Bio-processing of solid wastes and secondary resources for metal extraction – A review

Waste Management 32 (2012) 3–18

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Waste Management journal homepage: www.elsevier.com/locate/wasman

Review

Bio-processing of solid wastes and secondary resources for metal extraction – A review Jae-chun Lee a, Banshi Dhar Pandey a,b,⇑ a b

Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Gwahang-no, Yuseong-gu, Daejeon 305-350, Republic of Korea CSIR – National Metallurgical Laboratory, Jamshedpur 831007, India

a r t i c l e

i n f o

Article history: Received 11 March 2011 Accepted 9 August 2011 Available online 16 September 2011 Keywords: Bio-processing Wastes Secondary resources Metals Microbes

a b s t r a c t Metal containing wastes/byproducts of various industries, used consumer goods, and municipal waste are potential pollutants, if not treated properly. They may also be important secondary resources if processed in eco-friendly manner for secured supply of contained metals/materials. Bio-extraction of metals from such resources with microbes such as bacteria, fungi and archaea is being increasingly explored to meet the twin objectives of resource recycling and pollution mitigation. This review focuses on the bioprocessing of solid wastes/byproducts of metallurgical and manufacturing industries, chemical/petrochemical plants, electroplating and tanning units, besides sewage sludge and fly ash of municipal incinerators, electronic wastes (e-wastes/PCBs), used batteries, etc. An assessment has been made to quantify the wastes generated and its compositions, microbes used, metal leaching efficiency etc. Processing of certain effluents and wastewaters comprising of metals is also included in brief. Future directions of research are highlighted. Ó 2011 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Microbes in bioleaching of wastes and secondary resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Bioleaching of valuable metals from wastes and byproducts – the secondary resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Bio-processing of wastes/byproducts of metallurgical and manufacturing industries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.1. Copper smelter flue dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.2. Copper converter slag and dump/smelting slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.3. Filter dust of copper plant and filter press residue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.4. Industrial and metallurgical sludge/tailings/waste spillage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1.5. Steel/blast furnace slag/sludge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Bio-processing of sludge from tanning, electroplating and wastewater treatment units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2.1. Sludge from tanneries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2.2. Electroplating and wastewater sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3. Bio-processing of spent catalysts and used batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3.1. Spent catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3.2. Used batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4. Bio-processing of waste electronic equipment/e-waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.5. Bio-processing of sewage sludge and fly ash of municipal waste incinerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.5.1. Sewage sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.5.2. Fly ash/residue from incinerator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.6. Bio-processing of other secondary resources and wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.6.1. Molybdenite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.6.2. Copper containing byproduct of a uranium mill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.6.3. Waste waters/streams/effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.6.4. Leachate of secondary material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

⇑ Corresponding author at: CSIR – National Metallurgical Laboratory, Jamshedpur 831007, India. E-mail address: [email protected] (B.D. Pandey). 0956-053X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2011.08.010

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3.

Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1. Introduction Industrial production generates some kind of waste/byproduct/ toxic substance while contaminating the sites and often degrades surroundings of human habitation including air, surface and ground water. Increasing usage of consumer goods including electronic goods and consumption of food as well as essential items for sustenance contribute to this problem. In fact, at every stage of the production and consumption cycle of essential and consumer items, whether intermediate or the finished products, certain contaminants-organic and/or inorganic compounds are invariably generated which have various degree of toxicity to the living organisms including human beings. In general, the organic compounds are easy to degrade (Bosecker, 2001; Lewandowski and De Fillipi, 1998; Skippers and Turco, 1995) by biological, chemical and physical means, but inorganic substances which are the part of different wastes with heavy metals may not be decomposed by such methods. The mobility of the heavy metals from the wastes in aqueous system is a cause of concern because of their intrinsic solubility at varying pH, reduction–oxidation characteristics and complex formation tendencies. Immobilization of heavy metals from wastes by precipitation (from solutions)–storage–disposal or aggregate formation including vitrification is a debatable issue because of some degree of mobility of metal ions in the long run. Numerous industries e.g., electroplating, metal-finishing, electronic, steel and nonferrous processes, petrochemical and pharmaceutical, and the used electronic/household goods discharge a variety of toxic heavy metals like Zn, Cd, Cr, Cu, Ni, Pb, V, Mo, Co etc. threatening the public health and the environment, if improperly managed. In order to consider a waste to be toxic or not it must go through toxicity characteristic leaching procedure (TCLP) test to determine as to whether the waste has toxicity characteristics in amount that meet or exceed the regulatory limits causing it to be a hazardous waste (Salkin, 2003). Application of a suitable extraction process to recover some of the valuable metals present in the wastes is an appropriate approach to abate the toxicity. For sustainable development, biological processes are often recommended which are poised to play a significant role not only in the future metallurgical and chemical sector, but also for the treatment of metal containing wastes and by-products. In this direction, an approach (OECD, 2001) termed as ‘process-integrated biotechnology’ has been propagated and widely accepted. This has features such as use of microbes in the existing processes, substitution of industrial processes and developing altogether a new methodology (Brandl and Faramarzi, 2006), but has to compete with the prevalent physico-chemical processes in economic terms. The role of microbes in nature has been well documented particularly for the weathering of metal containing rocks/minerals/ metals and coals (Rossi, 1990; Torma, 1988) leading to the mobility/dissolution of metal ions, formation of acid mine drainage (AMD) through oxidation–reduction, and at times precipitation/ transformation of metals to a different state such as oxide, hydroxide, sulphate etc. The concept has finally matured into the development of commercially viable metal extraction technologies through microbial action in the last six decades. With the first exploitation of copper bio-leaching from the mine dump in early 1950s by Kennecott Copper Corporation (Zimmerley et al., 1958), minerals/ores containing copper, gold and cobalt are processed today on industrial scale (Brierley, 2008; Brierley and Brierley, 2001; Watling, 2006; Watling et al., 2010), with occasional operations for

uranium bio-leaching from low grade ores (Munoz et al., 1995). The microbial leaching has also shown a great promise for the processing of sulphides of Ni, Zn, Mo Co, Ga and Pb and the platinum group metals (Pt, Rh, Ru, Pd, Os, Ir) associated with sulphide minerals. In these processes, the chemolithotrophic bacteria mainly Acidithiobacillus ferrooxidans (A. ferrooxidans), Acidithioacillus thiooxidans (A. thiooxidans) and Leptospirillum ferrooxidans (L. ferrooxidans) are reported to convert metal sulphides into metal sulphates while generating sulphuric acid in the system (acidolysis), besides the involvement of biogenic ferric ion in oxidizing the metal sulphides (redoxolysis). On the other hand, heterotrophic bacteria and fungi can be applied to treat non-sulphides and acid consuming oxide materials for metal extraction (Brandl and Faramarzi, 2006) mostly through production/secretion of organic acids (acidolysis) and complexing compounds to form chelates (complexolysis), and through change in the oxidation state (redoxolysis). With the processing of sulphides on commercial scale, potential application of bio-hydrometallurgical methods to treat metal containing solid wastes has convincingly emerged and is being increasingly researched. From the view point of resource reclamation, several solid wastes and by-products, which are often rich in metal contents and at times even richer source of valuable metals than the naturally occurring ores/minerals, may be processed by biological method. The microbial treatment (Brandl and Faramarzi, 2006; Torma et al., 1996) of such secondary resources can have advantages such as low cost (one-third to one-half the cost of conventional processes), low hazardous emissions and low-tech systems with the use of naturally occurring biocatalysts. The merit of bio-processing in achieving twin objectives of ‘‘resource reclamation and low environmental degradation’’ can offset the disadvantages relating to the process control measures (nutrients, pH, temperature, oxygen) and slower kinetics. As regards the microbial reaction that can govern the process of metal solubilization/mobilization from the waste and secondary resources, popularly known as bioleaching, a combination of the three mechanisms viz. acidolysis, redoxolysis and complexolysis as mentioned above may be applicable (Brandl, 2001, 2002; Krebs et al., 1997). Similarly, metal immobilization in the wastes with the aim of remediation may be opted particularly for the clean-up of industrial sites, soils and sediments contaminated with toxic metal ions. In view of the proven strength of bio-processing (Brierley, 2008) and scope for its application to metal extraction from mining/metallurgical wastes including the man-made secondary materials or environmental protection (Bosecker, 2001; Rawlings, 1997), a need for a comprehensive review was felt. The present review focuses on analyzing and compiling researches relating to the bio-treatment of solid wastes and byproducts from the metallurgical and manufacturing units (dust, slag, sludge, tailings, by-products), petrochemicals/chemicals (hydrogenation/spent catalysts), galvanic, electroplating and tanning units (sludge), besides the sewage sludge and fly ash from municipal waste incinerators, electronic scraps/wastes (printed circuit boards-PCBs), spent batteries etc. Bio-processing of certain effluents and wastewaters mainly to produce intermediates/solids is also touched upon in very brief. Bioleaching of mining wastes and dumps is generally not covered here because of analysis of the subject in the recent past (Devasia and Natarajan, 2004; Ehrlich, 2002; Ehrlich and Brierley, 1990; Narayan and Sahana, 2009; Rawlings, 2002; Rawlings and Johnson, 2007; Rawlings and Silver, 1995).

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1.1. Microbes in bioleaching of wastes and secondary resources Among the major bacteria group that are involved in the bioleaching process are chemolithotrophic acidophiles namely, A. ferrooxidans, A. thiooxidans, and L. ferrooxidans and heterotrophs like Sulfolobus. Besides, fungal species such as Penicillium simplissimum and Aspergillus niger are some of eukaryotic bioleaching microorganisms that are applied in metal recovery from industrial wastes particularly the non-sulphides (Brandl and Faramarzi, 2006; Jain and Sharma, 2004). The acidophilic microorganisms facilitating dissolution of metals from the wastes are autotrophic in nature. They can grow in inorganic medium having low pH values and can tolerate high metal ion concentrations. The two main functions

of this type of bacteria are oxidation of Fe(II) to Fe(III) and S to H2SO4 which take part in leaching. Depending on their tolerance to temperature the acidophilic micro-organisms are categorized into three groups such as mesophiles, moderate thermoacidophiles and extreme thermoacidophiles. A. ferrooxidans, A. thiooxidans and L. ferrooxidans are the important mesophiles which are effective in the temperature range 28–38 °C, whereas Sulfobacillus thermosulfidooxidans, a moderate thermophile can function at 50 °C. The Sulpholobus species such as S. acidocaldarius, S. solfataricus and S. brierley are the examples of extreme thermophiles which can be used up to 70 °C. Besides the role of acidophilic autotrophs in bioleaching, some heterotrophic bacteria are also used for recovery of valuable metals like gold, silver, nickel, platinum from solid

Table 1 Bio-leaching of metals from the wastes and byproducts of metallurgical and manufacturing units. Specific type and source of waste/by-product

Microbe used

Leaching efficiency (%) and conditions

Type of reactors

Reference

Cu smelter flue dust

(i) Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans (ii) Mixed culture-A. ferrooxidans, A. thiooxidans and L. ferrooxidans

Cu-87% in 22 d at 1.8 pH and 50 g/L pulp density (PD) Cu-91% in 6.5 days at 1.8 pH

Shake flask Bioreactor

Massianaie et al. (2006) Oliazadeh et al. (2006)

Cu-86.8% in 6 days at 1.8 pH and 70 g/L PD

Two-stage stirred tank reactor (STR) Two-stage airlift reactors Shake flask

Bakhtiari et al. (2008a)

Shake flask by bio-oxidized Fe(III) Shake flask with culture filtrate Shake flask with culture Shake flask

Carranza et al. (2009)

Shake flask

Vestola et al. (2010)

Shake flask

Willscher and Bosecker (2003)

Shake flask

Cheng et al. (2009), Guo et al. (2010)

Bioreactor

Bosecker (1986) Schinner and Burgstaller (1989), Burgstaller et al. (1992), Muller et al. (1995) Hahn et al. (1993)

Shake flask

Bojinova and Velkova (2001)

Shake flask

Vanchon et al. (1994)

Bioreactor

Hsu and Harrison (1995)

Shake flask

Francis and Dodge (1990)

Shake flask Shake flask

Yang et al. 2002; Wu et al. (2009) Hita et al. (2008)

Shake flask

Ostrega et al. (2009)

Shake flask

Banerjee (2007)

Shake flask

Solisio et al. (2002)

Shake flask

Gahan et al. (2009)

Copper converter slag

(i) A. ferrooxidans and A. thiooxidans (ii) A. ferrooxidans (to produce ferric ion by biooxidation in solution) (iii) Aspergillus niger

Cu-86% in 5 days at 1.8 pH, 70 g/L PD, 34 °C and 91% at 38 °C Cu-66%, Ni-50%, Co-64% with <75 lm size slag Cu-93% in 4 h at 60 °C by ferric ion (11.5 g/L) leaching-BRISA process Cu-46.5%, Ni-28%, Co-38% in HCl

Cu-47%, Ni-50%, Co-23% Copper dump slag/slag/ tailings

(i) A. ferrooxidans (ii) A. ferrooxidans and Leptospirillum sp. (mixed culture) (iii) coryneform bacteria

Pb/Zn smelting slag

Filter dust of a copper plant/ filter press residue

Industrial waste-sludge/ tailings/red mud, etc.

Mixed culture isolate of moderate-thermophilic bacteria (i) A. thiooxidans (ii) Penicillium simplicissimum, Pseudomonas putida, etc. (iii) A. niger/heterotrophic micobe (i) A. thiooxidans in 9 K media with 0.2% (w/v) S (ii) Bacteria/fungi (iii) A. ferrooxidans/S. acidocaldarius

Mining/metallurgical-waste dump/pyrite sludge spillage/waste (Fe–Ni)

Steel/Blast furnace slag, sludge

(iv) Anaerobic bacteriaClostridium sp. (i) A. ferrooxidans (ii) Sulfolobus sp. (iii) Autotrophic and heterotrophic bacteria (i) A. ferrooxidans

(ii) Mixed culture of Fe and S oxidizers and archaea

Cu-8%, Ni-13% from dump slag in 15 d at 0.5 and 1.0 pH and 16% Cu , 45% Ni from flotation tailing Cu-100% and Ni-100% in 42 d at 10 g/L PD and 1.5 pH with 10 g/L S and 4.4 g/L Fe(II); Cu-45% and Ni-19% at 100 g/L PD in 79 days Recovery from alkaline slag (siliceous) dump38%Mn, 46% Mg, 68% Ca, 26% Ni, 40%, Co and 80% Pb Al-82–84%, As-85–91%, Cu-86–88%, Mn-85–95, Fe85–90%, Zn-95–97% at 65 °C, 1.5 pH and 50 g/L PD Cu/Zn-90% in the acidic pH range Zn-93% in 13 days in shake flask and 80–90% in 9 days within pH 2–7 Cu-95% and above Al-71% in 28 days with 4 h mechano-chem activated mass and K-78%, Na-91% in 7 days with 4 h activated mass Al-75% at 100 g/L PD by P. simplicissimum and 96% by citric + oxalic acid at 1.5 pH Cu-99% from tailing pond sediment in 12 h in bioreactor, Zn-90% from chert pile rock at 70 °C in 5 days with thermophile Ni-55%, Cd-48%, Zn-41%, Fe-59%, Cr-3.2% from the coprecipitated goethite of the waste stream Cu-30% in 1 year from the dump, 44% Zn-0.060 g/L, As-0.075 g/L, Fe-1.2000 g/L at 65 °C in 10 days Cu, Ni, Mn, Fe, Cr-80–100% leaching in six-step sequential manner High recovery of Fe and heavy metals from blast furnace (BF) sludge/flue dust in bacterial leaching than with fungus Zn and Al-76–78% at 10 g/L PD in 12 days, 72–73% at 20 g/L PD from electric arc furnace (EAF) sludge BF slag as neutralizing agent in pyrite biooxidation (75–80%) at 1.5 pH

Bakhtiari et al. (2008b) Mehta et al. (1997, 1999)

Sukla et al. (1992, 1995)

Munoz et al. (2009)

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wastes (Faramarzi et al., 2004). These heterotrophs, coined as cyanogenic bacteria, produce cyanide in the aqueous medium forming cyanide complex of respective metal ions. The well-known cyanogenic bacteria are Chromobacterium violaceum, Pseudomonas fluorescens, and Bacillus megaterium. Bioleaching of metals from the non-sulphide wastes/materials by Acidithiobacillus sp. may involve indirect leaching mechanism by the biogenic reagent (H2SO4)/oxidant (Fe3+) produced in the first stage. Bacterial oxidation of metal sulphides can however, follow a combination of direct and indirect leaching mechanisms. As no direct interaction between the bacterial membrane and the sulphide mineral in the enzymatic mechanism has been established (Sand et al., 2001), direct mechanism is suggested to be renamed as contact leaching (Tributsch, 2001). Indirect leaching occurs through microbial catalytic oxidation of aqueous ferrous to ferric ions and successive direct oxidation of sulphide by either ferric ions only, generating thiosulphate or both iron (III) and protons, resulting in the formation of elemental sulfur via polysulphides (Sand et al., 2001). 2. Bioleaching of valuable metals from wastes and byproducts – the secondary resources 2.1. Bio-processing of wastes/byproducts of metallurgical and manufacturing industries Generally, the metal containing wastes and byproducts of the metallurgical industries are in the form of slag, sludge, dust, tailings and byproducts generated either in the end or as intermediate product. Even the liquid wastes containing metals are treated to convert it to a solid sludge for disposal/storage. Many of these solid wastes are non-sulphides, but a few of these may be in the sulphide form also and can be treated by pyro-/hydro-metallurgical processes (Hoffman et al., 2007; Jha et al., 2001; Shamsuddin, 1986) or by the bio-hydrometallurgical processes involving the thiobacilli group of bacteria (Rossi, 1990; Rawlings and Johnson, 2007). Heterotrophic bacteria and fungi can also be used for the bioleaching (Jain and Sharma, 2004). The details on the bio-processing of metals from these wastes are summarized in Table 1. 2.1.1. Copper smelter flue dust The flue dusts containing 30–36% Cu are generated to the tune of 3–5% of the concentrate feed of copper smelters (Biswas and Davenport, 1976). These dusts mainly from the reverberatory/flash and converter furnaces are often recycled in the plant after blending with the concentrate to recover copper at the cost of plant productivity and causing environmental pollution, besides damaging the refractory bricks. The main copper sulphide minerals (Oliazadeh et al., 2006) in a typical smelting dust have been identified as chalcocite (16%), chalcopyrite (2–3%), bornite (2–3%) and covellite (1.0%), and 13% copper oxide. Recently, some efforts were made to bioleach copper from the Iranian (Sarcheshmeh) smelter dust on bench scale and also in bioreactor. The use of 10% inoculum of mixed culture of mesophilic bacteria (A. ferrooxidans and A. thiooxidans) isolated from the acid mine drainage of the deposit resulted in recovery of 87% copper in 22 days in shake flask experiments (Oliazadeh et al., 2006). As compared to the shake flask data, rate of bioleaching of copper improved (91%) in a bioreactor with 35 L pulp at a pulp density of 50 g/L, pH 1.8 and 31 °C in 6.5 days only (Massianaie et al., 2006). The high leaching of copper from the flue dust may be attributed to the presence of significant amount of secondary sulphides such as chalcocite and bornite which are easily leached by such bacteria (Watling, 2006). The bioleaching of copper from the flue dust of the same plant was also examined using GeoCoat technology (Bakhtiari et al., 2008a).

In order to develop the leaching process in a continuous mode, a two-stage stirred tank bioreactors (Bakhtiari et al., 2008a) and two-stage airlift reactors (Bakhtiari et al., 2008b) were applied. Apart from the A. ferrooxidans and A. thiooxidans, the mixed culture also had L. ferrooxidans and was inoculated after adaptation over the dust feed. The operation of laboratory (2.0 L) scale bioreactors (two-stage) arranged in a series for 180 days in continuous mode demonstrated the recovery of 86.8% Cu in 6 days at 70 g/L pulp density and 32 °C. Investigation in airlift reactor that was operated for 150 days to leach the acid treated flue dust using the adapted mixed culture, further confirmed the increased rate of dissolution (86% Cu) in 5 days at 34 °C and 70 g/L pulp density. These results have clear potential for bioleaching of copper from the copper smelter flue dusts if tested and implemented on large scale. 2.1.2. Copper converter slag and dump/smelting slag The generation of copper converter slag and dump slag in copper smelters is generally in the range 0.95–1.1 ton and 2.0– 2.5 tons/ton of copper produced (Agrawal et al., 2004). The converter slag is a rich source of some important non-ferrous metals such as nickel (1–2%), cobalt (0.4–0.8%) and copper (1–9%) depending upon the composition of the concentrate fed to the smelter and control in operation. The converter slag is recycled in the plant to recover copper and to some extent nickel but cobalt is totally lost in the dump slag. In order to extract metals from the converter slag of an Indian plant (ICC, Ghatsila), studies on the bioleaching of metals were carried out using either a fungus or bacteria (Table 1). The use of A. ferrooxidans and A. thiooxidans dissolved 66% Cu, 50% Ni and 64% Co from the particles of <75 lm size slag at pH 2.0 (Mehta et al., 1997, 1999). The iron is initially dissolved as FeSO4 from the slag containing fayalite (2FeOSiO2) and magnetite (Fe3O4) as the major phases and copper silicate, wustite (FeO) and digenite (Cu9S5) as the minor phases by chemical action of acid (reaction 1–2); sulphur is oxidized to sulphuric acid (3) by A. thiooxidans

Fe3 O4 þ 4H2 SO4 ! FeSO4 þ Fe2 ðSO4 Þ3 þ 4H2 O

ð1Þ

FeO þ H2 SO4 ! FeSO4 þ H2 O

ð2Þ

2S0 þ 2H2 O þ 3O2 ! 2H2 SO4

ð3Þ

A. ferrooxidans, the iron oxidizing bacteria oxidizes ferrous sulphate to ferric sulfate (4):

4FeSO4 þ 2H2 SO4 þ O2 ! 2Fe2 ðSO4 Þ3 þ 2H2 O

ð4Þ

Ferric sulphate chemically oxidizes metal sulphides (MS) and even metals in the slag (Mehta et al., 1999) as given in reactions (5–6):

MS þ Fe2 ðSO4 Þ3 ! MSO4 þ 2FeSO4 þ S0

ð5Þ

MðCu; Ni; CoÞ þ Fe2 ðSO4 Þ3 ! MSO4 þ 2FeSO4

ð6Þ

Recently, Carranza et al. (2009) reported the application of BRISA process in which metal from the copper converter slag (9.0% Cu) was leached chemically by ferric sulfate (ferric leaching stage) coupled with the biological generation (biooxidation stage) of the leaching reagent (reaction 4) by A. ferrooxidans. The regenerated ferric iron is reused in the leaching step (reaction 5–6) running in close circuit, resulting in the dissolution of 93% Cu in 4 h at 60 °C, 1.67 pH and 20 g/L pulp density with the fine size particles (D80–47.03) in presence of 11.5 g/L ferric sulphate. The use of a fungus, A. niger could dissolve 47% Cu, 50% Ni and 23% Co from the slag as compared to the low recovery of Cu (46.5%) and Ni (28%) when the material was leached in culture filtrate. The metabolic activity of A. niger secretes carboxylic acids such as

J.-c. Lee, B.D. Pandey / Waste Management 32 (2012) 3–18

citric, succinic and gluconic which are responsible for dissolution of metals during fungal bioleaching by the formation of metal complexes (Brandl and Faramarzi, 2006; Sukla et al., 1992, 1995). As regards the bioleaching of copper dump slag low metal dissolution (8.0% Cu, 13.0% Ni) was observed by Munoz et al. (2009) by A. ferrooxidans at pH 0.5 and 1.0 in 15 days whereas metal bioleaching from the flotation tailing was higher (16% Cu, 45% Ni) than the chemical leaching during the same period. However, use of a mixed culture of mesophiles (A. ferrooxidans and Leptospirillum sp.) leached 100% copper and nickel (Table 1) from a copper dump slag in 42 days at 1.5 pH in presence of 10 g/L S and 4.4 g/L Fe (II) at a low (10 g/L) pulp density (Vestola et al., 2010). A mixed culture containing mesophilic and a thermophilic bacterium has been found to be particularly effective in leaching of metals (86–88% Cu, and 95–97% Zn along with 85–95% other metals) from a Pb–Zn smelting slag at 65 °C and 1.5 pH (Cheng et al., 2009; Guo et al., 2010). Earlier, an alkaline dump slag of metallurgical processing plant in Germany containing siliceous material and glassy, and partially crystalline structure with 34% SiO2, 35% Fe2O3, 11.7% Cao, 7.8% Na2O, 4.3% Al2O3, 3.4% MgO, 1.17% MnO, 1.14% TiO2 and minor amounts of Cr, Zn, V, Ni, Co, Pb, Cu and Sr was leached by a bacterial isolate from the source (Willscher and Bosecker, 2003). The study shows the leaching of 26% Ni and 40% Co along with 38% Mn, 46% Mg and 68% Ca with the bacterial isolate – coryneform of the alkaline slag. The importance of the study is to extract valuable metals and to regulate the environmental pollution caused due to the weathering/microbial attack on the slag dumps. 2.1.3. Filter dust of copper plant and filter press residue The bioleaching of a filter residue from copper converter typically containing 58.6% Zn, 11.3% Pb, 6.1% Sn, 1.5% S. 0.478% Cu, etc. in presence of Penicillium sp. could dissolve 93% Zn in 13 days at 25 g/L pulp density in shake flask (Schinner and Burgstaller, 1989). The Zn leaching is associated with the complexation with citric acid generated by the fungus at higher pH (4–7) in contrast to the low pH (2.0) needed for A. niger. The filter dust of a copper converter when leached in a bioreactor dissolved 80–90% Zn by Penicillium simplicissimum in the pH range 2–7 in 9 days (Burgstaller et al., 1992; Muller et al., 1995). Hahn et al. (1993) also reported high recovery of copper (95% and above) from such materials by A. niger and heterotrophic bacteria. Earlier, about 90% Cu and Zn leaching from the filter residue was reported with A. thiooxidans in shake–flask experiments (Bosecker, 1986). 2.1.4. Industrial and metallurgical sludge/tailings/waste spillage With hardly any significant utilization of red mud which is generated to the tune of 66 million tons every year globally at the rate of 1.0–1.5 ton/ton of alumina production (Yang et al., 2008), bioleaching the same could be a challenging activity (Bojinova and Velkova, 2001). The leaching of the red mud after mechano-chemical activation for 4 h dissolved 78% K and 91% Na in 7 days in presence of 16 g/L S by A. thiooxidans thus reducing the alkalinity of the waste whilst 71% Al dissolution in 28 days was possible under this condition. The aluminium biorecovery by thiobacilli culture with 10 g/L S without activation was however, much low due to the growth limitation (Vanchon et al., 1994), whereas best leaching result was obtained (75% Al) at 100 g/L pulp density using the metabolite such as citric acid produced from P. simplicissimum. As a part of assessing the metal mobility Hsu and Harrison (1995) reported high leaching of copper (99%) and zinc (90%) from tailing pond sediment and a chert pile rock, respectively at 70 °C over 5 days in shake flask by a mixed culture of A. ferrooxidans and S. acidocaldarius. Francis and Dodge (1990) treated the solid waste generated during co-precipitation of metal ions from the effluents with iron as goethite (FeOOH) using anaerobic bacteria

7

such as Clostridium sp. The dissolution of 55% Ni, 48% Cd, 41% Zn, 59% Fe and 3.2 Cr in shake flask confirmed the instability of such dumps. Amongst a few industrial problems, bioleaching of copper locked in waste rock dump (1.2 million ton Cu) in China may be mentioned (Yang et al., 2002). With the possibility of leaching of 30% Cu per year by A. ferrooxidans from the dump, an effort was recently made by Wu et al. (2009) to convert the dump into an in situ bioleaching operation. Copper leaching of 44% on bench scale was obtained. Recently, as a part of study of geochemical aspects of pyrite sludge spillage in 1998 from a dam in Spain (2 million m3 pyrite sludge and 4 million m3 acidic water into a river and 4600 hectare of land), the leaching of toxic metals in the temperature range 22– 65 °C by mesophilic and thermophilic bacteria (Sulfolobus sp.) was investigated (Hita et al., 2008). The mobilization of 0.060 g/L Zn, 0.075 g/L As and 1.2 g/L Fe in 10 days in bench scale test at 65 °C shows that the spill would create pollution because of the oxidation of pyrite and leaching of toxic metals under moist conditions in summer. Study by Ostrega et al. (2009) on the high mobilization of toxic metals (80–100%) from the stored metallurgy wastes in Poland during a six-stage extraction with autotrophic and heterotrophic bacteria, suggests the sequential bioleaching of the metals from the material as an alternative before it is dumped safely. 2.1.5. Steel/blast furnace slag/sludge The generation of 250–300 kg of the blast furnace (BF) slag per ton of iron (450 million ton slag generated globally) and 300– 500 kg of slag per ton of the steel produced is a formidable amount of solid waste available for treatment, although fairly good amount of BF slag is utilized in cement making. Besides, the formation of flue dust and sludge from the blast furnace, flue dust/sludge from the electric arc furnace are rich in non-ferrous metals depending on the type of metal and alloys produced. There are limited studies on the bioleaching of metals from such wastes. Higher bio-recovery of iron and heavy metals from the BF sludge and flue dust by A. ferrooxidans as compared to that of a fungus was reported by Banerjee (2007). Technical feasibility for leaching of 76–78% Zn and Al from EAF sludge was also demonstrated (Solisio et al., 2002) at low pulp density (10 g/L) with A. ferrooxidans in presence of a sulphur containing substrate. The BF slag can be used as a neutralizing agent (to pH 1.5) in place of slaked lime during pyrite oxidation (75–80%) by a mixed mesophilic culture at 35 °C (Gahan et al., 2009). Even the slag originating from the stainless steel production with fluoride, chromium and vanadium was effective as neutralizing agent, though it could be less environmentally friendly. 2.2. Bio-processing of sludge from tanning, electroplating and wastewater treatment units 2.2.1. Sludge from tanneries A significant amount of sludge from tanning units is generated world over since chrome tanning is the predominantly used (90%) process to protect leather (Agrawal et al., 2006). During the tanning only 60% chrome feed reacts with the hides and the remaining amount of chromium in the spent bath/effluent is managed in the wastewater plant, which finally ends up as sludge (Wang et al., 2007). Because of high level of chromium (1–4%) compounds and other pollutants in the sludge it is considered a toxic waste. The chemical leaching of chromium from the tannery sludge by sulphuric acid is generally low (Tyagi et al., 1988) due to the poor leachability of 10–20% Cr(III) bound with the organic matter as compared to the remaining 80–90% Cr that exists as mixed oxides of Cr–Fe precipitate (Chuan and Liu, 1996). Through bioleaching (Table 2) it is possible to improve the dissolution of chromium.

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Table 2 Bio-leaching of metals from the waste sludge of the tanning/plating/wastewater treatment industries. Specific type and source of waste/by-product

Microbe used

Leaching efficiency (%) and conditions

Type of reactors

References

Tannery sludge

(i) S-oxidizing bacteria – A. thiooxidans (ii) Acidithiobacillus sp. (TS6)

Cr-87%, Al-73%, Fe-72%, Mg-62%, Zn-73% with 30 g/L sulphur at 40 g/L PD and 1.3 pH in 25 days Cr- 100% in 10 days in presence of 0.185 mL sulphuric acid/g dry sludge with final pH of 1.5

Shake flask

Shen et al. (2002, 2003) Zhou et al. (2005) Zhou et al. (2006) Fang and Zhou (2007) Wang et al. (2007). Zheng et al. (2009)

(iii) Indigenous S- and Fe-oxidizing bacteria (iv) A. thiooxidans TS6 and Brettanomyces B65 (v) A. thiooxidans

Shake flask Shake flask

Cr-95.6% in 12 days in presence of 6 g/L Fe (II) and 20 g/L sulphur, Residue can be used in agriculture Cr- >95% in 3 days with the combined species at 1.65 pH Cr-99.7% in 5 days in presence of 2 g/L sulphur

Plating sludge

(vi) A. ferrooxidans LX5 and A. thiooxidans TS and Pichia spartinae D13 A. ferrooxidans

Sludge from the waste water treatment

A. ferrooxidans (hybrid process  chem. + biol.)

Zn-97%, Cu-96%, Ni-93%, Pb-84%, Cd-67%, Cr-34% in 20 days Cd-63%, Cu-71%, Cr-49%, Zn-80% by bio-oxidized Fe(III) solution

Sludge-plating and pickling waste water pit of steel works

A. ferrooxidans from the source sludge

Oxidation of Fe(II) in the Fe(III) containing waste waters with other metals

The use of sulphur oxidizing bacteria such as A. thiooxidans alone or in combination with iron oxidizing bacteria has been investigated in presence of sulphur (Fang and Zhou, 2007; Shen et al., 2002; Wang et al., 2007; Zheng et al., 2009; Zhou et al., 2005, 2006). High chromium bioleaching (87%) along with the dissolution of Al (73%), Fe (72%) and Zn (73%) was noticed at 40 g/L pulp density in presence of 30 g/L sulphur in 25 days at the resulting pH of 1.3 (Shen et al., 2002, 2003). On the other hand the preacidified dry sludge with sulphuric acid (0.185 mL/g sludge) when leached with Acidithiobacillus sp. (TS 6) dissolved almost 100% Cr while attaining the final pH of 1.5 in 10 days (Zhou et al., 2005). The bioleaching of tannery sludge using the adapted mixture of ingenuous iron- and sulphur-oxidizing bacteria was investigated by Zhou et al. (2006). The pre-acidification of the sludge to pH 6.0 and the addition of energy source such as 20 g/L sulphur and 6.0 g/L Fe2+ accelerated the acid generation and increase of oxidation–reduction potential. Dissolution of chromium increased to 95.5% in 12 days due to decrease in pH and the residue with less amount of toxic metal was found suitable for use in agriculture. Chromium dissolution in an airlift reactor was kinetically favoured with a mixed culture of A. thiooxidans TS6 and Brettanomyces B65 and an acid tolerant yeast strain inoculated at pH 1.65 while degrading dissolved organic matter (DOM) (Fang and Zhou, 2007). Higher than 95% of Cr removal in only 3 days was noticed. The bioleaching of the sludge in a bubble column bioreactor shortened the period to 5 days with very high dissolution of chromium (99.7%) using A. thiooxidans and 20 g/L sulphur as energy source (Wang et al., 2007). Recently, Zheng et al. (2009) reported shortening in metal leaching of sludge by 3 days in successive multibatch reaction when a DOM – degrading heterotrophic microorganism, Pichia spartinae D13 was inoculated along with the Fe- and S-oxidizing strains. Above 90% Cr removal was obtained in 6 days with the addition of P. spartinae D13 in the first batch along with A. ferrooxidans and A. thiooxidans while replenishing it periodically in the fifth batch. 2.2.2. Electroplating and wastewater sludge The electroplating sludge particularly those from the chrome plating units that settles in the bath contains 7–11% Cr and 3–5% Fe along with other metals such as nickel, cobalt, zinc, cadmium, molybdenum, vanadium, copper etc. in variable amounts depending on the type of substrate used for the surface treatment (Agrawal

Airlift reactor Aerated bubble column bioreactor Multi-batch reactor

STR (3 L) Continuous stirred tank reactor (CSTR) – 5.2 L capacity Shake flask

Bayat and Sari (2010) Drogui et al. (2005a,b) Fujii et al. (1988)

et al., 2006). The treatment of spent plating solution/effluent also generates sludge with variable composition according to the prevailing practice of water treatment. A few literatures are available on the bio-treatment of plating sludge (Brar et al., 2006). Recently, Bayat and Sari (2010) reported the bioleaching of plating sludge in a completely mixed batch reactor (CMBR) of 3.0 L capacity using A. ferrooxidans. The results show the low recovery of chromium (34%) along with reasonably good leaching of other metals (97% Zn, 96% Cu, 93% Ni, 84% Pb and 67% Cd) in 20 days time at 20 g/L pulp density, 2.0 pH and 25 °C from the dewatered metal plating sludge with no sulphide or sulphate compounds. With lower removal of the hazardous heavy metals by ferric chloride and sulphuric acid leaching, the bioleaching is suggested to be an alternative or adjunct to the conventional physico-chemical treatment of the sludge. The hybrid process involving bio-chemical approach was suggested for the treatment of a wastewater sludge containing chromium and other metals (Drogui et al., 2005a,b). The chemical leaching of the sludge in a 5.2 L continuous stirred tank reactor (CSTR) with the bio-oxidized Fe(III) using A. ferrooxidans dissolved about 49% Cr along with 63% Cd, 71% Cu and 80% Zn. Earlier, Fuji et al. (1988) reported the potential application of a bacterial isolate (A. ferrooxidans) from the sludge of the plating and pickling wastewater pit of a steel works (Nippon Steel) for the oxidation of Fe (II). The oxidizability of Fe (II) from such waters containing heavy metals and chloride ions may be used to realize activated sludge treatment of the steel works water and for subsequent metal recovery by a suitable method. 2.3. Bio-processing of spent catalysts and used batteries 2.3.1. Spent catalysts The hydro-treating catalysts are extensively used in the petroleum refineries to produce clean fuels. These catalysts usually contain molybdenum supported on an alumina carrier with promoters such as cobalt or nickel to enhance removal of undesirable impurities such as sulphur, nitrogen, and metals (V and Ni) by promoting hydro-desulphurization (HDS), –denitrogenation (HDN) and – demetallization (HDM) reactions (Marafi and Stanislaus, 2008a,b). Presently, a large quantity of the waste hydroprocessing catalysts (150,000–170,000 tons/year, Dufresne, 2007) is generated worldwide which may further increase as a result of the processing of heavier feed-stocks with high level of impurities and steady

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increase in the production capacity to meet the demands. The spent HDS catalysts generally consist of 10–30% Mo, 1–12% V, 0.5–6% Ni, 1–6% Co, 8–12% S, 10–12% C and balance is alumina (Zeng and Cheng, 2009). The disposal of such wastes is the last option with the associated liability and therefore, several processing methodologies are reported including those industrially used (Marafi and Stanislaus, 2008a,b). For the bioprocessing of spent catalysts, bacteria such as A. ferrooxidans and A. thiooxidans or fungi such as Aspergillus and Penicillium sp. can be utilized (Aung and Ting, 2005). Studies (Table 3) were initially carried out by Blaustein et al. (1993) to treat waste catalyst of coal liquefaction to recover molybdenum using A. ferrooxidans and L. ferrooxidans. A low molybdenum recovery at 30 °C was observed in 42 days time which increased with decreasing particle size of the catalyst. In another study, Joff and Sperll (1993) found that nickel could be leached effectively from the waste coal catalyst initially with A. ferrooxidans and later on in presence of Sulfolobus, a thermophilic culture at higher temperature. The presence of Mo had adverse effect on leaching and consequently, the leaching process had to be extended to achieve the desired level of leachability (Joff and Sperll, 1993). Generally, the microbes which are tolerant to Mo or W can leach nickel from the waste catalysts. The use of heterotrophic denitrifying bacteria and thermophilic cultures such as Bacillus

stearothermophilus and Metallosphera sedula could yield high dissolution (>80%) of both Ni and Mo from the spent catalysts (Sandback, 1995; Sandback and Joffe, 1993). The application of a fungus such as A. niger was described by Aung and Ting (2005) to treat spent fluid catalytic cracking (FCC) catalyst at initial pH of 5.5. Low amounts of metals were leached out (9% Ni, 30% Fe, 23% Fe, 36% V) in 46 days even at a low pulp density of 10 g/L by the fungus which was accompanied by the formation of complexes with the secreted acids. Briand et al. (1999) reported high recovery of vanadium (70%) by the catalyst adapted and grown culture of A. thiooxidans. The experiments conducted by Santhiya and Ting (2005) in one- and twostage bioleaching from such a catalyst by A. niger show almost similar level of metal dissolution over a period of 70 days with the particles of 100–150 lm size. The metal recovery was found to be higher for Ni (65%) and Mo (80%) than that of Al (55%) for the same size particles. In another study by Santhiya and Ting (2006), the use of adapted culture of A. niger facilitated higher leaching of metals (79% Ni, 82% Mo and 65% Al) in two-stages and 30 days with the particles of <37 lm size. The high concentration of the secreted acids such as citric, oxalic and gluconic acids with the catalyst of particles size <37 lm in 30 days may be correlated with the higher metal recovery at a low pulp density of 10 g/L. The leaching of the metals from the waste catalyst

Table 3 Bio-processing of metals from the spent catalysts and used batteries. Specific type and source of waste/byproduct

Microbe used

Leaching efficiency (%) and conditions

Type of reactors

References

Waste catalyst of coal liquefaction

(i) A. ferrooxidans and L. ferrooxidans

Low Mo bioleaching at 30 °C in 42 days

Shake flask

(ii) A. ferrooxidans, Sulfolobus and a thermophilic culture (iii) Bacillus stearothermophilus and Metallosphera sedula

High nickel with A. ferrooxidans alone and longer duration required for Mo bio-leaching with others High nickel and Mo leaching

(i) Aspergillus niger

Ni-9%, Fe-23%, Al-30%, V-36% , Sb-64% at 10 g/L PD in 46 days

Shake flask

Blaustein et al. (1993). Joff and Sperll (1993) Sandback and Joffe (1993), Sandback (1995) Aung and Ting (2005)

(i) A. thiooxidans

High vanadium bioleaching in acidic region

Shake flask

(ii) A. ferrooxidans/A. thiooxidans

Ni-88%, Mo-58%, V-32% in one stage bioleaching in presence of sulphur

Shake flask

Ni-88%, Mo-46, V-95% in a two-stage bioleaching with Soxidizing bacteria Ni-65%, Mo-80%, Al-55% in 70 days in one- or two-stage leaching for 100–150 lm size Ni-79%, Mo-82%, Al-65% in 30 days

Shake flask Shake flask

Co-90%, Li-80% in 5 days at 1.0 pH

Shake flask

Co-65%, Li-10% in 18 days at 100 g/L PD

Shake flask

Briand et al. (1999) Mishra et al. (2007, 2008a, 2009) Pradhan et al. (2009) Santhiya and Ting (2005) Santhiya and Ting (2006) Mishra et al. (2008b), Xu et al. (2008) Xin et al. (2009)

Fe(II) oxidation – 99.8% in presence of 8.0 g/L Cr and 13.0 g/L Ni in 11 days at 10 (vol) % wastewater and 66.5% Fe(II) oxidation in 90 (vol) % waste water at pH 1.8

Shake flask

Yoo et al. (2010)

Ni-96.5%, 100% Cd, 95% Fe in 93 days

Percolator

Cerruti et al. (1998)

Cd-100% and Ni-75.6% in 50 days with thiobacilli in sewage sludge of 4 days retention from one-stage reactor Max. Cd and Co leaching at pH 3.0–4.5, Ni at 2.5–3.5 pH; Complete leaching in 20 days for two batteries by S-oxidizing bacteria Fe-oxidizing system more efficient than S-oxidizing for metal leaching

Bioreactor

Zhu et al. (2003)

Two-step continuous reactor Two-step continuous flow reactor

Zhao et al. (2008a)

Spent fluid catalytic cracking (FCC) catalyst Spent hydroprocessing catalysts

(iii) Aspergillus niger

Spent Li ion secondary batteries (LIBs)

Bio-treatment of waste water of spent primary batteries Used Ni–Cd batteries

Spent Ni–Cd batteries

(iv) Aspergillus niger adapted to single and mixed metals (i) A. ferrooxidans and A. thiooxidans in presence of 4 g/L S, 2 g/L each S + FeS2 or 4 g/L FeS2 (ii) Aspergillus niger in presence of 10 g/L S and 3 g/L Fe(III) Aspergillus niger

A. ferrooxidans to generate acid with 10 g/L S in bioreactor and then leaching (i) Acid of S-oxidizing microflora from sewage sludge grown with S (ii) Using acid from thiobacilli of sewage sludge on S or FeSO4 substrate in acidifying reactor

Shake flask

Zhao et al. (2008b)

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by A. niger was found to be favourable as compared to the leaching with the synthetic acids. The use of acidophilic bacteria such as a mixed culture of A. ferrooxidans and A. thiooxidans grown on the catalyst could leach out 88% Ni, 58% Mo and 32% V in one stage in presence of sulphur (Mishra et al., 2007, 2008a, 2009). However, when the bacterial culture (S-oxidizing) was used to produce sulphuric acid in presence of 20 g/L sulphur in the first stage and the acidic medium used for leaching in the second stage, the metal leaching approached 88% Ni , 46% Mo and 95% V in 7 days (Mishra et al., 2008a; Pradhan et al., 2009). It may be pointed out that the current level of development is based on the bench scale work and needs to be established on large scale before it is commercialized. 2.3.2. Used batteries With the production of the lithium ion secondary batteries (LIBs) to the tune of 500 million cells in the year 2002, generation of about 200–500 tons of waste LIBs per year was estimated (Xu et al., 2008) which might have crossed the figure of 700 tons by now. The LIBs consist of heavy metals, organic chemicals and plastics in the proportion of 5–20% Co, 5–10% Ni, 5–7% Li, 15% organic chemicals and 7% plastics, the composition varying slightly with different manufacturers. As regards the recycling of these batteries, an account of chemical based processes including the hybrid processes combining mechanical treatment, leaching/separation and preparation of the product has been reviewed by Xu et al. (2008). As such very limited studies on the bioleaching of cobalt and lithium metals which are the most valuable metals in the spent LIBs containing LiCoO2, are reported (Table 3) and these are based on the use of acidophilic bacteria in presence of energy source such as elemental sulphur and iron (II) salt. The recent study by Mishra et al. (2008b) discussed about the generation of metabolites such as sulphuric acid and ferric ions utilizing the elemental sulphur and ferrous ions by A. ferrooxidans and dissolving the metals from the spent batteries with the help of these lixiviants. Higher concentration of iron (>5 g/L) decreased the dissolution of metals due to the precipitation of Fe(III). Bio-dissolution of cobalt was found to be faster than lithium. In presence of 10 g/L sulphur and 3 g/L Fe(II) at initial pH of 2.5, cobalt leaching approached to 65% at 50 g/L pulp density and 56% at 100 g/L pulp density with lithium recovery of 10% in 18 days. While dealing with the mechanism of bioleaching of metals from the waste batteries using the mixed culture of sulphur-oxidizing and iron-oxidizing bacteria, Xin et al. (2009) reported the highest release of lithium at lowest pH of 1.54 with elemental sulphur as the energy source. In contrast, cobalt leaching occurred at higher pH and varied ORP with 2 g/L S + 2 g/L FeS2. The maximum recovery of cobalt and lithium was observed to be 90% and 80%, respectively in 5 days. The acid dissolution was the main mechanism for lithium leaching independent of energy matter type, whereas Fe2+ catalyzed reduction of Co3+ along with the acid dissolution in both FeS2 and FeS2 + S systems. In a recent study, Yoo et al. (2010) reported that the wastewater produced during the recycling of spent lithium primary batteries can be re-circulated in the recycling process on decreasing the pH and metal concentration by biological oxidation with A. ferrooxidans. The water recycling would be possible with decreased pH (1.8) and iron at 66.5% Fe(II) oxidation level in presence of high amounts of Cr, Ni and Li in 11 days in iron free 9 K media (composition – 3 g/L (NH4)2SO4, 0.5 g/L MgSO47H2O, 0.5 g/L K2HPO4, 0.1 g/ L KCl, 0.014 g/L Ca(NO3)24H2O) even for the high wastewater content of 90%. With a share of 28% of lithium batteries both primary lithium batteries (PLBs) and LIBs in 2003, rest being the Ni–Cd and nickel-metal hydride batteries world over (Xu et al., 2008) and with the production of 1 million tons/year of batteries by EU alone (Karnchanawong and Limpiteeprakan, 2009), the usage pattern

has reversed to 84% of lithium based cells in 2010. On an average, spent Ni–Cd batteries contain 15–25 wt.% Cd, 15–20 wt.% Ni, 15– 20 wt.% Fe and 6–7% plastics along with small amounts of other metals. Cathodic material is rich in Ni (55 wt.%) and anodic component (Cerruti et al., 1998) has high cadmium (56 wt.%). Some efforts were made to extract metals from the waste Ni–Cd batteries not only by conventional pyro- and hydro-metallurgical methods, but also by bioleaching using acidophilic bacteria (Cerruti et al., 1998; Zhao et al., 2008a,b, Zhu et al., 2003). As the Ni–Cd batteries contain nickel and cadmium in metallic form as well as Ni(OH)2, Cd(OH)2 and c-NiOOH phases, besides small amount of cobalt, the dissolution reactions are expected to be acid consuming. Therefore, Cerruti et al. (1998) used 10 g/L S as energy source to produce acidic metabolite by a pure culture of A. ferrooxidans in a percolator and then used the supernatant for bioleaching in another percolating column to recover 96.5% Ni, 100% Cd and 95% Fe in 93 days at 30 °C. Chinese researchers (Zhao et al., 2008a,b, Zhu et al., 2003) used sewage sludge as the source of Thiobacillus sp. as well as nutrients and added sulphur to produce acidic solutions of pH close to 1 in a bioreactor, and leached the spent batteries by the supernatant acid in a reactor The results show that nickel releases more slowly than cadmium and metal leaching depends on the sludge retention time in bioreactor (RTB). With the acid generated in 4 days of sludge retention time in bioreactor, 100% Cd release occurred in 35 days as compared to 75.5% Ni released in 50 days and the sewage sludge could also be acceptable for agricultural applications. Bioleaching of spent batteries in a continuous flow system shows that more hydraulic retention time (HRT) favoured metal dissolution (Zhao et al., 2008a). For an instance, 25, 30 and 40 days of HRT all the three heavy metals (Ni, Cd, Co) could be removed with the process load of two, four and eight batteries (wt.: 10.77 g  2 for cathode and anode) at 100 rpm agitation and 27–30 °C. Cadmium and cobalt required higher pH of 3–4.5 whereas nickel hydroxide and metallic form dissolved separately in different pH range (2.5– 3.5). Using the sewage sludge with ferrous sulphate and sulphur as the energy source of thiobacilli in an acidifying reactor, the biodissolution of metals in the continuous flow leaching reactor was found to be favoured by iron–oxidizing system (Maximum of Cd/ Co leaching in 6–8 days; Ni in 16 days) than the S-substrate system which had a lag phase of 4–8 days (Zhao et al., 2008b). 2.4. Bio-processing of waste electronic equipment/e-waste The steady generation of waste electric and electronic equipment (WEEE), or electronic waste (e-waste) is a cause of concern because the material is physically and chemically different from other form of municipal and industrial wastes. e-Waste contains valuable as well as hazardous metals/materials and requires a separate management procedure to obviate the environmental problem. As estimated by Robinson (2009), the annual e-waste production was 20–25 million tons at the lower end of the United Nations Environment Programme’s (UNEP, 2006) estimate of 20– 50 million tons per year. The printed circuit boards (PCBs) in the electronic waste are rich source of metals (30%) including base, precious and hazardous metals with about 70% non-metallic content. The metal contents of the PCBs are quite variable depending on the type and make of the equipments and are generally in the range 10–27% Cu, 8–38% Fe, 2–19% Al, 0.3–2% Ni, 1–3% Pb, 200– 3000 ppm Ag, 20–500 ppm Au, 10–200 ppm Pd etc. (Cui and Zhang, 2008; Huang et al., 2009). In majority of the cases, the values of precious metals are the prime economic diver to recycle the ewastes which is followed by the recovery of copper and zinc while the recovery of metals like Al, Fe and Pb is of much less value. Amongst the metallurgical recovery processes, the pyro-metallurgical approach used industrially for the extraction of precious

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metals and recent developments in the hydro-metallurgical based processes are reviewed by Cui and Zhang (2008). Though, the biotechnology has been proved to be one of the most promising technologies in the metallurgical sector as mentioned earlier, yet limited researches have been reported for the bioleaching of ewaste which are summarized in Table 4. With the presence of metals like copper, iron, nickel and precious metals such as silver, gold, palladium, etc. in the metallic state and other metals in the form of mostly metals or inorganic compounds, the use of thiobacilli group of bacteria and the thermophiles (Brandl et al., 2001; Choi et al., 2004; Ilyas et al., 2007, 2010; Vestola et al., 2010; Wang et al., 2009b; Yang et al., 2009), or fungi such as Aspergillus niger, Penicillium simplicissimum, etc. (Brandl, 2001) are reported. In order to recover gold from the e-wastes, a few studies are also described using cyanogenic bacteria such as Cyanobacterium violaceum, P. fluorescens, P. plecoglossicida (Brandl et al., 2008; Brandl and Faramarzi, 2006; Chi et al., 2010; Faramarzi et al., 2004; Kita et al., 2006). The mechanism of biodissolution of copper, the main constituent of the PCBs, by A. ferrooxidans is very similar to that of metal sulphides (Choi et al., 2004; Rossi, 1990). The leaching of copper proceeds through the oxidation reaction by Fe3+ formed by bacteria to convert metallic copper to the cupric state which is thermodynamically favourable (Wang et al., 2009a) as given in reaction (7):

2Fe3þ þ Cu0 ! Cu2þ þ 2Fe2þ

DG0 ¼ 82:9kJ=mol

ð7Þ

Thus, addition of Fe (II) ions along with the bacterial inocula, both mesophilic and thermophilic would play a crucial role to leach metals. Using a mixed culture of A. ferrooxidans and A. thiooxidans initial studies by Brandl et al. (2001) show the significant recovery of metals from the dust of the e-waste shredding in a two-stage leaching process. In view of the alkalinity of the electronic scrap and acid consumption at higher pulp density, and to reduce the toxic effects on the microbes, the biomass was produced in the first stage in absence of e-scrap over a period of 7 days. Electronic scrap was subsequently added in different concentrations and the cultures were incubated for an additional period of 10 days in the second stage. The mixed culture of Thiobacillus sp. was able to

leach above 90% of the metals such as Cu, Ni, Zn and Al at the pulp density in the range 5–10 g/L. As expected, the metal mobilization was reduced particularly for copper and aluminum at higher scrap concentration although, the mobility of Ni and Zn was still high of 60% and 90% respectively. No leaching of lead and tin was observed presumably due to their precipitation as PbSO4 and SnO. Because of the precipitation of the metals during microbial leaching process, subsequent treatment of the leach residue is essential to recover the metals already leached. The investigation by Choi et al. (2004) clearly shows a relation between the amount of copper dissolved in solution and that in the precipitate when Fe (II) is added in the leaching media. With 7 g/L Fe (II), copper leaching was 37% by A. ferrooxidans from the shreds of PCBs. The addition of citric acid as a complexing agent improved the copper leaching to 80%. The use of fungi (A. niger and P. simplicissimum) was also reported by Brandl (2001). The microbial growth was inhibited in one-step process with the scrap concentration of above 10 g/L in the medium. However, almost complete dissolution of the metals such as Cu, Pb, Sn and Zn was noticed in a two-step leaching process using a commercial gluconic acid solution. Recent research by Wang et al. (2009) demonstrates that the leaching of metals from the waste printed wire boards (PWBs) depends to a large extent on the particle size and the concentration of the scrap using the acclimatized acidophilic Thiobacillus sp. alone or in a combination (mixed culture) over PWBs in iron free 9 K (referred as 9 K), S-oxidizing and mixed media. Higher metal leaching is observed with the lower size of the particles and decreased pulp density. For an instance, 99–99.9% copper dissolution takes place with the particles of 0.5–1.0 mm size at 7.8 g/L pulp density in 9 days as compared to the leaching of 88.9% Cu in 5 days with the particles of size <0.35 mm along with the similar recovery of zinc. The leaching of the metals is reported to be governed by the bacterial oxidation of Fe(II) dissolved from the PWBs. Rise in pH because of alkaline nature of the substrate accounts for the precipitation of iron as Fe(OH)3 and subsequently as ammonium jarosite by the following reactions, which must be controlled under strongly acidic range to mobilize the metals from the waste.

Table 4 Bio-processing of waste electronic equipment/e-waste. Specific type and source of waste/by-product

Microbe used

Leaching efficiency (%) and conditions

Type of reactors

References

Waste PCBs (shreds)

(i) A. ferrooxidans and A. thiooxidans (ii) A. ferrooxidans and citric acid (iii) A. niger and P. Simplicissimum (i) A. ferrooxidans, A. thiooxidans and mixed culture

Al, Cu, Ni, Zn-90% at 0.5–10 g/LPD in two-stages for 7 and 10 days Cu-37% with 7 g/L Fe(II) and 80% in presence of citric acid Metal recovery 100% at 100 g/L PD by gluconic acid

Shake flask

Brandl et al. (2001).

Cu-99–99.9% with mixed culture at 7.8 g/L PD with 0.5– 1.0 mm size in 9 d; Cu, Pb and Zn 88.9% in 5 days with size <0.35 mm Cu-98% in 24 h at 1.5 pH with 100% inocula: Cu-72% without inocula Cu and Ni-100% in 42 days at 10 g/L PD, 1.5 pH in presence of 4.5 g/L Fe(II) + 10 g/L S Cu-89%, Ni-81% , Al-79%, Zn-83% at10 g/L PD in 18 d

Shake flask

(i) Printed wire boards

(ii)Waste PCBs

Waste PCBs/electronic scrap

(i)Waste PCBs sample (ii) Waste e-scrap, Agjewelry waste Ptcatalytic converter (iii) Waste mobile phone PCBs Electronic waste

(ii) A. ferrooxidans and 6.66 g/L Fe(III) (iii) Acidithiobacillus sp. and Leptospirillum sp. (i) S. thermosulfido-oxidans and Acidophilic heterotroph (ii) S. thermosulfido-oxidans and Thermoplasma acidiphilum (i) C. violaceum (ii) C. violaceum, P. fluorescens, P. plecoglossicida

Cu-86%, Ni-74%, Zn-80%, Al-64% in acid pre-leach (27 days) and for 280 days Au-14.9% in 7 days at a low pulp density Au-68.5% in 7 days, low Ag mobilization, low Pt recovery, low Au leaching

(iii) C. violaceum

Au-13% and Cu-37% in 8 days at 100 g/L PD

(i) Desulfovibrio desulphuricans to recover Au, Pd, Cu from nitrate liquor

High recovery of Au as powder and Pd 95% at pH 5

Choi et al. (2004). Brandl et al. (2001) Wang et al. (2009a)

Yang et al. (2009) Vestola et al. (2010) Shake flask Column rector Shake flask

Shake flask

Ilyas et al. (2007) Ilyas et al. (2010) Faramarzi et al. (2004) Brandl and Faramarzi (2006), Brandl et al. (2008), Kita et al. (2006) Chi et al. (2010) Creamer et al. (2006)

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Fe3þ þ 3H2 O ! FeðOHÞ3 þ 3Hþ

ð8Þ

3þ FeðOHÞ3 þ 4=3SO2 þ H2 O þ 2=3NHþ4 4 þ Fe

! 2=3ðNH4 ÞFe3 ðSO4 ÞðOHÞ6 þ Hþ

ð9Þ

Yang et al. (2009) reported the dependence of copper leaching on the pH, concentration of Fe(III) in the medium and the amount of bacterial stock culture used. A 100% stock culture (inoculum) of A. ferrooxidans with 6.66 g/L Fe(III) previously grown for 3–4 days, was strong enough to leach 98% Cu in 24 h at 1.5 pH, whilst 72% Cu leaching was observed in absence of inocula under the above conditions. The adverse effect of increased pulp density on the leaching of metals with a mixed culture (A. ferroxidans and Leptospirillum sp.) was clearly observed in a recent study (Vestola et al., 2010). Bioleaching in presence of 4.5 g/L Fe2+ and 10 g/L S could promote 100% dissolution of copper and nickel from the waste PCBs at 1.5 pH in 42 days at the low pulp density of 10 g/L; the leaching dropped substantially (45% Cu and 19% Ni) at 100 g/L pulp density under the same conditions. Ilyas et al. (2007) used a mixed culture containing a moderately thermophilic strain such as Sulfobacillus thermosulfidooxidans and an unidentified acidophilic heterotoph (code A1 TSB) isolated from the local environment to leach metals from e-waste. At a scrap (PCBs) concentration of 10 g/L, the metal adapted mixed consortium leached more than 89% Cu, 81% Ni, 79% Al and 83% Zn in 18 days in presence of 10 g/L S. The secreted carboxylic acids such as citric, oxalic and succinic acids from the heterotroph was able to increase the leaching rate in this case which could be due to the synergistic effect of the heterotroph on the growth of S. thermosulfidooxidans. Ilyas et al. (2010) further reported high recovery of metals in a column leaching of the spent PCBs using the metal adapted culture containing an identified heterotrophic bacterium, Thermoplasma acidiphilum, as a synergist along with S. thermosulfidooxidans. The acid pre-leach for 27 days followed by bioleaching for 280 days dissolved 86% Cu, 74% Ni, 80% Zn and 64% Al. The cyanide forming bacteria was also used to solubilize precious metals such as gold, silver, palladium etc. from e-wastes. Glycine is a direct precursor of cyanide which is formed by oxidative decarboxylation by cyanogenic bacteria during the short growth period in the early stationary phase of metabolism (Knowles and Bunch, 1986). The bacterial cyanide generation has the potential of replacing cyanide chemicals to leach gold under alkaline conditions making the metal recovery much easier, although, limited studies are reported (Table 4). Faramarzi et al. (2004) reported the feasibility of recovering gold from the waste PCBs by Cyanobacterium violaceum. The leaching of small pieces (5 mm  10 mm) of printed circuit boards containing 10 mg of gold in each piece could mobilize 14.9% gold as dicyanoaurate [Au(CN)2] in an alkaline solution. Microbe-mediated dissolution of gold in presence of C. violaceum with low metal mobilization was studied by Kita et al. (2006) without showing the formation of dicyanide complex of gold. Recent studies (Brandl et al., 2008; Brandl and Faramarzi, 2006) show the performance of cyanogenic bacteria particularly C. violaceum, P. fluorescens and P. plecoglossicida grown in presence of various metal containing solid wastes such as Au-containing e-scrap, Ag-containing jewelry waste and Pt-containing automobile catalyst converters. The C. violaceum was found to be the most effective cyanogen for mobilizing gold from the waste PCBs with the formation of higher amount of dicyanoaurate (68.5% in 7 days) at a low pulp density of 5 g/L as compared to P. fluorescens after a lag phase of 3 days. The gold complex was reported to be unstable in presence of P. fluorescens due to the sorption process onto the biomass or degradation of metal-cyanides (Brandl et al., 2008). The dissolution of substantial amount of copper from the e-waste was also ob-

served because of its high content in the scrap and rapid reaction with the cyanide in the solution. The recovery of silver and platinum from the waste materials was not very encouraging due to their resistance and toxic effects on the microbes. Recent results by Chi et al. (2010) show the extraction of 13% Au and 37% copper in 8 days time at 10 g/L pulp density from the pieces (1 mm  1 mm) of waste mobile phone PCBs containing 0.025% Au and 34.5% Cu when un-adapted C. violaceum was used. The role of copper in the dissolution of gold was particularly observed as the high concentration of copper present in the material could be leached out quickly with the small amount of cyanide (59– 69 ppm) bacterially generated as compared to that of gold. In presence of copper higher amounts of cyanides are needed to dissolve gold (Brierley and Brierley, 2001). For the recovery of precious metals from leach solutions, standard processes such as cementation and precipitation, and use of adsorption methods including those of several microbial species (bacteria, fungi, algae, proteins, alfalfa) have been explored (Cui and Zhang, 2008). Mention may be made of the bioseparation of Au(III), Pd (II) and Cu(II) from the nitrate/aqua regia leach liquor of waste PCBs in a three-step process (Creamer et al., 2006). The biomass of Desufovibrio desulfuricans was able to recover gold selectively as powder with hydrogen bubbling for 30 min in the first step from a solution at pH 1.5 and then settling for 24 h, followed by recovery of palladium with supplementation of pre-treated biomass containing bio-Pd0 and hydrogen sparging for 30 min plus settling over 24 h. Copper was recovered by the precipitaion method using biogas (ammonia) produced from the culture of Escherichia coli followed by centrifuging. Some research on biosorption of platinum group metals (PGMs) from the e-waste solution was also reported by Macaskie et al. (2005). 2.5. Bio-processing of sewage sludge and fly ash of municipal waste incinerators 2.5.1. Sewage sludge A huge amount of sewage sludge is produced (Pathak et al., 2009) by the treatment of municipal wastewater world over (estimated 8  106 tons on dry basis in USA, 4  106 tons in China, 2  106 tons in UK every year). Due to the presence of heavy metals which are found to be nearly 0.5–2% on dry basis and 6% in some cases, the most common use of the sludge for fertilizer/soil conditioner is considered unsafe with the release of metals. Therefore, decontamination of the sewage sludge prior to the land application assumes great importance. Recent reviews summarize the decontamination methodologies adapted for the sewage sludge by applying processes such as chemical leaching, electro-reclamation, supercritical fluid extraction and bioleaching (Babel and Dacera, 2006; Pathak et al., 2009). Bioleaching is projected to be an effective option to treat the sludge and using the treated sludge for land applications as fertilizer. The solubilization of metals from the sludge depends on the forms in which the metals are present according to the type of sludge, the metal and the method applied for the treatment. For example 40–70% of Cu and Cr are in oxidizable fraction (associated with organic matter) in the anaerobic sludge and exhibit low solubility, and most Ni and Zn in the exchangeable and reducible fractions have a tendency to dissolve easily. Analysis by Pathak et al. (2009) shows that anaerobically digested sludge can be leached by thiobacilli group of bacteria such as A. ferrooxidans/A. thiooxidans because of formation of metal sulphides in the sludge during the digestion process. No major difference is observed in the leaching of the organic bound metal and metal sulphide fractions present in the aerobic and anaerobic sludge. In view of the recent review available on the subject only the salient features of the bio-treatment process are summarized here.

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Use of Thiobacillus sp. sometimes in presence of ferrous sulphate and elemental sulphur has been reported to treat the sewage sludge (Table 5) in batch (Kim et al., 2005; Pathak et al., 2009; Viller and Garcia, 2006) and in continuous modes (Couillard and Mercier, 1991; Pathak et al., 2009; Seth et al., 2006). There is no report on the leaching of sewage sludge with heterotrophic microorganisms particularly bacteria such as Acetobactor, Acidophilum, Pseudomonas and Trichoderma, and fungi such as the genus Penicillium, Aspergillus, etc. With Thiobacillus sp. extraction of metals in presence of 30–100 g/L S varied in the range10–100% for different period of leaching in the batch mode. The leaching of metals in a bioreactor such as continuous stirred tank reactor (CSTR) with A. ferroxidans supplemented with the energy source shows higher dissolution of Cu (33–91%), Ni (48–98%), Cd (50–93%) and Zn (62–94%) in short time of 1–4 days as compared to the batch leaching. However, low leaching of lead (3%) was observed during the continuous run. The application of integrated technique termed as simultaneous sludge digestion and metal leaching (SSDML) in a single reactor is reported to be less costly as compared to conventional aerobic digestion and leaching. SSDML incorporates aerobic sludge digestion followed by leaching of metals due to rapid production of acid by the oxidation of sulphur. High recovery of copper (41–100%) and zinc (48–100%) was obtained with moderate to high leaching of Cd (25–100%) and Ni (15–84%) (Blais et al., 2004; Meknassi et al., 2000; Pathak et al., 2009; Tyagi et al., 1996). In a recent study Jain et al. (2010) reported high recovery of metals (76% Cu, 79.5% Ni, 84.2% Zn and 78.2% Mn) from a secondary activated sludge in a two-stage process. The sludge treated in the first stage using the less acidophilic thermophilic S-oxidizer in an autoheated thermophilic aerobic digestion reactor at 55–60 °C, was bioleached in second reactor (CSTR) at 30 °C by a mesophilic S-oxidizers. In spite of the high metal removal from the sewage sludge to decontaminate it, no large scale process development has been attempted so far perhaps due to the loss of fertilizer value, cost of dewatering and neutralization of the treated sludge after acid based leaching. Detailed economic analysis may be required to prove the viability of this process. An integrated approach involv-

ing biological and chemical process could be appropriately considered to reduce the operating cost and increase the efficiency of the bioleaching process (Drogui et al., 2005a). 2.5.2. Fly ash/residue from incinerator The incineration process is commonly used to treat municipal solid waste (MSW) and the ash both from the off-gas (top/fly ash) and the residue (bottom ash) contains significant amount of metals causing environmental degradation due to the leaching of heavy and toxic metals from the landfill disposal. Bottom-ash although, is sometimes used as a construction material by cementation or vitrification, but large portion is disposed-off in landfill sites. The composition of the fly ash varies from place to place, but often contains metals such as Cu, Al, Zn, Cr, Fe, Cd, Pb, Mn, Ni, As, Hg etc.; Al, Cu and Zn are present in sufficient amount that could be suitable for economic recovery (Ishigaki et al., 2005; Krebs et al., 1997). In most bioleaching studies the Thiobacillus sp. in presence of sulphur or Fe(II) or fungal species have been applied (Table 5). The investigation by Krebs et al. (1997, 2001) shows the treatment of a fly ash sample from the electrostatic precipitator unit with sulfuric acid generated (pH 1.0) using a mixed culture of A. thiooxidans. The growth medium consisted of 4% of anoxic sewage sludge as a source of nutrients and 10 g/L elemental sulphur to enrich the bacterial population while adding a pure culture of A. thiooxidans after 3 days and continuing the cultivation over a period of 1–3 months. The leaching was then carried out in batch experiments over a period of 2–3 weeks resulting in almost 80% leaching of Cu, Cd, Zn and 60% Al and 30% Fe/Ni with a low Pb dissolution. It may be pointed out that the leaching of the fly ash at higher pulp density may pose a problem due to the presence of high metal content and alkaline nature of the material. However, leaching of fly ash at a higher pulp density (40 g/L) in semi-continuous mode (STR) dissolved the same level of metal as that of batch reactor when a provision was made to decrease the pH by adding10 g/L S and sulphuric acid (Krebs et al., 2001). Almost similar trend of metal leaching from the fly ash was also observed by Tateda et al. (1998) with a Thiobacillus sp. strain (TM-32). The application of a mixed culture of sulphur- and iron-oxidizing

Table 5 Bio-processing of sewage sludge and fly ash of municipal solid waste (MSW) incinerators. Specific type and source of waste/ by-product

Microbe used

Leaching efficiency (%) and conditions

Type of reactors

References

Sewage sludge (anaerobic digested) Sewage sludge

(i) A. ferrooxidans, A. thiooxidans (Fe and S oxidizers)

Shake flask

Babel and Dacera. (2006), Kim et al. (2005), Viller and Garcia (2006), Pathak et al. (2009) Couillard and Mercier (1991), Seth et al. (2006), Pathak et al. (2009)

Sewage sludge

(i) A. ferrooxidans (aerobic sludge digestion-bioleaching)

Cu-34–100%, Ni-42–100%, Zn-38–100%, Cr-18–80%, Cd18–69%, Pb-10–58%, Mn-58–99% with S-30–100 g/L in 1–16 and 10 days (in one case) Cu-33–91%, Ni-48–98%, Zn-62–94%, Cr-6–8%, Cd-50– 93%, Pb- 3%, Mn-7–93% in 1–4 and 14 days (in one study) Cu-41–100%, Ni-15–84%, Zn-48–100%, Cr-9–50%, Cd25–100%, Pb-5–69%, Mn-77–100%

Secondary activated sludge Fly ash (MSW) of electrostatic precipitator unit

Less-acidophilic thermophilicand mesophilic-S-oxidizer

Cu-76%, Ni-79.5%, Zn-84.2%, Mn-78.2% in 24 h

Two-stage rector

(i) Mixed culture of A. thiooxidans

Cu, Cd, Zn -80%, Al-60%, Ni, Fe-30% in presence of sewage sludge suspension in 2–3 weeks Cu-57–85%, Cd-53–68.5%, Zn-39–56.4%, Pb-2–19.6% Cu-67%, Zn-78%, Cr and Cd-100% at 10 g/L PD in presence of 2–5 g/L S

Shake flask

Krebs et al. (2001)

Shake flask Shake flask

Tateda et al. (1998) Ishigaki et al. (2005)

Cd, Ni, Zn-60–80% in batch culture with 200 g/L pulp density Cu and Pb-60–70%, Al, Mn, Zn-80–100%, Fe-30% in 2 days at 10 g/L PD in 30 days

Shake flask

Krebs et al. (1997)

Shake flask in one-/ two-step Shake flask

Bosshard et al. (1996), Wu and Ting (2004, 2006)

Fly ash of MSW

(i) A. ferrooxidans (with ferrous sulfate/S addition)

(ii) Thiobacillus sp. strain TM-32 (iii) A. thiooxidans, A. ferrooxidans, (mixed culture of S and iron oxidizing bacterium) (i) Pseudomonas putida (ii) Aspergillus niger

Cd-96%, Mn-91%, Pb-73%, Zn-68%, Cr-35%, Fe-30% from 1% water-washed sample in 20 days

Continuous modebioreactor Bioreactor

Tyagi et al. (1996), Meknassi et al. (2000), Blais et al. (2004), Pathak et al. (2009) Jain et al. (2010)

Wang et al. (2009b)

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Table 6 Bio-processing of other wastes and secondary resources, and miscellaneous processes. Specific type and source of waste/ by-product

Microbe used

Leaching efficiency (%) and conditions

Type of reactors

Reference

Molybdenite containing concentrate/tails

(i) A. ferrooxidans

Mo-1% in 30–40 days from concentrate

Shake flask

Mo 1% in 30 days from concentrate in presence of pyrite

Shake flask

(ii) Mixed culture of A. ferrooxidans and Leptospirillum (i) A. ferrooxidans

Mo-50% after copper leaching first and 85% in 6 months at 40 °C from tails of Cu concentrator

Shake flask

Romano et al. (2001). Askari et al. (2005) Olson and Clark (2008)

Ni-85%, C0–85%, Cu-5% at 50 g/L PD, 1.7 pH and 30 °C

Shake flask

(i) Sulfate reducing bacteria (SRB)

Zn removal from waste solution as ZnS to sub-ppb level

(i) Desulfovibrio sp.

100% Removal of metal sulphides of Fe, Au and Cu by the biogenic H2S gas Effluent treatment by H2S bio-generated from sulphur-acetic acid system and metal removal as sulphides

Bioreactor – commercial scale (1800 m3) Shake flask

Byproduct of a uranium mill – dirty Cu concentrate Waste water of smelting work (zinc and sulfate) Wastewater/process streams/ effluent treatment

Water decontamination for Tc(VII), Cr(VI), Te(IV) and Se(IV)

Nano – Pt recovery in bact. biomass from PGM leachate of secondary source as a biocatalyst

(ii) SRBDesulphuromonas acetoxidans (i) SRB-D. sulphuricans and D. vulgaris (i) Desulfovibrio desulphuricans ATCC 29577 (ii) D. sulphuricans, Escherichia coli MC 4100

Reduction of 90% Tc(VII) as black precipitate of TcO2 (800 lmol Tc4+ g biomass1 h1); Cr(VI) reduction and removal as Cr(OH)3 (63 lmol Cr3+ g biomass1 h1); other metal ions reduced to base metals Producing bacterial hydrogenase to Bio-Pd0 nano-particles as bioinorganic catalyst Cr(VI) reduction to Cr(III) by catalytic action of Mixed metal-bioPd0 (MM-BIO-Pd0)

bacterium in presence of 2–5 g/L S shows almost complete leaching (100%) of Cr and Cd , and high recovery of Cu (67%) and Zn (78%) at 10 g/L pulp density (Ishigaki et al., 2005). Krebs et al. (1997) also applied Pseudomonas putida to treat the fly ash. Due to the formation of citric acid –a product of carbohydrate metabolism at a glucose concentration of 24–42 g/L with a drop in pH to 4.0 within a week, the leaching of metals proceeded through complex formation. The dissolution was found to be in the range 60–80% for most metals up to 20 g/L pulp density which dropped considerably to below 20% at a fly ash concentration of >30 g/L due to carbon limitation and low ratio of citrate to fly ash. In earlier studies, Bosshard et al. (1996) used fully grown A. niger for 2–3 weeks at 100 g/L sucrose concentration producing gluconic acid instead of citric acid which resulted in high recovery of lead (95%) at 40 g/L pulp density along with >80% leaching of Cd and Zn, and 60% Cu. The leaching of nickel and chromium was, however, very low (5–10%). The fly ash from an incinerator in Singapore was leached by A. niger (Wu and Ting, 2004, 2006). The metal leaching was mediated by gluconic acid formed by the fungus due to the presence of manganese and other metals in the material inhibiting the accumulation of citric acid. At 10 g/L pulp density, the dissolution pattern of metals was similar (80–100% Al, Mn, Zn; 60–70% Cu, Pb and 30% Fe) in both one-step and two-step bioleaching processes performed over a period of 30 days. A recent investigation by Wang et al. (2009) demonstrated the enhanced kinetics (20 days) of metal leaching with high metal recovery (96% Cd, 91% Mn, 73% Pb, 68% Zn, along with 35% Cr and 30% Fe) at 10 g/L pulp density in one-step or two-step process by A. niger from pre-treated (water washed) fly ash. Water washing at 10 g/L pulp density removed alkali chlorides (K, Na, Ca, etc.) from the fly ash before it was leached. 2.6. Bio-processing of other secondary resources and wastes 2.6.1. Molybdenite Although the concentrator plant tailings and byproducts of such plants have not been considered in this review, bioleaching of

Bioreactor (Thioteq process) Shake flask

Shake flask

Shake flask

Mehta et al. (2003) Barnes et al., 1991 Bhagat et al. (2004) Huisman et al. (2006) Lloyd et al. (2001)

Macaskie et al. (2005), Murray et al. (2007) Mabbett et al. (2006)

molybdenite is included (Table 6) as a special case because of low microbial dissolution of molybdenum. Generally, molybdenum is quite toxic to Thiobacillus sp. and its solubilization from molybdenite concentrate under the influence of A. ferrooxidans was found to be very low (1%) even after 30 days of leaching (Romano et al., 2001; Askari et al., 2005). However, leaching of 50% Mo from a tailing sample containing molybdenite and pyrite in 30 days was recently achieved only when very strong oxidizing condition at Eh above 750–800 mV (SHE) was maintained by amending the system with ferrous sulphate (6 g/L) using a mixed culture of A. ferrooxidans and Leptospirillum sp. at 1.4–1.6 pH and 40 °C (Olson and Clark, 2008). The leaching rate was found be high (4.91% per day) when size of the concentrate was reduced to 2.9 lm. Molybdenum recovery approached to as high as 85% in 6 months under this condition. Interestingly, molybdenum dissolution was almost nil until complete (100%) copper was leached with the thermophilic bacteria.

2.6.2. Copper containing byproduct of a uranium mill The bio-processing approach could be utilized for treating an Indian uranium mill byproduct such as a dirty copper concentrate which was rejected by the copper smelters due to high nickel content. The concentrate typically contained 16% Cu, 8% Ni, and 0.2% Co, and had pentlandite and chalcopyrite as the main nickel and copper bearing minerals, respectively. When the material was leached at 50 g/L pulp density, 30 °C and 1.7 pH using A. ferrooxidans it was possible to selectively dissolve above 85% Ni and Co with hardly 5% leaching of copper (Table 6) in 30 days on bench scale (Mehta et al., 2003). The galvanic interaction between pentlandite and chalcopyrite minerals, the latter being cathodically protected, played a major role in the selective bio-dissolution of nickel and cobalt over copper. The leach liquor containing Ni, Co and Cu could be treated for metal separation by solvent extraction and subsequent recovery in desired form. The chalcopyrite in the leach residue remained unaffected with less than 1.2% Ni. The material could be further bioleached to recover copper using the Thobacillus sp. or

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it could be transported as an acceptable feed to a normal copper smelter. 2.6.3. Waste waters/streams/effluents The wastewaters of metallurgical and process industries contain various heavy and toxic metals which are removed mostly as sludge through precipitation process. Because of reusability, low volume and effluent quality, the precipitation of metals as sulphides is considered superior than that of hydroxides (Lloyd et al., 2001; Poulin and Lawrence, 1996). Through biotechnological approach the precipitation of metals as sulphides to decontaminate the waste waters/solutions/effluents has assumed great importance (Barnes et al., 1991; Bhagat et al., 2004; Huisman et al., 2006). Biological treatment essentially consists of reduction of sulphur oxyanions (SO2 4 ) to sulphide by a sulphate reducing bacteria (SRB) followed by chemical precipitation of metal sulphides. Typical conversion reactions given below (Bhagat et al., 2004; Huisman et al., 2006) show that electrons are transferred from the organic compound (say acetic acid) to sulphate to produce hydrogen sulphide which reacts with divalent metal (M) ions to form metal sulphides:  þ þ SO2 4 þ CH3 COOH þ 2H ! H2 S þ 2HCO3 þ 2H

ð10Þ

M2þ þ H2 S þ 2HCO3 ! MS þ 2H2 O þ 2CO2

ð11Þ

This concept was first ever commercially exploited in an 1800 m3 concrete bioreactor using an undefined SRB for removal of zinc to a sub-ppb level from the waste water containing zinc and sulphate of a zinc smelter at Budel–Dorplein, Netherland (Barnes et al., 1991). Similarly, removal of metal sulphides from the synthetic waste water was reported by Bhagat et al. (2004) precipitating almost 100% Fe, Au and Cu as sulphides by the biogenic H2S produced (0.6 g/L sulphide) by Desulfovibrio sp. in a bioreactor and mixing with the model solution. SRB for effluent treatment under the Paques technology has been used world over with 500 installations for conversion of broad range of organic and inorganic compounds to metal sulphides (Huisman et al., 2006). Recovery of base metals as sulphides, and the combined removal of sulphate, nitrate and heavy metals, selenium and fluoride by applying biogenic sulphide and diffusiondialysis sequentially, are the examples in metal industries. Thioteq technology was developed later on by modifying the Paques technology for copper bleed solution with high acid content (Huisman et al., 2006). In this process biogenic H2S is produced in a separate reactor using a SRB such as Desulphuromonas acetoxidans in presence of sulphur followed by passing H2S with a carrier gas and the waste water in a chemical contactor (second stage) while neutralizing the solution; MS is then recovered as a product in a clarifier. Lloyd et al. (2001) reported the possible decontamination of waste water (Table 6) containing Tc (VII), Cr (VI), Te(IV) and Se(IV) by using SRBs, D. sulfuricans and D. vulgaris. Reduction of Tc (VII) and Cr(VI) to their lower valence states and precipitation as TcO2 and Cr(OH)3 was confirmed by EXAFS while Se and Te were reduced to the metallic state. 2.6.4. Leachate of secondary material As discussed above several microorganisms are capable of reducing toxic metals to their less harmful states by a variety of mechanism including hydrogenase activity (Lloyd et al., 2001). Metal reductase activity can be a potential tool to reduce and recover precious metals from the wastes at the expense of hydrogen. Murray et al. (2007) reported the treatment of a platinum group metals (PGMs) containing leachate from a secondary resource (Table 6) produced in microwave leaching using Escherichia coli (MC 4100) pre-metallised with Pt. Pd(II) was reduced to Pd0 (Macaskie et al., 2005) by Desulfovibrio desulfuricans (ATCC 29577) which was held

15

as bio-templated and supported metallic nano-particles on the cell surface. The microbially reduced Pd0 (Bio-Pd0) was found to have high catalytic activity comparable with a commercial supported metal catalyst in its ability to reduce Cr(VI) to less toxic form, Cr (III) at the expense of hydrogen. Further studies by Mabbet et al. (2006) show the removal of Pd and Pt from a mixed metal (MM) leachate of an industrial waste to prepare a bio-catalyst such as MM-bio-Pd0 using D. sulfuricans. The bio-inorganic catalyst was very effective in reducing 1 mM Cr (VI) solution to Cr (III) in a flow-through reactor and remained active for long time (3 months) as compared to the chemically reduced Pd0 catalyst which was effective for one week only.

3. Conclusion Processing of wastes and byproducts assumes great importance in view of the presence of several metals of economic interest which are at times richer than those of the available natural reserves (ores/minerals). Recycling of such resources is critical not only to supplement the secured supply of metals and materials thereby reducing the load on the limited natural/mineral resources, but also to reduce the environmental degradation due to disposal. With the success of bio-hydrometallurgical processes for copper, gold and uranium on large scale by heap/dump and bioreactor (STR/CSTR) leaching, extraction of valuable metals from the secondary resources with the aid of microbial action has gained attention in the past few decades. Considerable efforts have been made to develop methodologies for bioleaching of wastes and byproducts generated from the metallurgical and industrial processes, and the man-made resources. As many of these solid wastes are in the form of non-sulphides and at times in the metallic forms, the metals from these can be solubilized by either autoptrophic or heterotrophic microorganisms while providing an energy source. The current level of development is mostly confined to use the known microbial systems for the metal leaching largely on bench scale (shake flask experiments) and only in some cases column/ bioreactor leaching at the laboratory scale has been attempted. Excepting the waste water treatment in certain cases using sulphate reducing bacteria to produce metal sulphides, none of the processes investigated for the bioprocessing of such secondary resources has reached to the level of commercial production. As such, the bioleaching of the waste materials has obvious shortcomings like requirements of long time periods, very low pulp densities in tank reactor systems and low efficiency levels of extractions. These are all significant issues for developing a practical scale process for commercial scale production. Therefore, it is urgently required to address these issues, and pursue the activities on large/pilot scale to derive design data for further scale up and prove the techno-economic feasibility for eventual exploitation. Besides the use of known microbes, it is also desired to isolate new and genetically improve the microbes isolated from the dump/waste site, particularly those which can be used at a temperature of 50 °C and above with higher metal tolerance and better performance on industrial scale. There could be two-pronged strategies of such developments, one to recover metal values where the economics work out well for metallurgical production and second for the remediation of such wastes/secondary resources with the composite aim of managing/reducing environmental degradation along with the resource recovery to compensate for the economic return. To improve the kinetics and overall performance of metal extraction, use of hybrid processes consisting of biological and chemical approach may be considered. The metallurgical/industrial wastes and secondary materials with high or acceptable metal contents such as smelter flue dust of copper/other smelting operations and residues, spent

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catalysts of HDS, used batteries, e-wastes/waste PCBs, electroplating and tanning sludge, sewage sludge, etc. appear to have high potential for bio-processing in eco-friendly manner. Other wastes described above with low metal values may eventually be treated to address the problem of environmental degradation in conjunction with metal recovery. Although, very limited researches have been carried out to recover high value materials particularly nano-particles using microbes, the area needs to be intensely pursued to realize the potential of material synthesis. This may prove beneficial to treat such low metal containing secondary resources which have high values like PGM, Co, Ni, etc. and the conventional process is not justified for the production in metallic state. Acknowledgement The authors are thankful to the Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon for providing the necessary facilities. B.D. Pandey is also thankful to KOFST, Korea for financial assistance under the Brain Pool invited scientist scheme. References Agrawal, A., Kumar, V., Pandey, B.D., 2006. Remediation options for the treatment of electroplating and leather tanning effluent containing chromium – A review. Miner. Process. Extr. M. Rev. 27 (2), 99–130. Agrawal, A., Sahu, K.K., Pandey, B.D., 2004. Solid waste management in non-ferrous industries in India. Res. Conserv. Rec. 42, 99–120. Askari, M.A., Hiroyoshi, N., Tsunekawa, M., Vaghar, R., Oliazadeh, M., 2005. Rhenium extraction in bioleaching of Sarcheshmeh molybdenite concentrate. Hydrometallurgy 80, 23–31. Aung, K.M.M., Ting, Y.P., 2005. Bioleaching of spent fluid cracking catalysts using Aspergillus niger. J. Biotechnol. 116, 159–170. Babel, S., Dacera, D.D.M., 2006. Heavy metal removal from contaminated sludge for land application: a review. Waste Manage. 26, 988–1004. Bakhtiari, F., Atashi, H., Zivdar, M., Bagheri, S.A.S., 2008a. Continuous copper recovery from a smelter’s dust in stirred tank reactors. Int. J. Miner. Process 86, 50–57. Bakhtiari, F., Zivdar, M., Atashi, H., Bagheri, S.A.S., 2008b. Bioleaching of copper from smelter dust in a series of airlift bioreactors. Hydrometallurgy 90, 40–45. Banerjee, D., 2007. Metal recovery from blast furnace sludge and flue dust using leaching technologies. Res. J. Chem. Environ. 11, 18–21. Barnes, L.J., Janssen, F.J., Sherren, J., Versteegh, J.H., Koch, R.O., Schreeren, P.J.H., 1991. A new process for the microbial removal of sulphate and heavy metal from contaminated water extracted by geohydrological control system. Chem. Eng. Res. Des. 69A, 184–186. Bayat, B., Sari, B., 2010. Comparative evaluation of microbial and chemical leaching processes for heavy metal removal from dewatered metal plating sludge. J. Hazard. Mater. 174, 763–769. Bhagat, M., Burgess, J., Antunes, P.M., Whiteley, C.G., Duncan, J.R., 2004. Precipitation of metal residues from wastewater using biogenic sulphide. Miner. Eng. 17, 925–932. Biswas, A.K., Davenport, W.G., 1976. Extractive metallurgy of copper, 1st ed. Pergamon Press, New York. Blais, J.F., Meunier, N., Mercier, G., Drogui, P., Tyagi, J.F., 2004. Pilot plant study of simultaneous sewage sludge digestion and metal leaching. J. Environ. Eng. 130, 516–525. Blaustein, B., Hauck, J.T., Olsen, G.J., Baltrus, J.P., 1993. Bioleaching of molybdenum from coal liquefaction catalyst residues. Fuel 72, 1613–1618. Bojinova, D.Y., Velkova, R.G., 2001. Bioleaching of metals from mineral waste product. Acta Biotechnol. 21, 275–282. Bosecker, K., 1986. Microbial recycling of mineral waste products. Acta Biotechnol. 7, 487–496. Bosecker, K., 2001. Microbial leaching in environmental clean-up programmes. Hydrometallurgy 59, 245–248. Bosshard, P.P., Bachofen, R., Brandl, H., 1996. Metal leaching of fly ash from municipal waste incineration by Aspergillus niger. Environ. Sci. Technol. 30, 3066–3070. Brandl, H., 2001. Microbial leaching of metals. In: Rehm, H.J., Reed, G. (Eds.), Biotechnology, Vol. 10, 191–224. Special processes, Wiley-VCH, Weinheim, Germany. Brandl, H., 2002. Metal-microbe interactions and their applications for mineral waste treatment. Recent Res. Dev. Microbiol. 6, 571–584. Brandl, H., Bosshard, M., Wegmann, M., 2001. Computer munching microbes: metal leaching from electronic scrap by bacteria and fungi. Hydrometallurgy 59, 319– 326. Brandl, H., Faramarzi, M.A., 2006. Microbe-metal-interactions for the biotechnological treatment of metal-containing solid waste. China Particuology 4 (2), 93–97.

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