From Solid to Solution: How Surface-Active Agents Influence Bioleaching Efficiency and Bacteria–Mineral Interactions

Abstract

The search for sustainable methods of metal recovery has led to increased interest in bioleaching as a sustainable alternative to traditional mineral processing. Despite the ecological benefits, the low bioprocess efficiency is limiting industrial applications. Surfactants offer a promising solution by modifying solid–liquid interactions and improving metal extraction. The review summarizes the effect of surfactants, biosurfactants, polymers, and flotation reagents on the bioleaching efficiency of various mineral materials. It includes their impact on microbial activity, bacteria–mineral interactions, as well as mineral properties such as surface potential and hydrophobicity. Recent literature from the past decade is critically evaluated. Current knowledge limitations and future directions for the effective use of surface-active agents in metal bioextraction were discussed.

1. Introduction

Research on the use of microorganisms to recover metal(loid)s from various mineral materials has gained interest due to its potential to reduce the environmental impact of the mining industry. Nevertheless, the complexity of bioprocesses causes that strategies to improve their efficiency are still under development. The potential use of surface-active agents in metal bioextraction has focused researchers’ attention since the end of the 20th century. According to a bibliometric analysis based on the Scopus database using the keywords “biosurfactant” AND “bioleaching” and “surfactant” AND “bioleaching”, the first publication was documented in 1994 (Figure 1). Initially, the number of articles published annually was low, ranging from one to four per year. From 2010, there was an increase in research activity, primarily focused on the application of surfactants in mineral surface modification, as well as the potential replacement of chemical reagents with their biological counterparts. Between 2019 and 2022, the number of studies on biosurfactants was higher than in previous years. Recent works are focused on the application of sustainable leaching strategies (soil remediation, e-waste processing). This trend highlights sustained scientific interest in bioextraction using surface-active reagents.

Surfactants, biosurfactants, and polymers modulate the surface properties of minerals and the adhesion of microorganisms, which in turn affect biofilm formation and the oxidation of sulfides through direct or indirect mechanisms. These compounds, when present in the leaching solution, can also form complexes with metal ions, increasing their solubility [1]. The interactions between surfactants, microbes, and minerals are complex and still require detailed investigation to develop more reliable and scalable bioleaching.

Until now, surface-active reagents have been studied for their use in mineral processing (flotation and flocculation) [2,3,4] and in bioremediation processes [5]. Surfactants, such as Tween 80 [6], as well as biosurfactants [7,8], have been used to enhance bacterial transport in porous media. Considerably less research has focused on their use in metal bioextraction, especially considering the physicochemistry of such a process.

The main aim was to compare the effect of various surface-active additives on the bioleaching, with particular emphasis on their impact on the surface properties of minerals (zeta potential and hydrophobicity), bacterial adhesion, biooxidation activity, and leaching efficiency. Basic information on the surfactants most commonly used was included. Current research limitations were identified, as well as future research directions that should be considered to have a more in-depth understanding of how those reagents influence process efficiency and bacteria–mineral interactions in bioextraction.

2. Surface-Active Agents Used in Bioleaching

Surface-active agents, or more precisely, surfactants, are a distinct class of chemical compounds characterized, among other features, by the capacity to decrease the surface tension of solutions [6]. The key parameter affecting their performance in bioleaching systems is the critical micelle concentration (CMC), which is the concentration above which surfactant molecules begin to associate into micelles. It depends on molecular structure, temperature, and ionic strength.

2.1. Surfactants and Polymers

Surfactants used in bioleaching include anionic, cationic, and nonionic types. Nonionic surfactants are typically fatty alcohol ethoxylates, alkylphenol ethoxylates, or fatty acid alkoxylates [7]. In addition to their CMC, two other parameters define these molecules. The first, related to the compound’s solubility, is the cloud point, defined as the temperature at which the surfactant separates from the solution, forming a distinct phase. Observed turbidity results from the dehydration of oxyethylene groups [8]. The second parameter is the hydrophilic–lipophilic balance (HLB), an important factor in the design of emulsion systems, that represents the ratio of hydrophilic to hydrophobic functional groups within a molecule and is typically assigned only to nonionic surfactants [9]. For the reagents discussed below, the cloud point is approximately 70 °C and is not relevant to bioleaching systems.

Nonionic surfactants often applied in microbial leaching include Tween 20 and Tween 80, which are registered commercial trademarks for polysorbates widely used in the food, cosmetic, and pharmaceutical industries [10,11,12,13,14,15]. Polysorbates are ethoxylated fatty acids of a cyclic sugar alcohol—sorbitan [16]. The hydrophilic part consists of a 1,4-anhydrosorbitol backbone linked to a polyoxyethylene chain, while a long-chain fatty-acid ester forms the hydrophobic part. Tween 20 is polyoxyethylene (20) sorbitan monolaurate, containing a C12 lauric-acid chain in its hydrophobic moiety (Figure 2A). Tween 80 is polyoxyethylene (20) sorbitan monooleate, containing a C18:1 oleic acid chain (Figure 2B) [17].

Another group utilized in metal bioextraction is Tritons. A well-known example is Triton X-100, which contains on average ~9.5 ethylene oxide (EO) units [18]. The hydrophobic part consists of 4-(1,1,3,3-tetramethylbutyl)phenyl (a branched alkylphenyl) groups attached to polyoxyethylene chains via ether linkages, while polyether (polyoxoethylene) chains provide hydrophilicity (Figure 2C) [19].

Surface-active agents such as polyoxyethylene (12) nonyl phenyl ether (NP-12), and polyoxyethylene (15) nonyl phenyl ether (NP-15, Figure 2D) are nonionic surfactants differing in EO chain length, used for their strong wetting and dispersing properties [20]. They were also applied to the intensification of metal leaching using microorganisms.

In studies on bioleaching, polymeric nonionic additives such as polyethylene glycol (PEG; Figure 2E) [21] and polyvinylpyrrolidone (PVP; Figure 2F) [22] have also been employed. PEG is a linear polymer of ethylene oxide terminated with hydroxyl groups. Its hydrophilic character increases with the number of repeating oxygen atoms, which are capable of solvating and interacting with water molecules [23]. PVP, in turn, is a polymer derived from N-vinylpyrrolidone monomers containing a pyrrolidone ring—that is, a cyclic amide (lactam). The nitrogen atom of the lactam is substituted with a vinyl group. The amide moiety imparts polarity and enables hydrogen-bonding interactions with water, thereby conferring hydrophilic properties to the polymer [24,25].

Among anionic surfactants, sodium dodecyl sulfate (SDS; Figure 2G) has been most used [26,27,28,29]. SDS is a linear alkyl sulfate with a C12 hydrophobic tail and a negatively charged sulfate head group.

Cationic surfactants are less commonly studied [12,27], but cetyltrimethylammonium bromide (CTAB; Figure 2H), having a C16 alkyl chain and a quaternary ammonium headgroup bearing a permanent positive charge, is one of the representatives.

The electrical neutrality of nonionic surfactants results in lower CMCs compared to ionic analogs. For instance, CTAB and SDS exhibit CMCs of ~300 and ~2300 mg/L, respectively, whereas nonionic Tween 80 ~13 mg/L [26].

2.2. Biosurfactants and Lignin-Based Polymers

Biosurfactants are amphiphilic molecules synthesized mainly by bacteria, yeasts, and fungi, consisting of a hydrophilic head (based on phosphates, sugars, or amino acids) and a hydrophobic tail formed by long hydrocarbon chains. As with synthetic surfactants, this dual structure allows them to reduce surface and interfacial tension, supporting processes such as emulsification and dispersion [30].

Rhamnolipids are among the most studied biosurfactants (Figure 3A). They consist of one or two rhamnose units linked to β-hydroxy fatty acid chains, with anionic character at pH > 6 due to carboxyl dissociation [31,32,33]. Rhamnolipid mixtures contain structural homologues, influencing their CMC (e.g., 6.5–90 mg/L) and functional performance [34,35]. They have been used in bioleaching at both sub-CMC and supra-CMC [27,36,37].

This class also includes sophorolipids produced by yeast species, whose hydrophilic moiety is a sophorose disaccharide glycosidically linked to a hydroxylated fatty acid, forming the hydrophobic portion [38]. The structure of a sophorolipid is shown in Figure 3B. The application of sophorolipids in bioleaching is beneficial as they combine strong surface activity with unique metal coordination abilities, offering enhanced stability and efficiency for metal extraction [39].

In addition to biosurfactants, also biopolymers are utilized in metal extraction. These are macromolecules obtained directly from biomass, synthesized from renewable monomers, or produced by microorganisms. Their structure, often featuring multiple diverse functional groups (e.g., hydroxyl and carboxyl), makes them reactive and enables cross-linking capabilities [40,41]. Lignin, a naturally occurring aromatic biopolymer, component of plant biomass, serves as a valuable source of surface-active compounds, such as sodium lignosulfonate (NaLS), which is obtained as a by-product of the sulfite pulping process [42,43]. This reagent acts as a polyanionic polyelectrolyte, consisting of high-molecular-weight, heterogeneously linked phenylpropane units that are additionally sulfonated (Figure 3C) to enhance their solubility in water [44]. Another example of lignin-based polymer used to improve leaching efficiency is calcium lignosulfonate (CaLS) [45]. It has a similar molecular structure to NaLS, differing in the presence of a calcium cation bound to sulfonate groups.

Compounds of biological origin share key advantages over synthetic materials: they are biodegradable, low-toxicity, and degrade into simple, harmless compounds under microbial activity [46,47,48]. These environmentally friendly properties, combined with their functional versatility, make them valuable in a wide range of applications and an alternative to conventional synthetic counterparts.

2.3. Adsorption at Hydrophobic/Hydrophilic Surfaces

The ability of a solid surface to repel water is referred to as hydrophobicity, and it is typically expressed in terms of the contact angle [49]. From a theoretical thermodynamic standpoint, a surface is classified as hydrophobic when the contact angle is greater than or equal to 90°. In contrast, within the field of mineral processing, it is often assumed that a mineral surface is naturally hydrophobic if the contact angle is simply greater than 0° [50]. This discrepancy illustrates the distinction between fundamental definitions and applied, industry-oriented approaches. Importantly, because the contact angle provides a quantitative measure, it allows for meaningful comparison across different systems, including both unmodified minerals and those modified by surfactant treatment.

Most natural minerals exhibit hydrophilic surfaces, i.e., surfaces on which hydrogen bonds can readily form. Examples include oxides such as quartz and carbonates like calcite. In contrast, many sulfide minerals (e.g., chalcopyrite and molybdenite) display more hydrophobic character, as their surfaces generally lack free hydroxyl groups. Similar surface properties are also observed in hydrophobic raw materials such as elemental sulfur [51,52].

The adsorption of surfactants from solution varies depending on the surface’s hydrophobicity and the surfactant concentration. The degree of surface hydrophobicity plays a key role in determining the structure and organization of adsorbed surfactant layers.

On a hydrophobic surface, at low concentrations (Figure 4A1), the hydrophobic chains interact directly with the solid surface, while the polar head groups remain close to the interface. In this configuration, each molecule arranges itself to occupy the maximum possible area.

As the concentration—and thus surface coverage—increases, the interfacial layer becomes more compact, and the head groups gradually move apart (Figure 4A2). At a specific concentration, this leads to the formation of a monolayer (Figure 4A3). At still higher concentrations, adsorbed hemimicelles can form (Figure 4A4; [53]). The orientation of the head groups at the surface makes the interface progressively more hydrophilic with increasing surfactant concentration [54].

In the case of a hydrophilic surface, at low surfactant concentrations, the polar head groups adsorb directly onto the surface, while the hydrophobic chains extend outward into the liquid phase (Figure 4B1). As the concentration increases, and provided that the head group–surface interactions are sufficiently strong, a monolayer can form (Figure 4B2). A change in surface hydrophobicity thus occurs when a layer is formed in which the hydrophobic chains are oriented nearly perpendicular to the surface, i.e., in the case of monolayer formation. With further increases in concentration, bilayer structures may develop, leading to the rehydrophilization of the surface (Figure 4B3; [31]). It occurs when the head group–surface interactions dominate over the interactions between the hydrophobic tails. Conversely, when hydrophobic interactions are stronger, the system favors the formation of surface micelles (Figure 4B4 [55]).

Electrostatic, hydrophobic, and chemical interactions govern the surfactant adsorption. The latter includes hydrogen bonding, ion exchange, and coordination with functional groups present on the mineral surface [56,57]. In the case of ionic surfactants, both pH and the composition and concentration of electrolytes significantly affect adsorption, primarily through their effect on the surface charge and the thickness of the electrical double layer [58]. Electrostatic interactions are determined by the charge carried by the surfactant head group (anionic or cationic) and the charge of the solid surface. In such systems, attractive forces arise when the surfactant carries a charge opposite to that of the surface, whereas repulsion occurs when the charges are of the same sign.

3. Effect of Various Reagents on Bacterial Growth

The activity of microorganisms, especially acidophiles, is strictly connected to bioleaching efficiency. Therefore, studying the effects of various reagents on microbial growth is the first step toward their efficient application in metal bioextraction. Jafari et al. [59] analyzed the impact of conventional sulfide flotation reagents and frothers such as potassium-amylxanthate (KAX), potassium isobutylxanthate (KIBX), sodium ethylxanthate (NaEX), potassium isopropylxanthate (KIPX), and dithiophosphate (Aero3477), and frothers (pine oil (PO) and methyl isobutyl carbinol (MIBC)) on the activity of Acidithiobacillus ferrooxidans. It was found that these reagents, depending on their chemical composition and concentration, can have a positive or negative impact. During three weeks of biooxidation, an increased microbial growth was noted only for Aero3477 at a concentration of 0.01 g/L. Higher amounts of collectors (0.1 and 1.0 g/L) inhibited bacterial activity. The negative effect on the bacterial population after 15 days was as follows:

-

0.01 g/L: NaEX > KIPX > MIBC/KIBX > PO/KAX > Aero3477;

-

0.1 g/L: NaEX > KIPX > MIBC > KIBX > KAX > PO > Aero3477;

-

1.0 g/L: NaEX > KIPX > MIBC > PO > KIBX > KAX > Aero3477.

Except for the above, reagent concentrations of 0.1–1.0 g/L increased the rate of jarosite formation. The influence of the tested reagents on pH, redox potential, or total iron was not the same as in the case of bacterial growth. It follows that there is a need to determine which parameter has the most significant impact on process efficiency and to select the appropriate concentrations and type of surfactant accordingly.

The effect of PEG 2000 (30, 60, 90, and 180 mg/L) on the activity of A. ferrooxidans was studied by Zhang et al. [21]. The initial cell density was 5 × 10⁷ cells/mL. In the presence of polymer, the pH of the medium decreased more rapidly than without the reagent, regardless of the concentration. The maximum drop occurred at a concentration of 90 mg/L, suggesting that higher concentrations inhibit microbial growth. The addition of PEG increased bacterial cell attachment to sulfur by over 20% compared to the control, resulting in improved oxidation of this element. It suggests that PEG at levels below 90 mg/L can be utilized in the bioleaching to enhance process efficiency.

Nonionic surfactants carry no charge, which makes them less disruptive to microbial cell membranes. For this reason, they are more often chosen for experimental studies. Xiong et al. [60] investigated the adaptation of A. ferrooxidans to Tween 80 (1–250 mg/L) and Triton X-100 (1–100 mg/L). An approximately 40% decrease in cell population was observed after the application of 75 mg/L Triton X-100 or 175 mg/L Tween 80. It was found that iron biooxidation decreased when 50–100 mg/L of Triton X-100 and 175–250 mg/L of Tween 80 were added.

Increased bacterial growth in a certain range of surfactant concentration results in a more rapid decrease in dissolved oxygen concentration. It was also reported that the presence of reagents caused a change in the morphological structure of bacteria. With a higher Tween 80 concentration, bacteria become more elongated and thinner, which was explained by a decrease in surface tension. It was observed that with increasing surfactant addition, more protein was produced by microbial cells [61].

Bacterial activity is strongly connected with biofilm formation, which is the second key feature in improving metal dissolution from various solids [62]. It has been demonstrated that the leaching efficiency of metals decreases when microorganisms are not in contact with the treated solid material [63,64]. Su et al. [14] investigated the influence of Tween 20 on the adhesion and biofilm formation of Acidianus manzaensis YN-25 on chalcopyrite surfaces. It was reported that low concentrations of Tween 20 (2 mg/L) enhanced bacterial adhesion by altering Lewis acid-base interactions and electrostatic forces, thereby increasing the total interaction energy and, consequently, adhesion. The addition of surfactant also enhanced biofilm formation by reducing the elemental sulfur passivation layer and complexing more ferric ions, leading to increased erosion of chalcopyrite.

The presented studies emphasize the importance of selecting the appropriate type and concentration of reagents to maintain high microbial activity. Increased cell adhesion promotes biofilm formation, leading to enhanced metal dissolution.

4. Interactions with Minerals and Microorganisms

4.1. Mineral Surface Wettability

Surfactant adsorption at solid surfaces modifies the surface free energy and, consequently, the wetting behavior, which can be quantified by the contact angle [65]. In bioleaching, changes in surface wettability arise mainly from surface erosion and chemical alteration. Wettability is a key determinant of how effectively microorganisms and their metabolites interact with mineral surfaces during bioleaching. Hydrophobic surfaces hinder bacterial adhesion and limit solution penetration, whereas enhanced hydrophilicity improves mass transfer and accelerates leaching reactions. At the solution–mineral interface, processes such as dissolution, adsorption, desorption, and redox reactions, among others, occur simultaneously [66].

Ai et al. [67] reported progressive bioleaching of copper oxide ore, where an initially smooth surface rapidly became rough due to fissure formation. Addition of SDS (8.00 × 10⁻³ M) significantly enhanced this effect: after 192 h, the roughness factor increased to 1.1802 compared to 1.1640 without surfactant, while surface height variance was ~6.4 times higher. SDS reduced the leaching solution surface tension from 78 to 40 mN/m and lowered the contact angle to 58% of the surfactant-free value, facilitating better spreading of the solution (higher spreading coefficient), which intensified phase contact and thereby accelerated bioleaching.

Multiple studies have demonstrated that surfactants decrease the contact angle of minerals, improving wettability, thus facilitating bacterial adhesion and enhancing metal extraction (

Table 1. Contact angle values in various systems subjected to bioleaching.

Mineral	Surfactant		Changes in Contact Angle		Ref.
	Type	Concentration [mg/L]	From [°]	to [°]
Chalcopyrite	PVP	120	77	44	[22]
Chalcopyrite (fresh)	Triton CG-110	100	81	69	[68]
		500		54
		2000		41
Chalcopyrite (oxidized)		600	72	50
Sulfur (elemental)	Triton X-100	120	105	60	[69]
Cobalt ore	Tween 20	150	64	41	[70]
	Tween 60	150		43
	Tween 80	150		45
Lepidolite	SDS	100	75	7	[71]
	Tween 20	100		43
	Rhamnolipid	300		11

).

Zhang et al. [69] investigated the bioleaching of chalcopyrite, a sulfide mineral generally more hydrophobic than oxides. A nonionic surfactant, Triton X-100, was introduced to the system to examine its influence on surface properties. Instead of directly measuring the contact angle of chalcopyrite, the authors analyzed elemental sulfur, which forms a hydrophobic passivation layer during leaching and restricts bacterial access. Since chalcopyrite itself is not the primary barrier, assessing sulfur’s surface behavior provided more relevant insight into how the surfactant improves bioleaching efficiency. The results showed that sulfur hydrophobicity decreased markedly with increasing surfactant concentration: the contact angle dropped from 105° (without surfactant) to 60° at 120 mg/L Triton X-100. Concurrently, bacterial adhesion to sulfur increased from 18.37% to 45.76%. These findings indicate that Triton X-100 improved surface hydrophilicity and promoted microbial attachment, thereby facilitating the biooxidation process. In a related study, the influence of Triton CG-110 on chalcopyrite was investigated [68]. The contact angle decreased from 81° to 69°, 54°, and 41° with the addition of 100, 300, and 500 mg/L of Triton CG-110, respectively. The measurements were carried out on freshly polished mineral surfaces. For oxidized surfaces with an initial contact angle of 72°, exposure to a 600 mg/L surfactant solution further reduced it to approximately 50°. However, the optimal concentration for bioleaching was 20 mg/L, at which approximately 69% of copper was extracted.

Similarly, the addition of PVP enhanced the chalcopyrite hydrophilicity [22]. The contact angle without polymer was 77° in the pure 9K medium, and 45° with PVP addition. Leaching efficiency was improved due to better infiltration of the leachate and ion transport between phases. Surface tension decreased, and viscosity slightly increased with rising PVP concentration; at 120 mg/L, the surface tension and viscosity were 63.6 mN/m and 5.46 cp, respectively, compared with 68.5 mN/m and 5.07 cp for the pure culture medium. This modest viscosity increase had little effect on extraction efficiency.

Jia et al. [12] examined other reagents—CTAB, NaLS and NP12 (each at 100 mg/L) –which increased chalcopyrite hydrophilicity and sulfur desorption, facilitating bioleaching.

The contact angle of cobalt ore and liquid phase was also reduced in the presence of Tween 20, Tween 60, and Tween 80 at a concentration of 150 mg/L. The measured values were 64°, 41°, 43°, and 45° for the samples without surfactant, and with Tween 20, Tween 60, and Tween 80, respectively [70]. A decrease in the interfacial tension between the mineral and leachate increased bacterial contact with elemental sulfur, enhancing metal extraction yield.

Further insights were provided by studies on bacterial effects. Contact angle measurements revealed that indigenous mixotrophic bacteria exert contrasting effects on the wettability of pyrite. Experiments were conducted using “adapted cells”, which were previously cultivated in the presence of 20% w/v pure pyrite for 48 h, and with bacteria without adaptation to pyrite (“non-adapted”). Bacillus altitudinis showed little influence, with adapted cells maintaining high contact angles (initially 126°, then 109° after 48 h). Non-adapted cells caused an increase in contact angle from 72° to 116° after 48 h. Contact of adapted cells of Citrobacter freundii with mineral surface increased hydrophobicity (from 68° to 114°), whereas non-adapted cells significantly enhanced hydrophilicity. An approximate 50% reduction, from an initial 99° to 54°, suggested that adapted C. freundii produces surface-active compounds that promote pyrite wetting. Such bacterial modulation of wettability has direct implications for bioflotation, where selective bubble attachment depends strongly on mineral surface hydrophobicity [72].

4.2. Interactions and the Role of Zeta Potential

The role of surfactants in bioleaching is not limited to chemical facilitation of the process. As noted above, the most critical aspect is their ability to regulate the properties of the bacteria–mineral interface. Surfactants modify dispersion stability and the ability of bacteria to adhere.

Adhesion is governed by van der Waals forces, hydrophobic interactions, and electrostatic forces. The latter are particularly important because they are highly sensitive to the charges present at the solid–liquid interface. Also, adhesion is determined by extracellular polymeric substances and steric effects [73,74,75]. A key quantitative descriptor of the interfacial state is the zeta potential, which reflects the potential within the electrical double layer [76]. It can be used, analogously to studies of surfactant or polymer adsorption, to assess whether microorganisms and mineral particles will attract or repel one another. In general, particles and microbial cells bearing opposite charges exhibit adhesion, whereas those with like charges repel each other.

A detailed description of the influence of different types of surfactants was provided by Pawlowska and Sadowski [26], who investigated the bioleaching of schwertmannite, a mineral with an isoelectric point at approximately pH 3.5. Below this value, the mineral surface was positively charged; however, with increasing pH, the charge became progressively more negative, reaching approximately –44 mV at a pH of around 9.5. It clearly indicates that the solution chemistry can strongly determine adhesion phenomena. Surfactants further modify the charge state by altering the mineral–solution interface. For example, the cationic surfactant CTAB induced only a slight increase in zeta potential of about +10 mV, while the addition of SDS led to a gradual decrease. The strongest effect was observed for the biosurfactant rhamnolipid, which shifted the zeta potential from +20 mV to −43 mV at a concentration of 1 × 10⁻³ M. The zeta potential of microorganisms was also measured, and changes were observed under the studied conditions; for instance, after the addition of SDS, it decreased from approximately –4 mV to –12 mV.

The combined effect of these modifications ultimately determines the probability and efficiency of bacterial adhesion to the mineral surface. When surfactant adsorption causes the ζ-potentials of minerals and microorganisms to fall within complementary ranges, electrostatic barriers are reduced and colonization is facilitated. Conversely, when both mineral and microbial surfaces become strongly negative, as observed at high SDS concentrations, repulsive forces dominate, leading to a reduction in bioleaching efficiency [27,77,78].

The diverse influence of surfactants on bacteria–mineral interactions in bioleaching is closely related to their molecular characteristics. Nonionic surfactants, such as Tween 80, possess large hydrophilic head groups, whereas ionic surfactants contain functional groups that confer charge. The presence of such a charge complicates the formulation of a single, universal mechanism describing bioextraction. In these systems, in addition to the functional groups of the surfactant, the surface groups of the solid phase also play a significant role—for example, hydroxyl groups on silica surfaces, or metal-hydroxo and oxidized sulfur species on sulfide minerals. As the pH varies, surface reactions involving protonation and deprotonation occur, resulting in measurable changes in surface charge [15,79].

Although the zeta potential does not provide a direct measurement of surface charge, it serves as an effective quantitative tool for monitoring how surfactants and biosurfactants modulate the electrostatic compatibility between minerals and microorganisms. Such interfacial charge modifications directly affect wettability, microbial adhesion, and the overall efficiency of processes.

5. Influence on Metal Recovery

Surface-active reagents, by altering the surface properties of both bacteria and minerals, impact cell adhesion to solids and, thus, leaching efficiency.

Chalcopyrite is the most common copper-bearing mineral. However, its bioleaching rate is still insufficient for industrial applications in comparison to secondary copper sulfides. In such cases, surfactants appear to be a promising solution. A wide range of reagents was reported, mostly chemical surfactants. In the work of Yia et al. [12], seven surfactants at a concentration of 100 mg/L of different properties were tested during bioleaching of chalcopyrite using a mixed culture of bacteria, including Leptospirillum, Sulfobacillus, Ferroplasma, and Acidithiobacillus: NaLS, CTAB, NP-12, Tween 80, Span 80, PEG, and polyethylene glycol dimethyl ether (NHD). The copper recovery without additives was 55.9%. It was shown that NHD, PEG, Tween 80, and Span 80 inhibited bioextraction with leaching rates of 40.1%, 38.7%, 33.6%, and 26.7%, respectively. NaLS and CTAB, when present in leachate, promoted the process, allowing chalcopyrite leaching by 91.8% and 76.5%, respectively. In the case of pyrite oxidation, the iron recovery was higher than in the control sample only when the CTAB and NaLS were added (100 mg/L). NP-12 was shown to accelerate chalcopyrite leaching and, at the same time, decrease the pyrite oxidation. It follows that surfactants can have different effects depending on the mineral being leached. NaLS and CTAB had a minimal effect on cell growth, yielding results comparable to those of the control sample. The cell population was approximately 140 × 10⁶ cells/mL and 120–135 × 10⁶ cells/mL for CTAB and NaLS, respectively. This is noteworthy because it provides a basis for further research into the use of these compounds at a given concentration. The Span 80, NHD, and PEG inhibited microbial growth by around 80%. The cell number were for Tween 80: 85 × 10⁶ cells/mL, Span 80: 18–20 × 10⁶ cells/mL, PGA: 25–30 × 10⁶ cells/mL, NP-12: 85–98 × 10⁶ cells/mL, and NHD: 20–32 × 10⁶ cells/mL.

Chalcopyrite concentrate was processed using acidophiles, such as A. ferrooxidans, also in the presence of Triton CG-110 [68]. The surfactant addition at a concentration of 20 mg/L (0.15 mM) improved copper leaching efficiency by 3.1%, and when ferrous iron was present, by 12.5%. Reduced sulfur passivation caused improved bacterial adhesion to the chalcopyrite surface. Bioleaching efficiencies were 56.6%. 59.7% and 69.1% for biotic control, samples treated with CG-110 and CG-110 supplemented with Fe(II), respectively. It was also shown that surfactant concentration 20–100 mg/L significantly increased bacteria’s biooxidation ability. In the case of 500 mg/L, the effect was comparable to that of the control (without additives). Increasing the dosage to 2000 mg/L completely hindered the metabolic activity of acidophiles in the system.

Ghadiri et al. [79] investigated the effect of five nonionic surfactants, including Tween 20 (5, 10, and 20 mg/L), Tween 80, Plurafac LF 120, Plurafac LF 600, and Lutensol XL 90 (5 and 10 mg/L), on copper recovery from chalcopyrite concentrate. A mixed thermophilic culture, dominant in Metallosphaera hakonensis (96%) and a small amount of Acidiplasma cupricumulans (4%), and reagent concentrations of 5 mg/L and 10 mg/L were used. The copper recovery rates were 49.3% and 72.6% for the abiotic and biotic leaching processes without additives, respectively. The copper extraction yield in the presence of Tween 20, Plurafac LF 120, and Lutensol XL 90 at concentrations of 5–10 mg/L was similar to that of the biotic control. The authors also evaluated the surfactant’s influence on ferrous iron-oxidizing capacity by monitoring redox potential and cell concentration. It was shown that the addition of 5 mg/L Tween 80, Plurafac LF 120, and Lutensol XL 90, as well as 5–10 mg/L of Tween 20, did not have a measurable effect on ferrous ion oxidation and microbial activity. The redox potential of the abiotic control showed maximum values of 660–670 mV at the end of the process. The addition of 20 mg/L of Tween 20, Tween 80, and Plurafac LF600 inhibited bacterial growth and, consequently, final copper recovery. The redox potential was approximately 400 mV, the same as that of the abiotic control (chemical leaching). It indicates that the activity of microorganisms corresponds to the leaching yield. The greater the efficiency of converting the ferrous ion to the ferric ion, the more effective the process.

High-purity chalcopyrite was processed in the presence of CaLS by Liu et al. [80]. Bioextraction was conducted with varying concentrations of reagent, ranging from 0 to 35 mg/L, and strain A. ferrooxidans. In the absence of dispersant, the copper concentration showed a gradual increase during the first 12 days, followed by a sharp rise between days 12 and 18, and a slower increase thereafter, with a final value of 1648 mg/L. After the introduction of CaLS, the metal concentration increased much faster during the first week than in the control sample. At concentrations of 10, 20, 30, and 35 mg/L, the final Cu²⁺ recoveries were 1330, 1700, 1592, and 1565 mg/L, respectively. It follows that the most favorable amount was 20 mg/L. In systems without dispersant, the bacterial growth followed the process yield curve. In contrast, when CaLS was present, the microorganisms proliferated rapidly after 3 days. The maximum cell density in the control sample was 94.12 × 10⁷ cells/mL. For 10, 30, and 35 mg/L, the bacterial population decreased, and was 80.84 × 10⁷ cells/mL, 89.27 × 10⁷ cells/mL, and 85.74 × 10⁷ cells/mL, respectively. When 20 mg/L of reagent was used, the maximum bacterial count was observed (102 × 10⁷ cells/mL). This finding was consistent with bioleaching results, indicating that the bacteria population is the primary factor in chalcopyrite dissolution.

The positive effect of PEG 2000 on the bioleaching of chalcopyrite was reported by Zhang et al. [21]. The process was divided into two stages (days 0–3 and 3–21). The effect of polymer addition was observed only in the second stage, as the first was controlled by chemical reaction. The supplementation with PEG at a concentration of 90 mg/L led to a decrease in solution pH. The final copper extraction (after 21 days) was 187.35 mg/L and 411.04 mg/L in the absence and presence of surfactant, respectively. The polymer in the leaching environment increased bacterial attachment to sulfur, enhancing its oxidation and resulting in better leaching efficiency.

The influence of CaLS (10–35 mg/L), Tween 80 (20–100 mg/L), and dodecyltrimethylammonium bromide, DTAB (2–8 mg/L), on the growth of A. ferrooxidans and arsenic gold concentrate bioleaching was studied by Fang et al. [81]. In the case of CaLS addition, all arsenic leaching rates were higher than the control. When no reagent was added, the leaching rate was 86.1%. The highest efficiency was achieved for a concentration of 30 mg/L (99.8%). Tween 80 also had a positive impact on bioleaching. At a concentration of 80 mg/L, the leaching rate reached 95.2%, which was 9.1% higher than that of the control. The highest bacterial population was reported under these conditions. When the surfactant exceeded that concentration, bacterial growth was inhibited. The arsenic leaching in the presence of 2 mg/L of DTAB was only 1.8% higher than the control sample. Higher surfactant additions (4, 6, and 8 mg/L) decreased arsenic recovery, confirming that bioleaching efficiency is closely related to the activity of bacteria in the leaching environment.

The catalytic effect of a silver and surfactant catalyst on cobalt ore bioleaching was investigated by Liu et al. [70]. The bacterial consortium was a mixture of A. ferrooxidans and Acidithiobacillus thiooxidans. Tween 20, Tween 60, and Tween 80 were tested at concentrations of 150, 300, and 450 mg/L. It was shown that when 150 mg/L of surfactant was introduced, the cobalt leaching efficiencies were 93.7%, 94.3%, and 93.5% for Tween 20, Tween 60, and Tween 80, respectively. It was a rise of over 25% compared to the control (67.8%). Leaching efficiencies were lower at a reagent concentration of 300 mg/L. The percentages were 83.3%, 84.1%, and 82.9% for Tween 20, Tween 60, and Tween 80, respectively. The same trend was observed for copper extraction. The best results were obtained for 150 mg/L. It was 58.5%, 70.3%, 69.2%, and 70.5% for control, Tween 20, Tween 60, and Tween 80, respectively. Moreover, in the presence of a surfactant, the leaching time decreased from 20 days to 18 days. The effect of additives on the bacterial population during the process is presented in

Table 2. Effect of surfactants on bacterial population.

Reagent	Concentration [mg/L]	Bacteria Concentration ×107 cells/mL	Reference
Without reagent	0	27.8	[13]
Tween 20	100	26.5
	250	22.6
	500	2.2
Tween 80	100	27.8
	250	23.4
	500	2.90
Without reagent	0	25.6	[70]
Tween 20	150	23.8
	300	24.3
	450	8.50
Tween 60	150	25.4
	300	26.2
	450	7.8
Tween 80	150	24.3
	300	22.8
	450	8.30

.

For concentrations of 150 and 300 mg/L, the number of bacteria was comparable to the control. A significant reduction in bacterial cells was observed at a concentration of 450 mg/L. It suggests that for an efficient process using the above consortium, the surfactant concentration cannot exceed 300 mg/L.

Another study of Liu et al. [13], a mixture of A. ferrooxidans, A. thiooxidans, and Leptospirillum ferrooxidans was used. The effects of Tween 20 and Tween 80 on the bioleaching process were studied at concentrations of 100 mg/L and 250 mg/L. Results are presented in

Table 3. Effect of Tween 20 and Tween 80 on bioleaching of cobalt ore [13].

Reagent	Conc. [mg/L]	Co [%]	Cu [%]
-	0	71.3	56.2
Tween 20	100	93.2	65.7
	250	82.6	58.1
Tween 80	100	92.4	64.3
	250	84.4	62.9

. The efficiency of the process was increased by 21% for cobalt and over 8% for copper after adding 100 mg/L of surfactant (Tween 80 or Tween 20). Furthermore, as in the previous study, a reduction in leaching time of approximately 15% was observed.

In the author’s previous work, microorganisms were found to be resistant to surfactant concentrations of up to 300 mg/L. However, a cited study showed that bioleaching was more effective when the reagent dosage did not exceed 250 mg/L. Although this seems contradictory, it should be noted that a different consortium of microorganisms was used. These results indicate that both the concentration of surfactants and the composition of the consortium of microorganisms are factors determining the optimal leaching conditions. The effect of increased metal extraction in the presence of surface-active agents was a result of the dispersion of sulfur product from the mineral surface, which accelerated biooxidation. Increased sulfuric acid production enhanced the leaching efficiency.

Jafari et al. [82] investigated the effect of industrial collectors such as KAX, KIBX, NaEX, KIPX, Aero 3477, and frothers, including PO and MIBC, on bioleaching of zinc sulfide concentrate using a mixed culture of acidophiles (A. ferrooxidans, A. thiooxidans, and L. ferrooxidans). Surfactant dosage was 10, 100, and 1000 mg/L. Results indicated that an increase in concentration of the tested reagents, except for KIPX, KIBX and Aero 3477, led to a reduction in zinc extraction yield (

Table 4. Zinc recoveries depend on the reagent used. Based on [82].

Reagent	NaEX	KAX	KIBX	KIPX	MIBC	PO	Aero3477
Conc. [mg/L]	Zinc Recovery [%]
0	79.10
10	71.10	79.70	76.88	33.22	72.31	76.78	75.41
100	63.42	66.76	78.77	88.28	71.25	68.27	67.55
1000	37.55	60.23	64.31	24.43	6.92	6.59	81.61

). Negative impact on bacterial cell concentration was reported (

Table 5. Influence of various flotation reagents on cell concentration. Based on [82].

Reagent	NaEX	KAX	KIBX	KIPX	MIBC	PO	Aero3477
Conc. [mg/L]	Cell Concentration [cells/mL × 107]
0	12.0
10	8.0	12.4	11.2	10	13.2	10	9.2
100	6.0	12.0	7.20	8.4	8.0	12.8	10.8
1000	0.4	10.0	12.0	0.6	0.8	1.6	10.8

). Higher concentrations of additives hinder interactions between bacteria and energy resources in the culture medium, leading to a decrease in the bacterial population. A positive correlation was observed between cell numbers and zinc recovery.

The results indicated that the inhibitory effect of surface active reagents on bacterial growth and therefore leaching efficiency depends on their concentration, chemical composition, and type. Among the tested reagents, the cell concentration in the presence of KAX, KIBX, and Aero was comparable to that of the control sample in the tested concentration range (Table 2). NaEX, KIPX, MIBC, and PO inhibited bacterial growth at a concentration of 1000 mg/L.

The same mineral material was also processed in the presence of Tween 20 and O-phenylenediamine (OPD) [83]. The addition of Tween 20 in the range 50–900 mg/L had a negligible effect on vanadium leaching. The highest efficiency was for 300 mg/L (55.74%), which was only 1.22% higher than the control. In the case of OPD, the highest recovery was observed for 200 mg/L, which was near 60%. Further increase in concentration decreased efficiency. The OPD (0.5 mg/L) had an inhibitory effect on bacterial growth by prolonging the logarithmic phase from 8 to 14 days, resulting in lower final cell number (1.65 × 10⁹ cells/mL). The cell concentration decreased with increasing reagent concentration. In the case of 0.5 mg/L of Tween 20, it was comparable to that of the control sample, reaching 2.51 × 10⁹ cells/mL and 2.42 × 10⁹ cells/mL for the sample without additives and with OPD, respectively.

The effect of anionic surfactant, SDS, on bacterial growth during vanadium leaching using Bacillus mucilaginosus at pH 6.5 was studied by Cai et al. [84]. The cell concentration was gradually decreased with the increase in the SDS concentration. Despite this, the efficiency of the leaching process increased, since the presence of the surfactant caused a decrease in the pH of the solution. The effect was stronger with increasing concentration of the reagent. In addition, it was shown that SDS enhanced the adsorption of bacteria on the mineral surface, which enhanced leaching efficiency. The vanadium leaching efficiency reached 62.60% at a surfactant concentration of 500 mg/L, which was 8.08% higher than that of the control group.

Table 6. Comparison of selected studies on the use of surface-active agents in bioleaching of minerals.

Mineral Material	Conditions	Microorganisms	Reagent	Conc. [mg/L]	Metal Leached	Process Efficiency	Effect on Microorganisms	Ref.
Chalcopyrite	Solid 10% w/v; pH 1.12–1.20; inoculum 10% v/v (1.2 × 108 cells/mL); 35 °C; 200 rpm; time: 123 d	Sulfobacillus Ferroplasma Acidithiobacillus	SLS CTAB NP12 Tween 80 Span 80 PGA NHD	100	Cu	91.8% 76.5% 70.5% 33.6% 26.7% 38.7% 40.1%	SLS, CTAB—minimal effect on bacterial cells; SP80, NHD, and PEG inhibited activity of around 80%. Tween 80 and NP12 delayed cell growth not as significant as above.	[12]
Cobalt ore	inoculum 10% v/v; solid 10% w/v; pH 1.5; 45 °C; 180 rpm; time 15 days	A. ferrooxidans, A. thiooxidans, L. ferrooxidans	Tween 20 Tween 80	100 and 250	Co Cu	Control: Co 71.3% Cu 56.2% Tween 20 (100 mg/L): Co 92.4% Cu 64.3% Tween 80 (100 mg/L): Co 93.2% Cu 65.7% Higher surfactant concentrations gave lower results.	No visible negative effect on bacterial activity within tested surfactant concentrations.	[13]
Chalcopyrite	solid 1% w/v; pH 2.0; 160 rpm; 30 °C; inoculum 1 × 107 cell/mL; time: 21 d	A. ferrooxidans	PEG 2000	0–180	Cu	PEG (90 mg/L): 411.04 mg/L; control: 187.35 mg/L	The presence of PEG improved bacterial attachment to sulfur.	[21]
Chalcopyrite	solid 1% w/v; inoculum 1 × 107 cell/mL; 30 °C; 160 rpm; pH 2.0; time 30 d	A. ferrooxidans	PVP	100	Cu	Increased metal recovery in the presence of PVP (786.5 mg/L); control: 385.6 mg/L	PVP slightly inhibited bacterial growth compared to control sample.	[22]
Chalcopyrite concentrate	Solid 1 wt%; inoculum 10% v/v; 34 °C; pH 1.70; 300 rpm; time: 30 d	A. ferrooxidans	CG-110	10–2000	Cu	Bio-Fe-CG (69.1%), Bio-CG (59.7%) Bio-Fe (56.6%),	CG-110 20 and 100 mg/L increased biooxidation ability of bacteria. Reagent dosage over 500 mg/L inhibited metabolic activity.	[68]
Chalcopyrite concentrate	Solid 3% w/v; 65 °C; inoculum 1–4 × 108 cell/mL; time: 18 d	M. hakonensis A. cupricumulans	Tween 20 Tween 80 Plurafac LF 120 Plurafac LF 600 Lutensol XL 90	5–10	Cu	Tween 20 (10 mg/L) showed an enhancement of the copper recovery by 2.4% relative to the biotic control.	Microorganisms able to grow in the presence of: 5–10 mg/L Tween 20, 5 mg/L Tween 80, 5 mg/L Plurafac LF 120, 5–10 mg/L Lutensol XL 90. Inhibititory effect for: 10 mg/L of Tween 80, 10 mg/L of Plurafac LF 120, 5–10 mg/L of Plurafac LF 600	[79]
Chalcopyrite, high purity	Inoculum 5% v/v; solid 2% w/v; 30 °C; 180 rpm; pH 2.0; time 30 days	A. ferrooxidans	CaLS	0, 10, 20, 30, 35	Cu	CaLS (20 mg/L): 1700 mg/L; control: 1648 mg/L	Bacterial cell population was increased only when 20 mg/L CaLS were used: 102.45 × 107 cells/mL vs. 94.12 × 107 cells/mL for control.	[80]

provides an overview of surface-active compounds used in bioleaching, including the minerals investigated, leaching conditions, extracted metals, and the influence on microorganisms. In all the examples cited there and described above, the additives tested were introduced into the leaching solution. However, there are reports where the surface of mineral particles was modified by the adsorption of surfactants, biosurfactants, or polymers and then subjected to bioleaching. In such a case, the surface-active compounds were present only on the mineral surface. In the work of Pawlowska et al. [27], column bioleaching of arsenic-bearing waste in the presence of the ionic surfactants was introduced. The results showed improvement in arsenic recovery in the presence of CTAB. A slightly higher amount of metalloid leached was also observed for the SDS compared to the control.

Regarding the use of biosurfactants in mineral bioextraction, only a few studies have been published in the last decade. The main reason is still limited large-scale production, which results in the high cost of such a reagent. On the other hand, when attempting to use bacterial broth or crude extract as a source of biosurfactants, there are difficulties in maintaining the consistent quality of such a product.

Pawlowska and Sadowski investigated the effects of rhamnolipids and lipopolysaccharides on the bioleaching of arsenic-bearing waste [36]. It was found that rhamnolipids adsorbed on the solid surface enhanced bioextraction efficiency (final recovery 31%; control 19%) by promoting bacterial adhesion, while lipopolysaccharides decreased arsenic extraction (only 16%) and favored the adhesion of secondary products, such as scorodite.

Rhamnolipids were also used as bioleaching intensifiers by Castro et al. [85]. The study investigated the solubilization of monazite and the recovery of rare earth elements (REEs) such as Ce, La, and Nd using two strains of Burkholderia thailandensis. Bacteria were able to solubilize metals by increasing pH (from 5.3 to 8.6). Final recovery (after 21 days) was 8.0 and 7.1 mg/L for strain E264 and ED1023, respectively. Authors also conducted abiotic studies using commercial biosurfactant (produced by Pseudomonas aeruginosa), and those produced by B. thailandensis showed the best efficiency at CMC, with the commercial reagent being more effective for all tested metals. The extraction yield was 9.36 mg/L REE and 5.13 mg/L for P. aeruginosa and crude extract, respectively. Higher concentrations caused reduced leaching capacity.

A variety of sophorolipids and sophorosides were used to leach Al, Cu, Fe, Pb, and Zn from secondary materials, including zinc processing sludges and slags, copper sulfide tailings, stainless steel residues, and automotive shredder and sludges [39]. Acidic sophorolipids were found to be the most effective in leaching metals, particularly copper. Optimal copper leaching was observed from fayalite slag (2.12%–27.63%) and CuS tailing (36.73%–53.30%).

6. Summary

Based on the reviewed literature, it can be concluded that both surfactants and biosurfactants influence the wettability of mineral surfaces, bacterial adhesion, and, consequently, the efficiency of bioleaching processes. An excessive amount of surface-active reagent inhibited bacterial growth and bioextraction yield. However, there were divergences in the optimal surfactant types, their concentrations, and the final effect on bioextraction. Therefore, the concentration of the additive must be precisely adjusted. Much depends on the conditions under which the reaction was carried out.

Table 7. Effect of surface-active reagents on mineral surface, bacterial activity and leaching efficiency.

Surface-Active Reagent Effect
Mineral Surface	Bacterial Activity	Leaching Efficiency
Surfactants form a layer on the mineral surface, the thickness of which depends on the surfactant concentration in the solution and its molecular structure	Nonionic surfactants at lower concentrations are less toxic to microorganisms than ionic ones	A higher molecular weight surfactant resulted in the leaching efficiency reduction (e.g., copper)
Moderate bacterial cell attachment. Ionic surfactant enhances bacterial adhesion due to electrostatic interactions	High concentration inhibits bacterial growth and causes cell disruption	Improvement solution penetration into small pores and cracks of large particles by decreasing surface tension
Rhamnolipids increase the contact between bacteria and minerals by forming a wetting film on a solid surface	Acceleration of sulfur oxidation and dissolution generated during bioleaching, which provides an additional energy source for bacterial growth (e.g., PEG)	An increase in leaching efficiency is mainly attributed to its ability to enhance bacterial adhesion to the mineral surface
Its presence improves hydrophilicity and reduces surface tension (e.g., SDS, CTAB, NaLS)	The addition of PEG increased bacterial attachment to sulfur
	SDS reduce the EPS secretion of bacteria and weakens cell agglomeration

summarizes general findings on how surface-active reagents can influence the bioleaching.

6.1. Current Knowledge Limitations

The interaction of bacteria with surfaces, especially minerals, is a complex biochemical process. Chemical surfactants instantly adsorb on mineral surfaces. Bacterial cells, on the other hand, act through extracellular polymers that serve as mediators. For this reason, the mechanism of bacterial modification of mineral surfaces is mainly based on the process of adhesion [86]. The above literature provides empirical observations, and in most cases, there is a lack of detailed discussion on the physicochemical mechanisms lying behind the biosurfactant–bacteria–mineral interactions during bioleaching, which is crucial in view of further industrial applications [87]. It was demonstrated that the optimal surfactant concentration must be determined for a specific solid material and the microorganisms used. Variability in experimental parameters, including pH, temperature, concentration of surface-active agent, and mineral material, makes it difficult to unify the method. In addition, only a few studies quantitatively discuss the influence of surfactants on energy consumption during bioleaching [26,67].

The surfactants of chemical and biological origin change the electrical double-layer structure and surface potential. Each solid material, surface-active reagent, and microorganism cell exhibits various surface charges, depending on the pH of the bioleaching environment, which in turn influences the process. Therefore, the effect of such reagents on mineral surface should be interpreted taking into account not only concentration and process variables but also physicochemical aspects, such as electrostatic interactions.

There is also a need to explain how these compounds interact with a specific mineral surface. At present, it is not fully known whether the thickness of the surfactant layer is essential or whether its presence in the solution rather than directly at the surface of the mineral is more desirable. It is also necessary to scale up processes. A solid concentration up to 3%, as most studies have shown, is not sufficient for industrial applications.

Research often focuses on a limited range of surfactants, in most cases, those already applied in mineral processing. However, in most cases, the impact of the tested substances on the natural environment is not taken into account. For example, Triton X-100, often used in research, is heavily restricted to uses in the European Union [88], as the decomposition products of this substance have a harmful effect on the environment.

The last limitation, which provides an area for further research, concerns biosurfactants. Despite their advantages in terms of environmental protection, they were less often used as a reagent in bioleaching. As mentioned earlier, this may be due to their high production costs.

6.2. Future Directions

There are still many aspects that require further development and clarification. Above all, more detailed studies are needed on the surface-active substances to establish a specific database of reagents that can be used in bioleaching, along with their optimal concentrations for the process.

It is also worth considering the application of mathematical and statistical techniques, such as Response Surface Methodology (RSM) [89] or Bayesian approaches [90] to optimize complex processes, which can provide valuable predictive tools for optimizing process design.

Future research on the use of surfactants in bioleaching should focus on scaling up studies to fill the gap between laboratory research and industrial application. An important step is to increase the leached mineral content while maintaining acceptable process efficiency. The selective recovery of metals and the reusability of additives should also be considered to reduce process costs and improve sustainability.

In the reviewed literature, bioleaching referred in most cases to primary ores. Considering the principles of the circular economy, greater emphasis must be placed on processing, for example, mining wastes. According to Eurostat, the mining industry accounted for 22.7% of total waste production generated in the European Union in 2022 [91].

Research needs to concentrate not only on the mineralogical composition, but also on the physicochemistry of bioleaching, such as surface charge and hydrophobicity, which have a significant influence on the adhesion and activity of microorganisms.

The use of various types of microorganisms and surface-active reagents in bioleaching, although promising for improving process efficiency, also carries potential environmental risks. One alternative might be the application of biosurfactants and other bio-based compounds, which are biodegradable, less toxic, and more compatible with the metabolism of microorganisms. Although their use might reduce potential ecological impact and support circular economy principles, they are less often applied in bioextraction due to their high cost. Therefore, another key direction is to enhance the production of biosurfactants to expand their utilization. There is also a need for further research on the toxicity and biodegradability of surface-active additives, combined with strategies for monitoring and environmental protection. If economically justified, the design of closed reaction systems is recommended.