability of nutrients which need precise monitoring and control to achieve optimal microbial function. The performance of gold extraction becomes less efficient when these environmental conditions experience variations. It becomes difficult to keep parameters consistent when performing bioleaching at industrial scale¹⁶.

Some bioleaching procedures generate toxic byproducts like cyanide which needs proper management to stop environmental damage thus requiring specialized waste handling strategies. Safety measures should be implemented to protect against both exposure and environmental release of cyanide because it has the potential to form during bioleaching procedures. The bioleaching procedures become more complex and expensive to operate when production takes place on a large scale⁷.

9. Future trends in Bioleaching:

9.1. Genetic Engineering of Microorganisms:

Scientists explore bioleaching microorganism’s genetic modification to boost their gold dissolving capacity and improve their environmental durability. Science-based modification of microorganism genetics enables researchers to enhance their metabolic capabilities for extracting gold. The designated technique demonstrates exceptional strength for simultaneously improving both bioleaching system sustainability and efficiency levels^10,14.

Modifying microorganisms results in better gold dissolution power that both enhances process speed and reduces overall time requirements. The efficiency of bioleaching grows best when researchers improve cyanide or organic acid production or enhance the mineral attachment and oxidation capacity. Microbial genetic modification enables scientific researchers to develop cellular resistance against heavy metals and toxic compounds¹⁰.

Bioleaching enables maximum tolerance to adverse circumstances alongside minimal waste generation thus making it both sustainable and environmentally beneficial for the process. The enhancement of heavy metal resistance existed in microorganisms can be achieved by genetic engineering methods while toxin compounds decrease through modified metabolic pathways. The application of genetic engineering techniques represents an efficient way to improve bioleaching process environmental efficiency¹⁴.

9.2. Development of Novel Bioleaching Processes:

The development of modern bioleaching operations by research teams aims to enhance operational performance while looking for solutions to fix existing bioleaching method limitations. Modern high-tech methods apply in these processes work to maximize both extraction values and environmental preservation. New bioleaching methods need research development because this advancement will promote the implementation of sustainable technology throughout industries².

Bioleaching effectiveness increases through using mixed microbial cultures since these diverse microorganisms work together to produce efficient cooperation. A mixture of microorganisms achieves better metal dissolving outcomes which enhances recovery rates. Several microorganisms participating in a single microbial culture show enhanced resistance against environmental fluctuations⁴.

The technology applies improved bioreactor systems to bioleaching methods which integrate operations based on hybrid approaches between bioleaching and chemical leaching and solvent extraction per². The optimized characteristics of bioreactor systems enable improved bioleaching process control and optimization features. The combination of bioleaching with complementary extraction methods makes the processes more efficient and sustainable².

Table 2: Timeline of bioleaching innovations.

Period	Development	Commercial Adoption
1950s	Discovery of metal-oxidizing bacteria	Lab scale
1980s	First copper bioleaching plants	Industrial
2000s	Genetically enhanced strains	Pilot plants
2010s	E-waste applications	23% of gold recycling
2020s	Nanobiohybrid systems	Emerging tech

9.3. Application of Nanotechnology:

The field of nanotechnology produces new approaches and methods which enhance gold dissolution and recovery during bioleaching research. Nanomaterials offer distinctive features that researchers can exploit to advance the efficiency together with sustainability of bioleaching operations. Nanotechnology presents numerous possible functions within the field of bioleaching per²⁵.

The application of nanoparticles boosts both gold dissolution and recovery performance through improved microbial contacts with enlarged surfaces while stabilizing gold complex connections. Scientists use nanoparticles to transport vital nutrients along with essential compounds which results in increased growth rates and improved activity of microorganisms. The effectiveness of bioleaching procedures increases substantially through nanoparticle utilization²⁶.

Bioleaching operations become less environmentally harmful because the utilization of toxic chemicals decreases and waste production decreases. The bioleaching solution can administer heavy metal and pollutant extraction using nanoparticles through selective mechanism. When combined with gold nanoparticles the nanomaterials function to prevent their escape into the environment as well as to provide protective encapsulation^25,26.

10. Synergistic approaches and integrated biorefineries:

10.1. Combining bioleaching with phytoremediation:

The combination of bioleaching and phytoremediation produces a partnership which boosts gold extraction alongside environmental site renewal. The combined strategy optimizes the abilities of both methods to generate better sustainable and efficient results. Through the combination of bioleaching with phytoremediation operations convert sites with environmental problems into economically worthwhile resources^12,23.

The simultaneous use of Bioleaching for metal extraction from mineral wastes and phytoremediation for soil and water heavy metal cleanup provides an effective solution. Both techniques operate in a self-sustaining manner because the metals extracted from bioleaching provide essential nutrients for plant operation in phytoremediation. The combined methodology delivers substantial environmental advantages to mining operations according to¹².

The tissues of plants used for phytoremediation will accumulate heavy metals until they reach harvestable levels from which the metals can be recovered as value-added products. The outcome of this method known as phytomining produces financial gains that help pay for remediation expenses. The processed plant biomass produces additional income through two purposes: energy generation and various valuable industrial uses²³.

10.2. Integrated biorefineries for gold and other resources

When bioleaching integrates with multiple bioprocessing technologies inside an integrated biorefinery the system becomes capable of complete resource recovery and waste reduction for a sustainable circular economic model. Biorefineries feature designs that accept ores as well as electronic waste and agricultural residues simultaneously with other components. Bioprocessing refineries gain improved sustainability and performance through integrating various processing technologies according to research findings^{1, 2}.

The biorefineries generate various valuable production items including gold together with other metals along with biofuels and bioplastics which creates multiple income streams. The simultaneous manufacturing of multiple products enables biorefineries to distribute operation expenses while making their financial operations more profitable. The biorefinery becomes more resistant to market instability because its diversified product range decreases dependence on a single output².

Bioleaching when integrated with other bioprocessing technologies creates systems which generate less waste during natural resource extraction as well as processing operations. The integration of processes through waste stream recovery enables feedstock circulation which establishes a waste reduction system that supports maximum resource usage¹.

11. CONCLUSIONS:

The review highlights bioleaching as a sustainable and eco-friendly alternative to traditional gold extraction methods, which often involve high environmental and economic costs. Microbial processes, utilizing bacteria like C. violaceum, P. aeruginosa, and fungi such as A. niger, effectively solubilize gold through mechanisms like biooxidation, biocyanidation, and organic acid production. These methods are particularly advantageous for low-grade ores and electronic waste, offering cost-effective and environmentally benign solutions. However, challenges such as slow processing times, sensitivity to environmental conditions, and potential toxic byproducts like cyanide must be addressed. Future advancements in genetic engineering, novel bioleaching processes, and nanotechnology integration promise to enhance efficiency and scalability, positioning bioleaching as a key player in sustainable gold recovery and circular economy initiatives.

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Received on 19.08.2025 Revised on 04.09.2025

Accepted on 22.09.2025 Published on 06.11.2025

Available online from November 11, 2025

Asian J. Research Chem.2025; 18(6):409-419.

DOI: 10.52711/0974-4150.2025.00062

This work is licensed under a Creative Commons Attribution-Non Commercial-Share Alike 4.0 International License. Creative Commons License.

Microorganism	Gold Recovery %	Optimal pH	Temperature (°C)	Process Time
A. ferrooxidans	85-92	1.5-2.5	30-35	15-25 days
C. violaceum	70-87	7.0-8.5	25-30	10-20 days
A. niger	65-80	2.0-3.0	25-28	20-30 days
Mixed Culture	88-95	1.8-3.0	28-32	12-18 days