Bioleaching of Copper Sulfide Ores: From Microbial Mechanisms to Industrial Applications

Abstract

The global energy transition and rapid electrification are driving increased demand for copper. However, conventional pyrometallurgical and hydrometallurgical extraction routes are increasingly challenged by declining ore grades and stricter environmental regulations. Bioleaching involves the microbial catalysis of sulfide mineral dissolution and provides a sustainable method for copper recovery from low-grade ores, tailings and secondary resources. This review provides a critical and integrated analysis of copper sulfide bioleaching, covering microbial diversity, molecular mechanisms, mineralogical controls, operational parameters, and industrial applications. This review also examines the functional roles of prominent acidophiles, including the functional roles of prominent acidophiles, including Acidithiobacillus spp., Leptospirillum spp. and thermophilic archaea, in the oxidation of iron and sulfur, mitigation of passivation, and metal solubilization. The molecular underpinnings of these processes are explored by investigating iron and sulfur oxidation gene networks (the rus operon and sox cluster), copper resistance systems (CopA, CusCBA) and biofilm formation pathways. The mineralogical controls on the behavior of chalcopyrite (refractory/passivating), chalcocite (highly reactive) and bornite (intermediate) are critically assessed. The synergistic effects of key operational parameters (temperature, pH, redox potential, aeration and particle size) on leaching kinetics and microbial community dynamics are investigated. The scalability, efficiency and environmental footprint of industrial applications such as heap, dump, stirred-tank and in situ bioleaching are discussed. Despite more than four decades of commercial development, several challenges remain, such as slow chalcopyrite dissolution, passivation, metal toxicity, and scale-up limitations. Emerging solutions such as synthetic microbial consortia, multi-omics technologies, artificial intelligence-assisted optimization, and digital twins are identified as transformative approaches for next-generation biomining. In this review, microbiology, mineralogy, electrochemistry, and process engineering are integrated to demonstrate that biotechnological leaching is among the most promising technologies for the sustainable production of copper and to identify future directions for its industrial application.

1. Introduction

Copper is an essential metal for modern industry, and its unparalleled properties, such as conductivity, corrosion resistance, and recyclability, ensure the development of modern infrastructure. This metal is one of the main materials used in electrical transmission networks, environmental technologies, electric vehicles, and other modern industrial applications. With the acceleration of the carbon neutrality commitments from the nations, the global demand for copper is expected to increase by 40–60% in the next two decades [1].

In the last three decades, the average grade of newly discovered copper deposits has been steadily decreasing and, today, despite substantial remaining reserves, average ore grades have fallen below 0.5% Cu [1,2]. Mining operations are increasingly treating low-grade and mineralogically complex sulfide ores, primarily chalcopyrite (CuFeS₂), chalcocite (Cu₂S) and bornite (Cu₅FeS₄), which represent the bulk of the world’s copper resource. Among these, chalcopyrite presents the greatest processing challenge because of its slow dissolution kinetics and the formation of passivation layers (elemental sulfur, jarosite and iron oxyhydroxides) which do not allow continuous mineral oxidation [3].

Conventional copper extraction remains dominated by pyrometallurgical processing, which provides high recoveries (98–99%) but requires temperatures above 1200 °C, consumes 40–60 GJ per ton of copper and generates 2–3 tons of CO₂ per ton of copper [4]. Hydrometallurgical alternatives are conducted at lower temperatures but use concentrated chemical reagents and generate secondary waste streams. Interest in bioleaching is driven by stricter environmental regulations and decreasing ore grades that are increasingly challenging these technologies [4].

Bioleaching employs indigenous acidophilic microorganisms that derive metabolic energy metabolic energy obtained from the oxidation of ferrous iron and reduced sulfur compounds [5,6,7,8,9]. Iron-oxidizing microorganisms regenerate ferric iron (Fe³⁺), the main chemical oxidant for sulfide minerals [10,11], and sulfur-oxidizing consortia prevent the accumulation of reduced sulfur intermediates that cause passivation. This continuous interaction enables metal recovery at ambient temperature, with considerably lower energy requirements and environmental impacts [11].

The performance of a bioleaching system results from complex interactions among microbial community structure, mineralogical properties, and physicochemical conditions [5,6,7,8,9,12]. Microbial activity, mineral dissolution kinetics, and copper recovery are collectively regulated by key parameters such as temperature, pH, redox potential oxygen and carbon dioxide availability, particle size, and pulp density [13]. Present-day bioleaching systems are integrated biological, electrochemical, and geochemical systems that must be optimized as a whole [14].

However, despite four decades of industrial application, copper bioleaching faces persistent challenges, including slow dissolution kinetics (especially for chalcopyrite), passivation layer formation, metal toxicity, and environmental fluctuations affecting community stability [15]. Recent advances in comparative genomics, transcriptomics, proteomics and systems biology have begun to provide insights into the molecular mechanisms involved in microbial adaptation, iron oxidation, sulfur metabolism, biofilm formation and metal resistance [16]. These advances facilitate the engineering of robust microbial consortia and optimization of industrial processes [17].

Several reviews have addressed different aspects of copper bioleaching, such as microbial diversity, mineral dissolution mechanisms, reactor design, and industrial applications [4,13]. However, these reviews have largely focused on isolated topics and provide limited integration of microbial physiology, molecular pathways, mineralogical characteristics, and recent biotechnological advances [18]. Emerging research on synthetic microbial consortia, multi-omics technologies, artificial intelligence-assisted optimization, and metabolic engineering has yet to be comprehensively integrated [17,18].

This review provides a critical and comprehensive evaluation of copper sulfide bioleaching by linking the current understanding of microbial diversity, molecular mechanisms, mineral-specific leaching behaviors, operational factors, industrial applications and emerging technologies. This study emphasizes the dynamic interactions between microorganisms, minerals and process conditions that control bioleaching performance, critically reviews current limitations and highlights future research directions.

2. Mineralogical Controls on Bioleaching Behavior of Copper Sulfide Ores

There are significant differences in the chemical composition, crystal structure, surface reactivity and electrochemical properties of copper sulfide minerals that strongly influence bioleaching efficiency. Susceptibility to oxidative dissolution is controlled by mineralogical properties, as are the formation of a passivation layers and the accessibility of microbial oxidants to reactive surfaces.

2.1. Chalcopyrite: The Refractory Challenge

Chalcopyrite is the most abundant mineral in the world’s copper reserves [19,20]. It accounts for about 70% of the world’s copper resources. Paradoxically, chalcopyrite is the most refractory copper sulfide mineral to bioleaching because of its intrinsically slow dissolution kinetics and the formation of a passivation layers composed of elemental sulfur, jarosite, and iron oxyhydroxides. Such precipitates impede electron transfer between the mineral surface and ferric iron and thus greatly reduce leaching rates. Chalcopyrite is a p-type semiconductor with a band gap of ∼0.6 eV, and the electron transfer rate from chalcopyrite to the oxidant is rate-limiting. This electrochemical constraint explains the need for high temperatures or specialized microbial consortia capable of maintaining high redox potentials [21,22].

2.2. Chalcocite: The Ideal Candidate

Chalcocite is bioleached more efficiently because of its simpler crystal structure and higher chemical reactivity. The oxidation of chalcocite occurs mainly via the thiosulfate pathway, which produces soluble intermediates that are more easily oxidized than those generated during chalcopyrite dissolution [23,24]. Thus, chalcocite can achieve copper recoveries greater than 90% in days to weeks, with shorter leaching times and little passivation. The rapid dissolution kinetics make chalcocite particularly amenable to bioleaching and account for its preferential treatment in many industrial operations [25,26].

2.3. Bornite: The Intermediate Case

Bornite showed intermediate bioleaching behavior between chalcopyrite and chalcocite [27]. It is generally more readily leached than chalcopyrite; however, extended bioleaching may cause surface changes that decrease oxidation rates. The bioleaching performance of bornite is highly variable, depending on its complex stoichiometry (Cu₅FeS₄), mineral purity, grain size distribution, associated gangue minerals, sulfide phase composition, microbial community structure, and operating conditions. This variability is mainly attributed to the complex oxidation pathways and phase transformations during mineral dissolution [28].

2.4. Gangue Minerals Effects and Electrochemical Considerations

The association of copper sulfide minerals with gangue minerals significantly affects bioleaching behavior. Silicate and carbonate gangue minerals can consume acid, buffer pH, and promote ferric iron precipitation, thereby limiting the availability of oxidants and inhibiting micro-biological activity [29,30]. Pyrite (FeS₂), however, generates ferric iron and acid during oxidation, which promotes the bioleaching of nearby copper sulfide minerals through galvanic interactions. In heap bioleaching, the acid-consuming capacity of gangue minerals is particularly important, as the acid balance directly affects microbial activity and metal recovery.

Special attention should be paid to the electrochemical nature of sulfide mineral dissolution. Sulfide minerals are semiconductors, and their dissolution is controlled by electrochemical reactions involving electron transfer from the mineral to the oxidant. Band structure, conductivity, and surface states control electron transfer kinetics and oxidative dissolution rates. Chalcopyrite is refractory due to its high resistance and wide band gap, while the metallic conductivity of chalcocite permits fast electron transfer. Understanding these electrochemical principles enables the rational design of strategies for enhancing dissolution rates, e.g., through catalytic ion addition or the application of an external potential [31,32].

2.5. Mineral Liberation and Characterization

The bioleaching behavior of copper sulfide minerals varies with their crystal structure, chemical composition, oxidation states, and associations with gangue minerals. These mineralogical variations influence the surface reactivity, electron transfer, the dissolution kinetics, and the formation of secondary passivation products during bioleaching. Thus, the interactions between sulfide mineral phases and their physicochemical properties must be understood to predict the leaching performance and develop effective extraction strategies [33,34]. Liberation characteristics, i.e., the degree of exposure of valuable minerals at particle surfaces, determine the mineral’s accessibility to microbial attack. Poor liberation leads to encapsulated sulfides that remains undissolved even after long leaching times. In natural copper sulfide ores, incomplete liberation is frequently related to complex mineral intergrowths, where chalcopyrite, chalcocite, bornite or other sulfide phases occur as locked or partially locked grains within gangue matrices. These textural relationships directly affect the accessibility of sulfide surfaces to microbial cells, ferric iron and other oxidizing agents. The presence of sulfide minerals in silicate, carbonate or iron-bearing gangue phases limits the available reactive surface area, leading to slower dissolution kinetics and suboptimal copper recovery even under favorable bioleaching conditions. Mineral intergrowths can also create heterogeneous surface environments that affect the formation of secondary phases and passivation layers during oxidation. Quantitative X-ray diffraction, scanning electron microscopy, and automated mineralogy are important tools for predicting bioleaching performance.

In the mineral industry, understanding mineral liberation is critical for designing appropriate grinding and pretreatment strategies. Over-grinding increases energy consumption and can affect heap permeability, while under-liberation limits microbial access and copper extraction. Consequently, optimization of particle size distribution must consider mineral exposure and the downstream impact on solution flow, microbial attachment, and passivation behavior. More advanced characterization approaches, including time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), and synchrotron-based X-ray absorption spectroscopy, provide deeper insights into mineral surface chemistry, passivation layer composition, and reaction mechanisms [35]. These tools, together with computational modelling and machine learning, enable a predictive understanding of mineral behavior and the rational design of optimized bioleaching strategies [36].

The main mineralogical factors controlling copper sulfide bioleaching are summarized in Figure 1. The figure illustrates the relationships between mineral crystal structure, surface properties, and electrochemical behavior, and their effects on microbial colonization efficiency, passivation layer formation and electron transfer pathways by these properties. The distinct behaviors of chalcopyrite (refractory, strong passivation), chalcocite (highly reactive, weak passivation), and bornite (intermediate) are highlighted, emphasizing the importance of mineral-specific processing strategies is stressed. The key characteristics of these three minerals are summarized in

Table 1. Comparative characteristics of major copper sulfide minerals commonly processed by bioleaching.

Mineral	Chemical Formula	Crystal System	Relative Bioleaching Rate	Primary Passivation Products	Dissolution Pathways	Typical Cu Recovery	Industria Significance
Chalcopyrite	CuFeS2	Tetragonal	Low (0.5–1×)	S0, Jarosite (FeOOH),	Polysulfide	(30–60% (months-years))	Largest world reserve; most refractory [37,38,39]
Chalcocite	Cu2S	Orthorhombic	High (5–10×)	Limited S0	Thiosulfate	>90% (days–weeks)	High recovery; amenable to bioleaching [38]
Bornite	(Cu5FeS4)	Cubic/orthorhombic	Moderate (2–4×)	Variable	Mixed	50–85%	Intermediate industrial importance [40,41]

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3. Microbial Diversity and Function in Copper Bioleaching

The success of copper sulfide bioleaching relies on the metabolic activities of acidophilic microorganisms that facilitate mineral dissolution. These microbial communities are complex and dynamic, with different species performing complementary roles. Their diversity, physiology and ecological interactions are essential for optimizing bioleaching performance.

3.1. Acidithiobacillus Species: Primary Bioleaching Catalysts

The most studied microorganisms in biomining are Acidithiobacillus species, which are generally regarded as the principal biological catalysts for the bioleaching of copper sulfides. These chemolithoautotrophic bacteria are adapted to highly acidic environments (pH 1.5–3.0) and couple the oxidation of ferrous iron and reduced sulfur compounds with energy conservation [42,43]. Acidithiobacillus ferrooxidans is the main model organism among acidophiles because of its metabolic flexibility, well-characterized genetics and industrial relevance.

A. ferrooxidans is an energy-metabolizing acidophile that oxidizes ferrous iron (Fe²⁺ → Fe³⁺) and reduced sulfur compounds (S₀ → SO₄²⁻). The generated ferric iron acts as the main oxidant for the indirect dissolution of sulfide minerals. High redox potential and mineral oxidation are supported by the continuous regeneration of ferric iron. A. ferrooxidans produces extracellular polymeric substances (EPS) that mediate microbial attachment and biofilm formation. Biofilms create localized microenvironments with higher ferric iron concentrations that favor mineral dissolution.

The performance of Acidithiobacillus species depends on mineral composition, copper concentration and environmental conditions. High levels of dissolved copper (>5–10 g/L) inhibit microbial growth and thus require adaptation for stable industrial performance. Copper tolerance mechanisms include efflux pumps (CopA, CusCBA) and copper-binding proteins (CopZ)that help cells survive under copper stress. Bioleaching in cold environments has attracted interest because of the psychrotolerant species Acidithiobacillus ferrivorans, which is able to grow at 5 °C [44,45].

3.2. Leptospirillum spp.: Specialized Iron Oxidizer

Unlike Acidithiobacillus, Leptospirillum species are obligate iron oxidizers, deriving energy exclusively from the oxidation of ferrous iron [46,47]. Their high affinity for Fe²⁺ allows them to sustain high redox potentials, even at very low pH. This kinetic advantage allows Leptospirillum species to dominate many industrial bioleaching communities, especially at low pH and high redox potentials, where A. ferrooxidans is outcompeted.

Leptospirillum ferriphilum, in particular, is capable of maintaining high redox potentials (>800 mV) that favor the oxidation of refractory sulfide minerals. Rapid regeneration of ferric iron stimulates the indirect dissolution of minerals and sustains sulfur-oxidizing microorganisms [48]. Leptospirillum and Acidithiobacillus species are often found together and form metabolically complementary communities rather than competing for ecological niches [49].

3.3. Sulfur-Oxidizing Microbes

Sulfur oxidation is essential for the bioleaching of copper. The oxidative dissolution of sulfide minerals produces elemental sulfur and reduced sulfur intermediates that accumulate on mineral surfaces [50]. If not continuously oxidized, these compounds can form passivation layers that greatly reduce dissolution rates. Sulfur-oxidizing microorganisms prevent accumulation by oxidizing reduced sulfur species to sulfate [51].

Acidithiobacillus thiooxidans is one of the most efficient sulfur-oxidizing microorganisms in acidic environments. This organism oxidizes reduced sulfur compounds to sulfate, thus preventing the accumulation of sulfur and maintaining acidic conditions. The sulfur oxidation pathway includes several enzymes: the Sox multienzyme complex, sulfide: quinone oxidoreductase (SQR), sulfur dioxygenase (SDO), and tetrathionate hydrolase (TetH). Coordinated expression of these enzymes ensures efficient sulfur turnover and prevents passivation [52]. This complementary function greatly enhances the efficiency of iron-oxidizing microorganisms during long-term bioleaching operations.

3.4. Thermophilic Microbes

Thermophilic microorganisms have received considerable attention for processing refractory minerals such as chalcopyrite. Species of Sulfobacillus, Metallosphaera, and Acidi-anus exhibit higher metabolic activity at elevated temperatures (45–80 °C) and higher mineral dissolution rates than mesophilic microorganisms [53,54]. The thermophilic archaeon Acidianus brierleyi can oxidize chalcopyrite more rapidly than A. ferrooxidans at high temperatures.

Increasing temperature enhances microbial metabolism, accelerates mineral oxidation kinetics, and reduces the formation of passivation layers. However, thermophilic bioleaching requires a higher process control and energy input, which limit its application [55]. Sulfobacillus thermosulfidooxidans is a moderately thermophilic species with an optimum growth of 45–55 °C, whereas Metallosphaera sedula is an extreme thermophile that requires growth temperatures of 65–80 °C. Process economics and heat management, in addition to reactor design, should be taken into account when determining the most appropriate thermophilic consortia [56].

3.5. Microbial Communities and Dynamics

Contemporary bioleaching systems rarely rely on a single microorganism. Different species perform complementary metabolic functions. Hence, mixed microbial consortia can outperform pure cultures [57]. Iron oxidizers regenerate ferric iron, sulfur oxidizers remove passivating sulfur compounds, heterotrophs recycle organic metabolites, and thermophiles increase oxidation kinetics at elevated temperatures. This functional complementarity confers resilience to environmental fluctuations and enhances process stability.

Recent metagenomic and transcriptomic studies show that microbial community composition changes dynamically during bioleaching, allowing adaptation to changing mineral composition, oxygen availability, and metal concentrations [58]. Community structure is determined by pH, temperature, redox potential, substrate availability, and microbial interactions. Understanding the ecological drivers of community structure may enable rational manipulation of microbial communities through process control [59].

Heterotrophic microorganisms may also produce growth factors that support autotrophic community members. The rational design of microbial consortia is an emerging frontier. Researchers are investigating defined consortia containing organisms with specific, complementary metabolic capabilities optimized for iron oxidation, sulfur oxidation, metal resistance, and biofilm formation [60]. Ferric iron Fe³⁺ is the main chemical oxidant of sulfide minerals and is regenerated through microbial oxidation of Fe²⁺ by iron oxidizing microorganisms including Acidithiobacillus ferrooxidans and Leptospirillum spp. as shown in Figure 2. Acidithiobacillus thiooxidans and Sulfobacillus spp. utilize reduced sulfur compound as an energy source and oxidize them to sulfate, thereby limiting the formation of passivation layers. Thermophilic archaea, including Metallosphaera and Acidianus, enhance mineral dissolution at elevated temperatures, while heterotrophic microorganisms recycle organic metabolites. The substrate and product flows and the metabolic complementarity are indicated by arrows, emphasizing the cooperative nature of functional bioleaching consortia. The functional properties of the most important bioleaching microorganisms are presented in

Table 2. Functional characteristics of representative microorganisms involved in copper sulfide bioleaching.

Microorganism	Energy Source	Temperature Optimum (°C)	pH Range	Function	Metabolic Features	Relevance to Industry
Acidithiobacillus ferrooxidans	Fe2+, S0	S0 28–35	1.5–2.8	Oxidizes iron and sulfur	Rus operon, EPS production, versatile metabolism	Bioleaching bacteria [61]
Leptospirillum ferriphilum	Fe2+	30–37	1.0–2.0	Iron oxidation	High Fe2+ affinity; high redox potential	Dominant in low pH piles [62]
Acidithiobacillus thiooxidans	reduced S0 and S	28–35	0.5–2.5	Sulfur oxidation	Sox complex, SQR, and TetH	Prevents sulfur poisoning [63]
Sulfobacillus thermosulfidooxidans	Fe2+, SO42−	45–55	1.5–2.5	Thermophilic oxidation	Mixotrophic, moderately thermophilic	Improved chalcopyrite dissolution [64]
Metallosphaera sedula	Fe2+, S0	65–75	1.0–3.0	Oxidation at high temperature	Archaeal metabolism; hyper thermophilic	Industrial thermophilic [65]
Acidianus brierleyi	Fe2+, S0	65–70	1.0–2.5	Hochtemperatur-oxidation	Archaeal iron oxidation; tolerance to high metals	Chalcopyrite leaching enhancement [66]

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4. Molecular and Biochemical Mechanisms of Copper Sulfide Bioleaching

The bioleaching of copper sulfide is governed by the tight coupling of biological, chemical and electrochemical reactions that control mineral dissolution and copper recovery. Bioleaching is an integrated system in which microorganisms continuously regenerate oxidants, maintain an acidic environment, and catalyze electron transfer reactions that lead to mineral oxidation. Efficiency depends on the combined action of microbial metabolism, mineral surface chemistry and environmental conditions.

4.1. Iron Oxidation Pathway

Iron oxidation is the main energy-producing pathway in most acidophilic bioleaching microorganisms and is also responsible for the indirect dissolution of copper sulfides. Iron-oxidizing microorganisms, including A. ferrooxidans and L. ferriphilum, regenerate ferric iron (Fe³⁺), which acts as the major chemical oxidant during sulfide mineral dissolution [67,68]. During mineral oxidation, ferric iron is reduced to ferrous iron, which is subsequently re-oxidized by microorganisms, thereby sustaining the iron redox cycle.

At the molecular level, electrons released during Fe²⁺ oxidation are transferred through an electron transport chain that includes the outer membrane cytochromes (Cyc2), the periplasmic blue copper protein rusticyanin (Rus), cytochrome c (Cyc1), and the terminal oxidases (CoxABCD) and so on [69,70,71,72,73,74,75]. This electron transfer generates the proton motive force that drives ATP synthesis and carbon fixation, while simultaneously regenerating ferric iron for continued mineral dissolution [76].

4.2. Sulfur Oxidation Pathway

Elemental sulfur and reduced sulfur intermediates formed during the oxidation of sulfide minerals are deposited on mineral surfaces. In the absence of continuous oxidation of sulfur, these compounds form passivation layers that block electron transfer and mineral accessibility. Sulfur-oxidizing microorganisms play an essential complementary role by oxidizing reduced sulfur compounds to sulfate and sustaining acidic conditions.

Sulfur oxidation involves several enzymatic pathways: the Sox multienzyme complex (SoxABXYZ), Sulfide:quinone oxidoreductase (SQR), sulfur dioxygenase (SDO) and tetrathionate hydrolase (TetH) [50]. The coordinated oxidation of sulfur compounds prevents their accumulation and promotes mineral dissolution [24]. The efficiency of sulfur metabolism is particularly critical for refractory minerals such as chalcopyrite, for which sulfur passivation is a major limitation.

4.3. Direct and Indirect Mechanisms

Two main mechanisms have been proposed for microbial dissolution of sulfide minerals. In the so-called “direct mechanism” as historically described, microorganisms adhere to mineral surfaces via EPS and form biofilms. It is generally accepted, however, that the main function of biofilms is to create localized microenvironments with higher concentrations of ferric iron and improved mass transfer rather than to directly oxidize the mineral surface enzymatically [77,78].

The indirect mechanism is now considered to be the dominant pathway in industrial copper bioleaching. Microbial iron oxidation regenerates ferric iron. During the chemical oxidation of sulfide minerals, copper ions are released and ferric iron is reduced to ferrous iron, which is subsequently recycled through the microbial oxidation cycle. These coupled biological and chemical reactions maintain continuous mineral dissolution [29,79]. The indirect mechanism therefore depends on iron-oxidizing microorganisms in bioleaching. Microbial attachment primarily concentrates oxidants at the mineral surface.

4.4. Mineral Dissolution Pathways

The dissolution of copper sulfide minerals takes place through different pathways depending on the chemical composition and crystal structure [31]. The dissolution of chalcocite occurs mainly through the thiosulfate route, where sulfur-containing intermediates are relatively soluble and rapidly oxidized, resulting in rapid copper recovery. On the other hand, the dissolution of chalcopyrite is controlled by the polysulfide route, which produces intermediate sulfur species that readily accumulate on the mineral surface and promote passivation, greatly reducing the dissolution kinetics. Bornite shows intermediate behavior, with the dissolution rate depending on mineral purity, associated gangue minerals, and the composition of the microbial community [27].

The thiosulfate pathway proceeds via oxidation of sulfide to thiosulfate, which is further oxidized to tetrathionate and then sulfate. This pathway predominates for minerals containing sulfur as S²⁻ or S₂²⁻ and does not involve the breaking of strong sulfur–metal bonds. The polysulfide pathway comprises the stepwise oxidation of sulfur from S²⁻ through polysulfide intermediates (S_n²⁻) to elemental sulfur. The formation of elemental sulfur and its oxidation to sulfate is a major rate-limiting step [80].

4.5. Electrochemical Aspects of Mineral Dissolution

The dissolution of sulfide minerals is essentially an electrochemical process. Sulfide minerals are semiconductors. Oxidation involves electron transfer from the mineral to the oxidant (Fe³⁺) [32]. Electronic properties such as band structure and conductivity, for example, determine the rate of electron transfer. The refractory nature of chalcopyrite is due to its wide band gap (~0.6 eV) and p-type semiconducting behavior, whereas the metallic conductivity of chalcocite promotes rapid electron transfer and dissolution.

Mineral oxidation is driven by the electrochemical potential of the leaching solution, which is mainly determined by the Fe³⁺/Fe²⁺ ratio. Higher redox potentials tend to favor mineral dissolution but may also lead to passivation through the formation of secondary products. Thus, the role of microorganisms in maintaining appropriate redox potentials by continuously oxidizing iron is essential for sustaining mineral dissolution without excessive passivation. Such electrochemical principles provide a basis for the rational design of strategies to enhance dissolution rates [81].

4.6. The Integrated Mechanistic Perspective

Bioleaching systems are characterized by the interplay of iron oxidation, sulfur oxidation, microbial attachment and mineral dissolution. Iron oxidation provides a continuous supply of ferric iron, sulfur oxidation removes inhibitory sulfur compounds, biofilms concentrate oxidants at the surfaces of minerals, and mineral dissolution provides substrates for microbial metabolism [82]. The integration of these biological, chemical, and electrochemical processes during copper sulfide bioleaching is illustrated in Figure 3. These processes interact dynamically, and overall copper recovery efficiency depends on their coordinated activity.

Comparative genomic, transcriptomic, proteomic and electrochemical studies show that these pathways are co-regulated in response to environmental conditions and mineral composition [83]. Consequently, process optimization requires simultaneous consideration of microbial physiology, mineral-surface reactions, electron-transfer processes, and operational conditions rather than the isolated analysis of individual mechanisms. As summarized in Figure 3, electron transfer links ferric-iron-mediated mineral oxidation with microbial iron oxidation and respiratory electron transport, while sulfur oxidation limits the accumulation of passivating sulfur species and biofilm formation creates localized microenvironments that facilitate mineral–microbe interactions. This integrated mechanistic framework provides a basis for developing engineered microbial consortia and advanced process-control strategies for next-generation biomining.

5. Molecular Basis of Copper Bioleaching: Genes, Proteins, and Metabolic Networks

Recent advances in comparative genomics, transcriptomics and proteomics have revolutionized the understanding of the molecular mechanisms involved in copper sulfide bioleaching. Bioleaching is not regulated by individual enzymes but by integrated gene networks that coordinately control iron oxidation, sulfur metabolism, electron transport, biofilm formation, stress adaptation, and metal resistance. These interconnected pathways allow acidophilic microorganisms to grow in extremely acidic, metal-rich environments and continuously promote mineral dissolution.

5.1. Iron Oxidation Gene Network

In bioleaching microorganisms, iron oxidation is a major pathway for energy generation. In A. ferrooxidans, the pathway is mainly encoded by the rus operon, one of the best- studied genetic systems in biomining organisms [84].

The pathway begins with the outer membrane cytochrome Cyc2, which takes up electrons released during the oxidation of ferrous iron. Electrons are then transferred sequentially to the periplasmic blue copper protein rusticyanin (Rus), then to Cyc1, and finally to the terminal cytochrome oxidase complex (CoxABCD). The PetABC (bc₁ complex) participates in reverse electron transport, producing the reducing power necessary for carbon fixation and cellular metabolism. These genes are co-expressed to ensure a constant supply of ferric iron (Fe³⁺) for the indirect oxidation cycle of copper sulfide dissolution. Comparative genomic studies have shown that the overall architecture of the rus operon is conserved among iron-oxidizing acidophiles, but differences in gene organization and regulatory elements lead to species-specific variations in the efficiency of iron oxidation [45].

5.2. Sulfur Oxidation Gene Network

Sulfur oxidation complements iron metabolism by removing reduced sulfur compounds that accumulate during the oxidation of sulfide minerals. The soxABXYZ gene cluster encodes the Sox multienzyme complex, a major system involved in sulfur oxidation [50]. Other important enzymes are sulfide: quinone oxidoreductase (SQR), which oxidizes sulfide and transfers electrons to the respiratory chain; sulfur dioxygenase (SDO), which participates in elemental sulfur oxidation; and tetrathionate hydrolase (TetH), which is involved in tetrathionate degradation.

These enzymes collectively contribute to sulfur turnover and reduce the formation of passivation layers on mineral surfaces. Genomic comparisons show greater diversity of sulfur oxidation pathways than iron oxidation pathways, suggesting adaptation to different sulfur substrates and environmental conditions [51]. This metabolic versatility plays a role in the ecological success of sulfur-oxidizing microorganisms in various biomining environments.

5.3. Copper Resistance Systems

In bioleaching environments, microbial cells are subjected to high concentration of copper ions in solution, which can disrupt membrane integrity, denature proteins and affect their intracellular metabolism. Acidophilic microorganisms have developed different copper resistance systems to control metal homeostasis. The P-type ATPase CopA expels Cu⁺ ions from the cytoplasm thus decreasing intracellular toxicity [85]. The CusCBA efflux complex extrudes copper ions from the cell envelope and CopZ is a copper chaperone that shuttles Cu+ ions to target proteins. The multicopper oxidase CueO catalyzes the oxidation of Cu⁺ to the less toxic Cu²⁺ form, further protecting cells from oxidative damage.

The expression of resistance genes is regulated by copper-responsive transcription factors (CueRs) and the two-component regulatory system CopRS, which enable rapid adaptation to changing metal concentrations. The efficiency of these systems directly depends on microbial survival and bioleaching performance under industrial conditions. Knowledge of these resistance mechanisms provides opportunities for the selection or engineering of strains with increased metal tolerance for high copper environments [86,87].

5.4. Biofilm Formation and Exopolymer Substances

Successful bioleaching relies not only on metabolic pathways but also on the ability of microorganisms to colonize mineral surfaces efficiently. Acidophiles produce extracellular polymeric substances (EPS) that facilitate cell adhesion, stabilize biofilms, and create localized microenvironments that promote mineral oxidation [77,88]. Biofilms maintain ferric iron levels at oxidation sites, protect microbial cells from environmental stress and enable syntrophic interactions between community members.

Transcriptomic studies have shown that EPS-associated genes are upregulated during active mineral colonization, suggesting that biofilm formation is a tightly regulated process rather than a passive response to surface attachment [78]. The composition of EPS (polysaccharides, proteins, and nucleic acids) varies between species and environmental conditions. This regulation allows microorganisms to optimize their strategies for surface colonization depending on the mineral composition and physicochemical conditions.

5.5. Multi-Omics and Systems Biology Approaches

Microbial bioleaching systems have advanced considerably with the development of high-throughput sequencing technologies. Comparative genomics has revealed extensive conservation of iron oxidation pathways but a remarkable diversity in sulfur metabolism and metal resistance genes [89]. Transcriptomic analyses reveal that gene expression changes dynamically during bioleaching depending on mineral composition, oxygen availability and metal concentrations.

Moreover, proteomic and metabolomic studies have revealed that microbial adaptation does not result from the independent activation of individual genes but from the coordinated regulation of multiple metabolic pathways. These observations support the view that bioleaching is a systems-level biological process, governed by complex regulatory networks integrating energy metabolism, environmental sensing, stress adaptation, and mineral transformation [82,88]. Recent multi-omics studies have shifted the focus from single gene functions to integrated metabolic networks, paving the way for the design of synthetic microbial consortia, metabolic engineering, and artificial intelligence-based optimization of industrial biomining processes. The major molecular pathways and gene networks controlling the copper sulfide bioleaching are shown in Figure 4. The figure depicts the rus operon encoding the electron transport chain for iron oxidation (Cyc2 → Rus → Cyc1 → CoxABCD) and the sox cluster encoding the sulfur oxidation pathway. Copper resistance systems (CopA, CusCBA, CopZ, CueO) and biofilm formation pathways (EPS-associated proteins) are also depicted. Regulatory systems (CueR, CopRS) enable adaptation to changing environmental conditions. This molecular-level understanding provides a basis for rational strain development and metabolic engineering.

6. Environmental and Operational Elements That Regulate Copper Bioleaching

The efficiency of copper sulfide bioleaching is the result of complex interactions among microbial activity, mineralogical characteristics, and the physicochemical environment. Parameters studied individually include temperature, pH, redox potential, aeration and particle size, but their effects are highly interdependent. Changes in a single parameter tend to affect multiple biological and chemical processes concurrently, requiring a holistic approach to process optimization.

6.1. Temperature

Temperature is one of the most important parameters controlling microbial metabolism and mineral dissolution kinetics. Mesophilic microorganisms (A. ferrooxidans and L. ferriphilum) are generally active at 25–40 °C, whereas moderately thermophilic and thermophilic microorganisms generally exhibit optimal activity [55]. For example, thermophilic bioleaching systems operating at 65–80 °C have shown improved chalcopyrite dissolution relative to mesophilic systems, owing to faster mineral oxidation kinetics and increased sulfur oxidation capacity. However, improved extraction efficiency must be balanced against increased energy demands and the need to maintain thermophilic microbial populations. Higher temperatures increase oxidation kinetics and enzymatic activity and reduce passivation, particularly for chalcopyrite bioleaching. However, high temperatures can inhibit mesophilic microorganisms, change community composition and increase operational costs. The optimum temperature depends on the target mineral and the predominant microbial consortium employed [56,90].

6.2. pH and Oxidation Reduction Potential

Acidic conditions are essential for maintaining microbial activity and ferric iron stability during bioleaching. Most acidophilic microorganisms grow optimally at pH 1.5–2.5, where ferric iron remains soluble and functions as an effective oxidant [34]. Experimental studies generally indicate that high microbial activity and ferric iron stability are maintained at pH values between 1.5 and 2.0, but as pH increases above about 2.5, jarosite and iron oxyhydroxides precipitate, reducing the oxidant availability and limiting copper extraction. This precipitation diminishes the oxidizing capacity of the solution and consequently reduces copper recovery. pH also affects microbial community composition, as different species have different pH optima.

The redox potential (Eh) is an indicator of the oxidation state of the system and its bioleaching performance [81,91]. High redox potential indicates efficient regeneration of ferric iron and correlates with enhanced mineral dissolution. In many bioleaching systems, Eh values around 600–800 mV is associated with active ferric iron regeneration and sulfide oxidation. However, excessively high redox potentials may promote passivation during chalcopyrite bioleaching and reduce dissolution efficiency, indicating that an optimal rather than maximal Eh should be maintained. The Eh is controlled by the Fe³⁺/Fe²⁺ ratio, which is controlled by the balance between microbial iron oxidation and chemical iron reduction during mineral dissolution.

6.3. Oxygen and Carbon Dioxide Availability

Aeration is necessary for microbial respiration because oxygen is the terminal electron acceptor in the oxidation of iron and sulfur. Low oxygen availability reduces microbial oxidation rates, ferric iron regeneration, and overall copper recovery [92,93]. Oxygen diffusion in heap bioleaching is often restricted by particle size, moisture content, and heap height, and engineered aeration systems are required in large-scale operations.

Carbon dioxide is the primary carbon source for chemolithoautotrophic microorganisms; although atmospheric CO₂ is generally sufficient near the heap surface, its availability may become limited in deeper regions, thereby restricting microbial biomass production. Effective gas transport through the mineral bed is therefore critical for maintaining adequate oxygen and CO₂ availability throughout the heap [94].

One of the main rate-limiting factors in heap operations is oxygen transfer, as microbial oxidation needs a continuous oxygen supply. Insufficient aeration limits ferric iron regeneration and can have a dramatic effect on copper dissolution rates. Improved air distribution systems can improve oxidation efficiency in large scale operations.

6.4. Particle Size and Pulp Density

The surface area available for microbial colonization and chemical oxidation is strongly dependent on particle size. Finer particles have more mineral surface exposed, which improves copper dissolution [95,96]. However, fines in excess can decrease the permeability of the heap, limit the diffusion of oxygen, increase the hydraulic resistance and negatively affect the microbial activity. Therefore, the particle size distribution must be optimized to maximize the exposure of minerals while simultaneously maintaining the bed’s permeability.

Studies on sulfide ores have demonstrated that reducing particle size generally increases copper extraction by increasing exposed mineral surface area and improving microbial attachment. However, excessive grinding may provide limited additional recovery benefits while substantially increasing energy consumption and reducing permeability in heap systems.

Pulp density reflects a balance between metal availability and microbial tolerance. Higher pulp densities might improve potential copper recovery, but excessive solid loading can limit oxygen transfer, increase dissolved metal concentrations and inhibit microbial growth. The optimum pulp density is a function of the mineral composition, tolerance of the microbial community and reactor configuration [34]. In stirred-tank systems, pulp densities are commonly optimized according to ore mineralogy and microbial tolerance. Increasing solids loading may increase volumetric copper productivity, but excessive pulp density decreases oxygen transfer efficiency and increases metal toxicity.

6.5. Nutrient Availability and Solution Chemistry

Essential elements such as nitrogen, phosphorous, potassium, and other trace minerals are required for growth and metabolism of the bioleaching microbes. Ammonium is the usual form of supplied nitrogen, phosphate the form of phosphorus. Nutrient deficiency may limit microbial activity and copper recovery, especially in low-grade ore with low nutrient content [97]. However, excessive nutrient addition can promote undesirable microbial growth or precipitation reactions.

Microbial activity and mineral dissolution are also influenced by solution chemistry e.g., sulfate concentration, ionic strength and dissolved ions. Changes in solution chemistry can influence mineral surface reactions, microbial metabolism, and the formation or stability of secondary precipitates during bioleaching. Higher sulfate concentrations, common in bioleaching solutions, can affect mineral surface chemistry and passivation behavior. The presence of chloride or other anions may enhance or inhibit bioleaching depending on their concentration and mineral type [79].

6.6. Microbial Community Structure and Management

Modern bioleaching systems use cooperative microbial communities rather than individual species. The composition and stability of these communities directly affect iron oxidation, sulfur metabolism, biofilm formation, and metal resistance. Environmental fluctuation can alter community structure and metabolic activity and therefore influence the overall performance of the process. Functional microbial diversity is often more important than maximizing the abundance of any one organism [57,58].

Community management strategies include inoculation with selected consortia, optimization of operating conditions for favored populations, and periodic monitoring to detect community shifts. Functional community stability is necessary for consistent bioleaching performance. Understanding the ecological principles governing community assembly and succession allows for rational management of microbial populations in industrial operations [98].

6.7. Integrated Process Optimization

Operational parameters should not be optimized individually as each variable affects several biological and chemical processes concurrently [99]. Temperature fluctuations affect microbial metabolism, oxygen solubility, ferric iron regeneration, and dissolution kinetics. pH affects microbial physiology, iron chemistry, and mineral surface properties. Such interdependencies require integrated control that simultaneously monitor and adjust multiple parameters.

The optimization of modern bioleaching processes is increasingly supported by online sensors that provide real-time measurements of pH, redox potential, dissolved oxygen, temperature and metal concentrations. Data from these sensors can be integrated into models and control algorithms that keep conditions optimal. The use of emerging approaches combining online monitoring, process simulation and artificial intelligence are expected to further improve process stability, copper recovery and operational efficiency, while lowering costs [35,100]. The interactions among the major environmental and operational factors regulating microbial activity, mineral dissolution, and copper recovery are illustrated in Figure 5, while their optimal ranges, primary effects, and potential limitations are summarized in

Table 3. Major environmental and operational parameters influencing copper sulfide bioleaching performance.

Parameter	Typical Operating Range	Biological and Chemical Effect	Impact on Copper Recovery	Limitation
Temperature	Depends on the microbial consortium (25–80 °C)	Controls microbial metabolism and oxidation kinetics	Higher temperature can accelerate sulfide oxidation and chalcopyrite dissolution	Microbial inhibition at sub-optimal temperatures [65]
pH	1.5–2.5 (acidic)	Maintains acidophile activity and ferric iron stability	Low pH improves oxidant availability	Precipitation at elevated pH [63]
Redox potential	Optimum controlled (600–800 mV)	Controls Fe3+/Fe2+ balance	Determines oxidation capacity and dissolution rate	High passivation [63]
Oxygen	Sufficient aeration	Supports iron and sulfur oxidation	Improves ferric regeneration	Oxygen limitation in the heaps [62]
Carbon dioxide	Sufficient supply	Supports autotrophic growth	Maintains microbial biomass	Limitation in deep piles [63]
Particle size	Balanced surface area	Controls surface exposure	Smaller particles generally improve accessibility	Less permeable if too fine [101]
Pulp density	Specific to process	Affects oxygen transfer and microbial contact	Controls productivity	Metal toxicity [102]

.

7. Copper Bioleaching: Industrial Applications and Process Optimization

One of the major breakthroughs in biomining has been the transition of copper bioleaching from laboratory studies to industrial practice. Over the past four decades, bioleaching has evolved from a slow microbial process to an economically viable technology for the recovery of copper from low-grade sulfide ores, waste rock, tailings and secondary resources. Continuous improvements in microbial management, heap engineering, aeration systems, solution chemistry and process monitoring have contributed to its industrial success.

7.1. Heap Bioleaching

Heap bioleaching is the most widely used technology for copper recovery on a commercial scale. In this process ore is crushed into coarse particles, stacked on impermeable pads and then irrigated with acidic solutions containing acidophilic microorganisms [36,103]. Iron-oxidizing microorganisms produce ferric iron which chemically attacks sulfide minerals; sulfur-oxidizing microorganisms oxidize reduced sulfur compounds and limit passivation as the leaching solution percolates through the heap. The pregnant leach solution (PLS) containing dissolved copper is collected and treated by solvent extraction and electrowinning (SX-EW) to give high-purity cathode copper.

Heap bioleaching offers several advantages, including relatively low capital investment, reduced energy consumption, and the ability to process large volumes of low-grade ore that would be uneconomical by conventional methods. However, its limitations include slow kinetics (months to years for full recovery), non-uniform solution distribution, limited oxygen diffusion, and the temperature gradients inside the heap. Successful heap operation requires careful engineering of irrigation rates, aeration systems, and heap permeability to maintain optimal conditions throughout the ore bed [104].

7.2. Dump Bioleaching

Dump bioleaching is based on similar principles but generally involves coarse waste rock or run-of-mine material with limited crushing. This approach has lower operating costs, but is challenged by heterogeneous material distribution, poor solution percolation, limited oxygen transfer, and slower microbial colonization [84,105]. Copper recoveries are typically lower than those obtained in engineered heap systems. Dump leaching is often used in legacy mining operations when the waste material contains enough residual copper to make recovery worthwhile [106].

7.3. Stirred-Tank Bioleaching

Stirred-tank bioleaching is primarily applied to finely ground concentrates and high value refractory ores. Continuous mixing, proper aeration and precise control of temperature, pH, and redox potential provide ideal conditions for microbial growth and rapid mineral oxidation [107,108]. These reactors have higher metal recoveries and faster leaching rates than heap systems, but their greater capital and operating costs restrict their use to economically valuable concentrates.

Thermophilic stirred-tank bioleaching is especially suitable for processing chalcopyrite concentrates since high temperatures increase dissolution kinetics and decrease passivation [54]. The technology has been successfully applied at commercial scale for refractory gold ores (BIOX^® process) and is being considered increasingly for copper concentrates where conventional smelting is environmentally or economically constrained.

7.4. In Situ Bioleaching

In situ bioleaching has been identified as a low-impact mining method. Acidic leaching solutions are pumped directly into fractured ore bodies without physical extraction of the ore, allowing copper to be leached in situ [109]. This approach reduces the surface disturbance, waste production, and energy requirements. However, groundwater protection, containment of leachate, control of solution flow, and long-term monitoring of the environment are key barriers to commercial-scale application. In situ bioleaching is nowadays considered an emerging technology with potential for certain geological settings [110].

7.5. Linking Ore Characteristics with Bioleaching Strategies

The choice of a suitable bioleaching technology is highly affected by the ore characteristics such as copper mineralogy, ore grade, sulfide reactivity, gangue composition, mineral liberation, and permeability. Different Cu sulfide minerals show different dissolution behavior, which affects their suitability for specific bioleaching arrangements. Therefore, successful industrial bioleaching depends on an integrated understanding of ore properties, process engineering and microbiome characteristics.

Heap and dump bioleaching is generally suitable for low-grade copper ores containing reactive secondary sulfide minerals such as chalcocite and covellite, which have relatively fast oxidation kinetics and can be processed economically at large scales. In these systems, mixed acidophilic microbial communities play a central role in sustaining mineral oxidation through iron and sulfur metabolism. Iron-oxidizing microorganisms, such as Acidithiobacillus ferrooxidans, and Leptospirillum spp., are responsible for the continuous regeneration of ferric iron, while sulfur-oxidizing microorganisms, such as Acidithiobacillus thiooxidans, contribute to the removal of reduced sulfur compounds and help to limit sulfur-related passivation.

Chalcopyrite-dominant ores are more difficult to process because of their slow dissolution kinetics and strong tendency to form passivation layers. Such ores tend to require more controlled bioleaching approaches such as stirred-tank reactors or thermophilic bioleaching systems where temperature, redox potential, aeration and microbial activity can be tightly controlled. Moderately and extremely thermophilic microorganisms such as Sulfobacillus spp., Metallosphaera sedula and Acidianus spp. have been shown to improve chalcopyrite dissolution through higher oxidation kinetics and better sulfur transformation at high temperatures.

Ore texture and the composition of gangue further influence the choice of the bioleaching strategy. Although the chemical conditions are favorable, minerals locked or partially locked in gangue matrices may be poorly accessible to microbes, which reduces copper recovery. Ores containing acid-consuming gangue minerals or that have poor permeability characteristics may require additional grinding, pretreatment or controlled reactor-based treatment prior to bioleaching. Therefore, mineralogical characterization and liberation analysis should be considered together with microbial selection and process design for efficient copper extraction. The systematic association of copper sulfide mineral attributes, appropriate bioleaching technologies, and microbial communities is summarized in

Table 4. Relationship between copper sulfide ore characteristics, bioleaching technologies, and microbial communities.

Ore Type	Mineral Characteristics	Suitable Technology	Representative Microorganisms
Chalcocite/covellite	Highly reactive secondary sulfides; rapid oxidation kinetics	Heap or dump bioleaching	Acidithiobacillus ferrooxidans, A. thiooxidans, Leptospirillum spp.
Chalcopyrite	Refractory behavior; strong passivation tendency; slow dissolution	Stirred-tank or thermophilic bioleaching	Sulfobacillus spp., Metallosphaera sedula, Acidianus spp.
Bornite	Intermediate reactivity between chalcocite and chalcopyrite	Heap or controlled reactor systems	Mixed iron- and sulfur-oxidizing consortia
Complex intergrown ores	Poor liberation; strong mineral–gangue association	Pretreatment followed by controlled bioleaching	Adapted mixed microbial consortia

.

7.6. Processes Monitoring and Control

Engineering design and microbial ecology must be integrated for successful industrial bioleaching. High ferric iron regeneration and sustained sulfide oxidation require uniform irrigation, adequate aeration, controlled heap permeability, and stable microbial communities. Modern operations increasingly use online sensors to measure parameters such as pH, redox potential, dissolved oxygen, temperature, and copper concentration in real time so that the operators can optimize performance and respond quickly to process upsets.

7.7. Digital Technology and Process Optimization

Industrial biomining is being transformed by recent advances in digital technologies. The application of computational fluid dynamics, process simulation, machine learning and digital twin models to bioleaching operations is becoming increasingly common for predicting microbial activity, optimizing irrigation strategies, improving aeration efficiency and forecasting copper recovery in bioleaching processes. These technologies can optimize processes based on data, eliminating operational uncertainty, reducing operational costs, and lowering resource usage.

Artificial intelligence (AI) and machine learning (ML) algorithms provide powerful tools to analyze complex bioleaching datasets obtained from operational monitoring, microbial characterization and mineralogical analyses. ML models can help identify hidden relationships between process conditions and copper recovery performance by integrating variables such as pH, redox potential (Eh), temperature, dissolved oxygen, copper concentration, mineral composition, and microbial community structure. These models can be used for the early detection of process instability, optimization of operating parameters, and proactive control strategies instead of conventional reactive management approaches.

Digital twin technology is another development that integrates biological, chemical and engineering information into dynamic predictive models that continuously simulate real bioleaching systems [102]. In an industrial setting, digital twins can combine real-time sensor data and process simulations to predict changes in leaching performance, identify possible passivation events, and optimize parameters such as aeration, irrigation, and microbial management. The convergence of AI, digital twins, and multidimensional biological datasets provides a pathway to intelligent biomining systems that can improve process stability, reduce operational constraints and improve copper recoveries from increasingly complex ores.

7.8. Industrial Implementation and Future Perspectives

Industrial applications have demonstrated that the success of bioleaching depends not only on the performance of microorganisms but also on the interaction of ore mineralogy, reactor design, hydrodynamics, and process control [111]. Future developments are expected to be multidisciplinary, combining microbiology, mineral processing, process engineering, environmental monitoring and artificial intelligence. The integration will increase process stability, increase metal recovery from complex ores and reinforce the position of bioleaching as a sustainable technology for future copper production [112]. The major stages of industrial copper bioleaching and downstream metal recovery are illustrated in Figure 6, including ore preparation, heap construction, irrigation and aeration, pregnant leach solution collection, and solvent extraction–electrowinning (SX–EW). The key characteristics, advantages, limitations, and applications of the major industrial bioleaching technologies are compared in

Table 5. Comparison of major industrial bioleaching technologies used for copper extraction.

Technology	Feed Material	Advantages	Limitations	Typical Use
Heap bioleaching	Low grade ore	Easy to use, inexpensive, scalable	Slow kinetics, non-uniform conditions	Commercial copper production
Dump bioleaching	Pile	Low cost, little processing	Poor control, low recoveries	Legacy mining operations
Stirred-tank bioleaching	Concentrates	Fast kinetics, high recovery	High capital and operating costs	Refractory concentrates
In situ bioleaching	Underground ore	Less surface disturbance	Hydrogeological problems, containment	Emerging technology

.

8. Current Challenges and Future Perspectives

However, many scientific and technological barriers still exist that impede the broad industrial application of copper bioleaching despite the significant progress achieved in microbial biotechnology and process engineering. Meeting these challenges calls for an integrated understanding of microbiology, mineral processing, electrochemistry, and systems engineering, rather than incremental improvements of individual process components.

8.1. Mineral Passivation

The formation of passivation layers is still one of the major limitations in copper sulfide bioleaching, particularly during the dissolution of chalcopyrite. During oxidation, secondary products such as elemental sulfur, jarosite, and iron oxyhydroxides form on mineral surfaces and may impede electron transfer, ferric iron diffusion, and the access by oxidizing agents to reactive mineral sites [113]. Therefore, even when microbial activity remains high, mineral dissolution gradually declines. The passivation layers can be partially removed by the oxidation of the accumulated elemental sulfur by sulfur-oxidizing microorganisms. However, the removal of passivating layers under industrial conditions remains challenging [114]. Recent studies have revealed that chalcopyrite passivation is not controlled by the formation of a single reaction product, but results from combined effects of electrochemical reactions, mineral surface transformations and secondary phase formation.

However, relative contributions of individual passivating species remain controversial. Elemental sulfur, which is hydrophobic and limits contact between the mineral surface and oxidizing species, has been reported in some studies as the main passivating phase. However, other electrochemical and surface characterization studies point towards iron secondary phases such as jarosite and iron oxyhydroxides as the primary responsible for covering reactive sites and modifying interfacial electron transfer processes.

The composition and behavior of passivation layers are strongly affected by operational conditions such as redox potential, pH, temperature, solution chemistry and microbial activity. Sulfur-rich and iron-rich surface products may coexist and transform continuously at different stages of bioleaching, instead of forming a stable uniform layer. Thus, the chalcopyrite passivation should be viewed as a dynamic electrochemical–chemical–biological process, rather than as a simple physical barrier. Future strategies should be focused on improving microbial oxidation capacity and controlling mineral surface chemistry through catalytic ion addition, electrochemical regulation, surface modification and integrated process optimization.

8.2. Slow Dissolution Kinetics

Bioleaching operations have much longer processed times than conventional hydrometallurgical and pyrometallurgical processes. Economically viable copper recovery from heap bioleaching operations can take months to years. The kinetics are slow because of limited microbial growth rates, diffusion limits in the ore heaps, variable mineral compositions, and slow regeneration of ferric iron. More recent studies have examined ways to enhance reaction kinetics through microbial adaptation, better aeration, improved irrigation systems, and higher operating temperatures. However, a major engineering challenge is maintaining stable microbial activity while accelerating mineral dissolution. Overcoming kinetic limitations is key in developing more efficient microbial consortia and better reactor designs [115].

8.3. Microbial Response to Metal Stress

Bioleaching microorganisms are exposed to extreme conditions such as low pH, high metal concentrations, oxidative stress, and nutrient limitation. However, long-term exposure to high concentrations of copper may affect enzyme activity, membrane integrity, and microbial community composition, despite the presence of complex metal resistance systems. Genomic and transcriptomic analyses suggest that microbial adaptation involves modulation of stress-response genes, metal efflux systems, antioxidant pathways and biofilm associated proteins [86,116]. However, the mechanisms that control long-term adaptation are poorly characterized. Understanding these mechanisms will be important for the selection or engineering of more robust strains for industrial applications [42].

8.4. Scale-Up Challenges

Laboratory bioleaching systems for copper recovery are promising but often do not achieve comparable performance when scaled up to commercial operations. Scale-up challenges include non-uniform oxygen distribution, uneven irrigation, temperature gradients, poor solution permeability, and spatial variations in community composition. These factors reduce process stability and increase operational uncertainty. Future scale-up strategies should combine computational fluid dynamics, process simulation, online monitoring, and advanced reactor engineering to improve oxygen transfer, solution distribution, and microbial activity [112]. Predictive models that integrate biological, chemical, and physical parameters need to be developed for successful scale-up [117].

8.5. Cutting-Edge Technology

Recent advances in biotechnology and computational science are providing opportunities to improve the efficiency, predictability, and scalability of bioleaching processes. Comparative genomics, transcriptomics, proteomics, and metabolomics provide unprecedented insights into microbial metabolism, regulatory responses, and community interactions. Multi-omics approaches are able to identify functional genes, metabolic pathways, and stress-response mechanisms involved in iron oxidation, sulfur oxidation, metal resistance, and biofilm formation, thus providing useful information for enhancing microbial performance under industrial conditions. Systems biology approaches together with synthetic microbial consortium design offer new strategies to improve metabolic cooperation, optimize sulfur and iron oxidation activities, and improve microbial resistance to environmental stresses. Understanding the functional interactions among diverse microbial populations facilitates the design of optimized consortia for stable bioleaching performance under complex mineral and operational conditions [100].

Artificial intelligence (AI) and machine learning (ML) methods provide additional opportunities for improving process prediction and control. These approaches may utilize large datasets generated from operational monitoring, mineral characterization, and microbial analyses to identify relationships between process conditions and copper recovery. Machine learning algorithms can be used to predict microbial behavior, optimize operating parameters (pH, temperature, redox potential), predict copper extraction performance, and detect early warning signs of process disturbance before productivity declines. This allows a shift from traditional reactive management to predictive and adaptive control of bioleaching processes.

These capabilities are further enhanced by digital twin technology that integrates biological, chemical and engineering information into dynamic predictive models of industrial bioleaching systems. Digital twins combine real-time process monitoring with computational simulations to continuously evaluate changes in microbial activity, mineral dissolution behavior, and operational performance. The integration of multi-omics, AI, and digital twin technologies provides a promising route toward intelligent biomining platforms with enhanced process stability reduced operational constraints and improved copper recovery from increasingly complex ores.

8.6. Research Priorities

Future trends in copper bioleaching are related to the coupling of microbiology with mineral processing, artificial intelligence, and environmental engineering. Priority research areas are as follows: Include identifying microbial interaction networks and regulating community function; developing microbial consortia that are genetically engineered or selected for enhanced metal tolerance and oxidation capacity; using advanced characterization techniques to investigate mineral surface chemistry and passivation mechanisms; integrating AI and machine learning for process control, optimization, and prediction; developing sustainable bioleaching technologies for low-grade ores, tailings, and electronic waste; and establishing circular-economy routes for the recovery of valuable metals from secondary resources. This convergence of technologies will lead to more efficient processes with lower environmental impacts and lower operational costs. The future of copper bioleaching involves a transition from traditional microbial leaching to intelligent data-driven biomining platforms where biological processes, engineering design, and digital technologies operate as integrated systems [52,118].

The future integrated directions of copper bioleaching technology are illustrated in Figure 7. The figure shows the convergence of systems biology (genomics, transcriptomics, metabolomics), synthetic biology (engineered consortia, metabolic engineering), artificial intelligence (machine learning, predictive modelling, digital twins), and process engineering (advanced reactor design, monitoring, control) in the design of smart, sustainable biomining platforms for copper recovery from low grade ores and secondary resources. This convergence provides a pathway toward next-generation biomining.

9. Conclusions

Copper sulfide bioleaching has evolved from a laboratory concept into an established industrial biotechnology for the recovery of copper from low-grade ores, waste materials and secondary resources. Compared with traditional extraction routes it offers advantages in energy efficiency, environmental performance and the ability to process resources that are increasingly difficult to exploit by traditional methods. However, the bioleaching performance is not determined by the microbial activity alone but by the complex interaction among microbial communities, mineral surfaces, electrochemical processes and operational conditions. Acidophilic iron- and sulfur-oxidizing microorganisms, including Acidithiobacillus, Leptospirillum, Sulfobacillus, and Metallosphaera are involved in iron regeneration, sulfur oxidation, mineral dissolution, and adaptation to extreme environments. Progress in comparative genomics and multi-omics studies has increased the knowledge about functional systems related to iron oxidation (rus operon), sulfur metabolism (sox pathways), metal resistance (CopA and CusCBA), and biofilm formation, opening new possibilities for microbial selection and consortium optimization. Important challenges remain despite decades of industrial application, namely slow chalcopyrite dissolution, complex passivation-layers formation, metal toxicity, and limited process predictability during scale-up. The future development of copper sulfide bioleaching will depend not only on the discovery of more efficient microorganisms but also on a deeper understanding and improved control of mineral–microbe–process interactions. The combination of mineralogical characterization, electrochemical approaches, microbial ecology and process engineering will be crucial for overcoming current limitations.

In the future, the most important practical benefits are likely to come from better management of microbial communities, real-time monitoring and effective strategies to control passivation and maintain stable operating conditions. New opportunities for predictive and adaptive biomining are provided by emerging approaches such as multi-omics, artificial intelligence, machine learning, digital twins, and synthetic microbial consortia. However, their industrial utility will rely on successful integration with fundamental knowledge of mineral behavior and microbial function. Therefore, the next generation of copper bioleaching must transition from empirical optimization to an integrated and predictive framework that incorporates biology, mineral processing, and data-driven engineering. With the continued expansion of electrification and renewable energy technologies, the global demand for copper is expected to increase, and multidisciplinary approaches will therefore be essential for developing sustainable and efficient metal recovery strategies from increasingly complex mineral resources.