ПРИКЛАДНАЯ БИОХИМИЯ И МИКРОБИОЛОГИЯ, 2013, том 49, № 3, с. 279-284
UDC 576.80
METAL SOLUBILIZATION FROM POWDERED PRINTED CIRCUIT BOARDS BY MICROBIAL CONSORTIUM FROM BAUXITE AND PYRITE ORES
© 2013 N. N. Adhapure*, S. S. Waghmare**, V. S. Hamde***, and A. M. Deshmukh*
*Department of Microbiology, Dr. Babasaheb Ambedkar Marathwada University, Osmanabad 413 501 (M.S.) India **Department of Microbiology, Fergusson College, Pune (M.S.) India ***Department of Microbiology, Yogeshwari College, Ambajogai, Dist Beed (M.S.) India
e-mail: adhapurenn@gmail.com Received June 27, 2012
With the current rapid developments in technology, there is an increasing accumulation of outdated electronic equipment. The primary reason for this increase is the low rate of recycling due to the complex nature of such waste. Bioleaching offers a promising solution for this problem.
Study was conducted on the solubilization of heavy metals from electronic waste (e-waste). For this purpose, a microbial consortium from bauxite and pyrite ore samples was obtained using a simple "top down" approach. Essentially, printed circuit boards (PCB) were obtained and used as representative samples of e-waste. Various concentrations (1—5%) of PCB powder were subjected to bioleaching, and the effects on metal solubilization, changes in pH and concentration of ferrous iron produced were assessed. It was observed that a maximum of 96.93% Cu and 93.33% Zn was solubilized by microbial consortium from 10 g/l of PCB powder, whereas only 10.26% Ni was solubilized from 30 g/l of PCB powder. For lead, only 0.58% solubilization was achieved from 20 g/l of PCB powder. An analysis of the precipitate formed during bioleaching using scanning electron microscopy with energy dispersive x-ray analysis revealed the presence of Tin (59.96%), Cu (23.97%), Pb (9.30%) and Fe (5.92%).
DOI: 10.7868/S0555109913030033
Rapid advancements in technology have lead to an increasing amount of electronic waste (e-waste), as older electronic equipment, such as computers and mobile phones, are discarded or approach the end of their usefulness. Interestingly, e-waste has a low level of recycling because its complex composition makes the separation of each component difficult.
It is believed that biotechnology is one of the most promising technologies in metallurgical processing. For many years, bioleaching has been used for the sol-ubilization of metals from ores [1]. Bioleaching is useful for treating ores with low concentrations of metals; it is also simple and cheap to operate. It has been successfully applied toward the leaching of metals from ores, though it has not yet been commercially applied toward the recovery of metals from printed circuit boards (PCB) [2]. Several authors [2—7] have recently published studies on the bioleaching of metals from electronic waste. Xin and colleagues [8] and Mishra with coworkers [9] have reported attempts to extract metals from waste batteries.
The aim of the study was to formulate microbial consortium for solubilization of metals from waste PCB and evaluate its efficacy.
MATERIALS AND METHODS
Growth of consortium. The bioleaching consortium was selected using the simple "top down" approach following Rawlings and Johnson in 2007 [10]. In the "top down" approach ("see-who-wins" approach), a mixture of microorganisms is used to inoculate the test material (in laboratory or pilot-scale operations), and it is assumed that a limited number of these acido-philes will emerge as a stable and effective bioleaching consortium.
In the present investigation, the microorganisms were derived from natural environments. Bauxite (Radhanagari, Kolhapur, India) and pyrite (Chitra-durga, India) ore samples were used as a source of ac-idophiles. The acidophiles were cultivated in modified 9K medium having composition (g/l): part A — (NH4)2SO4 - 3.0; KCl - 0.20; K2HPO4 - 0.050; MgSO4 • 7H2O - 0.050; part B - FeSO4 • 7H2O - 45. Part A was sterilized by autoclaving at 121°C for 15 min at 15 lb pressure and part B was sterilized by filtration using 0.25 ^m filter (Merck Millipore, USA). After sterilization equal amount of part B was added to part A, aseptically. The pH of the medium (2.4) was adjusted using 1.0 N H2SO4. The enriched acidophiles obtained from both the ore samples were mixed and again cultivated in the same medium. The obtained consortium was used for the bioleaching studies. The complete bi-
Chemical analysis of metals (%) in waste PCB powder
Metal
Solvent use for extraction*
aqua regia nitric acid
Copper 19.10 28.68
Iron 0.002 0.002
Zinc 0.81 0.46
Lead 2.81 5.14
Nickel 0.042 0.432
Aluminum 1.383 Nil
Silver 0.0044 Nil
Chromium 0.029 0.0033
Manganese 0.0007 0.00133
Cadmium Nil Nil
Silicon 0.1678 N.D.
Calcium 2.8405 9.77
* N.D. — not performed; Nil — absence of metal.
oleaching experiments were carried out in non-sterile conditions to obtain a stable and compatible consortium and mimic the conditions of the commercial bi-oleaching process.
The individual organisms from the consortium were not purified. However, the presence of iron oxidizers and acidophilic heterotrophs was confirmed using solid Fe-TSB non-overlay medium [11]. The Fe-TSB non-overlay medium was prepared in 3 parts. Part A was prepared as follows: 0.36 g ammonium sulfate, 0.005 g di-potassium hydrogen phosphate, 0.14 g magnesium sulfate, and 0.070 g tryptone soya broth (TSB) was added to 140 ml of double distilled water in a 250-ml Erlenmeyer flask. The pH was adjusted to 2.3. For part B, 2.0 g of agarose was dissolved in 70 ml of double distilled water in a 150 ml conical flask and maintained at a neutral pH. Parts A and B were auto-claved at 121°C and 15 lbs of pressure for 15 min. For part C, 6.9 g of ferrous sulfate heptahydrate was added to 10 ml of double distilled water, pH 2.15—2.3, and the concentration of the final solution was 2.5 M. The solution was filter-sterilized using a 0.25-^m filter (Merck Millipore, USA). A volume of1.5 ml of part C was added to part A. Part B was added to the mixture of A and C, and the plates were poured. All reagents used were of analytical grade.
Preparation of the waste PCB sample. Printed circuit board assemblies (PCBA) were collected from a scrap market. The PCBA consisted of different electronic components, such as random access memory (RAM) chips, peripheral component interconnect (PCI) slot, condensers, transistors, heat sink, etc., attached to PCB. The PCB consisted of metals required for the functioning of electronic equipment and non-metallic support.
The attached plastic parts, including RAM, PCI slots, and chip slots were removed from the PCBA. The PCB were shredded using a file (a file is a metal and woodworking tool used to cut fine particles of material from a workpiece). The obtained powder was composed of metallic and nonmetallic components of varied composition (mainly epoxy or phenolic resin).
The powder was sieved using a —14/+20 mesh. It was not pretreated prior to the bioleaching studies.
Chemical analysis of the waste PCB powder. A chemical analysis of the waste PCB powder was conducted to determine the heavy metal content in it. The content was determined using 2 different methods:
I) Approximately 1.0 g of homogenized powder was added to 100 ml of aqua regia, heated at 100°C for 1 h, cooled and subsequently filtered through Whatman filter paper no. 1 (UK). The volume was increased to 100 ml with double distilled water. The prepared sample was used for the detection of heavy metals with an atomic absorption spectrophotometer (Perkin Elmer A Analyst 300, USA).
II) A few drops of HNO3 were added to 1.0 g of homogenized powder in a 100-ml volumetric flask, and the volume was made to 100 ml with double distilled water. The sample was filtered with Whatman filter paper no. 1 prior to heavy metal analysis. The heavy metals were detected using method of atomic absorption spectroscopy.
Determining the metal solubilization ability and tolerance of the consortium. The metal solubilizing ability was determined using pure copper and solder. This metal adaptation increased the efficiency of the consortium in solubilizing the metals from PCB powder.
Solubilization of elemental copper. To determine the solubilization of elemental copper, we used 0.1— 0.8% copper turnings of <0.5 mm in length and <0.05 mm in width. Copper turnings were added to a 500-ml Erlenmeyer flask containing 200 ml of modified 9K medium and a 15% inoculum of consortium. The flask was incubated at 30°C on a rotary shaker at 140 rpm for 10 days.
Solubilization of lead from solder. The solder was purchased from the market and shredded into pieces of 1—2 cm in length. Approximately 0.1% of these pieces were treated in the same way as copper chips.
Bioleaching of metals from PCB. PCB powder (10, 20, 30, 40 and 50 g/l) was added to separate 500-ml Erlenmeyer flasks containing 200 ml of non-sterile modified 9K medium previously adjusted to pH 2.4 and a 15% microbial consortium inoculum. The flasks were subsequently incubated at 30°C on a rotary shaker at 140 rpm for 10 days. Samples were collected every 48 h for 10 days to determine the pH, and the ferrous iron and soluble metal content. The pH was measured using a digital pH meter (Elico LI 127, India). The ferrous iron content was measured using phenyl anthra-nilic acid [12], and the soluble metal content (Cu, Pb, Zn and Ni) was detected using method of atomic absorption spectroscopy. After 240 h, the precipitate formed during bioleaching was collected, dried and
nPHKHA^HAH EHOXHMHH H MHKPOBHOHOrHH tom 49 № 3 2013
Cu, % 100
80 60 40
20 -
0
Pb, % 0.6
(а)
Zn, % 100 г
80 -
60
40 -
20 -
48 96 144 192 240 h 0 48
Ni, %
(c) 12
10
240 h 0
(b)
240 h
240 h
Fig. 1. Copper (a), zinc (b), lead (c) and nickel (d) mobilization under different concentrations of PCB powder (g/l: 1 - 10; 2 -20; 3 - 30; 4 - 40; 5 - 50) relative to the time of incubation with microbial consortium. The d
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