Microbial Leaching of Waste Solder for Recovery of Metal

Microbial Leaching of Waste Solder for Recovery of Metal

H. Hocheng & T. Hong & U. Jadhav

Received: 4 December 2013 /Accepted: 24 February 2014# Springer Science+Business Media New York 2014

Abstract This study proposes an environment-friendly bioleaching process for recovery ofmetals from solders. Tin-copper (Sn-Cu), tin-copper-silver (Sn-Cu-Ag), and tin-lead (Sn-Pb)solders were used in the current study. The culture supernatant of Aspergillus niger removedmetals faster than the culture supernatant of Acidithiobacillus ferrooxidans. Also, the metalremoval by A. niger culture supernatant is faster for Sn-Cu-Ag solder as compared to othersolder types. The effect of various process parameters such as shaking speed, temperature,volume of culture supernatant, and increased solder weight on bioleaching of metals wasstudied. About 99 (±1.75)% metal dissolution was achieved in 60 h, at 200-rpm shakingspeed, 30 °C temperature, and by using 100-ml A. niger culture supernatant. An optimumsolder weight for bioleaching was found to be 5 g/l. Addition of sodium hydroxide (NaOH)and sodium chloride (NaCl) in the bioleached solution from Sn-Cu-Ag precipitated tin (85±0.35 %) and silver (80±0.08 %), respectively. Passing of hydrogen sulfide (H2S) gas at pH 8.1selectively precipitated lead (57.18±0.13 %) from the Sn-Pb bioleached solution. The pro-posed innovative bioleaching process provides an alternative technology for recycling wastesolders to conserve resources and protect environment.

Keywords Solders . Bioleaching . Aspergillus niger . Acidithiobacillus ferrooxidans . Culturesupernatant

Introduction

The rapid progress of electronic technology elevated the manufacturing and use of electronicproducts such as computers, digital cameras, mobile phones, etc. [1]. The lifetime of thesedevices is reducing day by day, and the waste generated by all these discarded electronicproducts are generally termed as E-waste [2]. Solders are an important part in any electronicproduct. The solder alloys are used as an interconnecting material in electronic packaging.They provide both electrical connection and mechanical support in the packaging modules [1].Lead-bearing/tin-lead (Sn-Pb) solders are used extensively in the electronics industry due to itsoutstanding solderability and reliability [1, 3]. However, lead is known to be a toxic material[4]. Thus, Sn-Pb solder has been substituted by various Pb-free solders, such as the Sn-Ag-Cu

Appl Biochem BiotechnolDOI 10.1007/s12010-014-0833-2

H. Hocheng (*) : T. Hong :U. JadhavDepartment of Power Mechanical Engineering, National Tsing Hua University, No. 101, Sec.2, Kuang FuRd., 30013 Hsinchu, Taiwan, Republic of Chinae-mail: [email protected]

series [1]. Recent reports suggested that Sn-Ag-Cu alloy is more harmful to environment thanSn-Pb solder [5]. Therefore, sufficient attention should be paid on possible spreading of theseheavy metal elements leached from lead-free solders in a natural environment. Unfortunately,most researches up to now are focused on leaching performance of heavy metal elements inlead-containing solder [6], and only a few studies have reported leaching behavior of alterna-tive solders [7]. There is a need to recycle the waste lead and the lead-free solders as theycontain materials which are valuable and recyclable as well as toxic.

Various pyro- and hydrometallurgical processes are in use to recycle waste solders. A leadrefining process which consists of melting and re-powdering is used to recycle waste Sn-Pbsolder [2]. Similarly, a simple melting process is used for recycling lead-free solders. Nitricacid leaching process has been employed for recycling of waste lead-free solders [4, 8].However, these pyro- and hydrometallurgical processes have disadvantages, such as use ofharmful chemicals, the consumption of high energy, and emission of harmful gases generatedduring combustion. Besides, it is difficult to recover metals as individual components in thepyrometallurgical processes [2, 4].

It has been reported that bio-hydrometallurgical processes can replace the pyro- andhydrometallurgical processes [9]. Bioleaching of metals is one of the bio-hydrometallurgicalprocesses. Microorganisms (bacteria and fungi) produce metabolic products such as inorganicand organic acids and ferric ions which react with metals and help in the leaching of metalsfrom wastes and minerals [9–11]. Bioleaching process reduces the demand of resources,energy, and landfill space [12, 13]. Jadhav and Hocheng [11] used Acidithiobacillusferrooxidans culture supernatant for extraction of silver from waste button cell batteries.They showed the usefulness of a two-step bioleaching process. In this system, the microor-ganism will not come in direct contact with waste materials which will protect them from theadverse effects of metals present in waste. In a previous study, Aspergillus niger culturesupernatant containing citric acid was used for biomachining of various metals. During thisstudy, it was found that A. niger produced 20 g/l citric acid by using sucrose as a carbon sourceat pH 4.0, 25 °C temperature, and 120-rpm shaking speed in 10 days [14]. Also, in anotherstudy, it has been shown that At. ferrooxidans oxidized (97.86 %) 40 g/l FeSO4 in 54 h, andthis At. ferrooxidans culture supernatant was used for copper biomachining [15]. Consideringthe usefulness of these microorganisms and a two-step bioleaching process, the present studywas undertaken to develop an environmentally benign method for extraction of metals fromlead-containing and lead-free solders. Various process parameters were studied. The resultsobtained from laboratory scale studies will be useful to develop a non-toxic metal recovery/removal process from waste solders.

Materials and Methods

Materials

The solders Sn-Pb (63 %-37 %), Sn-Cu (63 %-37 %), and Sn-Cu-Ag (60 %-37 %-3 %) weresupplied by Taiwan-Solid Enterprises Limited (Fig. 1). All the chemicals were purchased fromSigma Aldrich.

Growth of Microbes

A. niger (BCRC 34770) was obtained from the Food Industry Research and DevelopmentInstitute (FIRDI), Taiwan. Sucrose medium was used for the growth of fungi. The medium

Appl Biochem Biotechnol

contains the following substances per liter of glass-distilled water: 100-g sucrose, 1.5-gNaNO3, 0.5-g KH2PO4, 0.025-g MgSO4.7H2O, 0.025-g KCl, and 1.6-g yeast extract [14, 16].

At. ferrooxidans (BCRC 13823) was obtained from the Food Industry Research andDevelopment Institute (FIRDI), Taiwan. Basal 9 K medium was used for the growth ofbacteria. In the present study, 40 g/l FeSO4 was used as an energy source [15].

Collection of Cell-Free Culture Supernatant

A. niger was grown in sucrose medium for 10 days at 120 rpm and 30 °C. The culture wascentrifuged at 10,000 rpm for 30 min to remove the cells in the bottom of the centrifugationtube. The supernatant was then filtered through a 0.22-μm filter. The cell-free culturesupernatant was used in further studies.

Oxidation of FeSO4 (40 g/l) by At. ferrooxidans was carried out at 150 rpm and 30 °C in9 K medium (pH 2.5). After 54 h, the culture was centrifuged at 10,000 rpm for 30 min toremove the cells in the bottom of the centrifugation tube. The supernatant was then filteredthrough a 0.22-μm filter. The cell-free culture supernatant was used in further studies.

Bioleaching of Solder Pieces by Culture Supernatant

For bioleaching experiments, two types of lead-free solders were used viz., Sn-Cu and Sn-Cu-Ag. Along with this, lead-containing (Sn-Pb) solder was also used. All samples were rinsedwith distilled water and 75 % ethanol and dried before use. The solder pieces were coveredwith 100 ml of above mentioned A. niger culture supernatant in a 250-ml flask. A controlusing sucrose medium alone was also included in these studies. In case of the leachingexperiment for Sn-Cu-Ag solder, the flasks were placed in a shaker at 30 °C for 60 h, whilefor Sn-Cu and Sn-Pb solders, flasks were incubated for 96 and 144 h, respectively. Afterexposure to the leaching solution, the solder pieces were removed and gently rinsed withsterile distilled water and placed in an oven at 50 °C to remove the remaining moisture. Overthe course of the bioleaching, the mass of solder pieces was measured with a precise electronicbalance machine (Precisa XS225A and d=0.0001 g) at constant time intervals. A similarexperiment was carried out for bioleaching of solder pieces by At. ferrooxidans culturesupernatant. In this case for Sn-Cu-Ag and Sn-Cu solders, the flasks were placed in a shakerat 30 °C for 96 h, while for Sn-Pb solder, flasks were incubated for 144 h.

Fig. 1 Three types of solders used during the bioleaching study


Experiment of Various Process Conditions

The effect of variable shaking speeds on bioleaching of metals from solders was studied.The culture supernatant was collected after growth of A. niger in sucrose medium for10 days. The solder pieces were covered with 100 ml of culture supernatant in 250-mlflasks separately. These flasks were incubated at 0, 50, 100, 150, and 200 rpm at 30 °C.After exposure to the leaching solution, the solder pieces were removed, and the metalremoved was determined.

Temperature optimum for bioleaching of solder pieces was quantified. The culture super-natant was collected after growth of A. niger in sucrose medium for 10 days. The solder pieceswere covered with 100 ml of culture supernatant in 250-ml flasks separately. For the study ofthe effect of temperature, the flask was incubated at various temperatures (20–50 °C) in ashaker incubator at 200 rpm.

The culture supernatant was collected after growth of A. niger in sucrose medium for10 days. The solder pieces were covered with variable volume (50–200 ml) of culturesupernatant in 500-ml flasks separately. These flasks were incubated at 200 rpm at 30 °C.After exposure to the leaching solution, the solder pieces were removed, and metal removedwas determined.

The culture supernatant was collected after growth of A. niger in sucrose mediumfor 10 days. Variable weights (5–30 g/l) of solder pieces were covered with culturesupernatant separately. These flasks were incubated at 200 rpm at 30 °C. Afterexposure to the leaching solution, the solder pieces were removed, and metal removedwas determined.

Selective Precipitation of Metals from Bioleached Solutions

Two types of bioleached solutions were used for recovery of metals viz. bioleachedsolution from Sn-Cu-Ag and Sn-Pb solder. Yoo et al. [4] showed the use of NaCl forselective precipitation of silver. In the present study, along with NaCl, NaOH hasbeen used for precipitation of tin. Also, H2S was used for precipitation of metals fromSn-Cu-Ag solder. The leach solution used in these tests was obtained under thefollowing leaching conditions: 200-rpm agitation speed, 100-ml A. niger culturesupernatant, 30 °C temperature, and 0.5-g solder. The H2S gas was passed throughthe bioleached solution at pH 1.5, 2.5, and 3.5. The NaCl and NaOH were added inbioleached solution separately until the precipitation stops. Alvarez et al. [17] showedselective precipitation of “Pb” by passing H2S gas at pH 8.1. In the present studyalso, H2S gas was passed through the bioleached solution from Sn-Pb solder atpH 8.1 for precipitation of lead.

Analytical Methods

The solder surface was investigated by a scanning electron microscope (SEM, JSM-5610 LV). The accelerating voltage used was 0.5–30 kV at a resolution of 500 μm(5 kV, WD 6 mm). Ferrous iron content was determined by titrating the sample withpotassium dichromate using n-phenyl anthranilic acid as an indicator [11]. The con-centration of metals in the leach liquors was analyzed by inductively coupled plasmaoptical emission spectrometry. The precipitates were characterized by an X-ray powderdiffractometer (TTRAX III, Rigaku, Japan, Co.)


Results and Discussion

Comparison of Bioleaching Potential of Culture Supernatant of At. ferrooxidans and A. niger

Figure 2 shows metal removal using the culture supernatant of At. ferrooxidans and A. nigerfor Sn-Pb, Sn-Cu, and Sn-Cu-Ag solder. The metal removal increased with an increase inincubation time for both the microorganisms. A. niger culture supernatant was found to be asuitable lixiviant for the leaching of metal as compared to At. ferrooxidans culture supernatant.It required 144, 96, and 48 h for metal removal from Sn-Pb (99.4 %), Sn-Cu (99.8 %), andSn-Cu-Ag (97.7 %) solder, respectively (Fig. 2). It took 60 h to remove 98.5 % metal fromSn-Cu-Ag solder (data not shown). This metal dissolution from solders by A. niger can beattributed to an excretion of organic acid in the culture broth [18]. An acidic environmentcreated by citric acid present in A. niger culture supernatant favors dissolution of metals.The citric acid provides both a source of protons which protonate anionic component andan organic acid anion [19]. The protonation of oxygen atoms occurs around the surface ofmetallic compounds. The protons and oxygen associated with water displace the metalfrom the surface [20, 21].

In case of At. ferrooxidans culture supernatant, it showed poor metal removal for Sn-Pbsolder (4.2 % in 144 h), while for Sn-Cu and Sn-Cu-Ag solder it showed 97.9 and 97.4 %metal removal, respectively, in 96 h (Fig. 2). The culture supernatant of At. ferrooxidanscontains Fe3+. This Fe3+ is a strong oxidant. It can oxidize the metals and convert them insoluble form [11, 15]. The role of Fe3+ in metal solubilization from solders has been proved bystudying conversion of Fe3+ into Fe2+. Figure 3 shows the metal removal from various solders

Fig. 2 Comparison of the bioleaching potential of At. ferrooxidans and A. niger culture supernatants for metalremoval from Sn-Pb, Sn-Cu, and Sn-Cu-Ag solder


by At. ferrooxidans culture supernatant and conversion of Fe3+ into Fe2+ during the process. Incase of Sn-Cu and Sn-Cu-Ag solders, the Fe2+ concentration increased with an increase inmetal removal. The Fe3+ reacted with metals present in solder, oxidized them, and themselvesgot converted into Fe2+. An increase in Fe2+ concentration was observed during thisbioleaching process. For Sn-Pb solder, the Fe2+ concentration remained the same after 72 h,since no metal removal was observed after 72 h. These results suggest that the Fe3+ isresponsible for metal removal from solders. These results also present a good comparisonbetween two mechanisms of bioleaching. Bioleaching of metals by A. niger culture superna-tant is based on the acidolysis/complexolysis process carried out by organic acid. In a previousstudy, authors showed that A. niger produces only citric acid by using sucrose as a carbonsource as per Eq. 1. They found that A. niger produced 20 g/l of citric acid after 10-dayincubation [14].

C6H

12O

6þ 1:5 O2↔

C6H8O7 þ 2H2OCitric acid

ð1Þ

Application of culture supernatant for the bioleaching process created an acidic environ-ment which favored dissolution of metals (Eq. 2). Also, acidic metabolites favored formationof soluble metal complexes (Eq. 3) [16].

C6H

8O

7↔ C6H7O7ð Þ− þ H

þ ð2Þ

C6H

7O

7ð Þ− þM

þ↔Metal citrate complex ð3Þ

In case of At. ferrooxidans, the bioleaching process is carried out based on the oxidationmechanism. At. ferrooxidans uses Fe2+ as an energy source and produces Fe3+ (Eq. 4). Fe3+ isa strong oxidizing agent. It reacts with metals and solubilizes them (Eq. 5) [15].

Fig. 3 Conversion of Fe3+ to Fe2+ during bioleaching of Sn-Cu-Ag solder by At. ferrooxidans culture supernatant


2 Fe2þ þ 2 Hþ þ 0:5 O2→2 Fe3þ þ H20

Fe3þ þ e

−→ Fe

2þ ð4Þ

Msð Þ−e

−→Mþ

aqð Þ ð5Þ

where M(s) is Metal.The results of the present study suggest that metal dissolution based on the

acidolysis/complexolysis mechanism is more efficient as compared to the oxidationmechanism. One possible reason behind this is the metals are present mainly asoxides, carbonates, and silicates rather than sulfides in industrial solid waste materials.It is easier to leach metals via acid generated by microorganisms rather than ferriciron [22].

Cheng et al. [8] and Yoo et al. [4] used nitric acid for leaching of metals fromsolders. They found that the concentration of Sn was low in all the nitric acidleaching experiments. In this sense, the solder bioleaching process used in the presentstudy was found to be advantageous over the chemical leaching process. The culturesupernatants of A. niger and At. ferrooxidans were able to dissolve solders. Alongwith nitric acid leaching, several other methods have been developed for waste solderrecycling. These processes need use of chemicals such as sodium hydroxide, sodiumpersulfate, nitric acid, and organic solvents [4, 23]. Though these methods areeffective, use of such chemicals is harmful to the environment. The process describedin the present paper is environmentally benign. Since A. niger culture supernatantshowed better performance and Sn-Cu-Ag solder was dissolved in less time, they wereused in further experiments to optimize process parameters for bioleaching.

Optimization of Process Parameters for Bioleaching of Metals from Sn-Cu-Ag Solder

Yoo et al. [4] suggested that an optimum agitation speed is required to ensure effective particlesuspension in the solution. The mineral particles remain suspended in the liquor at higheragitation speeds [24]. Therefore, an effect of agitation speeds on metal removal from Sn-Cu-Ag solder was studied. At static condition (0-rpm agitation speed), only 2 % metal removalwas observed. The metal removal efficiency increased with an increase in agitation speed. At200-rpm agitation speed, 99 % metal removal was observed in 60 h (data not shown). Thesufficient kinetic energy enhances various fluidic motions and consequently the mass transferof ionic products in the solution, of which the saturated concentration near the metal surface isadvantageously diluted that provides fast removal of metal [15]. In the present study, 200-rpmagitation speed was found to be optimum. Therefore, in all the subsequent leaching experi-ments, a working agitation speed of 200 rpm was employed. These results are contrary to aprevious study [25], which showed that the leaching rate is nearly independent of the stirringspeed. This discrepancy in the results is considered due to the use of gluconic acid for leachingof calcined magnesite in their study.

An effect of temperature on metal removal from solder was studied by varying thetemperature over a range of 20–40 °C. The 30 °C temperature is optimum for metal removal(data not shown). At this temperature, 98.5 % metal removal was obtained for Sn-Cu-Agsolder by using A. niger culture supernatant. With an increase in the temperature from 30 to40 °C, metal removal efficiency decreased to 69.8 %. These results are in contrast to Yoo et al.[4] who showed that higher temperatures yielded higher dissolution efficiencies of Ag and Cufrom the solder. It is always easy to control processes that use low temperature. Therefore, theprocess put forth in the present study is advantageous.


The Sn-Cu-Ag solder pieces were covered with variable volume (50–150 ml) of culturesupernatant, and the flasks were incubated at 200 rpm at 30 °C. It is found that the optimumvolume of culture supernatant for metal removal was 100 ml (data not shown). By using 50-and 150-ml culture supernatant, 97.8 and 93 % metal removal was achieved, respectively. Thelarge volume of the working medium facilitates the transport of an ionic product from themetal removal process. This effect saturated beyond a certain amount of the supplied volume[15]. This may be a reason for the obtained results in the present study.

An effect of increasing weight of solder ranging from 5 to 30 g/l was investigated under theleaching conditions of 200 rpm and 30 °C. The metal removal increased with an increase intime for all the weights used. Variations in leaching efficiency (after 60 h) were observed with

Fig. 4 Effect of increasing solder weight on bioleaching efficiency

Fig. 5 SEM analysis of the Sn-Cu-Ag solder surface during bioleaching by A. niger culture supernatant


an increase in solder weight. For 10 g/l solder weight, around 90.4 % metal removal wasobserved in 60 h, while only 45.3 % metal was removed in case of 30 g/l solder weight in 60 h.An optimum solder weight for bioleaching was 5 g/l (Fig. 4). An amount of citric acid may notmatch an acid requirement for metal leaching, thereby yielding lower extraction at highersolder weight. In agreement to these results, a decrease in bioleaching efficiency due to lack ofsufficient acid at increased pulp density was reported by Aung and Ting [26] and Mehta et al.[27].

Scanning electron microscopy was used to observe the surface of solders during thebioleaching process. The deterioration of the Sn-Cu-Ag solder surface clearly indicatesremoval of metals from solder surface (Fig. 5). Further experiments are underway to determinethe corrosion product that is formed on the solder surface during the bioleaching process.

Table 1 shows the bioleaching performance of A. niger reported in previous studies forvarious metal-containing solid waste materials. If one compares the results of the present studywith data shown in Table 1, then it is clear that the metal leaching is faster as compared to theprevious studies. In particular, Cu (97 % in 8 days) [28], Pb (95 % in 21 days), and Sn (65 % in21 days) [29] showed less metal removal compared to the present study, and also it took longer

Table 1 Performance of A. niger for bioleaching of metals from various industrial wastes

Reference Metals Metal extraction (%) Time (days)

Amiri et al. [29] W 100 30

Mehta et al. [27] Ni 98 24

Mehta et al. [27] Cu 97 8

Brandl et al. [28] Zn 95 21

Brandl et al. [28] Pb 95 21

Mehta et al. [27] Mn 91 8

Amiri et al. [29] Mo 90.9 30

Anjum et al. [30] Fe 83 24

Mehta et al. [27] Co 86 8

Brandl et al. [28] Sn 65 21

Aung and Ting [26] Sb 64 50

Aung and Ting [26] V 36 50

Fig. 6 XRD pattern showing the presence of SnO2 and Ag in precipitates obtained by addition of a NaOH and bNaCl in the bioleached solution of Sn-Cu-Ag solders


time. High metal concentrations may inhibit production of metabolic products, therebyaffecting bioleaching performance [30]. Therefore, it is beneficiary to use a two-stepbioleaching process described in the present paper, where organic acid production is separatedfrom the metal leaching process. Similar observations were reported by Brandl et al. [29]. Atwo-step process seems appropriate to increase leaching efficiency for an industrial application.According to this process, in the first step, organisms were grown in the absence of wastematerial followed by the second step where the formed metabolites were used for metalsolubilization. According to Brandl et al. [29], the two-step bioleaching process has thefollowing advantages:

(a) Microbial biomass does not come in direct contact with metal containing waste and mightbe recycled.

(b) There is no microbial contamination of waste material.(c) Metabolic product formation can be optimized in the absence of waste material.(d) Increased metal yields can be achieved by applying higher waste concentrations com-

pared to a one-step process.

Selective Precipitation of Metals from Bioleached Solution

Experiments were carried out for recovery of metals from the bioleached solution. To achieveselective precipitation, hydrogen sulfide gas, sodium chloride, and sodium hydroxide wereused. In case of Sn-Cu-Ag solder, passing hydrogen sulfide gas at pH 1.5, 2.5, and 3.5 was not

Table 2 Precipitation of various metals from the bioleached solution by NaOH, NaCl, and H2S

Precipitation agent Metal precipitated Metal extraction (%)

NaOH Sn 85 (±0.35)

NaCl Ag 80 (±0.08)

H2S Pb 57.18 (±0.13)

Fig. 7 XRD pattern showing the presence of “Pb” in the precipitate obtained by passing H2S gas through thebioleached solution of Sn-Pb solders


successful. All of the three metals were precipitated together at all pH (data not shown). Wewere able to separate silver (in the form of silver chloride) and tin (in the form of stannousoxide) from the bioleached solution, when sodium chloride and sodium hydroxide were addedinstead of hydrogen sulfide gas. Figure 6 shows the XRD patterns of the precipitate obtainedfrom the bioleached solution after addition of sodium chloride and sodium hydroxide. Around85 % (±0.35), and 80 % (±0.08), metal recovery was observed for tin and silver, respectively(Table 2). These results are in accordance to Yoo et al. [4] who showed the presence ofstannous oxide in the precipitate. They also showed precipitation of silver ion by adding Cl−

ion in the form of NaCl as per the following reaction:

Agþ þ Cl

− ¼ AgCl↓ ð6ÞThey required 34.8 mol/m3 of Cl− to precipitate Ag, and for this they added 0.2 and 0.4 g of

NaCl to 100 cm3 of the leach solution. Experiments are underway for separation of copperfrom other metals. These results support separation of the tin component from Ag and Cuduring bioleaching. A similar experiment was carried out for recovery of lead from Sn-Pbsolder. Hydrogen sulfide gas was passed through the bioleached solution for Sn-Pb solder. Thepresence of lead in the precipitate was observed (Fig. 7). Around 57.18 % (±0.13) of lead hasbeen recovered from the bioleached solution (Table 2).

Conclusion

The chemical leaching process has been in use for recycling of solders. In consideration of anenvironmental sustainability, there is, however, no report available on the effectiveness of thebioleaching process for metal removal from waste solders. The present work indicates that it isfeasible to remove metals from lead-containing and lead-free solders by the culture superna-tants of A. niger and At. ferrooxidans. The culture supernatant of A. niger showed better metalremoval than At. ferrooxidans. Metals were removed faster from Sn-Cu-Ag solder than Sn-Cuand Sn-Pb solder by A. niger culture supernatant. Considering the toxicity of metals towardmicrobial cells, a two-step bioleaching process described in the present work is found to beeffective. Future studies are aimed on production of organic acid by using cheaper carbonsources. This will make the process more economical. Also, the use of bioreactors canfacilitate large-scale production of organic acid. The process developed in the present studywill be applied on a large scale for recovery of metals not only from solders but also from otherindustrial sources. An application of NaCl, NaOH, and H2S precipitated selectively Ag, Sn,and Pb, respectively. This study puts forth an effective environmental-friendly alternative forrecycling of waste solders.

Acknowledgment The current research is supported by the National Science Council of Taiwan undercontract 100-2221-E-007-015-MY3.

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Microbial Leaching of Waste Solder for Recovery of Metal