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Factors influencing bioleaching copper from waste printed circuit boards by Acidithiobacillus ferrooxidans
Hydrometallurgy 97 (2009) 29–32
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Factors influencing bioleaching copper from waste printed circuit boards by Acidithiobacillus ferrooxidans Tao Yang ⁎, Zheng Xu, Jiankang Wen, Limei Yang National Engineering Laboratory of Biohydrometallurgy, General Research Institute for Nonferrous Metals, Beijing 100088, China
a r t i c l e
i n f o
Article history: Received 5 August 2008 Received in revised form 20 November 2008 Accepted 21 December 2008 Available online 6 January 2009 Keywords: Bioleaching Printed circuit boards Acidithiobacillus ferrooxidans Copper
a b s t r a c t The aim of this paper is to understand the factors that influence copper leaching from electronic scrap. It is revealed that the bioleaching is greatly influenced by process variables such as Fe3+ concentration, quantity of stock culture added, and pH. Before starting the leaching process, A. ferrooxidans was cultivated for 3– 4 days as stock culture, until the concentration of Fe3+ ions had reached about 7.00 g/L, which was considered strong enough to dissolve metallic copper. The results show that high leaching rates of copper could be achieved in the presence of 6.66 g/L of Fe3+, 100% addition quantities of stock culture, and pH 1.5. It is concluded that bioleaching copper from printed circuit boards (PCB) using Acidithiobacillus ferrooxidans (A. f.) is feasible. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In China, it is estimated that the number of personal computers now exceeds more than one billion. While computers have helped industry and commerce, there has been a dramatic increase in electronic waste. If obsolete computers are discarded randomly in waste dumps or just thrown out there is a risk of environmental damage and a loss of valuable metals. Printed circuit boards are part of computer and their compositions are quite varied, containing polymers, ceramics and metals which are distributed as follows: plastics 19%, bromine 4%, glass and ceramics 49%; the base metal content is around 28% – copper: 10–20%, lead: 1–5%, nickel: 1–3%. The precious metal content of circuit boards is 0.3 to 0.4% (Ludwig et al., 2003), made up of silver, platinum and gold. It is therefore essential to recover these metals before disposing of the circuit boards. The existing process for re-cyclicing printed circuit boards is either pyrometallurgical or hydrometallurgical both of which generate atmospheric pollution by releasing dioxins and furans into the atmosphere (Xu et al., 2005). In addition, such processes are costly due to high consumption of energy and should not be regarded as the most economical way to extract valuable components from circuit boards. The use of microorganisms to recover metals from wastes may well be a low cost practical alternate to these processes. Though this process has been successfully applied for the leaching of metals from ores (Olson et al., 2003), it has not been commercially applied to the recovery of metals from circuit boards. But recently, a few studies have ⁎ Corresponding author. E-mail address: [email protected] (T. Yang). 0304-386X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2008.12.011
been undertaken for the extraction of metals from electronic printed circuit boards (Brandl et al., 2001; Choi et al., 2004; Faramarzi et al., 2004; Ballor et al., 2006; Sadia et al., 2007; Willscher et al., 2007; Zhou et al., 2006). The present study was undertaken to evaluate the potential of pure mesophile bacteria culture Acidithiobacillus ferrooxidans to bring metals into solution from printed circuit boards, the aim being to study the influencing factors such as different addition quantities of stock culture, the initial concentration of Fe3+ and the effect of pH on bioleaching PCBs. 2. Materials and methods 2.1. Microorganism The microorganism A. ferrooxidans was obtained from Institute of Microbiology, Chinese Academy of Sciences. 2.2. Culture condition The standard used to grow A. ferrooxidans is 9 K medium, which was composed of the following mineral salts: (NH4)2SO4 3.0 g/L, K2HPO4 0.5 g/L, MgSO4·7H2O 0.5 g/L, KCl 0.1 g/L, Ca (NO3)2 0.01 g/L, 45 g/L of FeSO4·7H2O, which served as the energy source for A. ferrooxidans. The pH of the medium was adjusted at 2.0 using 1N H2SO4. To obtain metal adapted cultures, 100 mL of this solution was prepared in 300 mL Erlenmeyer flasks. Stock solutions (1 M) of each of Ag+, Al3+, Cu2+, Fe3+, Ni2+, Pb2+, Sn2+ and Zn2+ ions were prepared by dissolving appropriate amounts of the salts of these metals in distilled
30
T. Yang et al. / Hydrometallurgy 97 (2009) 29–32
Table 1 Content of heavy metal ions in the PCB. Heavy metal
Cu
Fe
Pb
Zn
Ni
Content (%)
25.06
0.66
0.80
0.04
0.0024
The values represent means of those obtained from duplicate experiments. The standard error is ≤±0.08.
water. Appropriate volume of each of these metal solutions was dispensed into the A. ferrooxidans 9 K medium in flasks to obtain 10 mM final concentration of each metal ion. The flasks were inoculated with 1 mL of inoculum containing 1 × 107 cells/mL and incubated at 30 °C in a rotary shaker at 165 rpm. While in the logarithmic phase of growth, 1 mL of culture from each flask was transferred to fresh medium containing 20 mM of each metal ion. In this way further step-wise transfers were made to the media containing next higher metal concentrations i.e., 30, 40, 50, 100, and 200 mM. Finally the medium in logarithmic phase of growth as stock culture was prepared for inoculation. 2.3. Waste PCB The PCB scraps used in this study were obtained from Powder Technology Laboratory of Beihang University. They were shredded using stainless steel blades with no special pre-treatment. The “b0.5 mm” fraction was used for all the bioleaching experiments. The metal content of this fraction is given in Table 1.
variations of pH, ORP, and concentrations of Cu in the solution were measured over time. All experiments were carried out in duplicate and the averaged results reported 2.5.3. Influence of different volume of A. ferrooxidans stock culture for inoculation on leaching In order to investigate the influence of different inoculation quantities of stock culture on leaching the PCB. A. ferrooxidans stock cultures were inoculated into 9 K medium in a rotary shaker at 165 r/ min at 30 °C at different volume rates, these varied from 20%, 40%, 60%, 80% to 100%. Then PCB shreds were placed into at concentration of 15 g/L. One flask was not inoculated as a sterile control test. The solution pH was controlled at 2.0 by adding 1N H2SO4. The variations of pH, ORP, and concentrations of Cu in the solution were measured with time. All experiments were carried out in duplicate and the averaged results reported. 3. Results and discussion 3.1. The influence of Fe3+ on leaching copper from waste PCB The effect of Fe3+ on leaching copper from waste PCB is shown in Fig. 1. It can be seen that the higher the concentration of Fe3+, the faster the rate of copper was leached. The time taken to bring the copper into solution is shown in Fig. 1(a), it took from 96 h, 60 h, 48 h and 36 h to
2.4. Analytical methods Fe2+ and Fe3+ ions were analyzed by o-phenanthroline method with an absorption wavelength of 510 nm. The copper and other heavy metal content dissolved in the liquor taken from the PCB shreds was measured by an atomic absorption spectrophotometer. The pH was recorded by a Thermo Orion model-868 pH-meter. The ORP (Redox Potential) was examined by High Resistance DC potentiometer (UJ34D). 2.5. Bioleaching test of waste PCB copper 2.5.1. Leaching experiment under different concentrations of Fe3+ At the beginning of the bioleaching experiment, 10 ml of A. ferrooxidans stock culture was injected into 90 ml of 9 K medium with the initial concentrations of Fe2+ at 3 g/L, 5 g/L, 7 g/L and 9 g/L, respectively. They were cultivated in a rotary shaker at 165 r/min at 30 °C for 3–4 days. After filtering and removal of the precipitate, the solution were analyzed to determine the concentrations of Fe3+ and were found to be t 2.63 g/L, 3.48 g/L, 4.72 g/L and 6.66 g/L, respectively. Then 25 g/L of PCB shreds were placed into filtered medium solution with different concentrations of Fe3+, and the initial pH of solution adjusted down to a pH of 2.0 by adding H2SO4. The variations of pH, ORP, and concentrations of Cu, Fe2+ and Fe3+ in the solution with time were examined. Each experiment was repeated twice and the average was reported. 2.5.2. Influence of pH on leaching rates In order to investigate the influence of pH on leaching PCB, 1.5 g of PCB shreds were respectively placed into three 250 ml conical flasks which contained 100 ml of mineral salts medium. The pH of solution was adjusted to 1.5, 1.7 and 2.0 by adding H2SO4. A. ferrooxidans stock culture was injected into the solution at quantity of 20%, and was cultivated in a shaking incubator (165 rpm) at 30 °C. The variations of pH, ORP, and concentrations of Cu, Fe2+ and Fe3+ in solution were measured with time. Another 15 g/L of PCB shreds were leached in non-ferrous sterile medium at a pH of 2.0 as a control test. The
Fig. 1. Changes of copper mobilization rate with time under different initial concentration of Fe3+ (a) and time profiles of Fe3+ concentration (b) in the leachate. The data presented in this figure represents mean of values obtained from duplicate experiments with a standard error of ≤±0.05.
T. Yang et al. / Hydrometallurgy 97 (2009) 29–32
dissolve the copper depending on the Fe3+ concentrations which varied from 2.36, 3.48, 4.72, to 6.66 g/L, respectively. After leaching for 12 h, there is a significant decreases in the concentrations of Fe3+ as observed in Fig. 1(b), in which the concentration of Fe3+ fell to 0.64 g/L, 1.06 g/L, 1.24 g/L and 2.13 g/L after 12 h. And at the same time, 34.53%, 47.88%, 59.10% and 79.75% of the copper was mobilized into solution after 12 h. This indicates that mobilizing copper into solution consumes Fe3+. It should also be noted that the consumption of ferric ion and the rate of copper being dissolved were in proportional. Fig. 1(b) shows that ferric ion played an important part in the process of dissolving copper. Ferric ion was consumed and the concentration of Fe3+ decreased. Once leaching stopped the concentration of Fe3+ increased by being oxidated with A. ferrooxidans. At that time, the leaching process no longer required such a high concentration of Fe3+ as most of copper had been brought into solution. In the leaching process, the leaching mechanism of copper from PCB shreds by A. ferrooxidans is similar to that of metal sulfide (Choi et al., 2004). Fe3+ can be formed by A. ferrooxidans from Fe2+ through reaction (1), and A. ferrooxidans is an aerobic and autotropic bacterium that uses ferrous ion as energy source in an acidic condition (Hu et al., 1996). Then Fe3+ formed by A. ferrooxidans oxidizes the elemental copper contained in PCB to Cu2+ through reaction (2) (Bard et al., 1985). The procedures for the solubilization of copper are straightforward, as the Gibbs free energy of the reaction is a subtractive value of −82.90 kJ/mol, which means the reaction (2) can take place in thermodynamics. 2 +
Fe
0
+ O2 + H 3 +
+
A: ferrooxidans 3 + Y Fe + H2 O 2 +
2 + Δ
Cu + 2FeðaqÞ Y 2FeðaqÞ + CuðaqÞ
θ
G = − 82:90 kJ = mol
ð1Þ ð2Þ
The regeneration of Fe2+ in reaction (2) suggests that this bioleaching process is cyclical. This leaching mechanism shows the importance of Fe3+ in the leaching reaction. 3.2. The influence of pH in leaching copper from waste PCB Fig. 2 shows the temporal changes of the mobilizing of copper into solution at different pH. After hours 48, the mobilization rate of copper in solution reached 99.06% at pH 1.5. At a higher starting pH of 1.7 the copper recovery was 93.25% after 48 h and at this pH it takes 66 h to exceed 99% copper extraction. When the initial pH starts at 2.0, then copper mobilization rate drops to 88.4% over the same period.
Fig. 2. The influence of pH on the copper mobilization rate. The data presented in this figure represents mean of values obtained from duplicate experiments with a standard error of ≤±0.05.
31
Table 2 Changes of pH value in the leaching process. Leaching time
pH = 1.5
0h 12 h 24 h 36 h 48 h 60 h 72 h 84 h 96 h
1.50 1.53 2.92 1.68 1.87 1.50 2.06 1.96 2.03
Adjust
1.50 1.50 1.50
pH = 1.7 1.70 1.79 3.01 2.26 2.98 1.77 2.30 1.65 2.09
Adjust
1.70 1.70 1.70 1.70
pH = 2.0
Adjust
2.00 2.31 3.17 3.22 3.08 2.38 2.38 1.90 2.12
2.00 2.00 2.00 2.00 2.00
The values represent means of those obtained from duplicate experiments. The standard error is ≤± 0.05.
And it takes 72 h to bring +99% of the copper into solution. This all shows that copper leaching is sensitive to pH. Without the addition of the microorganism and ferrous ion in the solution and relaying just on acid, it can be seen that when the pH is adjusted at 2.0, the mobilization rate of copper into solution was only 21% after 120 h of copper. This indicates that pH alone yields very slow leaching of copper. Showing that low pH itself is contributing factor in leaching copper. Electronic scrap has been reported to be alkaline in nature, resulting in increase in the pH of the solution (Brandl et al., 2001). This phenomenon was observed when the scrap was subjected to bioleaching of a solution at an initial pH of 2.0, suggesting that some alkaline materials and metals were brought into solution from the scrap due to the action caused by low pH. Table 2 shows the increases in the pH of the solution and adjusting back after each raise. As shown in Table 2, when starting at pH 1.5, the highest pH reached was 2.92 after 24 h and the pH had to be corrected three times in 96 h. At pH 1.7, the pH overshot 2.5 twice and was adjusted four times. When testing with an initial pH of 2.0, the test overshot pH 2.5 three times and was adjusted five times. The optimum pH for A. ferrooxidans is in the range 1.8–2.5 (Hu et al., 1996). So the growth of A. ferrooxidans would be restrained when this pH is exceeded. 3.3. The influence of quantities of stock culture addition on leaching copper of waste PCB Fig. 3 shows the influence of quantities of stock culture addition on leaching copper from waste PCBs. While the flasks were inoculated
Fig. 3. The influence of time on copper mobilization rates at different quantities of stock culture addition. The data presented in this figure represents mean of values obtained from duplicate experiments with a standard error of ≤± 0.05.
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T. Yang et al. / Hydrometallurgy 97 (2009) 29–32
with 80% and 100% stock culture, A. ferrooxidans were able to leach 98.06% and 98.59% of the available Cu after 24 h leaching, which means most of the copper have been mobilized. However, after 24 h leaching, the copper mobilization rates were just 84.64%, 58.74% and 49.13% in conditions in which the quantities of stock culture addition were 60%, 40% and 20%, respectively. And the complete mobilizations of copper need 120 h, 72 h and 48 h, respectively. Without A. ferrooxidans in the medium (inocula of 0%), the mobilization rate of copper was observed to be just 71.02% after 120 h leaching. It can be observed in Fig. 3 that the greater the stock culture addition the faster copper is leached, indicating that the large amounts of bacteria accelerate copper leaching. This results from the ion stabilization at higher concentration of bacteria on the oxidation of ferrous ion in solution. In other words, more Fe3+ ions can be formed by greater addition of stock culture which oxidize the elemental copper to cupric ion by reaction (1). It needs to be emphasized that without A. ferrooxidans ferrous ion can only be oxidized to ferric ion by oxygen from air, but this process was very slow. 4. Conclusions Present fundamental studies for the microbial leaching of copper from PCB generated from waste computers have demonstrated the possibility that the valuable metallic components contained in wastes can be mobilized effectively through a bioleaching process. Fe3+ and pH have a very significant effect on bringing copper into solution. The higher copper extraction rates can be achieved by adding more stock culture, greater concentration of Fe3+ in the range of 6.66–2.36 g/L and lower pH in the range of 1.5 to 2.0. Ferric
ion and acid may be the two main variables for the mobilization of copper. References Ballor, Nicholas R., Nesbitt, Carl C., Lueking, Donald R., 2006. Recovery of scrap iron metal value using biogenerated ferric iron. Biotechnology and Bioengineering 93 (6), 1089–1094. Bard, A.J., Parsons, R., Jordan, J., 1985. Standard potentials in aqueous solution. Marcel Dekker Press, Inc., New York, p. 288. 392. Brandl, H., Bosshard, R., Wegmann, M., 2001. Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi. Hydrometallurgy 59, 319–326. Choi, M., Cho, K., Kim, D., Kim, D., 2004. Microbial recovery of copper from printed circuit boards of waste computer by Acidithiobacillus ferrooxidans. Journal of Environmental Science and Health. Part A, Environmental Science and Engineering & Toxic and Hazardous Substance Control A39 (11–12), 2973–2982. Faramarzi, M.A., Stagars, M., Pensini, E., Krebs, W., Brandl, H., 2004. Metal solubilization from metal-containing solid materials by cyanogenic Chromobacterium violaceum. Journal of Biotechnology 13, 321–326. Hu, Y., Kang, Z., 1996. The bacteriological description of Thiobacillus Ferrooxidans. Hydrometallurgy 4, 36–40 (in Chinese). Ludwig, C., Hellweg, S., Stucki, S., 2003. Municipal Solid Waste Management: Strategies and Technologies for Sustainable Solutions. Springer, Berlin, pp. 320–322. Olson, G.J., Brierley, J.A., Brierley, C.L., 2003. Bioleaching review part B: progress in bioleaching: applications of microbial processes by the mineral industries. Applied Microbiology and Biotechnology 63, 249–257. Sadia, I., Anwar, Munir A., Niazi, Shahida B., Afzal Ghauri, M., 2007. Bioleaching of metals from electronic scrap by moderately thermophilic acidophilic bacteria. Hydrometallurgy 88, 180–188. Willscher, S., Katzschner, M., Jentzsch, K., Matys, S., Poellmann, H., 2007. Microbial leaching of metals from printed circuit boards. Advanced Materials Research 20–21, 99–102. Xu, Z., Shen, Z., 2005. Technology of recovery and reutilization of waste printed circuit board. China Powder Science and Technology 01, 6–9 (in Chinese). Zhou, P., Zheng, Z., Peng, X., Wang, Y., 2006. Leaching of copper from printed circuit board by Thiobacillus ferrooxidans. Environmental Pollution & Control 12, 126–128 (in Chinese).
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Factors influencing bioleaching copper from waste printed circuit boards by Acidithiobacillus ferrooxidans Tao Yang ⁎, Zheng Xu, Jiankang Wen, Limei Yang National Engineering Laboratory of Biohydrometallurgy, General Research Institute for Nonferrous Metals, Beijing 100088, China
a r t i c l e
i n f o
Article history: Received 5 August 2008 Received in revised form 20 November 2008 Accepted 21 December 2008 Available online 6 January 2009 Keywords: Bioleaching Printed circuit boards Acidithiobacillus ferrooxidans Copper
a b s t r a c t The aim of this paper is to understand the factors that influence copper leaching from electronic scrap. It is revealed that the bioleaching is greatly influenced by process variables such as Fe3+ concentration, quantity of stock culture added, and pH. Before starting the leaching process, A. ferrooxidans was cultivated for 3– 4 days as stock culture, until the concentration of Fe3+ ions had reached about 7.00 g/L, which was considered strong enough to dissolve metallic copper. The results show that high leaching rates of copper could be achieved in the presence of 6.66 g/L of Fe3+, 100% addition quantities of stock culture, and pH 1.5. It is concluded that bioleaching copper from printed circuit boards (PCB) using Acidithiobacillus ferrooxidans (A. f.) is feasible. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In China, it is estimated that the number of personal computers now exceeds more than one billion. While computers have helped industry and commerce, there has been a dramatic increase in electronic waste. If obsolete computers are discarded randomly in waste dumps or just thrown out there is a risk of environmental damage and a loss of valuable metals. Printed circuit boards are part of computer and their compositions are quite varied, containing polymers, ceramics and metals which are distributed as follows: plastics 19%, bromine 4%, glass and ceramics 49%; the base metal content is around 28% – copper: 10–20%, lead: 1–5%, nickel: 1–3%. The precious metal content of circuit boards is 0.3 to 0.4% (Ludwig et al., 2003), made up of silver, platinum and gold. It is therefore essential to recover these metals before disposing of the circuit boards. The existing process for re-cyclicing printed circuit boards is either pyrometallurgical or hydrometallurgical both of which generate atmospheric pollution by releasing dioxins and furans into the atmosphere (Xu et al., 2005). In addition, such processes are costly due to high consumption of energy and should not be regarded as the most economical way to extract valuable components from circuit boards. The use of microorganisms to recover metals from wastes may well be a low cost practical alternate to these processes. Though this process has been successfully applied for the leaching of metals from ores (Olson et al., 2003), it has not been commercially applied to the recovery of metals from circuit boards. But recently, a few studies have ⁎ Corresponding author. E-mail address: [email protected] (T. Yang). 0304-386X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2008.12.011
been undertaken for the extraction of metals from electronic printed circuit boards (Brandl et al., 2001; Choi et al., 2004; Faramarzi et al., 2004; Ballor et al., 2006; Sadia et al., 2007; Willscher et al., 2007; Zhou et al., 2006). The present study was undertaken to evaluate the potential of pure mesophile bacteria culture Acidithiobacillus ferrooxidans to bring metals into solution from printed circuit boards, the aim being to study the influencing factors such as different addition quantities of stock culture, the initial concentration of Fe3+ and the effect of pH on bioleaching PCBs. 2. Materials and methods 2.1. Microorganism The microorganism A. ferrooxidans was obtained from Institute of Microbiology, Chinese Academy of Sciences. 2.2. Culture condition The standard used to grow A. ferrooxidans is 9 K medium, which was composed of the following mineral salts: (NH4)2SO4 3.0 g/L, K2HPO4 0.5 g/L, MgSO4·7H2O 0.5 g/L, KCl 0.1 g/L, Ca (NO3)2 0.01 g/L, 45 g/L of FeSO4·7H2O, which served as the energy source for A. ferrooxidans. The pH of the medium was adjusted at 2.0 using 1N H2SO4. To obtain metal adapted cultures, 100 mL of this solution was prepared in 300 mL Erlenmeyer flasks. Stock solutions (1 M) of each of Ag+, Al3+, Cu2+, Fe3+, Ni2+, Pb2+, Sn2+ and Zn2+ ions were prepared by dissolving appropriate amounts of the salts of these metals in distilled
30
T. Yang et al. / Hydrometallurgy 97 (2009) 29–32
Table 1 Content of heavy metal ions in the PCB. Heavy metal
Cu
Fe
Pb
Zn
Ni
Content (%)
25.06
0.66
0.80
0.04
0.0024
The values represent means of those obtained from duplicate experiments. The standard error is ≤±0.08.
water. Appropriate volume of each of these metal solutions was dispensed into the A. ferrooxidans 9 K medium in flasks to obtain 10 mM final concentration of each metal ion. The flasks were inoculated with 1 mL of inoculum containing 1 × 107 cells/mL and incubated at 30 °C in a rotary shaker at 165 rpm. While in the logarithmic phase of growth, 1 mL of culture from each flask was transferred to fresh medium containing 20 mM of each metal ion. In this way further step-wise transfers were made to the media containing next higher metal concentrations i.e., 30, 40, 50, 100, and 200 mM. Finally the medium in logarithmic phase of growth as stock culture was prepared for inoculation. 2.3. Waste PCB The PCB scraps used in this study were obtained from Powder Technology Laboratory of Beihang University. They were shredded using stainless steel blades with no special pre-treatment. The “b0.5 mm” fraction was used for all the bioleaching experiments. The metal content of this fraction is given in Table 1.
variations of pH, ORP, and concentrations of Cu in the solution were measured over time. All experiments were carried out in duplicate and the averaged results reported 2.5.3. Influence of different volume of A. ferrooxidans stock culture for inoculation on leaching In order to investigate the influence of different inoculation quantities of stock culture on leaching the PCB. A. ferrooxidans stock cultures were inoculated into 9 K medium in a rotary shaker at 165 r/ min at 30 °C at different volume rates, these varied from 20%, 40%, 60%, 80% to 100%. Then PCB shreds were placed into at concentration of 15 g/L. One flask was not inoculated as a sterile control test. The solution pH was controlled at 2.0 by adding 1N H2SO4. The variations of pH, ORP, and concentrations of Cu in the solution were measured with time. All experiments were carried out in duplicate and the averaged results reported. 3. Results and discussion 3.1. The influence of Fe3+ on leaching copper from waste PCB The effect of Fe3+ on leaching copper from waste PCB is shown in Fig. 1. It can be seen that the higher the concentration of Fe3+, the faster the rate of copper was leached. The time taken to bring the copper into solution is shown in Fig. 1(a), it took from 96 h, 60 h, 48 h and 36 h to
2.4. Analytical methods Fe2+ and Fe3+ ions were analyzed by o-phenanthroline method with an absorption wavelength of 510 nm. The copper and other heavy metal content dissolved in the liquor taken from the PCB shreds was measured by an atomic absorption spectrophotometer. The pH was recorded by a Thermo Orion model-868 pH-meter. The ORP (Redox Potential) was examined by High Resistance DC potentiometer (UJ34D). 2.5. Bioleaching test of waste PCB copper 2.5.1. Leaching experiment under different concentrations of Fe3+ At the beginning of the bioleaching experiment, 10 ml of A. ferrooxidans stock culture was injected into 90 ml of 9 K medium with the initial concentrations of Fe2+ at 3 g/L, 5 g/L, 7 g/L and 9 g/L, respectively. They were cultivated in a rotary shaker at 165 r/min at 30 °C for 3–4 days. After filtering and removal of the precipitate, the solution were analyzed to determine the concentrations of Fe3+ and were found to be t 2.63 g/L, 3.48 g/L, 4.72 g/L and 6.66 g/L, respectively. Then 25 g/L of PCB shreds were placed into filtered medium solution with different concentrations of Fe3+, and the initial pH of solution adjusted down to a pH of 2.0 by adding H2SO4. The variations of pH, ORP, and concentrations of Cu, Fe2+ and Fe3+ in the solution with time were examined. Each experiment was repeated twice and the average was reported. 2.5.2. Influence of pH on leaching rates In order to investigate the influence of pH on leaching PCB, 1.5 g of PCB shreds were respectively placed into three 250 ml conical flasks which contained 100 ml of mineral salts medium. The pH of solution was adjusted to 1.5, 1.7 and 2.0 by adding H2SO4. A. ferrooxidans stock culture was injected into the solution at quantity of 20%, and was cultivated in a shaking incubator (165 rpm) at 30 °C. The variations of pH, ORP, and concentrations of Cu, Fe2+ and Fe3+ in solution were measured with time. Another 15 g/L of PCB shreds were leached in non-ferrous sterile medium at a pH of 2.0 as a control test. The
Fig. 1. Changes of copper mobilization rate with time under different initial concentration of Fe3+ (a) and time profiles of Fe3+ concentration (b) in the leachate. The data presented in this figure represents mean of values obtained from duplicate experiments with a standard error of ≤±0.05.
T. Yang et al. / Hydrometallurgy 97 (2009) 29–32
dissolve the copper depending on the Fe3+ concentrations which varied from 2.36, 3.48, 4.72, to 6.66 g/L, respectively. After leaching for 12 h, there is a significant decreases in the concentrations of Fe3+ as observed in Fig. 1(b), in which the concentration of Fe3+ fell to 0.64 g/L, 1.06 g/L, 1.24 g/L and 2.13 g/L after 12 h. And at the same time, 34.53%, 47.88%, 59.10% and 79.75% of the copper was mobilized into solution after 12 h. This indicates that mobilizing copper into solution consumes Fe3+. It should also be noted that the consumption of ferric ion and the rate of copper being dissolved were in proportional. Fig. 1(b) shows that ferric ion played an important part in the process of dissolving copper. Ferric ion was consumed and the concentration of Fe3+ decreased. Once leaching stopped the concentration of Fe3+ increased by being oxidated with A. ferrooxidans. At that time, the leaching process no longer required such a high concentration of Fe3+ as most of copper had been brought into solution. In the leaching process, the leaching mechanism of copper from PCB shreds by A. ferrooxidans is similar to that of metal sulfide (Choi et al., 2004). Fe3+ can be formed by A. ferrooxidans from Fe2+ through reaction (1), and A. ferrooxidans is an aerobic and autotropic bacterium that uses ferrous ion as energy source in an acidic condition (Hu et al., 1996). Then Fe3+ formed by A. ferrooxidans oxidizes the elemental copper contained in PCB to Cu2+ through reaction (2) (Bard et al., 1985). The procedures for the solubilization of copper are straightforward, as the Gibbs free energy of the reaction is a subtractive value of −82.90 kJ/mol, which means the reaction (2) can take place in thermodynamics. 2 +
Fe
0
+ O2 + H 3 +
+
A: ferrooxidans 3 + Y Fe + H2 O 2 +
2 + Δ
Cu + 2FeðaqÞ Y 2FeðaqÞ + CuðaqÞ
θ
G = − 82:90 kJ = mol
ð1Þ ð2Þ
The regeneration of Fe2+ in reaction (2) suggests that this bioleaching process is cyclical. This leaching mechanism shows the importance of Fe3+ in the leaching reaction. 3.2. The influence of pH in leaching copper from waste PCB Fig. 2 shows the temporal changes of the mobilizing of copper into solution at different pH. After hours 48, the mobilization rate of copper in solution reached 99.06% at pH 1.5. At a higher starting pH of 1.7 the copper recovery was 93.25% after 48 h and at this pH it takes 66 h to exceed 99% copper extraction. When the initial pH starts at 2.0, then copper mobilization rate drops to 88.4% over the same period.
Fig. 2. The influence of pH on the copper mobilization rate. The data presented in this figure represents mean of values obtained from duplicate experiments with a standard error of ≤±0.05.
31
Table 2 Changes of pH value in the leaching process. Leaching time
pH = 1.5
0h 12 h 24 h 36 h 48 h 60 h 72 h 84 h 96 h
1.50 1.53 2.92 1.68 1.87 1.50 2.06 1.96 2.03
Adjust
1.50 1.50 1.50
pH = 1.7 1.70 1.79 3.01 2.26 2.98 1.77 2.30 1.65 2.09
Adjust
1.70 1.70 1.70 1.70
pH = 2.0
Adjust
2.00 2.31 3.17 3.22 3.08 2.38 2.38 1.90 2.12
2.00 2.00 2.00 2.00 2.00
The values represent means of those obtained from duplicate experiments. The standard error is ≤± 0.05.
And it takes 72 h to bring +99% of the copper into solution. This all shows that copper leaching is sensitive to pH. Without the addition of the microorganism and ferrous ion in the solution and relaying just on acid, it can be seen that when the pH is adjusted at 2.0, the mobilization rate of copper into solution was only 21% after 120 h of copper. This indicates that pH alone yields very slow leaching of copper. Showing that low pH itself is contributing factor in leaching copper. Electronic scrap has been reported to be alkaline in nature, resulting in increase in the pH of the solution (Brandl et al., 2001). This phenomenon was observed when the scrap was subjected to bioleaching of a solution at an initial pH of 2.0, suggesting that some alkaline materials and metals were brought into solution from the scrap due to the action caused by low pH. Table 2 shows the increases in the pH of the solution and adjusting back after each raise. As shown in Table 2, when starting at pH 1.5, the highest pH reached was 2.92 after 24 h and the pH had to be corrected three times in 96 h. At pH 1.7, the pH overshot 2.5 twice and was adjusted four times. When testing with an initial pH of 2.0, the test overshot pH 2.5 three times and was adjusted five times. The optimum pH for A. ferrooxidans is in the range 1.8–2.5 (Hu et al., 1996). So the growth of A. ferrooxidans would be restrained when this pH is exceeded. 3.3. The influence of quantities of stock culture addition on leaching copper of waste PCB Fig. 3 shows the influence of quantities of stock culture addition on leaching copper from waste PCBs. While the flasks were inoculated
Fig. 3. The influence of time on copper mobilization rates at different quantities of stock culture addition. The data presented in this figure represents mean of values obtained from duplicate experiments with a standard error of ≤± 0.05.
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T. Yang et al. / Hydrometallurgy 97 (2009) 29–32
with 80% and 100% stock culture, A. ferrooxidans were able to leach 98.06% and 98.59% of the available Cu after 24 h leaching, which means most of the copper have been mobilized. However, after 24 h leaching, the copper mobilization rates were just 84.64%, 58.74% and 49.13% in conditions in which the quantities of stock culture addition were 60%, 40% and 20%, respectively. And the complete mobilizations of copper need 120 h, 72 h and 48 h, respectively. Without A. ferrooxidans in the medium (inocula of 0%), the mobilization rate of copper was observed to be just 71.02% after 120 h leaching. It can be observed in Fig. 3 that the greater the stock culture addition the faster copper is leached, indicating that the large amounts of bacteria accelerate copper leaching. This results from the ion stabilization at higher concentration of bacteria on the oxidation of ferrous ion in solution. In other words, more Fe3+ ions can be formed by greater addition of stock culture which oxidize the elemental copper to cupric ion by reaction (1). It needs to be emphasized that without A. ferrooxidans ferrous ion can only be oxidized to ferric ion by oxygen from air, but this process was very slow. 4. Conclusions Present fundamental studies for the microbial leaching of copper from PCB generated from waste computers have demonstrated the possibility that the valuable metallic components contained in wastes can be mobilized effectively through a bioleaching process. Fe3+ and pH have a very significant effect on bringing copper into solution. The higher copper extraction rates can be achieved by adding more stock culture, greater concentration of Fe3+ in the range of 6.66–2.36 g/L and lower pH in the range of 1.5 to 2.0. Ferric
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