Leaching of metals from printed circuit board powder by an Aspergillus niger culture supernatant and hydrogen peroxide

Abstract

An efficient and rapid metal leaching process for solubilisation of heavy and precious metals from waste printed circuit board (PCB) powder was investigated. The PCB powder was first treated with 0.1 M sodium hydroxide (NaOH) solution to remove the chemical coating. An incubation time of 8 h and 150 rpm shaking speed were found to be optimum for the removal of the chemical coating from the PCB powder (1 g). The NaOH treated PCB powder was further subjected for metal leaching using an Aspergillus niger (A. niger) culture supernatant and hydrogen peroxide (H₂O₂). 100% solubilisation of all the metals was observed. The shaking speed showed insignificant effect on metal solubilisation. The 3.18% H₂O₂ concentration was found to be optimum for metal solubilisation. An increase in temperature (80 °C) reduced the time required for metal solubilisation. Optimization of all these parameters considerably decreased the leaching period to 2 h.

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1. Introduction

In recent years, a drastic increase in diversity and consumption of electrical and electronic equipment has taken place. The rapid technological development has reduced the life span of this equipment. This resulted in concomitant generation of ever increasing amounts of end-of-life equipment, known as waste electrical and electronic equipment (WEEE) or E-waste.^1,2 PCBs are a basal and essential part of EEE. They constitute about 6% of the total weight of electronic scrap. Their main components are non-conducting substrates or laminates, conductive circuits printed on or inside the substrate and mounted components (chips, connectors, capacitors).^3,4 In PCBs, the main metallic elements include copper (Cu), aluminium (Al), nickel (Ni), iron (Fe), tin (Sn), lead (Pb) and precious metals such as silver (Ag) and gold (Au). PCBs also contain a large number of hazardous substances such as brominated flame retardants and other heavy metals. Therefore, these days the waste printed circuit boards (WPCBs) are responsible for metal pollution in the environment.⁵ On the other hand, WEEE can be considered as secondary ores since they contain metals in quantities even higher than the natural ores.⁶ Also the purity of the metals present is more as compared to ores. Therefore, recycling of WEEE is of prime importance from both environmental and economic points of view.^7,8

There are three technologies currently being used for treatment of WEEE. These are pyrometallurgical, hydrometallurgical and bio-hydrometallurgical (bioleaching) processes.⁹ Out of these three methods pyrometallurgical, and hydrometallurgical processes are commercially applied on large scale. However the pyrometallurgical processes generate secondary atmospheric pollution through the release of toxic gas, i.e. dioxins and furans. Also they are not suitable or economical to recycle the WEEE of low metal content.^5,10 The hydrometallurgical processes are fast and economical as compared to pyrometallurgical processes. But they generate high volume of acid wastewater. Also they require high energy.^5,11

Use of microorganisms (bio-hydrometallurgical approach) for the recovery of metals from WEEE is gaining increasing attentions, since it is simple, environmental friendly, and economical.¹² Several authors reported the bioleaching of metals from WEEE using a variety of bacterial and fungal cultures. Still, the Acidithiobacillus ferrooxidans (At. ferrooxidans) and Acidithiobacillus thiooxidans (At. thiooxidans) are the most widely used species in bioleaching process.^{3–5,12–18} Very few studies reported the use of fungus for recovery of metals from WEEE.^13,19–21 Because constant supply of nutrients for fungal growth, handling of fungi in turnover and long processing time restricts use of fungus.²² Besides these limitations, fungal bioleaching has several advantages over bacterial bioleaching. Fungi have the ability to grow at high pH, which makes them more effective for bioleaching of alkaline materials. They can also leach metals rapidly with a short lag phase. They secrete organic acids which are able to chelate metal ions thus being useful in metal leaching process. Despite such advantages, there is lack of information regarding metal leaching from WEEE using fungi.^22–25 The WEEE does not contain any source of energy for the growth of bacteria. Therefore external supply of nutrients is must in case of bacterial leaching of metals from WEEE. Hence the heterotrophic leaching which includes fungal assisted bioleaching is the suitable and effective process for recovery of metals from WEEE. Aspergillus niger is one of the most extensively studied fungi. It has proven advantages over bacterial leaching, such as its ability to grow under a wide range of pH, ability to produce high concentrations of organic acids and a faster leaching rate.^22–27

Besides many advantages of bioleaching process, commercial implementation of this process is in a stage of infancy. This is due to inherently slow nature of this process. Many of the reported bioleaching processes require 48 to 245 h to recover metals. Also these processes cannot recover all of the metals present in WEEE.²⁸ Therefore there is a need to develop a fast and economical bioleaching process that has industrial scale application.

Considering these limitation, present study was undertaken to develop a fast and industrially applicable process. To achieve this, a two-step bioleaching process was developed using A. niger. In the first step the fungus was grown for organic acid production. In the second step the culture supernatant without fungal cells was collected and applied for metal leaching process. Rasoulnia and Mousavi, (2016) also showed the usefulness of spent-medium leaching for V and Ni recovery from a vanadium-rich power plant residual ash.²⁵ They suggested several advantages of application of culture supernatants for metal leaching process. Such a process separates the biomass from solid waste. This facilitates the recycling of the biomass. Also it avoids the contamination of waste material being treated by the microbial biomass. The organic acid production by the microorganism can be optimized. This process avoid the metal toxicity towards fungus and hence higher pulp densities can be applied as compared to one-step bioleaching process.²⁵ Before the bioleaching process, the PCB powder was treated with sodium hydroxide solution. This step removed the chemical coating of metals and made them easily available for reaction with organic acid produced by the fungus. Lastly the rate of metal solubilisation was increased with addition of hydrogen peroxide (H₂O₂). The H₂O₂ can act as reducing or oxidizing agent. In alkaline media it acts as reducing agent while in acidic media it acts as oxidizing agent. The final decomposition products of H₂O₂ are oxygen and water. Hence, its use has no adverse effects on environment. It is easy to handle.^29,30 Compared to many other oxidants such as hypochlorite and permanganate, hydrogen peroxide does not carry any additional or interfering substance except water, for the reaction system.³⁰ Previously it has been used in effluent treatment, soil bioremediation, industrial gas treatment and recovery of metals from Li-ion batteries.^30,31 Therefore, it is important to study the effect of H₂O₂ addition on metal leaching from WEEE.

2. Materials and methods

2.1. Materials

All the chemicals were purchased from Sigma Aldrich. The deionized water was used in preparation of solutions. The waste PCB was obtained from local market. PCB scrap was crushed and then ground to fine powder. The “299.3 μm” fraction was used for all the metal leaching experiments.

2.2. Microorganism used and collection of supernatant

A. niger was obtained from the Food Industry Research and Development Institute (FIRDI), Taiwan. Sucrose medium was used for the growth of fungi. The medium contains the following substances per liter of glass-distilled water: 100 g sucrose, 1.5 g NaNO₃, 0.5 g KH₂PO₄, 0.025 g MgSO₄·7H₂O, 0.025 g KCl, and 1.6 g yeast extract.³² A. niger was grown in sucrose medium for 10 days at 120 rpm and 30 °C. The culture was centrifuged at 10 000 rpm for 30 min to remove the cells in the bottom of centrifugation tube. The supernatant was then filtered through 0.22 μm filter. The cell free culture supernatant was used for metal leaching studies.

2.3. Determination of metal content of PCB powder

The PCB powder (1 g) was digested with aqua-regia to fully leach the metals. The digestion solution was then filtered through a 0.45 μm membrane filter and diluted using deionized water. Inductively coupled plasma-optical emission spectrometry (ICP-OES, PerkinElmer) was applied to determine various metals present in the digestion solution. The metal content of the PCB is shown in Table 1.

Table 1 Metal content of PCB powder

Sr. No.	Metals	Metal content (mg g^−1)
1	Copper (Cu)	80.25
2	Aluminum (Al)	56.68
3	Tin (Sn)	9.8
4	Lead (Pb)	4.99
5	Boron (B)	4.0
6	Iron (Fe)	2.10
7	Silicon (Si)	1.10
8	Magnesium (Mg)	1.00
9	Titanium (Ti)	0.47
10	Nickel (Ni)	0.26
11	Strontium (Sr)	0.19
12	Zinc (Zn)	0.14
13	Arsenic (As)	0.022
14	Manganese (Mn)	0.015
15	Cobalt (Co)	0.0059
16	Cadmium (Cd)	0.0040
17	Palladium (Pd)	0.0025
18	Gold (Au)	0.0017
19	Silver (Ag)	0.0590

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2.4. Pre-treatment of PCB powder with sodium hydroxide

In the present study, the PCB powder was treated with sodium hydroxide prior to metal leaching to remove solder mask/chemical coating. To do this a pre-treatment method was adopted from the reported studies with little modification.^33,34 The PCB powder (1 g) was mixed with 0.1 M NaOH. The PCB powder floats on the surface of NaOH solution. During the NaOH treatment the metals present in PCB powder get free from chemical coating and settle down at the bottom of container (Fig. 1). After certain incubation time, the powder was washed repeatedly with water until it attains neutral pH. The effect of incubation time on pre-treatment of PCB powder with NaOH was studied. The PCB powder was treated with 0.1 M NaOH at 30 °C and 150 rpm for 4 and 8 h. The effect of shaking speed on pre-treatment of PCB powder with NaOH was studied. The PCB powder was treated with 0.1 M NaOH at variable shaking speed (0–200 rpm) and 30 °C for 8 h. The effect of increasing pulp density on pre-treatment of PCB powder with NaOH was studied. The variable pulp density of PCB powder (1–5 g) was treated with 0.1 M NaOH separately at 150 rpm and 30 °C for 8 h. During all above experiments, the amount of metals freed from chemical coating was determined by digesting with aqua-regia. Also the metals removed by NaOH solution from PCB powder were determined. After the NaOH treatment the PCB powder was washed thoroughly and then used in further metal leaching study.

Fig. 1 Process describing removal of chemical coating of PCB powder using 0.1 M sodium hydroxide solution.

2.5. Metal leaching procedure

The metal leaching system studied in the present work used A. niger culture supernatant. Organic acid production by this fungus was studied previously.³⁵ It produced only citric acid using sucrose as a carbon source. The study reported the citric acid production and change in pH at varying time periods. It was observed that the citric acid production increased with incubation time until 10 days. A. niger produced 8.65 (±0.305) g l⁻¹ citric acid after 10 day. This study also noted change in pH during citric acid production. An initial pH of sucrose medium was 6.0. The pH of the medium dropped significantly in first 24 h. It changed to 3.07 (±0.15) from pH 6.0. The pH decreased gradually until it reached 2.16 (±0.23).³⁵ Various culture conditions were optimized for citric acid production. Effect of initial pH of the medium on citric acid production was studied. The citric acid production increased with an increase in initial pH value of the medium. Maximum citric acid production (10.88 ± 0.29 g l⁻¹) was observed in the growth medium adjusted to pH 4.0.³⁵ An effect of temperature on the citric acid production by A. niger was studied. An optimum temperature for citric acid production was found to be 25 °C with 20 (±0.22) g l⁻¹ citric acid production.³⁵ Citric acid production is an aerobic process. Therefore effect of varying stirring speed on citric acid production was studied. It was observed that an amount of citric acid produced ranged from 8.76 to 20 g l⁻¹ when the stirring speed was maintained between 0 and 180 rpm. An optimum stirring speed was found to be 120 rpm. The 20 (±0.061) g l⁻¹ citric acid production was observed at this stirring speed.³⁵ Considering these results of previous study, the culture supernatant was collected in the present study after incubating A. niger in a sucrose medium of pH 4.0, for 10 days at 25 °C and 120 rpm. The A. niger culture supernatant collected in this way had a pH 1.97 and contained 20 g l⁻¹ citric acid.³⁵

A two-step bioleaching process was applied in the present study. The pre-treated PCB powder was mixed with A. niger culture supernatant (100 ml) in 500 ml beakers. To this 3.18% hydrogen peroxide was added and the beaker was incubated in an incubator at static condition and 30 °C. Aliquots of the samples were withdrawn at 12 and 24 h and filtered. The clear samples were sent for heavy metal content analysis by means of ICP. Various control experiments were carried out to study the metal solubilisation from PCB powder. For the first control experiment the leaching solution contained 100 ml deionised–distilled water and 3.18% H₂O₂. In this experiment the NaOH treated PCB powder was used for metal solubilisation experiment. In the second control experiment the leaching solution contained 100 ml A. niger culture supernatant only. In this experiment the NaOH treated PCB powder was used for metal solubilisation experiment. In the third control experiment the PCB powder (without NaOH treatment) was added in 100 ml A. niger culture supernatant only. In the fourth control experiment the PCB powder (without NaOH treatment) was added in 100 ml deionised–distilled water and 3.18% H₂O₂. In the fifth control experiment the PCB powder (without NaOH treatment) was added in 100 ml A. niger culture supernatant. To this 3.18% H₂O₂ was added. All the experiments were carried out in an incubator at static condition and 30 °C temperature for 24 h. The metal extraction percentage was calculated as follows,

2.6. Effect of H₂O₂ concentration on metal solubilisation

The effect of H₂O₂ concentration on the metal solubilisation from pre-treated PCB powder was studied. The pre-treated PCB powder was added in A. niger culture supernatant (100 ml) in various beakers. Then various concentrations of H₂O₂ (1.66–4.56%) were added. The beakers were incubated at static condition and 30 °C temperature for 24 h.

2.7. Effect of shaking speed on metal solubilisation

The effect of shaking speed on the metal solubilisation from pre-treated PCB powder was investigated. For this study the pre-treated PCB powder was mixed separately with A. niger culture supernatant (100 ml) and 3.18% H₂O₂. Then the beakers were incubated at 30 °C under two conditions. For one set of experiments the beakers were kept in a static incubator while for other set of experiments the beakers were kept in an orbital shaking incubator at 150 rpm for 24 h.

2.8. Effect of temperature on the time required for metal solubilisation

The effect of temperature on the time required for the metal solubilisation from pre-treated PCB powder was studied. The pre-treated PCB powder was mixed separately with A. niger culture supernatant (100 ml) and 3.18% H₂O₂ in 500 ml beakers. These beakers were incubated at various temperatures (30–90 °C) and at static condition. The time required for the metal solubilisation was determined during the experiment.

2.9. Effect of pulp density on metal solubilisation

The effect of pulp density on the metal solubilisation from pre-treated PCB powder was studied. The pre-treated PCB powder was mixed separately with A. niger culture supernatant in varying pulp densities (10–40 g l⁻¹) in 500 ml beakers. To these beakers 3.18% H₂O₂ was added. These beakers were incubated at static condition and 80 °C temperature for 2 h.

2.10. Analytical techniques

During the metal solubilisation experiments, samples were taken at scheduled intervals and sent for ICP analysis to determine the metal content. All tests were performed in three replicates.

3. Results and discussion

3.1. Dissolution of metals during NaOH treatment of PCB powder

The PCBs have a chemical coating (solder mask). Solder mask covers the metals mounted on PCBs. The metals remained covered with solder mask even after crushing to the powder form. This mask does not allow the leaching agent to penetrate through it and reach the metals effectively for their solubilisation. Therefore it is necessary to remove the chemical coating. The NaOH (0.1 M) was used for the treatment of 1 g PCB powder to remove this chemical coating. The results show that after NaOH treatment, the weight of powder reduced from 1 g to 0.2016 g (Fig. 1). During this pre-treatment process some of the metals from PCB powder were dissolved in NaOH solution and removed. Around 0.9, 0.5 and 0.3 mg g⁻¹ of Pb, Al and Sn were removed respectively. Along with these metals 28, 26, 14 and 2 μg g⁻¹ Zn, Fe, Cu and Ni were removed respectively from PCB powder. No dissolution of other metals was observed during the NaOH treatment (data not shown). A small fraction of the Zn, Sn, Fe, Pb, Cu and Ni was lost during the NaOH treatment as compared to the metal content of PCB powder (Table 1). Hence, the NaOH treatment of PCB powder prior to metal leaching process is very useful to remove the chemical coating of the metals. Other studies have reported similar effect.^33,34 But these studies used large pieces of PCBs. The present study is the first report showing the effect of NaOH treatment on PCB powder.

3.2. Effect of process parameters on NaOH treatment of PCB powder

Fig. 2 show the effect of incubation time on NaOH treatment of PCB powder. In first 4 h the chemical coating of metals was removed and over 60% of all the metals were recovered except Al (54%), Sn (43%), Pb (33%) and Si (42%). The PCB powder was further treated for 8 h. It was observed that over 95% of all the metals were recovered in 8 h. Further incubation to 12 h showed insignificant effect on metal recovery. These results show that 8 h is an optimum incubation time for NaOH treatment of PCB powder.

Fig. 2 Effect of incubation time on removal of chemical coating of PCB powder using 0.1 M sodium hydroxide solution.

The effect of shaking speed on NaOH treatment of PCB powder revealed that, at static condition very small fraction of metals were released from the chemical coating. The metal release from chemical coating was increased with an increase in shaking speed. Over 95% of all the metals were released from chemical coating at 150 rpm. Further increase in shaking speed to 200 rpm had no effect on metal release from chemical coating (Fig. 3).

Fig. 3 Effect of shaking speed on removal of chemical coating of PCB powder using 0.1 M sodium hydroxide solution.

The increasing pulp density severely affected (except for Mn) the metal release from chemical coating. The metal release was decreased with an increase in pulp density. Around 50–80% of metals were released from chemical coating at 5 g 100 ml⁻¹ pulp density (Fig. 4). The concentration of NaOH may not be sufficient to release the metals at higher pulp densities. Therefore 1 g 100 ml⁻¹ pulp density was selected to treat the PCB powder.

Fig. 4 Effect of pulp density on removal of chemical coating of PCB powder using 0.1 M sodium hydroxide solution.

3.3. Leaching of metals from PCB powder

The A. niger culture supernatant in combination with H₂O₂ was used for the metal leaching from NaOH treated PCB powder. The 100% metal solubilisation was achieved for all the metals tested in 24 h at 30 °C temperature and at static condition (Fig. 5f). The acidolysis and chelating mechanisms of citric acid present in A. niger culture supernatant caused the metal solubilisation from NaOH treated PCB powder. The presence of H₂O₂ accelerated the metal solubilisation process. The NaOH treatment facilitated the removal of chemical coating from metals present in PCB powder. This helped the culture supernatant to reach to the metals present in PCB powder. The presence of chemical coating interfere the contact of metals with leaching reagent and delay the metal solubilisation. To avoid this effect, the PCB powder was treated with 0.1 M NaOH solution. Few control experiments were carried out using NaOH treated and untreated PCB powder, to explain further the effect of chemical coating on metal solubilisation and the effect of H₂O₂ and culture supernatant on metal solubilisation.

Fig. 5 Metal solubilisation under various experimental conditions: (a) NaOH treated PCB powder + 100 ml deionised–distilled water and 3.18% H₂O₂; (b) NaOH treated PCB powder + 100 ml A. niger culture supernatant; (c) without NaOH treated PCB powder + 100 ml deionised–distilled water and 3.18% H₂O₂; (d) without NaOH treated PCB powder + 100 ml A. niger culture supernatant; (e) without NaOH treated PCB powder + 100 ml A. niger culture supernatant and 3.18% H₂O₂; (f) NaOH treated PCB powder + 100 ml A. niger culture supernatant and 3.18% H₂O₂.

For the first control, NaOH treated PCB powder was added to distilled water containing 3.18% H₂O₂ and incubated for 24 h, at static condition and 30 °C. A very small fraction of metals were dissolved using this system ranging from 0.7% (Mg) to 0.0002% (As) (Fig. 5a). For the second control, NaOH treated PCB powder was added to the culture supernatant and incubated for 24 h, at static condition and 30 °C. The metal extraction increased slightly, but still the fraction of metals dissolved was very low ranging from 1.45% (Sn) to 0.0004% (Co) (Fig. 5b). For the third control, PCB powder without NaOH treatment was added to distilled water containing 3.18% H₂O₂ and incubated for 24 h, at static condition and 30 °C (Fig. 5c). For the fourth control, PCB powder without NaOH treatment was added to culture supernatant and incubated for 24 h, at static condition and 30 °C (Fig. 5d). The results of Fig. 5c and d showed that the metal dissolution was lower as compared to Fig. 5a and b. The comparison of Fig. 5a–d and f suggest that the NaOH treatment enhanced metal solubilisation. Also, for the fifth control, PCB powder without NaOH treatment was added to 100 ml A. niger culture supernatant and 3.18% H₂O₂ and incubated for 24 h, at static condition and 30 °C. The metal solubilisation increased as compared to other controls, but still no significant metal solubilisation was achieved. The metal solubilisation was in the range from 5.6% (Cu) to 0.001% (Co) (Fig. 5e). The results also suggest that the combination of A. niger culture supernatant and H₂O₂ is required for an effective metal solubilisation (Fig. 5a–f). These results also support the assertion that the chemical coating doesn't allow the leaching reagent to reach the metals embedded within it. Hence a pre-treatment of PCB powder with NaOH support the better solubilisation of metals.

3.4. Possible mechanisms for metal dissolution

Several mechanisms can be put forth to explain the metal dissolution process described in the present study. Organic acids dissolve metals by supplying protons and ligands. They can dissolve the metallic fractions by acidification and complexation. In acidification an organic acid dissociate to donate H⁺ for proton-promoted dissolution processes.^36–39

RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺ (1)

The reduction of protons generates hydrogen and oxidizes the metal,

2H₃O⁺ + 2e⁻ → H₂ + 2H₂O (2)

M → M²⁺ + 2e⁻ (3)

In a complexation mechanism, the ligands from organic acids, for example citrate (Cit) from citric acid, forms stable complexes with metals present in PCB powder. The complexation reaction can increase the solubility of metals in solution:^36–39

M = metal; R = organic substituent group.

The H₂O₂ can also take part in metal dissolution. The H₂O₂ can act as an oxidant or a reductant in its reactions with metals. A generalized mechanism for metal oxidation by H₂O₂ can be proposed as per the following reaction,⁴⁰

M_(s) + H₂O_2(aq) → M_(aq)²⁺ + OH + OH⁻ (5)

M_(s) + OH → M_(aq)²⁺ + OH⁻ (6)

M: metal.

In the present study the H₂O₂ acted as oxidant since use of citric acid created acidic conditions. The results in Fig. 5 show that a combination of A. niger culture supernatant and H₂O₂ showed better metal dissolution compared to individual use of these systems. This suggest that all of the above mentioned mechanisms act together when the combination of A. niger culture supernatant and H₂O₂ was used and favoured better metal dissolution.

3.5. Effect of H₂O₂ concentration on time required for metal solubilisation

The effect of H₂O₂ concentration on metal leaching was studied. It was observed that the metal solubilisation in the range of 64% (Cu) to 78% (Ag) was observed in 24 h, at static condition and 30 °C using 1.66% H₂O₂ concentration. The metal solubilisation increased with an increase in H₂O₂ concentration. The 100% solubilisation of all the metals present in NaOH treated PCB powder was observed at static condition and 30 °C, using 3.18% H₂O₂ concentration. Insignificant effects were observed by further increasing the H₂O₂ concentration to 4.56% (Fig. 6). Therefore, 3.18% concentration was considered to be optimum for metal solubilisation from NaOH treated PCB powder. These results suggest that the metal solubilisation efficiency depends on the H₂O₂ concentration which is similar to the other results obtained.^29,41 Fig. 7 shows the H₂O₂ concentration as a function of time. The concentration of H₂O₂ decreased with incubation time. In absence of citric acid and PCB powder the H₂O₂ concentration decreased from 3.18% to 1.06%, in 24 h at 30 °C temperature. It can be predicted that the natural decomposition of H₂O₂ caused this decrease in the concentration. In the presence of citric acid and PCB powder the H₂O₂ concentration decreased from 3.18% to 0.2%, in 24 h at 30 °C temperature. This caused due to reaction of H₂O₂ with metals present in PCB powder and natural decomposition.

Fig. 6 Effect of H₂O₂ concentration on metal solubilisation from NaOH treated PCB powder at 30 °C temperature and static condition in 24 h.

Fig. 7 Effect of incubation time on H₂O₂ concentration during the metal leaching from NaOH treated PCB powder at 30 °C temperature and static condition in 24 h.

3.6. Effect of shaking speed on metal solubilisation

The results of present study showed that the shaking speed has insignificant effect on metal leaching from NaOH treated PCB powder, using 3.18% H₂O₂ at 30 °C temperature. The 100% metal solubilisation was achieved for all the metals tested in 24 h, at static as well as shaking speed (data not shown). Negative effect of increase in shaking speed on metal solubilisation process was observed by other authors.⁴²

3.7. Effect of temperature on time required for metal solubilisation

The effect of various temperatures on the time required for metal solubilisation from NaOH treated PCB powder was studied using 3.18% H₂O₂, at static condition. The results show that the increase in temperature accelerated the metal solubilisation process. The time required for metal solubilisation decreased with an increase in temperature. Only 2 h incubation was sufficient for 100% solubilisation for all of the metals at 80 °C temperature. Further increase in temperature to 90 °C showed insignificant effect (Fig. 8). These results are in accordance to Li et al. (2010).²⁹ According to him, at high temperatures the dissociation process of citric acid undergoes an endothermic reaction. This releases more H⁺ in the solutions. Therefore, the leaching velocity of metals increases. Similar explanation can be given for the reduction of time required for metal solubilisation from PCB powder with an increase in temperature.

Fig. 8 Effect of temperature on time required for metal solubilisation from NaOH treated PCB powder at 3.18% H₂O₂ concentration and static condition.

3.8. Effect of pulp density on metal solubilisation

The pulp density is an important parameter to establish a metal leaching process. Effect of pulp density on the metal solubilisation from NaOH treated PCB powder was studied. In all above mentioned experiments the pulp density used was 2 g l⁻¹. At this pulp density 100% metal solubilisation was observed for all metals. The metal solubilisation was significantly affected by increasing the pulp density to 10 g l⁻¹ except for Cu, Mg and Ti. Also it was observed that the metal solubilisation decreased with an increase in pulp density (Fig. 9). The H₂O₂ added externally and chelating agents supplied by A. niger culture supernatant may not be sufficient to solubilise the metals at higher pulp densities.

Fig. 9 Effect of pulp density on metal solubilisation from NaOH treated PCB powder at 3.18% H₂O₂ concentration and static condition in 2 h.

3.9. Significance of the present study

The chemical coating present on PCBs prevents access of microorganisms or leaching reagents to the metals present in PCBs.^33,34 Due to this the metal solubilisation processes need longer incubation time. In the present method the PCB powder was treated with NaOH solution. This removed the chemical coating and the metals in the PCB powder were made accessible to the A. niger culture supernatant and H₂O₂. The results of present study showed that the metal solubilisation efficiency enhanced due to pre-treatment of PCB powder with NaOH solution.

Most of the bioleaching studies reported application of At. ferrooxidans and At. thiooxidans for solubilisation of metals from PCB powder. Very few studies reported the metal bioleaching from PCB powder by fungi. In these reports, the metal solubilisation was either low or required long time. It was found in other study that, A. niger is able to solubilize 41% Cu and 80% Ni from WEEE powder in 21 days.¹³ Two Aspergillus strains were used for metal solubilisation from waste cellular PCBs and computer goldfinger motherboards (GFICMs).²¹ They found 5 and 29% Cu recovery using various conditions. They also found that the fungi showed limited ability to dissolve Ni. Biometabolized acids were also used for leaching of metals from E-waste.¹⁹ They suggested that the organic acids metabolised by heterotrophic fungi solubilized Cu more effectively as compared to sulphuric acid generated by Acidithiobacillus bacteria at pH 2.0. Solubilisation of 68.3% of Cu and 27.9% of Pb were obtained after 42 days bioleaching treatment using fungus A. niger.²⁰ The presence of metals in PCB powder inhibited the growth of fungi.²¹ The method described in the present study is far superior as compared to above mentioned reports with respect to types of metals, amount of metals solubilised and time required for solubilisation (Table 2). Also in present study a two-step bioleaching process has been applied. The organic acid production phase is separated from metal leaching process. This helps to avoid the metal toxicity towards fungi and to increase the efficiency of metal leaching process. The addition of H₂O₂ to A. niger culture supernatant facilitated reduction in time required for metal solubilisation from NaOH treated PCB powder. The 100% solubilisation of all the metals present in PCB powder in 2 h showed the potential application of presented method on industrial scale.

Table 2 Results for metal leaching from PCB powder

Fungus	Citric acid concentration (g l^−1)	pH	H2O2 concentration (%)	Stirring speed (rpm)	Leaching temperature (°C)	Pulp density (g l^−1)	Metals and their recovery (%)
A. niger	20	1.97	3.18	0	80	10	Cu (99), Mg (99), Ti (98), Mn (84), Zn (82), Sn (78), Ni (76), As (76), Sr (73), Cd (72), Co (72), Ag (72), Al (70), Si (70), Pb (65), B (63), Fe (61), Pd (40), Au (30)

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The bacterial bioleaching processes used for solubilisation of metals from PCB powder face certain problems. In bioleaching processes based on ferrous ion oxidation the jarosite precipitation is unavoidable due to increase in pH. This lowers the Fe³⁺ concentration in the solutions and cause low metal solubilisation. The maintenance of low pH is must to avoid this precipitation.¹⁶ The precipitate formed during the bioleaching process is composed of Sn, Cu, Pb and Fe.^14,33,43,44 Occurrence of such precipitate makes it difficult to distinguish between the precipitate and residual PCB powder.^33,44 The formed precipitate contaminate the PCB powder and make the overall metal solubilisation process more complicated. It also creates problems for the final separation of non-metallic fraction of PCB sample and metal purification.^33,34 The present method is based on microbial production of organic acid therefore the iron based precipitate formation is avoided. The NaOH treatment removes the chemical coating of metals in PCB powder. Hence, the final metal purification becomes easy.

4. Conclusion

The present study showed that the presence of chemical coating slow down the metal solubilisation from PCB powder. The treatment of PCB powder with NaOH facilitated the efficient contact between metals present in PCB powder and the lixiviant used. Also, a new approach of using combination of A. niger culture supernatant and H₂O₂ was demonstrated for the solubilisation of metals from PCB powder. The results showed that such combination effectively solubilized more metals as compared to individual culture supernatant and H₂O₂. The concentration of H₂O₂ showed significant effect on metal solubilisation from NaOH treated PCB powder. The time required for metal solubilisation was reduced from 24 h to 2 h with an increase in temperature from 30 °C to 80 °C. The removal of chemical coating from metal in PCB powder prior to metal leaching process may improve the subsequent purification of metals from leach liquors. The presented study has great potential for large scale application.

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