Sethurajan ET AL., 2012

Environmental Engineering and Management Journal October 2012, Vol.11, No. 10, 1839-1848

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

BIOLEACHING OF COPPER FROM BLACK SHALE ORE

USING MESOPHILIC MIXED POPULATIONS

IN AN AIR UP-LIFT BIOREACTOR

Manivannan Sethurajan1, Rajasekar Aruliah1, Obulisamy Parthiba Karthikeyan1, Rajasekhar Balasubramanian1,2

1Minerals, Metals and Materials Technology Centre

2Department of Civil and Environmental Engineering, National University of Singapore, 4 Engineering Drive, Singapore 117576

Abstract Black shale (BS) deposits found in Europe (Kupferschiefer type) contain base (e.g., copper and aluminium) and precious (e.g., gold and silver) metals and also PGE (platinum group elements), but also high contents of organic matter that potentially impede with metal recovery by conventional techniques. The aim of the present investigation was to recover copper (Cu) from BS using microbial leaching. BS deposits were collected from a mining site in Lubin, Poland which consisted of 14 ± 2 mg/g of Al, 13 ± 2 mg/g of Cu, 10 ± 1 mg/g of Fe and relatively significant amounts of Mn and Zn. Several chemolithotrophic bacteria (Leptospirillum ferrooxidans, Leptospirillum ferriphilum, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Sulfobacillus thermosulfidooxidans) were evaluated for their ability to extract Cu selectively from BS with a 2% pulp density. Among these bacteria, the iron oxidizing L. ferrooxidans, L. ferriphilum, At. ferrooxidans were identified as the potential candidates, and their Cu extraction efficiencies were 62% ± 3%, 55% ± 3%, 25% ± 2%, respectively. Using a mixed population of these microorganisms, the bioleaching process was performed at different pulp densities of BS and their corresponding Cu leaching efficiency was evaluated. Following the batch experiments, the bioleaching process was scaled-up to a 5 liter laboratory scale custom-designed Air Up-lift Bioreactor (AUBR). The Cu extraction could be increased to > 70% at a higher pulp density (10% wt/v) under the experimental conditions set at pH 2.0 and 37oC with the aeration rate of 150 liter per hour for 15 days of continuous bioleaching operation. Key words: acidophiles; air up-lift bioreactor (AUBR); bioleaching; black shale; mixed consortia Received: April, 2012; Revised final: September, 2012; Accepted: September, 2012

Author to whom all correspondence should be addressed: [email protected]; Phone: (65) 6516-5135; Fax: (65) 6779-1635

1. Introduction The use of bioleaching for metal extraction

from sulfidic mineral ores by chemolithotrophic bacteria such as Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans has been extensively studied for many years (Anjum et al., 2012; Olsen et al., 2003; Rawlings, 2002, 2004, 2005; Tributsch, 2001; Spolaore et al., 2009). Bioleaching can be employed effectively and economically to high grade

ores, low grade ores and also slag wastes from mining industries for the solubilization and selective extraction of base (Cu and Fe) and precious metals (Au and Ag) (Chi et al., 2011; Frı́as et al., 2002; Rawlings, 2002). In addition, bioleaching processes involve low energy consumption and do not have significant adverse environmental impacts when compared to those of the conventional chemical leaching (Olson et al., 2003; Calmoi and Cecal 2007). In view of the depletion of mineral ores (high and low

Sethurajan et al./Environmental Engineering and Management Journal 11 (2012), 10, 1839-1848

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grade) and other major metal reserves, the current focus of bioleaching has now been shifted to BS and brown shale (BrS) deposits and metal schists for the extraction of base and precious metals (Anjum et al., 2012; d'Hugues et al., 2008; d'Hugues et al., 2007a; Rawlings, 2004). The BS deposits are exceptionally rich in organic matter and sulfides, and contain unusually high concentrations of certain trace elements, base and valuable metals (Oszczepalski, 1996). However, their specific chemical composition depends on the nature of the mining site. Therefore, selective extraction of metals of interest from the complex matrix of BS remains an analytical challenge.

Selection of suitable microorganisms is important for effective metal recovery from mineral ores. The commonly used bacteria for the extraction of copper and other metals from mineral ores are iron- and sulfur-oxidizing Acidithiobacillus ferrooxidans, sulfur-oxidizing Acidithiobacillus thiooxidans and Acidithiobacillus caldus, and iron-oxidizing Leptospirillum ferriphilum and Leptospirillum ferrooxians (Coram and Rawlings, 2002; Fouchera et al., 2003; Okibe et al., 2003; Rawlings, 2005). Bioleaching of mineral ores using microorganisms occurs via their disintegration as induced by (i) acids, (ii) alkali, (iii) inorganic complexes, and (iv) redox reactions (Bosshard et al., 1996; Schinner and Burgstaller, 1989; Brandl H., 2001, Wu and Ting 2006).

Mixed cultures of these bacteria showed successful bioleaching of mineral ores and also proved to be more effective compared to the use of pure strains in many instances, due to a synergistic combination of these mechanisms (Bakhtiari et al., 2011; Fu et al., 2008; Sandstrom and Petersson, 1997). Based on the specific bacteria selected for the mixed consortia, the bioleaching process would be either mesophilic (10 to 37oC, optimum 30 to 35oC), or thermophilic (40 to 80 oC). Bioleaching operations with thermophilic bacteria are relatively faster as compared to those with the mesophilic bacteria, but the higher energy utilization increases the overall operational cost in the case of the former one (Woranart, 2003).

In recent years, different bioreactors such as the stirred tank reactor (STR) (Spolaore et al., 2009) and the airlift bioreactor (AUBR) (Chen and Lin, 2004; Cozma and Gavrilescu, 2010; Giaveno et al., 2007; Shi and Fang., 2005) have been explored to achieve a homogenous reacting mass and control the main process variables effectively so that a high yield of metals can be obtained from the bioleaching of mineral ores. The aerated STR configurations are most widely used in the mining industry, but they are very expensive because of their complex design and operations (Bakhtiari et al., 2008). As a result, their use is limited only to high grade ores and high metal concentrates (Rawlings, 2002).

The AUBR configuration is relatively simple, requires low power input, and provides uniform mixing, effective heat and mass transfer and most

importantly negligible shear stress (Chisti, 1989; Chisti and Yong 1987). However the use of AUBRs in metal extraction industries is still limited as rigorous laboratory testing and pilot scale development are required prior to large-scale application.

The objective of the present study is to investigate the possibility of extracting Cu, a representative member of base metals, from the polymetallic BS, collected from a local mining site, in Lubin, Poland, by chemolithotrophic bacteria.

The BS samples used in this study are polymetallic and rich in Cu and Al, but also contain organic carbonates and sulfides which could potentially interfere with the extraction of Cu. The bioleaching process was carried out by individual bacterial strains (Leptospirillum ferrooxidans, Leptospirillum ferriphilum, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Sulfobacillus thermosulfidooxidans) as well as by their mixed consortia. The effect of pulp density of BS on the performance of the mixed consortia was studied. By maintaining the optimum pulp density of the BS, the bioleaching process was further investigated using an AUBR for selective extraction of Cu in the presence of other metals in the BS matrix.

2. Materials and methods

2.1. Commercial isolates and growth conditions

Pure cultures of mesophilic chemolithotrophic

bacteria, namely, Leptospirillum ferrooxidans (2391), Leptospirillum ferriphilum (14647), Acidithiobacillus ferrooxidans (11477), Acidithiobacillus thiooxidans (11478), and Sulfobacillus thermosulfidooxidans (9293) were purchased from the German Collection of Microorganisms and Cell Cultures (DSMZ), Germany. Pure cultures were initially grown in their respective growth media under optimum pH and temperature conditions as per the DSMZ’s recommendation (http://www.dsmz.de/catalogues/ catalogue-microorganisms/culture-technology/list-of-media-for-microorganisms.html). All the cultures were then adapted to 9K medium (Silverman and Lundgren, 1959; Olubambi et al., 2009) which consisted of (grams per liter), 3.0 g of (NH4)2SO4, 0.5 g of K2HPO4, 0.5 g of MgSO4•7H2O, 0.1 g of KCl, 0.01g of Ca(NO3)2, and 44.2 g of FeSO4•7H2O. The pH was adjusted to 2.0 with 18 M H2SO4. The culture was initially enriched in 1% and then with 2% (wt/v) BS in the 9K medium.

The cultures were again sub-cultured through several transfers in the mineral medium in order to adapt the bacteria to the experimental conditions. The stock cultures were sub-cultured at two-week intervals. Cell growth and total viable counts were measured by epi-fluorescence microscopy (Nikon eclipse Ci E-Ni L) (by acridine orange dye stained direct counting).

Bioleaching of copper from black shale ore using mesophilic mixed populations in an air up-lift bioreactor

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2.2. Chemical characterization of black shale The BS samples were collected from a local

copper mining site in Lubin, Poland. The samples were ground to ensure that the particle size was less than 2 mm for this study. For the total metal extraction and analysis, a modified method of Yafa and Farmer (2006) was used. To the BS sample (1.0 g) taken in a 100 mL Erlenmeyer flask, 9 mL of HCl and 3 mL of HNO3 (3:1) were added, heated to a temperature of 100 oC for 2 hours, covered it with a watch glass and then allowed to cool at room temperature for 2 hours. Then, 20 mL of 2% v/v HNO3 was added on the sides of the conical flask and the solid residues were separated by filtration through Whatman (No 1) filter paper. Then the filtrate was made up to 100 mL by deionized water.

The final solution was used for elemental analysis by inductively coupled plasma–optical emission spectroscopy (ICP-OES, USEPA SW-846 Method). Reference material for copper ore (331a) was obtained from the National Institute of Standards and Technology (NIST, MD, USA) and used as an internal standard for acid leaching experiment to calculate the recovery efficiency. Scanning electron microscopy- energy dispersive X ray spectroscopy (SEM-EDAX, JOEL JSM-5600LV), X-Ray diffraction (XRD, JEOL Model JDX—8030) were used for the characterization of BS. Fourier transform infrared spectroscopy (FTIR, Shimadzu corporation) analysis was also carried out. The distribution of metals in the BS sample is shown in Table 1.

Table. 1. Chemical characterization of selected metals per

gram of black shale

Metals Mass fraction (mg g-1) Aluminium 14 ± 2

Copper 13 ± 2 Iron 10 ± 1 Zinc 3.5 ± 1

Manganese 1.5 ± 0.5 Vanadium 0.4 ± 0.2

Cobalt 0.2 ± 0.2 Nickel 0.2 ± 0.1

2.3. Bioleaching experiments on black shale for copper extraction

2.3.1. Pure cultures studies

Bioleaching experiments were carried out in 250 mL Erlenmeyer flasks, each containing 2% (wt/v) BS in 100 mL of the 9K medium and 2% v/v inoculum of pre-adapted culture of the individual bacterial strains (1 X 105 cells/mL), on a rotary shaker at 180 rpm at 37 oC for 15 days.

The initial pH of the bioleaching medium was adjusted to 2.0 using 1 M H2SO4. Abiotic control was also maintained with the addition of 2% thymol (in 4% methanol) in 9K medium with 2% BS to understand the medium effect on bioleaching (Bakhtiari et al., 2011).

2.3.2. Effect of mixed consortia and pulp density Bioleaching experiments were carried out at

37oC, pH 2.0 and 150 rpm with the inocula containing a mixed population of L. ferrooxidans, L. ferriphilum, At. ferrooxidans in the ratio of 1:1:1. The initial concentration of the biomass of the mixed culture was approximately 1 x 105 cells/mL. Cell numbers were monitored during the bioleaching experiments using a Thoma counting chamber and an epi-fluorescence microscopy. Culture samples were collected in a microcentrifuge tube, vortexed for a short time and then centrifuged at low speed (3000 rpm for 5 min) to remove BS particles prior to counting. The mixed population used for the bioleaching studies was previously sub-cultured several times with 1% of BS and later by 2% BS as the sole energy source. Various pulp densities i.e. 2, 5, 7.5 and 10% of BS with the 2 % v/v mixed consortia were performed to understand the bioleaching efficiency for Cu under shake flask conditions which were then optimized for bioreactor studies.

2.3.3. Bioreactor experiments for copper bioleaching from black shale

Scale-up experiments were carried out in 5 L AUBR (Winpact, USA) in a batch regime for 15 days of continuous operation. 4.95 L of the 9K medium was added to the reactors and 50 mL (approximately 1 X 105 cells/mL in the total working volume) of the enriched mixed culture was transferred aseptically to the reactor.

The process temperature was maintained at 370C while a constant pH of 2.0 was maintained with the addition of concentrated H2SO4. The oxygen supply was maintained at 150 L/hour, and was pumped from the bottom to ensure proper mixing. Sterile distilled water was added to the reactor through a peristaltic pump in order to compensate for evaporation losses. 10% of BS was used as a substrate. Liquid samples were withdrawn on a regular basis to analyze cell growth and metal recovery. After the batch run, the BS samples were collected from the reactors, fixed in 0.2% glutaraldehyde in 0.25 mM sodium cacodylate buffer, dehydrated by increasing concentrations of ethanol to ensure proper fixation and were again studied using SEM-EDAX and FT-IR.

3. Results and discussion

3.1. Mineralogy and characterization of Black shale

Table 1 shows that the BS sample considered

in this study is a polymetallic deposit, which contains Cu (13 ± 2 mg/g), Al (14 ± 2 mg/g and Fe (10 ± 1 mg/g). It also contains small amounts of Zn (3.5 ± 1 mg/g), Mn (1.5 ± 0.5 mg/g) and trace amounts of V (0.4 ± 0.2 mg/g), Co (0.2 ± 0.2 mg/g) and Ni (0.2 ± 0.1 mg/g). Occurrences of rare earth elements (REE) and PGE were also reported in Kupferschiefer type BS obtained in Rote FaÈule near the Lubin mining district, Poland (Bechtel et al., 2001).


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Uraniferrous BS was found in China, which contained more than 20 mg/g of uranium and its mineralization was demonstrated by Qiuying (2000). The BS found in Estonia was found to contain uranium, molybdenum and vanadium at higher concentrations (Tasa et al., 1997). It is thus clear that the composition of BS depends on the location and nature of the mining site.

The XRD profile (Fig. 1) reveals the presence of chalcopyrite, pyrite, tennantite, troilite, franklinite, vallerite, Quartz, muscovites and dolomites in BS (Gouin et al., 2007). The BS morphological characteristics were analyzed by SEM (Fig. 2a and 2b), and the surface of the BS was found to be smooth and polished after 15 days of bioleaching. Changes in the functional groups of BS before and after bioleaching are shown in Fig. 3.

Compared with the raw BS FT-IR spectrum, some peaks became less intense (3650 cm-1 (O-H stretch, free), 2900 cm-1 (C-H stretch), 2500 cm-1 (O-H stretch), 750 cm-1(aromatic hydrocarbons)) while new small characteristic peaks (3400 cm-1 (N-H

stretch), 2250 cm-1 (C-N stretch), 1700 cm-1 (C-O stretch), 1100 cm-1 (alcohols, ethers, acids, and esters stretch), 750 cm-1 (aromatic hydrocarbons)) appeared in bioleached samples. These spectral features reveal that the bacteria were indeed adsorbed onto BS. Chen et al. (2008) also reported that At. ferrooxidans and At. caldus follow similar patterns on chalcopyrite surface, suggesting enhancement in its bioleaching efficiency for the extraction of copper.

3.2. Bioleaching Experiments

3.2.1. Pure culture experiments

Bioleaching efficiency of pure commercial strains (L. ferrooxidans, L. ferriphilum, At. ferrooxidans, At. thiooxidans, and S. thermosulfidooxidans), as observed at 37 oC, 150 rpm and pH at 2.0, is shown in Fig. 4a. The pure cultures of iron-oxidizers, namely L. ferrooxidans and L. ferriphilum, extracted 156 mg/L of Cu (62% per gram of BS used in the medium) and 136 mg/L (54%), respectively.

Fig. 1. X-Ray Diffraction spectrum of BS before bioleaching

Fig. 2. Scanning electron micrographs (SEM) of BS ore (a) before bioleaching and (b) after bioleaching


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Fig. 3. Fourier transform infrared (FTIR) spectrum of black shale before bioleaching (RBS, raw black shale), and after bioleaching (M2, 2% pulp density of BS bioleached by mixed consortia) and M10,

10% pulp density of BS bioleached by mixed consortia)

The other iron-oxidizing bacterium, At. ferrooxidans, and mixotrophic bacterium, S. thermosulfidooxidans, were able to extract 64 mg/L (25%) and 37 mg/L (15%), respectively. Farbiszewska-Kiczma et al. (2004) studied the bioleaching of BS for the extraction of Cu by heterotrophic bacteria (Bacillus spp) in a neutral medium, but the bioleaching process was not effective even after 24 days of operation; the maximum concentration of Cu that resulted from bioleaching was 90 mg/L. The leaching efficiency of heterotrophic bacteria (e.g. Bacillus spp) was found to be less than that of chemolithotrophic bacteria in an organic carbon rich BS. The preliminary work conducted in this study with the use of heterotrophic bacteria, indicated that as much as 60% (data not shown) of the organic carbon (Total organic carbon content 8%-12% per gram) could be degraded. Similar research findings have already been reported in the literature (Matlakowska and Sklodowska, 2009 and 2011).

The bioleaching efficiency of sulfur oxidizer, At. thiooxidans, was very low, which might be due to the presence of the inorganic sulfur moieties in the BS samples that were not readily accessible for its growth. The modified 9K medium which contains an initial concentration of 8 g/L of inorganic sulfur favours the growth of At. thiooxidans in the presence of BS (data not shown) than the sulfur deficient one, the 9K medium (Silverman and Lundgren, 1959) (Fig. 4a). S. thermosulfidooxidans is a thermophilic mixotrophic bacterium which requires yeast extract (as suggested by DSMZ) and high temperature (optimum temperature 50oC) for its growth. The lack of these favourable conditions might be responsible for the lower bioleaching efficiency of S. thermosulfidooxidans. All strains showed either constant cell numbers or a significant increase in cell

numbers throughout the 15 day incubation period (Fig. 4b). Cell numbers of L. ferrooxidans increased rapidly initially before levelling off, followed by L. ferriphilum, At. ferrooxidans, S. thermosulfidooxidans and At. thiooxidans. Among the five strains tested, L. ferrooxidans grew particularly well, increasing in number to > 4 x 107 cells/mL after 170 h. Of all the strains tested, L. ferrooxidans and L. ferriphilum were observed to be well adapted to the BS. From Fig. 4a, it is clear that the pure cultures of iron oxidizing bacteria recovered a considerable amount of Cu from the BS samples. Hence, we used the mixed cultures of the L. ferrooxidans, L. ferriphilum and At. ferrooxidans for improving the bioleaching of Cu from BS. 3.2.2. Effect of mixed culture of acidpophiles on black shale bioleaching

Fig. 5a shows that the copper recovery increased (86 %) considerably with the use of the mixed population of L. ferrooxidans, L. ferriphilum and At. ferrooxidans. After 15 days of incubation, the final concentration of Cu in the aqueous phase was found to be 219 mg/L (86 %), which was relatively greater than what was achieved in the pure culture bioleaching experiments. Cell numbers in the mixed microbial population in BS are presented in Fig. 5b., which were found to be greater than 107 cells/mL throughout the incubation period. As shown in Fig. 5b, after repeated sub-culturing, the cells were able to grow well in the presence the BS. The ferrous ion/ferric ion concentration in the medium plays an important role in the bioleaching efficiency which was confirmed from the experiments that used iron deficient medium (0K medium, Munoz et al., 2007). The metal recovery in the iron deficient medium was negligible when compared with the iron rich (44.2 g/L) 9K medium (data not shown).


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(a) (b)

Fig. 4. (a) Copper extraction profiles from 2% pulp density of BS by the pure acidophilic strains (Leptospirillum ferrooxidans, Leptospirillum ferriphilum, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Sulfobacillus

thermosulfidooxidans); (b) Growth curve of pure cultures of mesophilic bacterial strains with their respective growth medium on 2% wt/v Lubin black shale at pH 2.0, incubated at 37oC

(a) (b)

Fig. 5. (a) Bioleaching efficiency for copper by mixed population of L. ferrooxidans, L. ferriphilum and At. Ferrooxidans from 2% pulp density of the BS ore, (b) Growth curve of mixed consortia on 2% wt/v Lubin black shale in 9 K medium

at pH 2.0, incubated at 37o C. Averaged data are shown with errors representing standard deviations

It was clearly evident that even though the BS was rich in ‘Fe’ (XRD spectrum); it was not easily available for the bacteria for its metabolism. The bioleaching processes might follow the sequential reactions depicted below. The acidic pH of the bioleaching medium was maintained by sulfuric acid production by acidiophilic bacteria such as At. ferrooxidans, as shown in the reaction 4. The regeneration of the ferric ion and sulfuric acid keeps the environment in acidic conditions.

2 3

2 22 1/ 2 2 2 1/ 2 Fe O H Fe H O

(1)

3 22 2 2 4 4 15 2 4 8 4 FeS O H O Fe SO H

(2)

3 2 2 02 4 5 2 CuFeS Fe Cu Fe S (3)

º2 2 2 42 3 2 2 S O H O H SO (4)

3.2.3 Effect of pulp density in black shale bioleaching

A comparison of BS leaching by the mixed population (9K medium) with different pulp densities (2%, 5%, 7.5% and 10%) is shown in Fig. 6.

The results showed that the mixed cultures of L. ferrooxidans, L. ferriphilum and At. ferrooxidans with different pulp densities leached Cu in the range of 75% to 85% within 15 days of incubation. As expected, the highest liquid-solid phase ratio i.e. 10% pulp density gave the lowest extraction efficiency of Cu (74%) compared to those observed at lower pulp densities, in accordance with the findings reported by Zhou et al. (2009) and Guo et al. (2010) on the Pb/Zn smelting slag. However, the extraction efficiencies observed with 5% pulp density and 7.5% pulp density were not significantly different (at confidence level 95%) from that of the 2% pulp density.


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Fig. 6. Comparison of percentage of copper extraction with different pulp densities (2%, 5%, 7.5% and 10%) by the mixed bacterial consortia containing L. ferrooxidans,

L. ferriphilum and At. Ferrooxidans

The decrease in the Cu recovery to 74% with

10% pulp density might be attributed to the bacterial activity inhibition (Mousavi et al., 2005), bacterial cell disruption and its impaired metabolic activity (Nemati et al., 2000) and also the deposition of high metal (Cu and Al) concentrations as well as the distribution of toxic metals such as Zn and Cr within the system. For large-scale bioleaching operations, higher pulp densities are preferred in order to get higher yields of desired metals within a short span of time. Hence, we investigated the efficiency of Cu extraction from BS samples in a laboratory scale bioreactor, (5L AUBR) with high pulp density (10% wt/v).

3.3. Bioleaching studies in AUBR

Copper recovery from BS samples as a

function of time is shown in Fig. 7, and the leaching ability of mixed cultures of other base metals (Al, Zn and Mn) from BS is presented in Fig. 8. Fig 7 shows that the copper solubilization from BS by the mixed consortia in AUBR (71%, 900 mg/L) was relatively

comparable to that of the shake flask test (74%, Fig. 6). During the initial 3 - 4 days of bioleaching in AUBR, the bacterial population adapts in the lag phase and consequently the rate of copper recovery was low. After adaptation, the copper solubilization was in the exponential phase from 4 to 9 days, and the subsequent leaching rate was relatively slow, which might be due to the passive jarosite layer formed on the BS surface.

Jarosite is a passivation layer (mainly hydroxy iron sulfate complexes) formed due to the excess production of ferric ions and precipitates on the surface of BS, and subsequently inhibits the dissolution of BS, leading to low copper leaching rate (Acevedo, 2000; Zhang et al., 2008; Zhou et al., 2009). At the 15th day, the final soluble copper concentration in the aqueous phase was about 900 mg/L in the bioreactor.

During the lag phase, H2SO4 was added continuously to maintain the pH below 2.0 to neutralize the dissolution of carbonates contained in the BS, which would otherwise increase the pH and reduce the bioleaching of Cu.

Fig. 7. Copper extraction from BS (10% pulp density) by mixed bacterial consortia containing L. ferrooxidans, L. ferriphilum and

At. ferrooxidans in air up-lift bioreactor (AUBR)


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Fig. 8. Percentage of extraction of other metals (aluminium (Al), zinc (Zn) and manganese (Mn) by mixed bacterial consortia from 10% pulp density of BS in an air up-lift bioreactor (AUBR)

However, in the later stages of bioleaching the

increase in pH, caused by the carbonated dissolution, was neutralized by the release of sulfuric acid from At. ferrooxidans (Gomez et al., 1997) and also by the conversion of Fe2+ to Fe3+ ions (Sandstrom and Petersson, 1997). From Fig. 8, the leaching ability of the mixed culture towards the other base metals is also quite significant; about 89% of Mn, 60% of Al and 54% of Zn were leached. Anjum et al. (2011) and Nouren et al. (2011) studied the extraction of several base metals by fungi from BS and BrS, respectively. Some of the base metals such as Zn (Anjum et al., 2011) and Al (Nauren et al., 2011) were extracted with higher efficiency (> 80%), but the leachability of metals is highly dependent on the chemical composition of the shales used and the incubation period maintained during bioleaching experiments.

4. Conclusions

Lubin BS deposits consist of valuable metals

such as Cu. Pure culture experiments using chemolithotrophic bacteria suggested that L. ferrooxidans, L. ferriphilum and At. ferrooxidans are the potential bacterial species for selective extraction of Cu from the complex matrix contained in the BS. With the mixed population of these bacterial strains, more than 70% of Cu can be extracted with the AUBR configuration. Thus, employing the mixed consortia would be an efficient, less time consuming, economic and eco-friendly approach for the significant extraction of metals from BS deposits. Acknowledgment We gratefully acknowledge the support of KGHM Cuprum Ltd. Research and Development Centre, Poland (Dr. Agnieszka Schbery) for providing us with the black shale samples used in this study. The authors wish to express their gratitude to the Singapore Economic Development Board (EDB) and Minerals, Metals and Materials Technology Centre (M3TC), Faculty of Engineering, National University of Singapore for funding this work (Project R261-501-007-414).

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