Ecological Engineering

Microbiological Approach for Leaching Out Metallic Elements from Electric and Electronic Waste

  • John ANGEL SNEHA , 1 ,
  • Gurumurthy KALAICHELVAN , 2, *
  • 1. School of Biosciences and Technology, Vellore Institute of Technology, Katpadi, Tamil Nadu 632014, India
  • 2. School of Agricultural Innovations and Advanced Learning, Vellore Institute of Technology, Katpadi, Tamil Nadu 632014, India
*Gurumurthy KALAICHELVAN, E-mail:

John ANGEL SNEHA, E-mail:

Received date: 2021-07-20

  Accepted date: 2022-06-29

  Online published: 2023-04-21


After the computer and mobile revolution, electric and electronic waste had become a serious threat to urban and rural communities equally. Prevention of the hazardous exposure and proper management are challenging in developing nations. One way to turn the crisis to opportunity is to extract metals from this Waste Electronic and Electric Equipment (WEEE) is making waste into a source of metal ores. The involvement of microbes in this technology could increase the boons by being an eco-friendly technique for reducing the hazardous nature. This article reviews the mechanisms involved in the process of bioleaching, the microorganisms employed, methods used and various developments as well as limitations along with recent advances and future prospects of the process of bioleaching of metals from WEEE.

Key words: bioleaching; WEEEs; MPPCBs

Cite this article

John ANGEL SNEHA , Gurumurthy KALAICHELVAN . Microbiological Approach for Leaching Out Metallic Elements from Electric and Electronic Waste[J]. Journal of Resources and Ecology, 2023 , 14(3) : 667 -674 . DOI: 10.5814/j.issn.1674-764x.2023.03.020

1 Introduction

Electronic waste is becoming a very serious issue that negatively affects environment and public health in India due to introduction of various metals and heavy metals in the environment. Sixty five Indian cities including the four metropolitan cities (Mumbai, Delhi, Chennai and Kolkata) contributes to about 60% of nation’s total e-waste generated (Joon et al., 2017). In Delhi alone people are dismantling about Ten to Twenty Thousand tons of electric and electronic waste per year with bare hands (Monika, 2010). Ecologically sound management of electric and electronic waste is very challenging for developing nations and therefore it is very limited or absent (Herat and Agamuthu, 2012). Exposure to these e-wastes could cause health hazards such as changes in functioning of thyroid, neonatal complications such as sudden abortions, still births and preterm births, etc. (Grant et al., 2013). Electric and electronic waste can be treacherous if the elements like Pb, Hg, As, Cd, Se, Cr(VI) and flame retardants are present beyond allowable quantities (Pant et al., 2012).
According to US Geological survey 2020, a considerable quantity of metals is recovered through recycling of scraps, including 3.4 million tons of Al, 0.87 million tons of Cu, 130 tons of Au, 0.11 million tons of Mg, 1100 tons of Ag, 11000 tons of Sn and 62000 tons of Zn.
The process of involving microbes in leaching out metals from low-grade ores, in order to replace the uneconomical conventional methods is known as bioleaching and it occurs in conditions which are suitable for the growth of the leaching microorganisms (Bosecker, 1997). This technique is also useful for detoxication of waste products from industries, sewage sludge and soils which were contaminated due to the presence of heavy metals (Narayan and Sahana, 2009). Although the technology of bioleaching has reached the level of initial maturity, in the case of WEEEs, the techniques for its practical implementation is still at the stage of infancy (Ilyas and Lee, 2014).
The process of leaching out of metals from electronic scraps could serve both as a way to reduce environmental contamination and as a secondary source metal resource (Awasthi et al., 2019). Compared to chemical leaching techniques, the process of bioleaching can be considered as an effective and environmentally sound strategy for recovering metallic elements from e-wastes (Cui and Zhang, 2008). In the process of bioleaching, both bacteria and fungi can be used. A two-step leaching method can be applied wherein, from metal leaching biomass growth is separated (Brandl et al., 2001). Involving acidophilic mesophile organisms, thermophilic species and some species of fungus, recovery of valuable metals like Au, Ag and Cu is possible from WEEEs in high concentrations through bio-hydrometallurgical process of bioleaching (Annamalai and Gurumurthy, 2019). Nowadays, researchers are working for optimizing the conditions for maximizing the efficiency of yield. By continuous and effective optimization, the process can be expected to be scaled to industrial level soon (Liang et al., 2014).

2 Pathways involved in microbial leaching

Biological leaching of metallic sulphides is carried out using varied bacterial groups. The process involves two different pathways, namely the thiosulfate and polysulfate pathways. Here the definite decisive factor is the acidic-solubility of sulphides. This process of sulphide dissolution can be determined by ‘direct’ and ‘indirect’ mechanisms (Mishra et al., 2005).

2.1 Direct mechanism

While direct oxidation, initially the bacteria will get attached onto the surface of sulphide mineral. Then it will do direct solubilization of the surface through hypothesized enzymatic oxidation reactions (Fig. 1).
Fig. 1 Direct and Indirect mechanisms of Bioleaching (Mishra and Rhee, 2014)

2.2 Indirect mechanism

Indirect oxidizing process take place through the process of catalyzing aqueous Iron[II] (Fe2+) to Iron[III] (Fe3+) ion using microbes after which the sulfide will get oxidized directly by ferric ion (Boon, 2001) (Fig. 1).
The pathways of indirect oxidation could be classified as the thiosulphate and polysulphide pathways.

2.2.1 Thiosulfate pathway

Thiosulfate pathway of bioleaching can also be called as pyrite leaching. The sulfur of pyrite, after getting attacked initially by the oxidizing agent, ferric ion (Fe3+), will get oxidized into the intermediates of sulfur which are soluble in nature. The bonds between S2-2 and Fe2+ can be cleaved, and as a result, hydrated ferrous ions and thiosulfate will be formed. Then the soluble intermediates of thiosulfate will get converted to tetrathionate through oxidation. Thereupon, the tetrathionate will undergo decomposition and degradation to form elemental sulfur and sulfite, as well as trithionate along with pentathionate. Finally, these Sulphur compounds will undergo complete oxidization to (SO4)2‒ in the solution. The related equations are as follows:
Spontaneous oxidation of disulfide to thiosulfate by ferric ion
$\text{Fe}{{\text{S}}_{2}}+6\text{F}{{\text{e}}^{3+}}+3{{\text{H}}_{2}}\text{O}\to 7\text{F}{{\text{e}}^{2+}}+{{\text{S}}_{2}}\text{O}_{3}^{2-}+6{{\text{H}}^{+}}$
Oxidation of ferrous ion by bacteria using oxygen
$4\text{F}{{\text{e}}^{2+}}+{{\text{O}}_{2}}+4{{\text{H}}^{+}}\to 4\text{F}{{\text{e}}^{3+}}+2{{\text{H}}_{2}}\text{O}$
Oxidation of thiosulfate by bacteria to give sulfate
${{\text{S}}_{2}}\text{O}_{3}^{2-}+2{{\text{O}}_{2}}\text{+}{{\text{H}}_{2}}\text{O}\to 2\text{SO}_{4}^{2-}+2{{\text{H}}^{+}}$
The intermediates of this pathway on pyrite are given in Fig.2
Fig. 2 Intermediates of thiosulfate pathway on pyrite (Huang and Li, 2014)

2.2.2 Polysulfide pathway

The process of bioleaching using this pathway can also be called as chalcopyrite leaching. The general mechanism of this pathway is represented through these equations (Huang and Li, 2014).
$\text{MS}+\text{F}{{\text{e}}^{3+}}+{{\text{H}}^{+}}\to {{\text{M}}^{2+}}+\frac{1}{2}{{\text{H}}_{2}}{{\text{S}}_{n}}+\text{F}{{\text{e}}^{2+}}\left( n>2 \right)$
$\frac{1}{2}{{\text{H}}_{2}}{{\text{S}}_{n}}+\text{F}{{\text{e}}^{3+}}\to \text{F}{{\text{e}}^{2+}}+\frac{1}{8}{{\text{S}}_{8}}+{{\text{H}}^{+}}$
$\frac{3}{2}{{\text{O}}_{2}}$+$\text{ }\!\!~\!\!\text{ }\frac{1}{8}{{\text{S}}_{8}}$.+$\text{ }\!\!~\!\!\text{ }{{\text{H}}_{2}}\text{O}\to \text{SO}_{4}^{2-}+2{{\text{H}}^{+}}$
The schematic representation of these pathways is given in Fig. 3.
Fig. 3 Illustrative representation of (a) thiosulfate pathway (b) polysulfide pathway (Vera et al., 2013)

3 Microbial participants of the process

Though they have several common characteristics, that makes it appropriate for leaching out metals from WEEEs, the microbes that are engaged in bioleaching process are of different types including, acidophilic, cyanogenic and fungal bioleachers.

3.1 Acidophilic bioleachers

Bioleaching with acidophilic microbes has been considered as a subsequent way for treating waste products which exhibits comparatively low content of precious metals, or are not so easy for handling or treating. The commonly used species includes At. thiooxidans and At. ferrooxidans, along with At. caldus belong to γ-proteobacteria. Acidiphilium, Leptospirillum, Acidimicrobium, Ferromicrobium, Sulfobacillus along with archaeabacteria of the genus Sulfolobales also helps in the process of leaching (Rohwerder et al., 2003).

3.2 Cyanogenic bioleachers

Cynogenic type of bioleachers mainly consist of bacterial species that leaches out the metals by producing hydrogen cyanide. Species that are commonly used in the process of microbial leaching are P. fluorescens, P. aeruginosa, P. putida, C. violaceum, B. megaterium, P. chlorophis, P. aureofaciens and E. coli. Bioleaching using cyanogenic microorganisms are generally carried out at pH 7.0-11.0 and 25-35 °C temperature (Srichandan et al., 2019).

3.3 Fungal bioleachers

Fungal leaching of metals is done on the basis on three processes. They are acidolysis, complexolysis and redoxolysis. Usually, fungi utilize a high amount of energy for carrying out their microbial activities. On comparing the bacterial and fungal leaching, the latter is slower than the former. Fungal bioleaching is better than bacterial leaching as the material possesses huge amount of primal matter content, similar to the case of black shale. As the result of Fungal bioleaching, soluble complexes will be formed with metallic ions in environments with neutral nature or is with minimum toxicity level, which could also be an advantage. Most actively used organisms are usually of Penicillium and Aspergillus genera (Anjum et al., 2012).
The involvement of different types of microbes in leaching out metals from different industrial wastes is represented in Table 1.
Table 1 Types of microbes involved in bioleaching from Industrial waste (Mishra and Rhee, 2010)
Types Acidophilic bioleachers Cyanogenic
Fungal bioleachers
Micro-organism involved Acidithiobacillus sp At. thiooxidans Sulfobacillus Chromobacterium violaceum Pseudomonas fluorescens Bacillus
Aspergillus niger
Types of wastes
Flue ash,
Electronic scraps,
Exhausted battery,
Exhausted refinery
Exhausted petroleum catalyst
Sewage sludge, Sediment,
Tannery sludge
Electronic scrap Waste electric device,
Jewelry waste/
Jewelry waste/
Automobile catalyst
Jewelry waste/
Fly ash, Spent fluid
cracking catalyst
Metals leached out Al, Zn, Cu, Cd, Co, Li, Ni, V, Mo Cu, Mn, Zn, Ni, Cr Cu, Ni, Sn, Al, Zn Au, Ag, Pt Ag, Pt, Au Ag, Pt, Au Al, Zn, Cu, Cd, Mo, V

4 Factors affecting microbial leaching

Microbial leaching is a process involving the action of several microorganisms. So, several factors are noticed to determine the process of leaching. Those influencing factors can be mainly categorized into physicochemical and microbiological features of the environment of leaching, characteristics of minerals to undergo leaching, and the methods of processing (Jafari et al., 2019).

4.1 Physicochemical parameters

The physicochemical nature of leaching environment is a determining factor on the efficiency of leaching. Some of the common physicochemical factors includes temperature, pH, redox potential, nutrient availability, etc., An optimum range of temperature is necessary for the bacteria to work efficiently. This range is different for different types of bacteria. In a mixed culture, Mesophilic MES1 showed an optimum temperature of 35 ℃ and moderately thermophilic MOT6 showed an optimum temperature of 50-55 ℃. At temperatures beyond the optimum level the oxidative activity of bacteria would decrease and it could result in protein denaturation which will affect the oxidizing system of bacteria.
The pH is also a determinant for the activity of microbes in bioleaching process. The optimal value pH could be marked by the optimum growth of bacteria and efficient oxidation of minerals. The consistent range reported was 1.5-2.3. In commercial application the operating pH will be less than optimum and it may vary. For BIOX® process, it is 1.2-1.8 and for BacTech® process, it is 1.3-1.5.
The culture medium which has been chosen for the growth and isolation of bacteria should contain certain chemical compounds which are necessary for providing all the required elements and energy for cell mass production as well as biosynthesis and maintenance. The major chemical composition of a nutrient media includes an ammonium salt, potassium salt, magnesium sulfate and other salts such as calcium nitrate or chloride. Chemical composition of some culture media is mentioned in Table 2.
Table 2 Chemical composition of different culture media for bioleaching studies (Deveci et al., 2003) (Unit: g L-1)
Culture media MgSO47H2O (NH4)2SO4 KH2PO4 KCl Ca(NO3)2 H2O
T&K media 0.4 0.4 0.4
9K media 0.5 3 0.5 0.1 0.01
ES media 0.4 0.2 0.1 0.1
Leathen media 0.5 0.15 0.01 0.05 0.05
Norris media 0.2 0.2 0.2
Availability of carbon dioxide and oxygen is a determining factor of the oxidizing activity of bacteria. Both these CO2 and O2 are required for the microorganism for completing the respiratory cycle as carbon dioxide is essential during the synthesis of biomolecules and oxygen acts as the terminal electron acceptor. Bioleaching system maintain a lowest limit, which is named as critical dissolved oxygen concentration. Below this the bacterial activity is limited because of the inadequate amount of dissolved oxygen. Along with oxygen, adequate amount of carbon dioxide also has to be supplied for bacterial cell growth. Carbon dioxide enriched air shows positive effect particularly on bioleaching using thermophilic bacteria.
For carrying out effective extraction the particle size should be reduced accordingly. The biooxidation of refractory gold concentrates were done at a particle size of 75 µm. The optimum particle size should be based on considering the size reduction costs and improved kinetics and recoveries. Along with particle size, pulp density too could act as an important factor that affects the efficiency of leaching. The pulp density of ore determines the surface area availability for leaching. Due to process economics, it is preferred to operate the process at high solid concentration. Increasing pulp densities involves some practical limitations (Deveci et al., 2003).

4.2 Microbiological parameters

The bacterial attachment to metallic waste could cause dissolution of metallic fraction by promoting a sequence of electrochemical reactions. Bacterial adhesion to surfaces is influenced by different parameters including roughness of surface, surface chemistry of metals, medium, etc., With greater surface roughness, the adhesion possibility of bacteria can be increased, because they will get more contact points for adhesion. Old waste products with corrosion including Cr2O3 and Fe2O3 possess surface roughness as well as positively charged surface, which could increase the chance of adhesion. This property increases the possibility of bioleaching from e-waste. Attaching of bacteria to surface can be dissuaded through the presence metals which are toxic in nature, on the surface of the waste. For example, some metals like Copper, Silver, Nickel, Magnesium, and Chromium on stainless steel could resist bacterial attachment (Valix, 2017).
The metal tolerance of different organisms also could affect the efficiency of leaching. When metal sulfides are leached, there would be an increment in the metal content of leachate. Normally, the bioleaching organisms shows high metal tolerance, but different strains possess different tolerance capacity (Bosecker, 1997).

4.3 Mineralogical properties

The composition of minerals in the bioleaching substrate is a determining factor of the efficiency of leaching. When the ore or gangue have high content of carbonates, it could result in the increase of pH which would inhibit or completely suppress the activity of bacteria. Surface area of the substrate too can be a determining factor. When the size of particle shows reduction, the total surface area will show increment which results in higher yield without changing the total mass of particles. Certain metals in low concentration are inevitable for the growth of microbes, but when it is in high concentration it could act as protoplasmic poison which induces denaturation of nucleic acid and proteins which are needed for metabolic activities.
The organic compounds and surfactants used while solvent extraction also could inhibit the bioleaching bacteria. This can be caused by the lowering of surface tension and decrease of mass oxygen transfer. When bacterial leaching is coupled with solvent extraction, then the solvents will get enhanced in aqueous state and it has to be detached prior to the recirculation of barren solution to bioleaching operation (Bosecker, 1997).

5 Methods of microbial leaching

Bioleaching techniques can be of different types, including heap, dump and in-situ bioleaching (Narayan and Sahana, 2009).

5.1 Heap bioleaching

This is one of the most cost-efficient method of bioleaching in which the reactors used are cheaper to construct and operate. Due to this reason, they are normally used for treating low grade ores. Rather than leaching out from existing dumps, construction and irrigation of heaps that are especially designed will be more effective (Pradhan et al., 2008).

5.2 Dump bioleaching

This could be considered as a subtype of heap leaching. In this type the solution will percolate through ore bed and metal can be recovered from the leachate. Just as heap leaching, this method also is typically used for separating metals from ores of lower grade. Dump leaching is not an efficient method of bioleaching as particles with large size will present small surface areas to lixiviants and at the same time, small particles will block the flow of solution and the aeration will be impeded. Not only that, freightage vehicles and time will make the ore to condense which will cause the reduction in permeability (Watling, 2006)

5.3 In-situ bioleaching

For reducing the footprints of mining, this type of bioleaching is an effective approach. With this approach the substantial diminishing of the surface impacts, that are caused due to mining, will be possible. This method can be used for only certain metals, in which the potential for microorganism usage exist (Brierley, 2010).

6 Advantages and limitations of using microbes for leaching

The three major techniques that are applicable for leaching out metals from e-wastes include, Pyro-hydrometallurgy, Hydrometallurgy, and Biohydrometallurgy. In the process of pyrometallurgy, steps including incinerating, amalgamation in a plasma arc or blast furnace, drossing, sintering, melting, and reactions at risen temperatures (up to 2732°F) in gas phase are involved and it is one of the traditional methods to recover metals which doesn’t belong to ferrous category and those are valuable, from electric and electronic waste. In this method, the scraps are crushed and are burned in a molten bath for removing plastics, and the refractory oxides would form a slag phase along with some metallic oxides. Hydrometallurgy involves steps such as leaching through oxidation for extracting, purifying, and recovering metals. At the same time, in biohydrometallurgy the chemicals are replaced by microorganisms and these microbes interact with the metallic elements, which could then be easy for extraction and purification. This is eco-friendlier than other techniques as for mineral oxidation mainly, autotrophs are used and they are capable for fixing of CO2, when the process of smelting causes large amount of carbon dioxide emission. A comparative analysis of three techniques is represented in Table 3 (Habibi et al., 2020).
Table 3 Pros and cons of different metal leaching techniques (Habibi et al., 2020)
Pros and cons Pyro-hydrometallurgy Hydrometallurgy Biohydrometallurgy
Pros The energy obtained is also utilised separately Shortest process time
The recovered metals are used in the manufacturing of other products
Environmentally friendly
Simpler and cheaper operation
and maintenance
Lower energy requirement.
Operation at surrounding pressure and temperature that is not excess
Cons Requires high energy input to sustain the temperature at which the process takes place
Emission of harmful gases, such as polychlorinated dioxins and furans (PCDD/PCDF)
Emission of metals, including Pb, As, and Cd.
Strong gas emissions, including CO2 and CO used as reducer
Requires high concentration of metals in ores
Requires complicated process plants as well as maintenance
Requires large amounts of leaching agents
Cost of operation is high
Liability due to using of dangerous chemical during the treatment
Can’t be applied for materials with high contamination
The dangerous acid waste may find its ways in to the local water sources
Longer period of operation compared with hydrometallurgy
Besides being an environmental benign procedure, the major hitch of this procedure is the kinetics of leaching out using microbes is not so fast. In order to overcome this limitation novel techniques should be implemented. These techniques should target towards the development of novel catalysts that are capable of improving the microbial mineral interactions as well as accelerating the kinetics. The three main functions that the specific catalyst should be able to conduct are:
Activation of the mineral surface for faster microbial interactions
Prevention of mineral surface passivation during the process
Providing the microbes with continuous nutrients or electrons supply
These could accelerate the kinetics (Borja et al., 2016).

7 Microbial leaching from mobile phones

Nowadays, in this modern society, the use of movable electronic devices such as laptops, mobiles, tablets, camera, etc., has risen to unprecedented levels (Jha et al., 2013). This could give space for these products to be the major part of total e-waste generated. When it comes to the case of mobile phones, the presence of metals in different parts are analysed quantitatively. The metallic composition of Lithium- ion battery of waste mobile phones were analysed through X-ray Fluorescence method and chemical digestion. The results are shown in Table 4.
Table 4 Metallic composition of Lithium-ion mobile battery (Naseri et al., 2019)
Manganese Cobalt Aluminium Copper Nickel Lithium Iron Sulfur Silicon Magnesium Sodium Titanium Niobium
X-ray fluorescence 22.0 17.11 9.45 6.60 2.82 - 0.19 0.17 0.11 0.06 0.06 - 0.01
Chemical digestion 21.31 16.54 9.12 5.93 2.56 2.22 0.04 - - 0.04 Not tested 0.02 -
When it comes into the case of LCD screens of mobile phones, it is also of short life span and becoming a major part of e-waste generated by mobile phone parts as it was not used for more than five years in normal case (Yang, 2012). When bioleaching experiments were conducted using At. ferrooxidans and At. thiooxidans, for leaching out Indium and Tin from LCD screens, two types of mediums including 9K medium and H2SO4 medium were used. The results pointed out a higher level of efficiency in 9K medium with 55.6% leaching rate for Indium and more than 90% efficiency for Tin. The study also observed the role played by the presence of ferrous/ferric iron in leaching out tin from LCD and a simultaneous reduction of Tin content with increment in the recovery rate of Indium (Willner et al., 2018).
A comparative study on leaching out metals from MPPCB and computer PCB using chemical leaching process and bio-hydrometallurgical process proves that the latter could be more efficient, ecologically sound and cost-efficient than the former (Shah et al., 2014). Besides the biological leaching of Copper and Nickel, a study conducted on MPPCB appealed Gold as the key to the microbial leahing of mobile phone PCBs. The gold content in Mobile phone PCBs that A. ferrooxidans are not capable for recovery amounts up to Two hundred times that in a normal gold mine. It opens up the space for future study regarding leaching out gold from MPPCBs, where cyanogenic bacteria are a possible choice (Arshadi and Mousavi, 2015).
Further studies based on leaching out gold from waste MPPCBs resulted in finding out the possibilities in different micro-organisms including, the use of Chromobacterium violaceum under YP medium through cyanide generation for leaching out gold along with copper (Chi et al., 2011) and use of Bacillus megaterium, where the bioleaching process was analysed statistically to study the interaction between parameters. In the latter case, under optimal conditions, approximately 65 g of Au per ton of MPPCBs were obtained (Arshadi et al., 2016). Au can be leached out from MPPCBs using Aspergillus niger also. Here A. niger MXPE6 and a fungal consortium formed by A. niger MXPE6 + A. niger MX7 were used for bioleaching and the results proved that leaching out using this organism and fungal consortium is possible without agitation, where the use of fungal consortium without the step of agitating, where the use of fungal consortium without agitation will increase the rate of Au bioleaching from MPPCB (Argumedo-Delira et al., 2019)

8 Recent advances and future prospects

The technique of bioleaching is exhibiting a rapid development and beyond research centre scale, pilot-scale implementation is also progressing. Some scientists are focussing on comprehending water systems and bio-load colonization. Bioleaching of uncommon components and from polymetallic sources are under scientific thoughts and discussions. Through specific targeting of the uncovered mineral grains, feasibility of extracting minerals could be expected. Manufacturing of specific proteins for bioleaching is also under scientific concern (Kumar and Yaashikaa, 2020).
Besides being cost-efficient and eco-friendly technique, the efficiency of bioleaching appears to be a negative against this technique. But for increasing the efficiency, some factors are altered and achieved maximized leaching rate. Cobalt and Nickel leaching rate from spent electric vehicle Li-ion battery was increased with the help of pH adjustments (Xin et al., 2016). By enhancing the effect on cell growth, the release efficiency of Cobalt increased from 43.5% to 96% and of Nickel shows a rise from 38.3% to 97% (Wu et al., 2019). By regulation of electron transfer from Iron mediated bacterial community to metals, biochar presence could lead to increased efficiency of bioleaching (Wang et al., 2016). Liu et al. (2019) proposed a novel strategy of three-step leaching involving consortium construction, directed evolution, and chemostat selection to increase the leaching efficiency and as a result increased iron recovery rate was observed.
Bioleaching when coupled with biomineralization, the application of these technique could be effective in nanoparticle synthesis and further related applications (Yu et al., 2020)

9 Conclusions

The behaviour of use and throw of electric and electronic materials has a significant impact on the increasing amount of e-waste produced. As reduction of e-waste generation is not practical, to reduce the level of accumulation, an effective treatment plan is required. Leaching out of metals from WEEEs is an efficacious way for the reduction in quantity. Chemical leaching of metals is possible but when compared to bioleaching, the latter is better choice as it is environmentally benign. Bioleaching of metals from WEEEs could serve as a method for reducing the problems due to e-waste as well as it could serve as a secondary metallic resource. The limitations of this process due to its slow kinetics and less efficiency is there. For overcoming this application of right techniques involving the use of new strains of microorganisms, special designed bioreactors, and some other novel innovations will be useful and for that purpose proper scientific studies have to be conducted in this field.
During the last few decades, bio-hydrometallurgical studies were carried out on different types of e-waste samples, including PCBs of television, computer, mobile phone, LCD screen of mobile phones, batteries, chargers, etc. Even though several works were carried out on different parts of mobile phones, up to the available literary knowledge, no works were identified regarding specific type of waste mobile phones. As in current society, the patterns of mobile phones used and later wasted are changing, future works could be focused on specific type or pattern of waste mobile phones.
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