The pyrometallurgical processes involve smelting, combustion, incineration, and pyrolysis in a furnace or blast furnace. Pyrolysis is carried out at elevated temperatures of up to 900 °C (or even higher in some instances) to treat e-waste, which generates 70% metal-rich residue, 23% oil, and 5% gases ^[1][2][19,36]. These processes are associated with high-energy consumption, are expensive and generate toxic compounds, including dioxins and furans. This is because e-wastes, especially PCBs, contain halogenated flame-retardants ^[3][35].

The hydrometallurgical process primarily involves the use of chemical reagents, where strong acids are used to leach base metals while chemical reagents such as cyanide, halide, thiourea, and thiosulfate are used to leach out precious metals from e-waste ^[2][3][35,36]. These processes require large quantities of chemical reagents and produce a variety of by-products and high volumes of effluent waste. It is noteworthy to mention that the recovery of precious metals from ores and e-waste is preferred via cyanide leaching due to its potential to yield high recovery and economic cost. However, this method has some disadvantages, such as increasing the additional work of treating the effluent before disposal, and cyanide is one of the controlled chemicals due to its toxicity ^[4][37].

Biometallurgical processes are mostly referred to as bioleaching. This is a process that has received significant attention as a promising sustainable technology to recover metals from e-waste due to its advantages such as low operational cost, low energy consumption, reduced chemical reagents usage, and easy manageability of secondary waste effluents ^[5][6][7][8][3,4,5,6]. However, this process is slow to leach metals; hence, it has not been fully industrialised for ores with high metal content and e-waste ^[7][9][5,38]. Table 12 shows the metallurgical process used to leach metals from e-waste, including their advantages and disadvantages.

Bioleaching of metals from e-waste is divided into two processes: (1) the One-step and (2) the Two-step bioleaching process.

In one-step bioleaching, the microbial inoculum sourced from the exponential growth phase is added to a suitable bioleaching medium and the e-waste. The process produces ferric iron (Fe³⁺) through the oxidation of ferrous iron (Fe²⁺) and protons, which slowly solubilises the embedded metals within the e-waste. The one-step bioleaching process can only be operated at low pulp density ranging between 1 and 10% (w/v) due to the presence of the toxic substances in e-waste that inhibit microbial growth ^[6][4]. Hence, the direct culturing of microorganisms in the presence of e-waste is not advisable due to the presence of toxic substances in e-waste that inhibits microbial growth, which in-turn lowers the availability of the extractant, leading to low metal extraction efficiencies ^[14][41]

In the first step of the two-step bioleaching process, microorganisms are cultured in the absence of e-waste in their respective culture medium under their optimum conditions. When the cells have reached their maximum cell growth and lixiviant production (Fe³⁺ and protons), the e-waste is then added to the medium as a second step ^[6][4]. This process reduces the inhibition of microbial growth caused by the toxic substances available in the e-waste. The two-step bioleaching process has been found to be more attractive as it results in rapid and high metal extractions, and it can be carried out at high pulp density, unlike the one-step bioleaching process. A large quantity of Fe³⁺/protons is readily available to react with the added e-waste and dissolution metals at the fastest possible rate ^[15][42].

The well-known acidophilic microorganisms Acidithiobacillius ferroxidans, Leptospririllum ferrooxidans, Acidithiobacillus thiooxidans and others play a vital role in the extraction of metals from e-waste. These are characterised by (1) survival or operation at low pH values (<2), (2) high metal concentration tolerance, and (3) they assist in the generation of lixiviants for the solubilisation of metals from the solid phase into the aqueous phase ^[8][16][6,43]. The metals embedded within the e-waste are in their insoluble form, whereas in sulphide minerals, they are present in the form of metal sulphides, where these iron and sulphur-oxidising organisms derive their energy from. Microorganisms in the bioleaching system use pyrite/sulphur as substrates to continually produce Fe³⁺ and protons as lixiviants. Therefore, in the bioleaching system of e-waste, it is necessary to add ferrous iron (Fe²⁺) and a source of sulphur, since such substrates are not available within the e-waste ^[8][6]. The acidophilic microorganisms oxidise Fe²⁺ to produce Fe³⁺, and sulphur oxidation to produce protons which are responsible for the conversion of insoluble metals (Cu⁰, Zn⁰, and Ni⁰) to their respective water-soluble metals (Cu²⁺, Zn²⁺, and Ni²⁺) as represented from Equations (1)–(6) ^[6][17][18][4,34,44].

Copper, Zinc, and Nickel dissolution from e-waste occurs through the action of Fe³⁺ in acidic conditions, maintained by the oxidation of sulphur by the microorganisms (see Equations (2)–(5)). All these chemical reactions occur at an ambient temperature and atmospheric pressure ^[6][4]. Biohydrometallurgical studies on recovering metals from e-waste have been widely reported with the use of mesophilic and chemolithotrophic bacteria such as Acidithiobacillus ferroxidans and Acidithiobacillus thioxidans, and acidophilic, moderately thermophilic microorganisms such as Sulfobacillus thermosulfidooxidance and Thermoplasma acidophilum, including cyanogenic microorganisms such as Chromobacterium violaceum and Pseudomonas fluorescens ^[9][17][34,38].

Pradhan and Kumar ^[14][41] studied the bioleaching of metals from personal computer e-waste with both one and two-step bioleaching processes using cyanogenic microorganisms (Chromobacterium violaceum, Pseudomonas aeruginosa and Pseudomonas fluorescens) at a pH of 7.2. They were incubated at 30 °C with an incubator shaker speed at 150 rpm. The researchers also studied the impact of pulp density of 10 and 20% w/v and discovered that the bioleaching rate depends on the pulp densities. The authors reported that C. violaceum and the mixture (C. violaceun and P. aeruginosa) achieved maximum bio-leachability efficiency of more than 79, 69, 46, 9, and 7% (C. violaceum) and 83, 73, 49, 13, and 8% (C. violaceun and P. aeruginosa) of Cu, Au, Zn, Fe, and Ag, respectively ^[14][41]. The mentioned studies are a demonstration that the two-step bioleaching process is more effective compared to the one stage process.

Different cyanogenic bacterial strains such as Chromobacterium violeaceum, Pseudomonas fluorescens, Pseudomonas aeruginosa, Bacillus megaterium have been exploited to extract precious metals and metalloids. These cyanogenic bacteria can detoxify and degrade the excess cyanide to β-cyanoalanine by β-cyanoalanine synthase during the late stationary and/or death growth phase, which makes the bio-cyanidation process attractive due to significant environmental and health risk mitigation, thus making the bio-cyanidation process eco-friendly ^[8][19][20][6,48,49]. Cyanogenic bacteria are effectively used to recover precious metals and metalloids such as Au, Ag, Pt, Pd, Ti and Mo from e-waste through a process referred to as alkaline bioleaching or the heterotrophic bioleaching process ^[13][40]. The cyanogenic bacteria are all capable of producing hydrocyanic acid (HCN)/cyanide ion (CN⁻) as their secondary metabolite at their late stationary phase during the decarboxylation of glycine, which serves as a lixiviant for the dissolution of the solid metals ^[12][21][39,50]. Gold cyanidation is an electrochemical process that consists of anodic and cathodic reactions, where gold is dissolved in alkaline cyanide solution to form a gold cyanide complex, as summarised by Elsner’s equations (see Equations (7)–(9)) ^[2][4][22][36,37,51]. This cyanide can be produced by the mentioned organisms and the extraction process can be carried out as demonstrated by Elsner’s equation ^[21][23][24][50,52,53].

It is essential to extract base metals from e-waste before the recovery of precious metals since they are present in high quantities and can quickly form cyanide complexes at higher concentrations than precious metals, thus lowering the recovery of precious metals ^[19][25][26][46,48,54]. Li et al. ^[2][36] investigated the bio-cyanidation of gold using a two-step bioleaching process, where P. fluorescens was used to recover gold from mobile phone e-waste powder in a 250 mL stirred airlifted tank reactor incubated at 30 °C with an incubator shaker speed at 150 rpm. The authors observed a 54% bioleaching efficiency of gold when they added glycine and methionine in the growth medium. In addition, these authors ^[2][36] also studied the effect of pulp density (0.33% w/v, 0.67% w/v, 1% w/v, 1.5% w/v and 1.67 w/v) on the bioleaching efficiency of gold and achieved 42% bioleaching efficiency of gold with a pulp density of 0.33% w/v and also noticed that when the pulp density increased, the bioleaching efficiency decreased ^[2][36]. Işıldar et al. ^[25][46] used Pseudomonas fluorescens and Pseudomonas putida to extract gold from desktop computers, laptops, and mobile phone e-waste in 300 mL stirred tank bioreactors incubated at 30 °C and achieved the highest gold recovery of 44%, using P. putida at 0.5% w/v pulp density ^[25][46].

Natarajan et al. ^[20][49] studied the bio-cyanidation of gold from electronic strap material (ESM) powder with a particle size of less than 100 µm in a two-step bioleaching process using Chromobacterium violeaceum and engineered strains and achieved the highest gold recovery efficiency of 30% with engineered strains at a pulp density of 0.5% w/v, compared to 11% achieved by C. violeaceum. The study also demonstrated that the increase in pulp density decreased gold recovery efficiency. This was due to the increased toxicity levels of the metals ^[20][49]. Marappa et al. ^[27][55] studied bio-cyanidation of gold and other precious metals from PCBs powder in a one and two-step bioleaching process using two Frankia bacterial strains (Frankia casuarinae and Frankia sp.). They observed that Frankia casuarinae achieved the highest gold bioleaching efficiency of 75% compared to Frankia sp. In contrast, Frankia sp. achieved the highest copper recovery efficiency of 94% compared to Frankia casuarinae ^[27][55]. Arshadi and Mousavi ^[28][56] investigated the biocyanidation of gold and copper simultaneously from computer e-waste using the central composite design of response surface methodology (CCD-RSM) method to achieve maximum metal bioleaching efficiency and the optimum conditions. The authors evaluated four factors that affect bioleaching activity using Bacillus megaterium in a 250 mL stirred tank reactor and achieved maximum gold extraction efficiency of 36.8% at an initial pH of 10 and 2.5% w/v pulp density and glycine concentration of 0.5 g/L. The authors pre-treated the e-waste with A. ferrooxidans to bioleach copper as a first step, while the second step involved the extraction of gold by cyanide-producing Bacillus megaterium at an extraction efficiency of 63.8% ^[28][56]. Pourhossein et al. ^[26][54] investigated the bioleaching of precious metals from spent light diode lamps (LED) using A. ferrooxidans as the first process to pre-treat LED e-waste to extract base metals and thereafter, used Bacillus megaterium to bioleach precious metals from the residue that was generated in the first process. In this study, A. ferrooxidans bioleached 80% Cu, 94% Ni, 93% Sn, 68% Al, 51% Pb, 46% Cr and 35% Fe, whereas Bacillus megaterium produced a maximum cyanide concentration of 15 g/L and achieved a high dissolution efficiency of 93% Au, 91% Ag, 98% Ni, 87 Cu, and 84% Ga after 4 days. The authors also noticed that there was a low dissolution of precious metals when untreated LED powder was used ^[26][54]. These studies demonstrate the efficacy of using the two-step process for the bioleaching of base and precious metals ^[29][30][57,58], which is proposed in the subsequent section as the only viable extraction process of base and precious metals from e-waste. These studies are summarised in Table 23.