Introduction

The continual and unprecedented consumer demand for electrical and electronic equipment (EEEs) with rapid technological advancement and accelerated product obsolescence has led to build up of electronic waste (e-waste) worldwide. The rate of waste electrical and electronic equipment (WEEE) generation is so massive that it constitutes about 8% of municipal solid waste (MSW) and is the fastest growing waste stream in the world (Widmer et al. 2005). A major portion of the global WEEEs seems to originate from developed economies with highly saturated markets for EEEs. In the year 2014, the worldwide generation of e-waste was reported to be around 41.8 Mt. (Baldé et al. 2015). The worldwide e-waste generation is high and is expected to increase with growth rate of 4–5% per annum reaching to 49.8 Mt. by 2018 (Baldé et al. 2015). Country-wise e-waste generation linking to their per capita purchasing power has been reviewed in detail elsewhere (Priya and Hait 2017). Printed circuit boards (PCBs) make the foundation of EEEs constituting about 3–6% of their total weight and a significant portion of metal content of e-waste (Basdere and Seliger 2003; Das et al. 2009). New technological innovations, short lifespan of electronic products continue to accelerate e-waste generation, leading to a significant increase in waste PCBs (Huang et al. 2009; Zhu and Gu 2002).

PCBs have complex and heterogeneous matrix with significantly high content of metals, particularly heavy and base metals such as Cu, Zn, Fe, and Pb as well as precious metals like Au, Ag, Pd, and Pt in quantities even higher than their natural repositories making them a potential secondary reservoir of recyclable metals (Cui and Zhang 2008). Apart from the rich content of valuable and base metals, there are also different toxic elements present in PCBs (Cui and Zhang 2008; Huang et al. 2009). However, the elemental composition of PCBs varies depending on type, class, and application. In general, PCBs have a complex composition of materials and constitute approximately 28% of metals, 23% of plastics, and remaining percentage of glass and ceramic (Zhou and Qiu 2010). The principal support material for PCB laminate is glassfiber or silica onto which metals are coated (Hall and Williams 2007).

Majority of metals in PCBs are Cu, Pb, Zn, Fe, Ni, and Sn in which content of Cu is around 10–20%, Pb is 1–5%, and Ni is 1–3% of PCB (Maragkos et al. 2013; Wang et al. 2009; Zhou and Qiu 2010). The toxicity of e-waste is connected with the presence of metallic constituents, such as As, Hg, Cd, Cr, and Pb beyond total threshold limit concentration (TTLC) (Li et al. 2004). PCBs of EEEs such as computers, laptops, and mobile phones have rich metallic content. A single mobile phone is reported to contain more than 40 elements of the periodic table including metals like Ga, In, Ti, Si, Ge, As, Au, Pt, and Ag apart from base and toxic metals such as Cu, Zn, Fe, Pb, Ni, and Cr (Maragkos et al. 2013; Li et al. 2004). PCB of end-of-life computer of various brands also contain up to 20–24% of Cu, 0.2–20% Fe, and 0.6–6.3% of Pb which are significantly high when compared with the natural deposits (Bandyopadhyay 2008; Kang and Schoenung 2005; Zhou and Qiu 2010).

High metal content of PCBs have necessitated a dedicated recycling process for the recovery of metals for economic development and also for waste treatment to avert environmental pollution. Various metallurgical techniques for recycling of metals from e-waste has been reviewed in great detail elsewhere (Priya and Hait 2017). The prioritization of target metals to be recycled is motivated by the economics as well as environmental metrics and is largely influenced by the proportion of the metals in waste PCB. Comminution of PCB to appropriate size range for efficient metals liberation followed by a good digestion method for complete solubilization preceding metal recycling are the premise to further metal recovery. Though, a wide range of the United States Environmental Protection Agency (USEPA) acid digestion methods exist at the research and development level for sample preparation thereby improving solubilization of various metals from analytes (Agazzi and Pirola 2000). But most of these methods aim at analytes decomposition and elemental dissolution from matrices such as soil, sludge, sediments, and siliceous, organic-based materials. While no standardized qualitative and quantitative metal liberation procedure exploiting chemical digestion in correlation with particle size exists for accurate determination of metal content in waste PCBs. Although studies (Grosser et al. 2007; Mello et al. 2015) have been conducted on solubilization and quantification of elements from bulk e-waste, scarce information is available on metal liberation and dissolution focussing on complex and heterogeneous matrix of PCBs. Due to lack of effective chemical digestion method, accurate, reproducible, and efficient metals, liberation and determination in e-waste such as PCBs is difficult. Therefore, the task of identifying the most suitable acid digestion method for metal characterization from PCBs is indispensable for accurate metal quantification and recovery. However, the degree of metal liberation also depends on PCB comminution. Metals liberation from PCBs is reported to increase with decreasing size range (Cui and Forssberg 2003; Wen et al. 2005). In view of increasing concerns over eco-profitable recycling of metals from e-waste, the objectives of this research are to assess metal liberation upon comminution using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) analysis followed by comparative evaluation of the existing USEPA digestion procedures viz., USEPA 3050B, USEPA 3051A, and USEPA 3052 for effective dissolution of metals from comminuted particles of waste PCBs of computer, laptop, mobile phone, and TV so as to recommend the most appropriate method for sample preparation for their characterization.

Materials and methods

Waste PCBs

The e-waste used for this study is PCB scraps from end-of-life EEEs, viz., computer, laptop, mobile phone, and TV set (Fig. 1). A total of 24 PCBs with six each from each type of obsolete EEEs were collected from local scrap dealers.

Fig. 1
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Typical waste PCBs of a computer, b laptop, c mobile phone, and d TV used for the study

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Sample preparation

Mounted components such as capacitors, resistors, and batteries were removed manually from the PCBs before comminution. The PCBs were then dismantled and fragmented before comminution. Further, particle size reduction of the PCBs was carried out by means of cutting mill (SM200, Retsch GmbH, Germany). The comminution was obtained through shearing and cutting of the PCBs. The comminuted waste PCBs were used for further physical and elemental characterization. Physical characterization was conducted for each of the PCBs from each type of four EEEs (n = 6 each), and chemical characterization of the PCBs was conducted from each of the EEEs (n = 18 each).

Metals liberation and dissolution assessment

Particle size, morphology, and liberation assessment of comminution fines

Particle size distributions of the comminuted waste PCB samples were observed under optical microscope (BX51, Olympus, Japan). The qualitative metal release assessments of waste PCBs upon comminution were conducted by SEM-EDS (EVO 18, Zeiss, Germany) at magnification of 100× operated at 20 keV to generate elemental lines under EDS mode to characterize morphology and element liberation from PCBs.

Metal dissolution by the USEPA digestion procedures

For quantitative metal dissolution and liberation assessment, the samples were subjected to three different USEPA acid digestion procedures: USEPA 3050B, USEPA 3051A, and USEPA 3052 (USEPA 1995, 1995, 1997). USEPA 3050B was conventional thermal extraction with acids mixture in an open vessel with reflux, while the other two methods, USEPA 3051A and 3052, were microwave-assisted digestions conducted in closed vessels with different acid mixtures in microwave digester (Ethos Easy, Milestone, Italy) using high-pressure SK-15 rotor as per the USEPA prescribed procedure. Along with the samples taken in triplicates, one blank determination was also performed for every digestion procedure. Details of the three USEPA digestion procedures for sample preparation are mentioned in Table 1. The solutions obtained after acid digestions were transferred to volumetric flasks to make up to 100 ml volume with distilled water. The resulting solutions were filtered through 0.22 μm Millipore filter paper. The corresponding filtrates were subjected to analysis by atomic absorption spectroscopy (AAS) (iCE3500, Thermo Scientific, USA). All digestions and analysis were conducted in triplicates.

Table 1 Details of the USEPA digestion methods employed for sample preparation
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Metals analysis by atomic absorption spectroscopy

The samples generated after acid digestion were investigated for the content of eight metals, Cu, Zn, Fe, Pb, Ni, Cd, Cr, and Mn using atomic absorption spectroscopy. Calibrations for the analyzed metals were based on linear working graphs made from four different concentrations of standards of the respective metals. Detailed conditions for AAS analysis are given in Table 2.

Table 2 Operating conditions of the AAS for metal analysis
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Statistical analysis

One-way analysis of variance (ANOVA) was used to analyze significant differences between different USEPA digestion procedures for obtained metal dissolution, since the experimental investigation fits the classical one-way factorial design. Tukey’s HSD (honestly significantly differences) test was also performed as a post hoc analysis to identify the homogenous type of digestion conditions for the obtained results. The probability level used to analyze statistical significance was P < 0.05 for the tests. All statistical analyses were performed using the SPSS Package (Version 22).

Results and discussion

Metals liberation analysis by SEM-EDS and particle size distribution

The PCB samples analyzed under SEM depicted the distribution of metals in waste PCB comminution fines. Figure 2 shows SEM micrograph of comminuted PCB of end-of-life computer, laptop, mobile phone, and TV set. The lustrous particles were analyzed to be metal composites. As evident from the SEM micrographs and EDS analysis, most of the metallic particles are unliberated from the PCB matrix and are attached to silica. The heterogeneity in shapes and sizes of particles is symptomatic of various shearing, tensile, and composite forces under which the comminution of PCBs was achieved. The general SEM morphology provided a qualitative assessment of sections showing abundance of composite particles. The amalgam of silica content with metals in the SEM micrograph confirmed that majority of metallic particles are unliberated and complexed to PCB laminate.

Fig. 2
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SEM images of comminuted waste PCBs of a computer, b laptop, c mobile phone, and d TV

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The SEM-EDS compositional analyses of comminuted PCBs are presented in Table 3. The EDS analysis shows chemical compositions of few constituent parts annotate in Fig. 2 of comminuted PCB samples under investigation. Comminuted fractions of size < 1 mm contained metals like Cu, Zn, Pb, Fe, Ni, Cd, Cr, and Mn in ample; however, the metals are present in amalgamation with each other. Additionally, the PCB samples were observed to have high silica content complexed to metallic particles. The fiberglass/silica laminate locks up the metals which upon physical and chemical treatment get liberated. This leads to comparatively high metal liberation in fine comminuted fractions of PCBs as compared to the coarser ones. However, ultrafine PCB particles have lower metallic deportment, because metallic substances are generally ductile in nature and do not shatter easily into dust during comminution in contrast to their non-metallic counterparts.

Table 3 The SEM-EDS analysis of comminuted waste PCBs for metal composition (n = 6 each)
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A similarity concerning diversity in particle shapes and sizes was observed in the comminuted samples of the waste PCBs observed under optical microscope. Majority of the particles of waste PCBs are in range of 75–180 μm, while very few particles are of size < 75 μm and > 180 mm up to 2 mm (Fig. 3). Thus, determination of effective metal dissolution from waste PCB comminution fines of diversified range was found to require comparison of data from more than one digestion condition.

Fig. 3
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Particle size distribution of comminuted waste PCBs of a computer, b laptop, c mobile phone, and d TV

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Comparative assessment of digestion methods based on metal dissolution

The metal content of the PCBs of computer, laptop, mobile phone, and TV determined by the AAS after exposure to three different USEPA digestion conditions are presented in Tables 4, 5, 6, and 7, respectively. The USEPA digestion methods showed statistically significant (P < 0.05) effect on metal dissolution from the comminuted waste PCBs. Among the USEPA digestion methods, the USEPA 3052 method showed maximum content of all the eight metals in waste PCBs. Cu was determined to be the most abundant metal in all the four PCBs analyzed. Cu solubilized in the laptop PCB samples by the three USEPA methods were significantly different from each other with the content of 16.38 ± 0.08 wt% by the USEPA 3050B, 18.70 ± 0.14 wt% by the USEPA 3051A, and 20.13 ± 0.04 by the USEPA 3052 (F 2, 24 = 3486.289, P = 0.000). Similarly, the content of Zn (1.89 ± 0.05 wt.%) as determined following the dissolution by the USEPA 3052 was also maximum in laptop PCB and significantly varied (F 2, 24 = 176.412, P = 0.000) with other two USEPA methods. The USEPA digestion methods showed statistically significant (P < 0.05) variation in dissolution of the toxic metals, viz., Pb, Cd, Ni, and Cr from waste PCBs. Solubilization of Pb from the TV PCBs by the three USEPA digestion methods was significantly (F 2, 24 = 207.756 P = 0.000) different from each other with the maximum content of 2.26 ± 0.08 wt.% by the USEPA 3052 method. Ni content of 0.100 ± 0.002 wt.% obtained through the USEPA 3052 digestion method was maximum in mobile phone PCBs, while Cd content of 0.0812 ± 0.0008 wt.% was found to be the highest in the USEPA 3052 digestate of computer PCBs. The contents of both Ni and Cd also varied significantly in mobile phone and computer PCBs with the USEPA digestion methods (mobile phone: F 2, 24 = 4013.331, P = 0.000; computer: F 2, 24 = 14,198.050, P = 0.000). The variability in metal contents achieved by the three different USEPA methods can be explained on the basis of factors such as acids and digestion conditions employed for metal dissolution from waste PCBs. High-efficiency digestion of samples by the USEPA 3052 and USEPA 3051A can be attributed to the disintegration of PCB particles due to the high-pressure generated in microwave digester leading to exposure of unliberated metallic particles of the samples to acid attack leading to greater solubilization as compared to hot plate USEPA 3050B digestion method (Nadkarni 1984). Metal dissolution trend by the digestion conditions in the present study is in consistent with Agazzi and Pirola (2000), who reported that the microwave-assisted digestion yields higher recoveries comparable to hot plate digestion. In addition to the digestion conditions, acids used in sample digestion are also important in determining metal dissolution. It can be inferred that HF being strong acid aided to efficient dissolution of metals complexed to silica matrices in PCBs as evident from the SEM-EDS metal liberation analysis and lead to better metal solubilization by the USEPA 3052 method as compared to the acids used in the USEPA 3051A method. The trend of better dissolution of metals from PCBs by the USEPA 3052 digestion method using HF as compared to other USEPA digestion methods is in agreement with the digestion of other silica-rich materials such as soil and sediment (Bettinelli et al. 2000; Chen and Ma 1998, 2001).

Table 4 Metals content (wt.%) in waste computer PCBs using the USEPA digestion methods
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Table 5 Metals content (wt.%) in waste laptop PCBs using the USEPA digestion methods
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Table 6 Metals content (wt.%) in waste mobile phone PCBs using the USEPA digestion methods
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Table 7 Metals content (wt.%) in waste TV PCBs using the USEPA digestion methods
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The difference in metal solubilization by the three USEPA digestion methods is large as evident from Tables 4, 5, 6, and 7. This entails the importance of selection of an appropriate digestion method for metal dissolution prior to quantification. Microwave-assisted USEPA 3052 method was found to be the most suitable method for digestion of waste PCBs for metal dissolution. In general, microwave-assisted method in comparison to conventional thermal method is rapid, safe, efficient, and unsusceptible to losses of volatile metals.

Cu being highly conductive metal was highest in percentage among the other analyzed metals in all the four types of waste PCBs. Cu concentration was found to be superior in laptop PCBs (20.14 ± 0.04 wt.%), followed by computer PCBs (19.35 ± 0.13 wt.%) and mobile phone PCBs (18.40 ± 0.07 wt.%) as compared to TV PCBs (10.31 ± 0.06 wt.%) with significant variation with the USEPA digestion methods (laptop: F 2, 24 = 3486.289, P = 0.000; computer: F 2, 24 = 1128.581, P = 0.000; mobile phone: F 2,24 = 33,214.351, P = 0.000; TV: F 2,24 = 72,700.894, P = 0.000). Being the base metal, the USEPA digestion methods showed significant effect on the dissolution of Zn with its content of 19.35 ± 0.13 wt.% in computer PCBs (F 2, 24 = 81.007, P = 0.000), 1.89 ± 0.06 wt.% in laptop PCBs (F 2, 24 = 176.412, P = 0.000), 1.27 ± 0.24 wt.% in mobile phone PCBs (F 2, 24 = 24.170, P = 0.000), and 1.15 ± 0.09 wt.% in TV PCBs (F 2, 24 = 1268.770, P = 0.000). The results are predictable considering the diversity in types of PCBs, i.e., single layer, multilayer (Ladou 2006).

Fe having strongly magnetic properties was found in majority in all the four PCBs showing the viability of its recovery from PCBs in abundance. The high content of Fe could be attributed to that Fe has good electrical conductivity and mechanical strength. Pb concentration in all the PCB samples was greater than 1.5 wt.% except in mobile phones. This metal is used extensively in soldering of components on PCB laminate (Zhang and Forssberg 1997). As there are more electrical and microelectronic components welded onto the PCB platform in case of computer, laptop, and TV than in mobile phone, the content of Fe was higher in them. The bulk presence of these four metals, viz., Cu, Zn, Fe, and Pb in PCBs was normal because of their conductivity, mechanical strength, and welding properties. In contrast, it can be observed that the higher Ni content of 1.00 ± 0.02 mg/g was observed in mobile phone PCBs as compared to other PCBs. This is probably because of the use of Ni film in metallic contact with the key pad in mobile phones (Svoboda and Fujita 2003; Veit et al. 2005). The content of other metals such as Cd, Cr, and Mn was observed to be particularly low as compared to other heavy metals. The possible reason might be the use of these metals in PCBs in traces (Zhang and Forssberg 1997).

The content of all the eight metals analyzed in computer, mobile phone, and TV PCBs in the present study was in the range as reported in literature (Table 8) (Baba et al. 2010; Bandyopadhyay 2008; Deveci et al. 2010; Hall and Williams 2007; Hageluken 2006, 2007; Hageluken and Art 2006; Hino et al. 2009; Kang and Schoenung 2005; Kasper et al. 2011; Yamane et al. 2011; Yazıcı et al. 2010; Zhou et al. 2007). However, the comparison of metal content in waste laptop PCBs with literature could not be possible, as there has been no prior study concerning quantification of metals in laptop PCBs to the best of our knowledge. The slight variation in selected metal content in the present study with the available scientific literature can be attributed to differences in type, make, and model of PCBs under investigation.

Table 8 Metal content (wt.%) in waste PCBs in the present study in comparison with literature data
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It is evident from the metal quantification in waste PCBs that the base metals like Cu and Zn and toxic metals such as Pb and Cd were present in abundance and in general exceeded the TTLC limit (Table 9) prescribed by the Department of Toxic Substances Control (DTSC), USA (2004). However, most of the metals that are present in PCBs do not pose toxicity to the environment in their low concentrations within the TTLC limit. As the content of metals, viz., Cu, Zn, Pb, and Cd, in all the four types of PCBs under investigation have exceeded the TTLC limit, the e-waste is aptly categorized as hazardous waste. This has, therefore, necessitated the task of designing sustainable EEEs with reduced environmental impact and eco-friendly e-waste recycling options.

Table 9 The total threshold limit concentration (TTLC) for metals under investigation (DTSC 2004)
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Conclusions

Metals liberation upon comminution waste PCBs of computer, laptop, mobile phone, and TV was assessed using SEM-EDS analysis followed by comparative evaluation of the USEPA digestion methods, viz., USEPA 3050B, USEPA 3051A, and USEPA 3052, for their efficacy in solubilization of eight metals such as Cu, Zn, Fe, Pb, Ni, Cd, Cr, and Mn from comminuted particles for characterization of e-waste for recycling. Upon comminution, partial release of metals from the silicate matrix of PCBs was observed as evident from the SEM-EDS analysis for metal liberation. It implies that the requirement of aggressive treatment like digestion procedure for greater metal dissolution from e-waste. However, the metal solubilization is not only dependent on the digestion conditions but also on the degree of metal liberation upon comminution so as to expose the metals completely from the PCB matrix.

The USEPA digestion methods showed statistically significant (P < 0.05) difference with greater dissolution of metals by the USEPA 3052 method. Apart from other conditions, the use of strong acid like HF in the USEPA 3052 method ensures effective dissolution of metals complexed to silica matrices in PCBs. The metal quantification in waste PCBs demonstrated that the base metals like Cu and Zn and toxic metals such as Pb and Cd were present in abundance and in general exceeded the TTLC limit. Following the USEPA 3052 method, the maximum contents of Cu (20.13 ± 0.04 wt.%) and Zn (1.89 ± 0.05 wt.%) in laptop PCBs, Pb (2.26 ± 0.08 wt.%) in TV PCBs, Cd (0.0812 ± 0.0008 wt.%) in computer PCBs, and Ni (0.100 ± 0.002 wt.%) and Cr (0.0099 ± 0.0004 wt.%) in mobile phone PCBs were observed. It is recommended from the present study that the USEPA 3052 digestion method to be used for effective dissolution metals from e-waste for its characterization prior to metal recovery and recycling.