Bioleaching of Printed Circuit Board Waste to Obtain Metallic Nanoparticles

Abstract

In this work, a biological recovery of metals (copper and gold) from computer printed circuit board (PCB) waste was carried out by bioleaching using Aspergillus niger. Three bioleaching methods comprising one or two steps or using spent medium were tested in an incubator shaker at 30 °C and 160 rpm with different PCB waste concentrations (2.5 to 10 g/L). Glucose was used as the carbon source. The best condition evaluated was carried out in a stirred tank reactor. The FTIR spectrum confirmed the presence of oxalic, citric, and gluconic acids. A. niger showed an efficiency of bioleaching of up to 100% and 42.5% for copper and gold, respectively, using the two-step method with 2.5 g/L PCB waste after 14 days of the process. The efficiency of bioleaching in a stirred tank reactor was 83% for copper and 24% for gold. The mean metallic particle size obtained after bioleaching varied according to the PCB waste concentration (2.5–10 g/L) added in the experiments. A transmission electron microscope analysis confirmed the synthesis of metallic nanoparticles with spherical morphology. The results indicated that the PCBs bioleaching process with A. niger can be an environmentally friendly alternative to current mechanical and metallurgical processes for metal leaching.

1. Introduction

Most electronic equipment has printed circuit boards (PCBs) as an essential component, comprising ~3 wt.% of electronic waste [1], reaching a yearly amount worldwide of 62 million tons in 2022 [2]. When environmental legislation is not strictly enforced, electronic equipment is often disposed of in landfills or in inappropriate places where it can contaminate the soil with heavy metals [3]. Thus, PCB waste became a global environmental challenge due to its complex and hazardous composition.

On the other side, PCBs contain metals with purity levels that are sometimes higher than those of minerals, e.g., copper (12–30 wt.%), nickel (1–5 wt.%), iron (1–3 wt.%), silver (0.05–0.15 wt.%), and gold (0.01–0.07 wt.%) [4]. Nevertheless, hazardous materials such as epoxy resin, other plastics, glass fibers, and ceramics are also present. In addition to the environmental issues related to the waste, the recycling of metals from PCB waste has also become economically viable once the metals represent more than 95% of the total value of the board [5].

Mechanical [6], pyrometallurgical [1], hydrometallurgical [7], and biohydrometallurgical processes can be used to extract metals from PCBs [8,9]. During the recovery process of electronic waste, the mechanical processes are applied as pre-processing, aiming to separate the components present in the PCBs and pre-concentrate the metallic fraction, being subsequently forwarded to refining processes [10].

The pyrometallurgical and hydrometallurgical processes present some environmental disadvantages due to pollutant generation (such as dioxins, heavy metal pollution, and sludge generation). Thus, there is a requirement to develop eco-friendly treatments, which are intended to increase the sustainability of recovery processes [3]. Consequently, studies’ interests are moving towards biohydrometallurgical processes as an alternative promising for recovering metals from PBCs [8,11,12,13,14].

Biohydrometallurgical processes can be performed through bioleaching, biosorption, bioprecipitation, biooxidation, bioreduction, and bioaccumulation. Bioleaching refers to the ability to solubilize metals present in the solid matrix using microorganisms, whereas biosorption refers to the ability of the microorganism to absorb metals (metals present in the aqueous solution) in the microbial biomass [15,16]. Bioleaching is performed under conditions at room temperature, generally without the addition of toxic substances, as the metabolites produced by the microorganisms—chemical substances, organic acids, polymers, and enzymes—are used to recycle metals from waste [17].

The amount of metals recovered by bioleaching depends on the microorganism used and the growth conditions applied. Bioleaching of PCB waste by microorganisms that produce organic acids, amino acids, and other metabolites has been attracting growing interest [18]. Fungi show great potential for the recovery of metals due to fast leaching rates, tolerance of a wide pH range (1.5–9.8), the ability to use a wide variety of substrates, ease of handling, organic acids production, and selective metal leaching [19,20,21,22]. Heterotrophic fungi, such as Aspergillus niger, have shown satisfactory results in the dissolution of metals from electronic waste [23,24,25,26]. However, few studies are currently available on the bioleaching of precious metals using filamentous fungi; therefore, more studies are needed on this topic.

Bioleaching can be carried out in one-step or two-step processes or with a spent medium. In the one-step method, the PCBs are mixed immediately with the microorganisms in the culture medium. In the two-step procedure, PCBs are added after the microorganisms reach the maximum growth (logarithmic growth phase). In the spent medium method, bioleaching experiments are carried out in cell-free acidic filtrates after the production of metabolites [20,22,27].

In this work, to enhance the metal dissolution, different methods were evaluated to compare the efficiency of the bioleaching of copper and gold present in computer PCBs using Aspergillus niger. A scale-up of the process using the best method was also performed in a stirred tank reactor.

2. Materials and Methods

2.1. PCB Preparation and Metals Quantification

PCBs from desktop computers from different manufacturers were manually stripped down to remove capacitors, resistors, processor carriers, and batteries. PCB waste was cut out using a guillotine into 3 × 3 cm pieces and reduced in size using a hammer mill (Servitech CT-058–Santa Gertrudes–SP, Brazil) followed by a knife mill (Marconi MA 340–Piracicaba–SP, Brazil). A dense medium separation process separated the polymers and ceramic fraction from the metal fraction. A concentrated water-based solution of sodium silicate (9.20 wt.% Na₂O, 29.50 wt.% SiO₂, and 61.30 wt.% H₂O; pH: ~11.25; Quimidrol) was used as dense liquid media in the separation process. The dense medium separation process was performed using a ratio of 1 to 10 solid:sodium silicate solution in a 250 mL graduated cylinder at 25 °C. The decanted fraction obtained after separation was washed twice with distilled water and dried until constant weight at 60 °C [28].

Aluminum (Al), copper (Cu), gold (Au), iron (Fe), nickel (Ni), palladium (Pd), silver (Ag), and zinc (Zn) contents from the PCBs were measured by inductively coupled plasma mass spectrometry (ICP-MS, NexION 300 D, Perkin Elmer, Waltham, MA, USA) after digestion in the microwave (MLS 1200, Milestone–Sorisole–Italy). Microwave digestion was performed using 120 mg of metal fraction, added with 4 mL of nitric acid (HNO₃), 2 mL of hydrochloric acid (HCl), and 1 mL of hydrogen peroxide (H₂O₂) in closed Teflon flasks. The mixture was kept in the microwave oven for 5, 4, and 3 min, respectively, at 250, 400, and 650 W. After cooling, ultrapure water was added to the mixture until a final 50 mL volume was reached. The metal fraction (d50 ~ 128 μm) was sterilized for 20 min at 121 °C and 1 atm for bioleaching.

2.2. Production and Characterization of Metabolites

The metabolites produced by A. niger were collected every 48 h and vacuum-filtered with Whatman 42 filter paper to determine pH, reducing sugars, total acid, and dry biomass. The dry biomass was determined by mycelium obtained from vacuum filtration; the mycelium was dried until constant weight at 60 °C. The concentration of reducing sugars was quantified by the dinitrosalicylic acid method [29]. The total acid was quantified by titration according to Standard Methods for the Examination of Water and Wastewater [30]. At the end of the metabolite production process (10th day), the sample within KBr pellets was analyzed by Fourier transform infrared spectroscopy (FTIR, Shimadzu–Kyoto–Japan, Model IRPrestige-21, 8400S). A resolution of 4 cm⁻¹ with scanning in the 500–4000 cm⁻¹ range was applied to determine active functional groups.

2.3. Bioleaching Procedure

Aspergillus niger was isolated and identified by Treichel et al., [31]. The microorganism was inoculated in 100 mL of potato dextrose agar medium (PDA) in 1 L Erlenmeyer flasks and incubated at 30 °C for 7 days [32]. After the fungal growth on the agar surface, the spore suspensions were prepared with 50 mL of Tween 80 solution (0.1%), followed by scraping the spores and filtering in cotton to retain the hyphae. Spore counting was performed using a Neubauer counting chamber.

Three different bioleaching methods were tested (one-step, two-step, and spent medium) (Figure 1) in 300 mL autoclaved Erlenmeyer flasks with 125 mL of mineral medium containing (g/L): CaCl₂ (0.1), KH₂PO₄ (0.5), NH₄Cl (1.5), MgSO₄·7H₂O (0.025), and glucose (50.0), according to the literature [33]. The pH was adjusted to 4.4 with 1 N of H₂SO₄. The medium was sterilized for 20 min at 121 °C and 1 atm. After sterilization, the inoculation was performed in the mineral medium at room temperature. Different metal concentrations (0.0, 2.5, 5.0, 10.0 g/L) were used to evaluate the influence of metal concentration on the bioleaching process. The experiments were performed in duplicate.

In the one-step method, A. niger was inoculated with 4 × 10⁶ spores/mL_medium in 125 mL of mineral medium containing the metal and incubated in a shaker incubator at 30 °C at 160 rpm for 24 days. Samples were collected after 0, 14, and 24 days.

In the two-step method, A. niger was inoculated with 4 × 10⁶ spores/mL_medium in 125 mL of mineral medium (without a metal fraction) up to the logarithmic growth phase, which corresponds to the beginning of metabolites production responsible for the bioleaching process. After 4 days, the autoclaved metal fraction was added to the culture medium, and the bioleaching process was carried out for up to 24 days in the incubator at 30 °C and 160 rpm. Samples were collected at 0, 14, and 24 days.

In the spent medium method, A. niger was inoculated with 4 × 10⁶ spores/mL_medium in 125 mL of mineral medium without a metal fraction until the maximum production of metabolites, which took 10 days. Then, the suspension was filtered through the Whatman 42 filter paper to obtain a cell-free spent medium. The metal fraction was added to the filtrate and bioleached for up to 14 days in the incubator at 160 rpm and 30 °C. Samples were collected at 0 and 14 days.

For all evaluated methods, the samples were collected on the first day and at the end of the process. After the bioleaching process, the cultures were first filtered in Whatman 42 filter paper, then in a 0.22 μm syringe filter, and finally characterized. The bioleaching efficiency was calculated according to Equation (1) [24]. The bioleaching yield was calculated considering the increase in metallic concentration in the solution (2.5; 5.0 or 10 g/L), the initial mass of the metal (copper or gold) added to the culture, and the concentration of the metal detected after the bioleaching in the solution.

$$\mathrm{B}\mathrm{i}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{C}}_1}{{\mathrm{C}}_0}}\times 100$$

(1)

where:

• C₀: Concentration of metal (Cu or Au) in the solution before bioleaching;
• C₁: Concentration of metal (Cu or Au) in the solution after bioleaching.

2.4. Bioleaching in a Stirred Tank Reactor

The scale-up of the bioleaching process was accomplished in a 5.0 L stirred tank reactor (Tec-Bio-Flex II, Tecnal–Piracicaba–SP, Brazil), with an initial working volume of 3.0 L. A. niger was inoculated with 4 × 10⁶ spores/mL_medium in 3 L of mineral medium. The metal fraction was only added when A. niger entered the growth log phase. The pH was set to 4.4 with 1 N of H₂SO₄. After 4 days, the autoclaved metal fraction was added to the medium, and the bioleaching process was carried out. The reactor was operated with a controlled temperature (30 °C) and agitation rate (160 rpm) for 14 days. The aeration rate was automatically adjusted at 0.5 vvm (1.5 L.min⁻¹) during the experiments. Samples were collected after 0 and 14 days. After the bioleaching process, the culture was filtered with Whatman 42 filter paper and a 0.22 μm syringe filter and intended for characterization.

2.5. Characterization of Bioleached Products

Bioleached copper content was measured by atomic absorption spectrometry (AAS, Shimadzu model AA 6300, Kyoto, Japan) using an air-acetylene flame, and the gold content was measured using inductively coupled plasma mass spectrometry (ICP-MS, NexION 300 D, Perkin Elmer, Waltham, MA, USA).

A particle size analyzer by laser scattering instrument was used to determine the average particle size distribution (Zetasizer Nano ZS, Malvern, UK). The stability of the metallic particles after the bioleaching was evaluated by the zeta potential (Stabino 2.00.23, Particle Metrix–Ammersee–Germany). The stability of bioleached metallic particles is based on electrostatic attraction (aggregation) or repulsion (stability) between particles. A transmission electron microscope (TEM, JEM-1011 JEOL, Akishima, Tokyo, Japan) at 100 kV was used to investigate the morphology of the metallic particles after bioleaching. A drop of leached solution was placed in 300 mesh copper grids coated with a carbon film and dried at room temperature for 48 h.

3. Results

3.1. Metabolites Production by Aspergillus niger

The effect of initial glucose concentration on substrate consumption and product synthesis during the fermentative process was evaluated (Figure 2). As observed from the results, in the lag phase, low carbon content was consumed for the respiratory chain until the second day. Then, on the fourth day, glucose was quickly spent, and the synthesized products (such as organic acids) increased. A. niger consumed up to 72% of the initial glucose supplied at the beginning of the fermentative process.

The mechanism of bioleaching using A. niger is related to the production of low-molecular weight metabolites, such as gluconic, citric, oxalic, malic, and succinic acids, which are responsible for bioleaching metal ions [34,35]. Metabolite production (Figure 2b) is indicated by increased acidity—or decreased pH—throughout the fermentation process.

On the second day of the process, the pH was reduced from 4.13 to 2.24 (Figure 2a), mainly due to the enzymatic action corresponding to the beginning of the production of organic acids (Figure 2b). In this case, the dry biomass increased from 0.01 to 2.30 g/L. Values of pH between 1.5 and 2.0 indicate high production of citric acid; the optimal pH for citric acid production is below 3.5 [36]. The pH variation and microbial growth showed a reverse behavior, with a microbial growth increase for pH reduction. After 10 days, the highest microbial growth was reached (4.74 g/L). After 10 days, the growth shows a small decrease, probably due to inhibition by citric acid, the primary metabolite to be secreted.

The FTIR spectrum after 10 days (process end) confirms the organic acid production (Figure 2c). The peak located at 3429 cm⁻¹ is assigned to water. Furthermore, the small peak identified at 2300–2400 cm⁻¹ is ascribed to CO₂ in the air. The peaks observed at 1639 and 2934 cm⁻¹ are associated with the stretching of C=O and O-H, as is typical for D-gluconic acid. Citric and malic acids show intense and characteristic bands ranging from 1500 to 900 cm⁻¹. This confirmed the presence of oxalic, citric, and gluconic acids [37,38,39].

3.2. Copper and Gold Bioleaching

The chemical composition of the metallic fraction used in this study was analyzed to evaluate the content of metals present in the waste (

Table 1. Metal content in PCB waste.

Metal	Concentration (wt.%)
Cu	15.25 ± 0.35
Fe	1.40 ± 0.06
Al	0.960 ± 0.009
Zn	0.760 ± 0.011
Ni	0.210 ± 0.004
Ag	0.740 ± 0.015
Au	0.02400 ± 0.00001
Pd	0.00100 ± 0.00001

). Cu, Fe, and Al were observed to be the predominant metals in PCBs, with a maximum content of 15.25, 1.40, and 0.96 wt.%, respectively. The total metal content for the samples analyzed in this work was ~20 wt.%. Small amounts of precious metals, including Au (0.024 wt.%), Ag (0.74 wt.%), and Pd (0.001 wt.%), were also detected.

The efficiency of Cu bioleaching depends on the metallic fraction concentration and process time. In this study, copper was bioleached from the waste PCBs with efficiency of up to 88% (one-step), 88% (two-step), and 100% (spent medium) at a 2.5 g/L metallic concentration after 14 processing days (Figure 3a). Higher metallic concentrations showed lower bioleaching efficiency with the one-step and spent medium methods in 14 days. Figure 3b shows that after 24 days, copper was 100% bioleached by the one-step method, regardless of the initial metallic fraction concentration. The decline in bioleaching efficiency in the two-step approach using metallic fraction concentrations of 2.5 and 10 g/L may be related to the precipitation of insoluble materials after the maximum leaching of metals [40].

It was observed that the efficiency of bioleaching after 14 and 24 days (Figure 3c,d) followed a downward trend for all experiments, resulting in a maximum leaching of Au of up to 42.5%, depending on the method, at 2.5 g/L PCB waste concentration. Increasing the waste concentration would be expected to increase the surface area considering a larger number of particles, which would accelerate bioleaching. On the other hand, higher waste concentrations result in more dissolved compounds, which may inhibit microbial activity and lower the amount of metals leached.

The highest percentages of bioleaching of metals were obtained using the methods with one and two steps. Nevertheless, it is known that heavy metal ions produced by the dissolution of industrial waste can inhibit the growth and metabolism of strain due to the toxicity of heavy metal ions. In the one-step method, it was possible to visually observe the growth inhibition of A. niger in the experiments with higher metallic fraction concentrations (10 g/L). The one-step process is a classical method conducted via the inoculation of spore suspension to the solid waste culture medium. The use of this method implies the reduction of operating and capital costs because fermentation and leaching processes are performed in one step [35,41].

In the two-step method, it was possible to visually observe no inhibition in fungal growth, regardless of the metallic fraction concentration used in the bioleaching process. In the two-step method, the fungi grow in the culture medium until the logarithmic growth phase, and later, the solid waste is added to the medium. In this method, the growth of the fungus until the logarithmic phase limits the inhibition effect of the solid waste on microorganism growth, the production of metabolites and, correspondingly, metal extraction. This method for an industrial application can be appropriate due to the enhanced bioleaching efficiency caused by metabolites generated before the addition of solid waste to the medium [35,42].

In a biological process, the pH can be influenced by several factors, such as the number of anions and cations (ionic strength) present in the culture and also by the metabolites produced by A. niger during the process [43]. The addition of different metallic fraction concentrations for all experiments in an increased pH level compared with the control, as shown in Figure 4, was observed. The pH follows the same behavior in all experiments: it increases and stays constant until the end of the bioleaching process. The experiments with higher metallic fraction concentrations (10 g/L) showed the highest pH values. The experiments with pH values above 5.4 showed the lowest percentages of bioleaching. When the metallic fraction of 2.5 g/L was used, the pH values were lower than 4.5, and consequently, the bioleaching percentages were higher.

The pH variation can be correlated with the consumption of organic acids during bioleaching and with the presence of metal fractions in the medium [44]. The higher the concentration of the metallic fraction, the greater the content of solids present in the culture medium, thus requiring a greater production of metabolites responsible for decreasing the pH of the leaching medium [43,45]. This behavior was also reported by Madrigal-Arias et al. [30], who used either A. niger MXPE6 or MX7 in PCB waste. The pH value increases due to the addition of PCBs, as also reported by Brandl et al. [46], who observed that the pH of the culture medium increases at a high metal concentration.

Related to the morphological analysis, the mean particle size obtained at different PCB concentrations decreased as the PCB concentration increased. For the one-step method (Figure 5a), a modal distribution with a mean diameter of 857 nm and 423 nm, respectively, at 2.5 and 5 g/L and a trimodal distribution with a mean diameter of 324 nm at 10 g/L PCB waste were obtained. The TEM images showed that the one-step particles obtained were almost spherical, with the observation of some agglomeration points.

The results of the two-step method (Figure 5b) showed a bimodal distribution with a mean diameter of 662 and 490 nm, respectively, for 2.5 and 5 g/L and a trimodal distribution with a mean diameter of 574 nm for 10 g/L PCB waste. The particle size distribution for the two-step method showed the same behavior as observed for the one-step method. As observed, spherical particles were detected by TEM with particle sizes of less than 100 nm for all two-step experiments. The results of the spent medium method showed a bimodal distribution with a mean diameter of 347, 352, and 524 nm, respectively, for 2.5, 5, and 10 g/L PCB waste. As previously observed, the mean metallic particle size obtained after the bioleaching process varied according to the metallic fraction concentration added to the culture (2.5–10 g/L), and the mean metallic particle size increased as the PCB concentration increased. The TEM images of the samples showed that the particles were almost spherical, with a slight variation in particle size.

The results of the one-step procedure (Figure 6a) after 24 days of bioleaching showed a bimodal distribution with a mean diameter of 131 nm for 2.5 and 5 g/L PCB waste and trimodal distribution with a mean diameter of 332 and 324 nm, respectively, for 5 and 10 g/L PCB waste. TEM images showed individual particles as well as aggregates with most spherical nanoparticles. The results of the two-step method (Figure 6b) showed a modal distribution with a mean diameter of 327 and 1925 nm, respectively, for 2.5 and 10 g/L PCB waste and a trimodal distribution with a mean diameter of 688 nm for the 5 g/L PCB waste. The size and shape of metallic particles were evaluated by TEM; the nanoparticles are spherical, and the average size of metallic nanoparticles is ~50 nm.

The zeta potential as a function of pH (Figure 7) was determined for the bioleached samples using the metallic fraction concentration of 2.5 g/L, respectively, for (a) one-step, (b) two-step, and (c) spent medium methods.

When the zeta potential of suspended particles is greater than approximately 25 mV (positive or negative), it is considered stable as the particles resist one another and do not agglomerate. Regardless of the pH area, the attraction appears to exceed the repulsion in all cases (Figure 7a–c), and the particles tend to aggregate, lowering the zeta potential values. The isoelectric point (IEP) is around pH 2 for the three suspensions analyzed, meaning that the trend to agglomerate is the highest at lower pH values. Additionally, several factors can influence the zeta potential of suspended particles, such as the particle concentration in the suspension, the particle morphology, and the leaching of soluble silica from glass containers [47]. Thus, the results confirmed high instability and particle aggregation, as previously observed by TEM analysis.

Nanoparticles are thermodynamically unstable, mainly due to their high contact surface, and therefore have a natural tendency to aggregate; the main challenge is in the synthesis of stable nanomaterials that maintain the same size, shape, and degree of dispersion [48,49]. The aggregation can occur both in the synthesis stage and later. The stability of colloidal solutions will depend on the nature and magnitude of the interaction between the nanoparticles and the medium [50]. Physicochemical instability limits the applicability of nanoparticle suspensions; however, strategies such as adding molecules arranged around the particles cause a steric effect, preventing their approximation [51]. An alternative is an addition of stabilizing agents (e.g., sodium citrate, cyclodextrins) [52,53], polymer stabilizers (e.g., polyvinyl alcohol, polyethylene glycol, polivinilpirrolidona) [53,54,55], or surfactants [56,57] that allow for the formation of emulsions and, consequently, increase the stability of the suspensions, thus allowing for the application of nanoparticles in different areas.

3.3. Bioleaching in a Stirred Tank Reactor

The scale-up of the bioleaching process was accomplished based on the previous study (item 3.2). The previous study showed that bioleaching using the two-step method with a 2.5 g/L metallic concentration was the best condition for Cu and Au leaching from PCB waste.

The copper and gold bioleaching efficiency in a stirred tank reactor was up to 83 ± 0.2% and 24 ± 0.1%, respectively. In this study, the mechanisms of metal dissolution consider that the main route in the dissolution of the metal proceeds through organic acids that can provide protons and complex anions. The metallic fractions of PCBs can be dissolved by organic acids by the mechanisms of acidification and complexation [36,58,59,60]. In the complexation process, metal ions bond to organic ligands produced by microorganisms to generate stable metal complexes that can help in metal solubilization and mobilization. In the acidification mechanism, an organic acid dissociates to donate H+ for proton-promoted dissolution (Reaction 2). The acid dissolution process involves the reduction of protons to H2 at the cathode (Reaction 3) and the oxidation of the metal at the anode (Reaction 4). The ligands from organic acids will then form stable complexes with metals present in the metallic fraction from PCBs (Reaction 5) [58,60]:

$$\mathrm{R}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{R}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$$

(2)

$$2{\mathrm{H}}_3{\mathrm{O}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+2{\mathrm{H}}_2\mathrm{O}$$

(3)

$$\mathrm{M}\to {\mathrm{M}}^{2+}+2{\mathrm{e}}^{-}$$

(4)

$$\mathrm{R}\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+\mathrm{M}\mathrm{O}\stackrel{\left({\mathrm{H}}_3{\mathrm{O}}^{+}\right)}{\leftrightarrow }\mathrm{R}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{M}+{\mathrm{H}}_2\mathrm{O}$$

(5)

where R is an organic substituent group and M is a metal.

Figure 8 shows particles with trimodal distributions and a mean diameter of 498 nm. Nevertheless, TEM confirmed the presence of the spherical nanoparticles with some agglomeration and an average size of <100 nm. The comparison of the obtained size from DLS with direct microscopic (TEM) measurements is complex as the hydrodynamic diameters of nanoparticles are a function of nanoparticle concentration and laser power. Moreover, this difference between particle size distributions may also be related to the solvation effects and the agglomeration behavior of the particles in suspension [61,62,63].

Metabolites from microorganisms were successfully used to recover metals from urban mining. The bioleaching process is an efficient, environmentally friendly, and economical substitute for the traditional recycling processes of metals from industrial waste [21]. Nevertheless, most studies in the literature on the fungal leaching of waste have been performed in the laboratory. Therefore, pilot-scale investigations should be performed toward the application of the process at an industrial scale. Moreover, other factors such as microorganisms, growth medium composition, bioleaching period, and composition of metal elements in the PCBs can affect the bioprocess and should be considered.

Economic development and the rapid advancement of technology result in the frequent obsolescence of electronic equipment, making e-waste the fastest-growing waste stream globally. The continuous increase in e-waste levels, coupled with inadequate and unsafe treatment and disposal by incineration or landfill, poses significant challenges to human health, the environment, and achieving the Sustainable Development Goals (SDGs). Target 12.5 of the SDGs establishes the need to substantially reduce waste generation through prevention, reduction, repair, recycling, and reuse strategies. Currently, recycling, recovering, and reusing obsolete electronic devices are recognized globally as a major challenge, considering the potential for adding value to the metals in these materials, such as gold, silver, copper, palladium, and rare metals. Recovering these components has turned recycling into a lucrative business opportunity in developed and developing countries. The significant volume of waste generated due to current consumption patterns presents substantial challenges regarding storage, handling, and final disposal. The efficient extraction of metallic components and the separation of non-metallic constituents are central issues in processing this waste. In this context, biotechnological processes, such as microorganism-mediated bioleaching, have emerged as a promising technology for the recycling, recovery, and reuse of electronic waste. This approach is attractive due to its low energy consumption and lack of toxic reagents, making it a sustainable alternative to conventional methods.

4. Conclusions

In this study, Aspergillus niger was successfully applied to recover copper and gold from PCBs. The highest efficiency of bioleaching was achieved with the one-step (100% Cu and 41% Au) and two-step methods (100% Cu and 42.5% Au). Bioleaching efficiency reached 100% and 35.16% in the spent medium method for copper and gold, respectively. A. niger produced metabolites that were effective in leaching copper (100% efficiency), regardless of the method used. As the concentration of PCB waste increased, the metal leaching efficiency decreased. In the two-step method, using the stirred tank reactor, bioleaching efficiency reached up to 82.4% and 23.3% for copper and gold, respectively. Concerning the particle size distribution (DLS analysis), particles with ~130 nm size were obtained at lower PCB concentrations (2.5 g/L), indicating that the biological leaching of PCBs depends on physicochemical factors. TEM micrographs revealed particles with a spherical morphology of smaller than 100 nm in the two-step method. In conclusion, it is possible to confirm that bioleaching is a green technology for the leaching and synthesis of metal nanoparticles as an alternative method of urban mining.