E-WASTE TREATMENT BY THERMAL PLASMA TECHNIQUE

There is a report on e-waste (cell phone) treatment by thermal plasma technique where the particularly high temperature in an oxygen-starved environment was used for the complete decomposition of input plastic waste into syngas (fuel gas). The generation of hydrogen gas of high concentration is from the hydrogen of the plastic part of the cell phone waste (Ruji and Chang, 2013). Plasma treatment of cell phone waste in a reduced environment produces gaseous components, viz. hydrogen (H,), carbon monoxide (CO), and hydrocarbons. It reduces the quantity of the e-waste from which recovery of precious metals is possible. E-waste treatment by thermal plasma technique decreases the requirement of the landfill and is one of the safe disposal techniques for cell phone waste. The principal advantage of plasma technology includes the low exhaust gas flow rates, installations with smaller footprints and low investment costs, and faster start-up and shut-down times.

MANAGEMENT OF E-WASTE

Non-renewable nature of the metallic elements necessitates their recovery from the e-waste stream. To reuse e-waste in an eco-friendly, efficient, and proper way is a need of the hour by recycling of valuable metals from e-waste stream. The e-waste management is mostly done by bioleaching. This method employs leaching of metals from e-waste using microorganisms. In microbiological leaching, the natural tendency of microorganisms is used for the transformation of the metals present in the waste in a solid form to a dissolved form. Among the chief groups of bacteria, the most frequently used bacteria for e-waste management are Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Leptospirillum ferrooxidans and heterotrophs, for example, Sulfolobus sp. Fungi such as Pemcillium sp. and Aspergillus niger are used in the leaching of metals from industrial wastes.

There is a report on the bioleaching of metals (Cu, Ni, and Zn) by Acidiphilium acidophilum using computer PCBs. A. acidophilum has been reported to leach Cu and Zn from shredded PCBs (Hudec et al., 2009). Bioleaching is the method where precious metals are recovered using microorganisms. PCBs are the main component of most e-waste, and some metals in PCBs are hazardous. If not disposed-off properly, these metals can leach into the soil and water and leak into watersheds. According to the literature, metal extraction from e-waste and PCBs has been studied using mesophilic chemolithotrophic bacteria (.Acidithiobacillus thioxidaus and Acidithiobacillus ferroxidan) (Choi et al., 2004), cyanogenic bacteria {Chromobacterium violaceum and Pseudomonas fluorescens) (Ting et al.,

2008) and acidophilic moderately thermophilic bacteria (Sulfobacillus thermosulfidooxidans and Thermoplasma acidophilum) (Ilyas et al., 2010). Study has been done on metal bioleaching from e-waste by Chromobacterium violaceum and Pseudomonas sp. (Pradhan and Kumar, 2012).

PHYTOREMEDIATION

Phytoremediation involves the use of vegetation or plant for removal of toxicity from soil and water. After disposal of e-waste hi landfills, heavy metals and persistent organic pollutants are found in the leachate. E-waste generated leachate can be decreased by using aquatic weed water hycianth (Eichhomia crassipes) (Omondi et al., 2015). Microorganisms colonizing with plants have a major contribution in removal of polychlorinated biphenyls from e-waste leachate in landfills (Chen et al., 2010). Plant species widely used for the phytoremediation include Arabidopsis sp., Alyssum bertolonii,Amaranthus retrofJexus, Chenopodium album, Brassica juncea and Helianthus annum (Rajiv et al., 2009).

VERMI REMEDIATION

Landfilling of e-waste leads to a decrease in the nutrient levels, which is the primary requisite of the microorganisms for their activity. Microbial activity can be obtained through the activity of earthworms. Earthworms are responsible for the soil aeration and casting of nutrients. Earthworms accumulate chemicals and cause their biotransformation to less harmful products (Sinha et al., 2010). Some of the earthworm species used for venniremediation are Aporrectodea tuberculata, Eisenia fetida, Lumbricus terrestris, Dendrobaena rubida, and Eiseniella tetraedra.

RECOVERY OF METALS FROM E-WASTE

The metal fractions recovered from e-waste are further processed using hydroinetallurgical, pyrometallurgical, electrometallurgical, biometallur- gical processes, and their combinations. The major methods for processing e-waste are hydroinetallurgical and pyrometallurgical processes, which are followed by electrometallurgical/electrochemical processes (e.g., electrorefining or electrowinning) for the separation and recovery of selected metal. At present, there are limited laboratory studies for e-waste processing through biometallurgical routes like bioleaching of metals from e-waste. The pretreatment of e-waste is not always mandatory for pyrometallurgical routes. For example, mobile phones and MP3 players can be treated directly using smelting processes. Preprocessing is required to segregate metal fractions from other fractions for hydroinetallurgical routes (Klialiq et al., 2014).

5.10.1 PYROMETALL URGICAL PROCESS

The pyrometallurgical processing has been a conventional technology for the extraction of precious metals from e-waste. The smelter processes and recycles 100,000 tonnes of electronics per annum, representing 14% of total throughput. In the reactor, materials are kept in a molten metal bath (1250°C), and churned by a mixture of 39% oxygen. Combustion of plastics and flammable materials in the feeding reduces energy cost. Iron, lead, and zinc are converted into oxides in the oxidation zone where it becomes fixed in a silica-based slag. Before disposal, the slag is cooled and milled to recover metals. The precious metal like copper is separated and transferred to the converter. Liquid blister copper is upgraded in the converters and then refined in anode furnaces, which will cast into anodes with the purity of 99.1%. The residual 0.9% contains valuable metals like gold, silver, platinum, and palladium, as well as recoverable metals, such as tellurium, selenium, and nickel (Cui and Zhang, 2008).

5.10.2 HYDROMETALLURGICAL PROCESS

The hydrometallurgical process involves a series of acid or caustic leaching of solid stuff. The solutions are then subjected for isolation and purification procedures like impurities precipitation, adsorption, solvent extraction, and ion-exchange to concentrate the metals. Concentrated metals are then treated by chemical reduction, electrorefining process, or crystallization for the recovery of metals. The patents available on hydrometallurgical processing of ores are shown in Table 5.2. The hydrometallurgical recovery of valuable metals from e-waste is shown in Table 5.3.

TABLE 5.2 Patents on Biohydrometallurgical Processing of Ores

Ores

Metals Extracted

Microbes

References

Iron

Fe

Pseudomonas sp.

Hoffmann et al., 1989

Gold

Au

Chromobacterium violoceum, ChloreUa vulgaris

Kleidet al., 1990

Sulfidic ore

Au, Ag, Cu

Thiobaciltus sp., Leptospirillum ferrooxidans

Hill, 1992

Sulfidic ore

Au, Ag, Pt

Bacteria reducing sulfate and hydrogen

Hunter et al., 1996

TABLE 5.3 Hydrometallurgical Process for Extraction of Metals

Agent for Leaching

Process Conditions

Metals Extracted

H,S04, chloride, thiourea, and cyanide leaching

Leaching and metal extr action using processes viz., cementation, precipitation, ion exchange, and carbon adsoiption

Au, Ag, Pd, and Cu

HC1, MgCl,. h,so4 and H,0,

Dissolution of electronic wastes in solvents and leaching and metal extraction

Al, Sn, Pb, and Zn (stage 1); Cu and Ni (stage 2) and Au, Ag, Pd, and Pt (last stage)

HC1, H,S04 and NaCloj

Burning e-waste at high temperature, i.e., 400-500°C and then leaching

Ag, Au, and Pd

Aqua regia and H,S04

Mechanical treatment and dissolution of e-waste in solvents

Cu

Source: Khaliq et al., 2014.

5.10.2.1 CYANIDE LEACHING

Mining industries use cyanide as leaching lixiviant for gold extraction because of its high efficiency and low cost (Syed, 2012). The mechanism involved in the dissolution of gold in cyanide solution is an electrochemical process. The overall reactions are (Dorin and Woods, 1991):

5.10.2.2 HALIDE LEACHING

All halogens have been tested for gold extraction, except fluorine and astatine (Hilson and Monhemius, 2006). With chloride, bromide, and iodide, gold forms Au(I) and Au(III) complexes. Aqua regia is used as a traditional medium for dissolving gold. Aqua regia is a mixture of hydrochloric acid and concentrated nitric acid in 1:3 proportions. The reactions involved in the aqua regia leaching are as follows (Slieng and Etsell, 2007):

5.10.2.3 THIOUREA LEACHING

Thiourea can dissolve gold under acidic conditions and lead to the formation of a cationic complex. A rapid reaction occurs, which extracts gold up to 99% (Hilson and Monhemius, 2006). The following reactions occur:

5.10.2.4 THIOSULFATE LEACHING

An electrochemical reaction for the dissolution of gold in an ammonical thiosulfate solution is catalyzed in the presence of cupric ions. On the anodic surface of gold, ammonia or thiosulfate ions react with Au+ ions and form either Au(NH3),~ or Au(S,03),3 Cu(NH3)2+ converts to Cu(S,03)35_ ions, and the same is for Au(NH3),+. The Cu(S,03)35_ and Cu(NH3),+ species in solution form oxidized product, i.e., Cu(NH3)42+. It is reported that at higher temperatur es and low pH values, thiosulfate stability is low.

BIOMETALLURGY PROCESS

Microbes utilize metal species for structural and catalytic functions. Microbes can bind with the metal ions present in the external environment at the cell surface or to transport them into the cell for various intracellular functions. Bioleaching and biosorption are the main areas of biometallurgy to recover the metals.

BIOLEACHING PROCESS

The acidophilic microbes which take part in the dissolution of metals from the e-waste grow in an inorganic medium having low pH values and can tolerate high metal ion concentrations. The first mechanism is redoxolysis (direct and indirect). The second mechanism of metal solubilization is by the formation of organic or inorganic acids. For example, production of citric acid or gluconic acid by A. niger and P. simplicissimum, and H,S04 by T. ferrooxidans and T. thiooxidans. The acid supplies the protons which contribute to the solubilization process. The third mechanism of metal extraction involves complex formation between metabolites produced by the microorganisms and the metal ions, which can increase their mobility. Bioaccumulation is another important mechanism in fungal bioleaching (Burgstaller and Schinner, 1993). Oxidation of Fe (II) to Fe (III) and S to H,S04 are the main functions of the acidophilic microorganisms. These acidophilic microorganisms are Acidithiobacillus, Sulfolobus, Acidianus, and LeptospirUlium, which oxidizes Fe (II) to Fe (III) and S to H,S04 (Okibe et al., 2003). Bioleaching has been successfully applied for the extraction of valuable metals and copper from ores (Ehrlich, 1997). Bioleaching is bacteria assisted reaction for the extraction of metals from metallic sulfides (Morin et al., 2006). In the case of Cu:Fe,(S04)3 created by Acidithioba- cillus ferrooxidans oxidizes elemental copper contained in the waste to the copper in the foim of ion, according to the reactions:

The e-waste containing base metals subjected to bioleaching is represented in Table 5.4. Bacterial metal sulfide oxidation involves both direct and indirect mechanisms. In direct mechanism, oxidation of minerals and solubilization of metals is achieved by microorganisms, whereas, in the indirect mechanism, ferric ion (Fe3') is the oxidizing agent for minerals and microorganisms regenerate Fe3+ from Fe:+. The reactions are,

TABLE 5.4 E-Waste Subjected for Bioleaching

Metals Extracted

Microbes

at, Sn, Ni, Pb, Zn, Al

A. ferrooxidans, A. thiooxidans,A. niger,P. simplicissimum

Cu, Zn, Al, Ni

SulfobaciUlus thennosulfidooxidans + Thermoplasma acidophilum

at, Pb, Zn

A. ferrooxidans, A. thiooxidans and mixture

Cu

A. ferrooxidans, A. ferrooxidans + A. thiooxidans

All

Chromobacterium violaceum

Source: Willner et al., 2015.

Thiosulfate and polysulfide pathways are operated in the leaching of metal sulfides. Proton attack and oxidation processes are involved in the dissolution of metal sulfides.

BIOSORPTION

Biosorption is the physico-chemical interaction between the charged surface groups of microorganisms and ions in solution. Physico-chemical mechanisms like ion-exchange, coordination, complexation, and chelation between metal ions and ligands. This interaction depends on the specific properties of the biomass (alive, or dead, or as a derived product). Other metal-removal mechanisms include metal precipitation, sequestration by metal-binding proteins or siderophores, transport, and internal compartmentalization (White et al., 1995). Biosorption of metals from solutions includes chemical and physical sorption mechanisms. Chemical sorption mechanisms involve the complexation, chelation, microprecipitation, and microbial reduction, and physical sorption mechanisms involve electrostatic forces and ion exchange (Kuyucak and Volesky, 1988).

E-waste is considered as a secondary ore for the recovery of gold, and cyanidation is widely used for gold recovery. In current years, the gold cyanidation process is carried out by using cyanogenic bacteria such as Chromobacterium violaceum, Pseudomonas fluorescens, Escherichia coli, and Pseudomonas aeruginosa. All these species are involved in the gold dissolution with their metabolic processes. The mechanism of gold recovery involves assemblage of the chemical knowledge (interaction of metals and cyanide) with microbiological principles (biological cyanide formation) for solubilization of metals from waste PCBs and the formation of water-soluble cyanide complexes.

CONCLUSION

The extraction of precious metals like copper, gold, silver, and palladium is the incentive for e-waste recycling. Cyanogenic bacteria, in combination with the hydrometallurgy process, will be useful in the extraction of metals from electronic waste and thus will help in the management of e-waste. Pollution of soil due to the dumping of e-waste in landfills can be prevented through the synergistic action of plants, earthworms, and their associated microorganisms. Compared to biosorption and bioaccumulation, bioleaching will be most effective in the reuse and recycling of e-waste.

KEYWORDS

  • • bioaccumulation • landfill
  • • bioleaching • phytoremediation
  • • biometallurgy • plasma technique
  • • biosorption • pollution
  • • chelation • pyrometallurgical
  • • cyanogenic • scrap
  • • E-waste • solubilization
  • • extraction • toxicity
  • • hydrometallurgical • vermiremediation
  • • incineration

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