Environmentally friendly synthesis of copper nanoparticles from waste printed circuit boards

Separation and Purification Technology 230 (2020) 115860

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Environmentally friendly synthesis of copper nanoparticles from waste printed circuit boards

T

Rania Seif El-Nasra, S.M. Abdelbasirb, , Ayman H. Kamela, Saad S.M. Hassana ⁎

a b

Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt Electrochemical Processing Dept., Mineral Processing Div., Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87, Helwan 11421, Cairo, Egypt

ARTICLE INFO

ABSTRACT

Keywords: Waste printed circuit boards (WPCBs) Alkaline leaching Ammoniacal/ammonium salts Copper nanoparticles Ascorbic acid

A facile environmentally friendly hydrometallurgical approach to recover copper as nanoparticles from electronic waste (e-waste) has been presented. Selective leaching of copper from powdered old computers waste printed circuit boards (WPCBs) was executed in ammoniacal/ammonium salt solutions at room temperature. The effect of parameters controlling copper recovery has given due consideration. Copper nanoparticles were prepared from leachant solutions by reduction using L-ascorbic acid reductant and (CTAB) as a modifier at room temperature. Characterization tests for the produced copper nanoparticles were performed to confirm its structure. XRD analysis manifested the pure crystallinity phase of copper nanoparticles and TEM images revealed its spherical shape with particle size in the range of 5–32 nm. It is considered that the studied process is merely efficacious and environmentally sustainable for preparing copper nanoparticles from WPCBs with the intention of recycling e-waste to gain a higher valued product.

1. Introduction With the speedy progress of electronic products, massive quantities of waste electric and electronic equipment (WEEE) have been engendered in the late years. For all electronic devices printed circuit boards (PCBs) are most required constituents [1–3]. The common and important constituents in PCBs are ceramics, metals, and polymeric parts [4]. About 30% mass of wasted PCBs (WPCBs) includes wide array of important metals, for example, copper, gold and silver and furthermore unsafe materials, for example, tin, lead and brominated fire inhibitors [5]. Copper is the essential metal in electronic appliances. It is the highest content in the WPCBs that ranges from10 to 30 masses % among the metallic elements [6]. If these WPCBs are disposed inappropriately, noxious materials would cause harsh environmental problems and countless high-valued metals would be lost. Therefore, Recycling of waste PCBs increased extraordinary considerations for the waste handling, as well as for the recovery of valuable materials [4,5,7,9]. Presently, many technologies were suggested for recovering copper from WPCBs. The suggested technologies include primarily pyro metallurgy [10,11], hydrometallurgy [7,8,12], bio-technology [13–15] and mechanical methods [16,17]. Amongst these technologies, hydrometallurgical processes are fairly low expenses, producing no fumes or dusts, selective and appropriate for traditional applications, are considered promising choices for the handling of WPCBs [18,19]. ⁎

Nevertheless, most of the hydrometallurgical processes principally stressed on leaching copper. After hydrometallurgical processes, the leach liquor containing copper of WPCBs needs subsequent treatment or purification for gaining highly valued products that can be reprocessed in various industrial fields. Accordingly, the recovery of copper from Cu-rich WPCBs will be a good impact. Hydrometallurgical treatments are more pliable throughout the upscaling and management processes. They are also considered more environment friendly and lesser energy consuming when compared to other WPCBs recycling methods [20]. Acids suchlike sulfuric, hydrochloric and nitric are frequently used as leachants in hydrometallurgical treatments to simultaneously leach numerous metals, especially iron and lead, from the WEEE and not give pure copper [21]. The separation process has tendency to become intricate due to inferior selectivity [22–24], and removing impurities out of the leach solution can be pricey resulting in possible waste of copper [6]. Selectively leaching copper from WPCBs was lately improved based upon ammonia leaching process [25]. The effect of ammonium salts on the copper recovery was described by Oishi and his coworkers [26]. They found that ammonium sulfate was more selective than ammonium chloride in the leaching and also in the purification step by solvent extraction. Impact of several ammonium salts on the copper recovery was also examined [27] and a high leaching efficiency from ammoniacal solution of ammonium persulfate was reported. A 90%, 60% and

Corresponding author. E-mail address: [email protected] (S.M. Abdelbasir).

https://doi.org/10.1016/j.seppur.2019.115860 Received 30 June 2019; Received in revised form 24 July 2019; Accepted 25 July 2019 Available online 25 July 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

Separation and Purification Technology 230 (2020) 115860

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Table 1 Metal content in waste PCBs specimens determined by XRF. Element

Cu

Sn

Pb

Fe

Ni

Au

Ag

Zn

Cr

Mn

Ti

Others

Content (wt%)

26.00

10.50

1.56

2.40

0.61

0.07

0.13

0.46

0.07

0.04

0.60

64.01

120

0.5M 1.0M 2.0M

Copper recovery (%)

100 80 60 40 20 0

0

30

60

90

120

150

180

Time (min) Fig. 3. Effect of time on the copper recovery percentage using different concentrations of ammonium citrate at room temperature, liquid-to-solid ratio of 10, ammonia concentration of 8%, and constant stirring rate of 400 rpm.

Fig. 1. The system used for leaching experiments.

100

60

40

20

0

0

30

60

90

120

150

0.5M 1.0M 2.0M

80

Copper recovery (%)

80

Copper recovery (%)

100

0.5M 1.0M 2.0M

60

40

20

0

180

Time (min)

0

30

60

90

120

150

180

210

240

Time (min)

Fig. 2. Effect of time on the copper recovery percentage using different concentrations of ammonium carbonate at room temperature, liquid-to-solid ratio of 10, ammonia concentration of 8%, and constant stirring rate of 400 rpm.

Fig. 4. Effect of time on the copper recovery percentage using different concentrations of ammonium chloride at room temperature at room temperature, liquid-to-solid ratio of 10, ammonia concentration of 8%, and constant stirring rate of 400 rpm.

9% of Cu, Zn, and Ni respectively were extracted from the WPCBs in 10 h at the most favorable conditions (2 kmol m−3 NH4Cl and 5 kmol m−3 NH3 solution with 0.1 kmol m−3 CuCl2 at 200 rpm and 30 °C with 1% pulp density). Pure copper (99.9%) was accomplished by electrowinning subsequent stage. Lim and coworkers studied the ammonia/ ammonium salt leaching of some valuable metals from an alloy obtained from smelting reduction process of mobile phone WPCBs [28]. The sort of ammonium salt, ammonia concentration and the interaction of WPCBs metals were investigated. The metal percentages leached were noticed to increase with the increase in ammonia concentration and the content of Cu could increase to reach 98% when using ammoniacal ammonium chloride leachant. On the other side, substantial interest was concentrated on metal nanoparticles referring to the intriguing characteristics and potential

roles in various areas. Copper nanoparticles have drawn considerable concern regarding their astonishing catalytic, optical, and electrical conducting properties [29,30]. Lately, different methods were suggested by various researchers for the preparation of ultra- fine copper powders from WPCBs [31–33]. Those methods included chemical reduction [34], cementation [21], electrochemical process [29,35], and electro kinetic process [36,37]. In such processes, it was difficult to manipulate the size of the prepared ultrafine copper. In this work for the first time, a simple, non-expensive, environmentally friendly process for the recovery of pure copper as nanoparticles from leachant solutions of WPCBs is presented. L- ascorbic acid was nominated as a reductant and also as a stabilizing agent for copper nanoparticles. Copper was first extracted from WPCBs powder 2

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120 100

Copper recovery (%)

[(C16H33)N(CH3)3Br; CTAB], Sigma-Aldrich] for the preparation of copper nanoparticles. Highly pure water and ethanol were utilized for solutions preparation, synthesis and washing of nanoparticles respectively. Approximately 2 kg of WPCBs of old computer monitors were obtained from a local computer mall shop. The WPCBs were slashed to pieces of about 5 cm × 5 cm using pliers. The WPCBs slashes were then crushed using a laboratory-scale crusher and further ground in a disc mill (HERZOG Maschinenfabrik GMBH Co.) to reduce their size to 0.6 mm. X-ray fluorescence spectrometer (XRF) (Axios Advanced WDXRFP analytical, Netherland) was used to determine the chemical composition content of WPCBs and confirmed by analysis using Atomic Absorption Spectrometer (Savantaa, Australia). The content of main elements in PCBs sample is shown in Table 1.

Amm. Citrate Amm. Chloride Amm. Carbonate

80 60 40 20 0

20

30

40

50

60

70

80

2.2. Leaching experiments

Temperature,°C

Ammonia–ammonium salt leaching experiments were performed in 500 ml double-necked glass vessel equipped with a condenser to prevent vaporization loss and a thermometer for controlling temperature. The vessel was put in a water bath placed on a stirring hotplate. All experiments performed with constant stirring (400 rpm). A draw of the used leaching system is displayed in Fig. 1. The leaching solutions (8% ammonia solution, ammonium salt solutions with concentration ranging from 0.5 to 2.0 M) were firstly added into the reactor then, 5 g of WPCBs powders were added to the solution and this was tagged as the experiment commence. For all experiments, 20 ml of leaching solution was used and the L/S ratio was kept constant at 10 except mentioned. Varieties in temperature (25–80 °C) and alkaline salt concentration (0.5–2 M) were considered. For the optimal conditions, experimentations with L/S of 20 and 30 were also conducted. The leaching residue was filtered and rinsed carefully with pure water. Atomic absorption spectrometer (AAS) was used for analyzing metal concentration in solutions. The copper recovery percentage was estimated by mass balancing obtained from the analysis of the raw WPCBs powder and the leach residue. Copper recovery prescribed as the copper percent leached into ammoniacal solution from the raw sample, computed by the equation:

Fig. 5. Effect of temperature on the copper recovery percentage using different leachants of ammonium salts (0.5 M), ammonia concentration of 8%for time period of 90 min and constant stirring rate of 400 rpm.

120

Amm.Citrate Amm. Chloride Amm. Carbonate

Copper recovery(%)

100 80 60 40 20 0

0

10

20

Liquid/Solid ratio

30

40

Copper recovery (%) =

Fig. 6. Effect of liquid/solid ratio on the copper recovery percentage using 0.5 M of different ammoniacal/ammonium salts for 90 min at room temperature.

Copper leached into ammoniacal solution × 100 Total copper in WPCBs original sample

2.3. Synthesis of copper oxide nanoparticles 5 g of WPCBs powder was leached using 0.5 M Ammonium Chloride in 8% ammonia solution with S/L ratio (1/10) for time periods (1–4 h). The solution was then left for 24 h. A black powder of copper oxide (CuO) was formed. The particles were centrifuged and washed many times with pure water followed by pure ethanol to expel unreacted ions, then dried.

using ammoniacal–ammonium carbonate, chloride and citrate solutions. The influence of different parameters as leaching time, temperature, solid/liquid ratio and concentrations of these alkaline solutions on the extraction of copper was examined. 2. Experimental procedure

2.4. Copper nanoparticles synthesis

2.1. Materials and chemicals

For copper nanoparticles (CuNPs) production, a proper weight of CTAB (0.01 g) was dissolved in 10 ml water, then, 20 ml of leached copper solution and L-ascorbic acid were added in two ways; either once or gradually with rate 3 ml/min. The full process was conducted in a closed two- necked bottle and the solution heated for 30 min at 70 °C, the solution became reddish in color confirming nanoparticles formation. The solution was then left to cool for 24 h before filtering the

All chemicals used were pure grade. The leaching solutions were prepared by dissolving ammonium salt [ammonium chloride (NH4Cl), ammonium carbonate (NH4)2CO3 and ammonium citrate dibasic ((NH4)2 C6H6O7) from Alfa Acer Chemicals] in 8% ammonia solution (NH3, 25% Adwic Co, Egypt.). L-ascorbic acid (C6H6O8, Alpha Aecar) was used as reducing agent and Cetyltrimethylammonium bromide

3

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Cu CuCl

B)

Cu

Amm. carbonate

(220)

(200)

Intensity (a.u.)

(220)

(200)

Intensity (a.u.)

(111)

(111)

A)

Amm. carbonate

Amm. citrate

Amm. citrate Amm. chloride

20

Amm. chloride

40

60

80

20

2-Theta (degree)

40

60

80

2-Theta(degree)

Fig. 7. XRD pattern for copper nanoparticles synthesized from different 0.5 M leachants at the optimum leaching conditions using L-ascorbic acid added: (A) gradually with rate 3 ml/min; (B) immediately.

particles out, washing repeatedly with pure water and ethanol then drying in vacuum. Fig. S1 in supplementary file shows the system used for synthesis.

where X depicts an anion from the salt (carbonate or chloride or dihydrogen citrate for instance). The net reaction can be written as:

1 Cu + 2NH3 ·H2 O + 2NH 4 + + O2 2

2.5. Characterization of the prepared nanoparticles

3. Results and discussion 3.1. Ammoniacal/ ammonium salt leaching study With the aim of extracting copper selectively from the WPCBs, a leaching reagent perfectly needed to react specifically and only with copper and be inert to other metals. Copper ions, in solutions with acidic pH values, are stable and also form stable complexes at alkaline pH (6.5–11.0). Hence, it can be removed from other reactive metals in electronic waste by leaching with ammonia or ammonium salt solutions. During the extraction process, the ammonium salt’s role is providing anions to the copper ammine complex [Cu (NH3)n2+] and also protons to react with OH− anion liberated during the reaction. Two main steps are intricate in the reactions of copper and the leachant solution as [38,39]:

1 O2 2

CuO

CuO + 2NH3⋅H2O+(NH4)2X → Cu(NH3)4X + 3H2O

(3)

Copper recovery from WPCBs sample was performed at ordinary temperature (~25 °C). Many influences are impacting the leachability or copper recovery such like ammonium carbonate, ammonium chloride and ammonium citrate concentration, solid–liquid ratio and leaching temperature. Ammonia solutions are highly particular and it is an economical choice in terms of usage for specific metals dissolution. Additionally, its price is low with respect to many solvents [34,40]. Fig. 2 shows the influence of time on extracting copper from WPCBs using ammonia/ammonium carbonate leachant at a temperature of 25 °C, liquid-to-solid ratio of 10, 8% ammonia, and stirring at 400 rpm. According to this figure, the impact of ammonium carbonate concentration on copper recovery is inconsequential, even though the recovery percentage decreases with increased concentrations. This can be credited to the interaction of ammonia and ammonium carbonate [41]. Ammonia and ammonium carbonate solution dissociate under equilibrium at a specific temperature as revealed by Eqs. (4) and (5). We can infer that by raising concentration of the ammonium carbonate, concentration of [CO32−] will increase and the overall [NH4+] concentration will decrease to keep K2 as a constant. Consequently, even if ammonium salt concentration was increased, the final recovery will still humbler.

Composition and crystal shape of the produced particles were confirmed by X-ray diffractometer (XRD) “Bruker AXS- D8, Germany” in the range 2θ from 20 to 70° using CuKα as a radiation source (λ = 1.54 Å). Fourier Transform Infrared Spectroscopy (FT-IR) was accomplished by means of JASCO-3600 spectrometer over the wavenumber spans from 4000 to 400 cm−1. Transmission electron microscope (TEM) “JEOL-JEM1230” was used to inspect the morphology of the synthesized particles and defining selected area electron diffraction (SAED) patterns. The absorbance spectra of the prepared particles were distinguished by a UV/Vis spectrophotometer Varian Cary 100 Scan system at 190–2200 nm with BaSO4 as a reference.

Cu +

Cu(NH3 )24 + + 3H2 O

NH3·H2 O

(NH 4 ) 2 CO3

NH+4 + OH 2NH+4 + CO23

K1 =

[NH4][OH] [NH3]

K2 =

[NH4]2[CO3] [(NH4)2CO3]

(4) (5)

In addition, diffusion can affect the leaching kinetics since the reactivity of the leachant solution becomes lower at low ammonia concentrations. The NH4+ ions dissipation on the metal surface might be influenced by high concentration of [CO32−] ions thus hindering the diffusion [38]. As revealed in Fig. 3, at constant temperature of 25 °C, and ammonium citrate concentration of 0.5 M, the Cu percentage in solution

(1) (2)

4

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R. Seif El-Nasr, et al.

Fig. 8. TEM images of copper nanoparticles prepared by gradual addition of L-ascorbic acid (3 ml/min) from different 0.5 M ammoniacal leachant solutions:(A, B) amm. citrate; (C, D) amm. carbonate; (E, F) amm. chloride at the optimum leaching conditions. Additional high resolution TEM images can be found in the supplemental section.

increased to 97% as leaching time reached 120 min. As can be noticed from the figure, the copper percentage in filtrate reduced distinctly from 97% to 65% as ammonium citrate concentration raised from 0.5 to 2 M. The dissociation reaction of ammonium citrate in solution is illustrated by Eqs. (6)–(9), with H+ originated from dissociation [42]. It is supposed that, by increasing the concentration of ammonium citrate, hydrogen ions (H+) generated from citric acid in solution will encourage the consumption of ammonium citrate [43]. Nevertheless, if the ingesting of ammonium citrate in reaction rules, the complex reaction may be diminished or detained thus inhibiting the leaching progression. (NH4)2 C6H6O7 + 2H2O = C6H8O7 + 2NH4OH

C6H8O7 = C6H7O7− + H+ C6H7O7

2−

C6H6O7

2−

=

C6H6O72−

=

C6H5O73−

(7)

+H

+

(8)

+H

+

(9)

Concentration range of ammonium chloride (0.5–2 M) were used to explore how the concentration affects the leaching progress at normal temperature (25 °C) and stirring of 400 rpm. Fig. 4 displays the changes in the copper recovery percentage with changing concentration of ammonium chloride. As was conspicuous that copper dissolution boosted by higher concentrations of ammonium chloride. The dissolution of copper was lower by using 0.5 M of ammonium chloride

(6) 5

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Fig. 9. TEM images of copper nanoparticles prepared from different 0.5 M leachant solutions by immediate addition of L-ascorbic acid: (A, B) amm. citrate; (C, D) amm. carbonate; (E, F) amm. Chloride. Additional high resolution TEM images can be found in the supplemental section.

(43–57%). This inferior dissolution might be referred to the deficiency of (NH4 +) as ligand [44]. When using 2 M ammonium chloride solution, the copper recovery increased. As anticipated, the copper recovery was highly increased from 58% to 75% by doubling the concentration of the ammonium chloride reaching a maximum percent of 79% with 2.0 M ammonium chloride solution. This was a supplementary confirmation that copper dissolution depends on the amount of (NH4+) in solution. Copper recovery increased using ammoniacal/ ammonium chloride solution because the slow change in pH of buffer solution during leaching ( ± 0.5) and also because of stable cuprammine complexes formation [26]. Temperature is a critical feature affecting the leaching reaction. In case of ammoniacal/ammonia salts leaching, temperature can highly affect the ammonia vaporization in the solution. Particularly, ammonium carbonate readily decomposes beyond 35 °C according to Eq. (10):

2NH4+ + CO32− Δ ⇄ 2NH3 + H2O + CO2

(10)

As given in Fig. 5, at high temperatures (50–80 °C), the ammonia losses due to vaporization from solution would be obtained because of its high vapour pressure and also its volatile nature. These ammonia losses during the extraction progress would impact the copper recovery percentage. In this regard, a closed reactor (where pressure is under control) might be adopted for industrial scale application of this process to prevent ammonia losses via evaporation [45]. The liquid -to-solid ratio could clearly affect the recovery process. Usually, a high recovery is obtained at a high ratio of liquid/solid [46]. The liquid/solid ratio through the current leaching study is reliant upon the ratio of the leachant volume and the solid WPCBs powders’ weight. The solution’s volume is increased whereas the ammonia and 6

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Transmittance (a.u.)

Amm.citrate Amm. chloride Amm. carbonate

606 1631

2918

1051 620

2916.8 3431

1630

3500

614 1632

2918 2918 3428.8

3431

1630

2915

3426

1625

1109

622

1109 624

1112

1629

3430

4000

1060

Amm. citrate Amm. chloride Amm. carbonate

1109

2918

3430

B)

Transmittance (a.u.)

A)

3000

2500

2000

1500

1000

4000

500

3500

-1

Wavenumber (cm )

3000

2500

2000

1500

-1

Wavenumber (cm )

1000

500

Fig. 10. FTIR for copper nanoparticles synthesized from different 0.5 M leachant solutions at the optimum conditions using L-ascorbic acid added: (A) gradually with rate (3 ml/min); (B) immediately.

A)

B)

Absorbance (a.u.) 450

citrate chloride carbonate

Absorbance(a.u.)

citrate chloride carbonate

500

550

600

650

700

750

800

450

500

550

600

650

700

750

800

Wavelength (nm)

Wavelength (nm)

Fig. 11. UV–Vis. spectra of copper nanoparticles synthesized from different leachant solutions at the optimum conditions using L-ascorbic acid added: (A) gradually with rate (3 ml/min); (B) immediately.

ammonium salt concentrations are kept constant. In Fig. 6, experiments were executed for 90 min at normal temperature (25 °C), ammonium salt concentration of 0.5 M, ammonia concentration of 8% and constant stirring of 400 rpm. An increase in liquid-to-solid ratio from 10:1 to 20:1 considerably increase the recovery of copper from the WPCBs by ammonium chloride and ammonium carbonate from 78 to 85% and from 58 to 79% respectively. The recovery decreases inconspicuously as the liquid-to-solid ratio increases to 30. It might be ascribable to the conviction that decreasing the pulp consistency helps the diffusion of reactants and products in lower liquid-to-solid ratio. However, in case of ammonium citrate, the change in copper recovery at liquid-to-solid ratios 10:1 is almost 98% and increasing the ratio to 20:1 and 30:1, the copper extraction decreases to 95% and 94% respectively. An increment in the liquid/solid ratio results in the consequent increase of the reaction reagent initial concentration and also contributes to higher mass transfer motivating cuprammine complex formation [47]. Accordingly, a relevant liquid/solid ratio of 10: 1 is thereby nominated for experimentations to synthesize copper nanoparticles from ammoniacal ammonium citrate leachant solution.

3.2. Synthesis of copper nanoparticles The structure and phase composition of the produced particles using (0.567 mol.) L-ascorbic acid (added either gradually with rate 3 ml/ min. or immediately) were established from XRD pattern displayed in Fig. 7(A, B). Bragg reflections at 2θ values 43.047°, 50.189° and 73.910° characterize (1 1 1), (2 0 0) and (2 2 0) crystallographic planes of face centered cubic structure of copper (JCPDS card No 85-1326). In Fig. 7(A), three noticeable diffraction peaks indexed to the (1 1 1), (2 2 0) and (3 1 1) planes of cubic CuCl phase at 2θ = 28.6°, 47.5°, 56.3° are displayed in case of synthesis from ammonium chloride leachant solution. The lattice constant calculated from its XRD data is 5.414 Å, which agrees with the cubic CuCl phase in the JCPDS card No. 77-2383. In Fig. 7(B), no distinguishing peaks of impurities found, indicating the purity of Cu nanoparticles. The well-defined peaks confirm the high crystallinity of the product and assure that the changing of precursors such as copper chloride, copper carbonate and copper citrate have no influence on crystal phase of the Cu. The XRD patterns in Fig. 7(A) assured the formation of very minor amounts of CuCl. When L-ascorbic acid added immediately to the

7

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Ascorbic acid

Cuprous ion

Ascorbate radical

Ascorbate radical

Cu NPs

Ascorbate radical

Dehydroascorbic acid

Fig. 12. Proposed mechanism for copper nanoparticles formation by L-ascorbic acid.

three precursors give nanoparticles with two different morphologies: one is composed of nanospheres with a size range 5–25 nm (Fig A, C) in case of citrate and carbonate solutions, while chloride solution gives nanoparticles comprising nanorods (NRs) having thickness range 10–12 nm (Fig. 9E). We can claim that the nature of counter anion in each precursor plays a major role to size of the produced copper nanoparticle along with the alkaline reaction media and the way of adding the reductant to it. Additionally, these anions are usually stabilizing metal nanoparticles [51] leading in our case to increasing the copper yield (see Table S1 in supplemental). Obviously, as reported by many authors, Cl− ions here play a significant role to control the crystal morphology as halogen ions (specifically Cl− and Br− ions) have been used to confine the (1 0 0) facets of many nanometals structures controlling their shapes [52,53]. The Cl− ions with CTAB effectually confined the (1 0 0) facets of Cu, resulting in formation of Cu NRs. The insufficient chloride ions (0.5 M) used for stabilizing the growth process of the copper nanoparticles from aggregation and act on growth into Copper NRs through Oswalt ripening [54]. Fig. 10 displays the FTIR spectra of the produced Cu NPs from different leachant solutions using L-ascorbic acid added either gradually with rate (3 ml/min.) (A); or immediately (B) using CTAB as surface modifier. We can see that FTIR spectra of Cu NPs displayed vibration peaks observed at 1109 cm−1 and 2926.58 cm−1 (CeO and CH stretching), 1630 cm−1 (C]C stretch) and 3430 cm−1 (OeH stretch, H bonded). The bands at 1630 cm−1 and 1598 cm−1 (Cu NPs) emerge owing to carbonyl stretching. The FTIR bands of a Cu nanoparticles reveals broad absorption from 2900 cm−1 to 3430 cm−1 mostly recognized to OeH and CeO groups on the copper nanoparticles surface. The peak observed at 624 cm−1 is a CeO bond stretching and it indicates the formation of copper nanostructure [55].

solution, it reduced CuCl2/ammine complex to CuCl [48] which in turn should be reduced to copper metal at alkaline pH range from 9 to 11. Since the redox potential of ascorbic acid changes with pH, whereas that of CuCl does not, alkaline pH will upgrade the driving force for CuCl reduction [49,50]. As L-ascorbic acid was added immediately in a sudden way, the pH of the solution drops to almost 7 leading to the reduction of most CuCl to copper nanoparticles (Cu NPs) and slight amount of CuCl remains unreduced. The crystal shape and microstructure of Cu NPs prepared by immediate and gradual addition of L-ascorbic acid were investigated by TEM microscopy as displayed in Figs. 8 and 9. TEM analysis in Fig. 8(A, C, E) reveal the polydisperse character of Cu NPs with spherical shape having average size of 5–25 nm which agrees with the XRD measurements. The consistent selected area electron diffraction (SAED) pattern in Fig. 8(B, D, F) with shining rounded spots that assures the Cu NPs crystallinity. The diffraction lines in Fig. 8(B, D) associate only to metallic copper nanoparticles with no oxide phase detected. TEM analysis also displays the generation of spherical shape nanoparticles of Cu and CuCl (Fig. 8D). It can be suggested that the nature of counter anion of each precursor plays a serious function in building the size of nanoparticle. As revealed in Fig. 8(A, C and E), the size of copper particles tends to lessen and the average diameter considerably decreases from citrate to carbonate solution with no aggregation. The particle size of samples ranged from 15 nm to 25 nm, 8 nm to 12 nm and 5 nm to 32 nm for Cu nanoparticles synthesized from the ammoniacal copper citrate, carbonate and chloride respectively. The TEM analysis revealed in Fig. 9(A, C and E) displays the establishment of pure monodispersed Cu nanoparticles. The SAED pattern of the nanoparticles demonstrates the number of rings pointing the crystallinity of the material (Fig. 9(B, D and F)). It can be seen that the 8

Separation and Purification Technology 230 (2020) 115860

This work [29] [30] [31] [34] [32]

The absorption peaks of copper nanoparticles appear at 500–600 nm and usually become broadened with smaller particle size [56]. Herein, the distinctive peaks displayed at about 570 nm (Fig. 11), but displayed a broadened peak at a shorter wavelength in case of very small Cu NPs (separated from carbonate and chloride solutions by L-ascorbic gradual addition and from citrate solution by immediate addition). Hence, we can guess that the way of adding the reductant along with the solution anions lead to a more efficacious capping ability of reductant and consequently producing so tiny Cu nanoparticles. In case of Cu nanoparticles synthesized from chloride solution by gradual addition of Lascorbic acid, absorption peak broadening is due to both Cu and CuCl particles. The precise position of the absorption depends mostly on nanoparticle size, used solvent and surface modifier. In this case, some modification in arranging CTAB molecules around the copper particles (as a consequence of the anion variation) may be present.

5–50 nm 5 nm 20–50 nm 7 nm 7–14 nm 50–180 nm 81–86% 84% 88% 90% 99.3% 85% Cost effective/ecofriendly Cost effective/ecofriendly Ecofriendly Ecofriendly – Ecofriendly Ascorbic acid reduction in presence of CTAB Sol gel/ electrospinning Reduction with NaBH4 in presence of Triton X-100 Ascorbic acid reduction in presence of cyclodextrin Slurry electrolysis Supercritical methanol process Ammoniacal ammonium salts leaching Copper foil liberation by chemical-ultrasonic treatment and dissolution in HNO3 acid Micro emulsion at controllable ambient temperature and pressure Copper foil liberation by chemical-ultrasonic treatment and dissolution in H2SO4 acid Concentrated metal powders of WPCBs by digested by microwave aided HNO3-H2O2 -HF system Nitric acid leaching Cu Cu Cu Cu Cu Cu/Cu2O

Yield Advantages Synthesis method Leaching process used Nano particles

Table 2 Comparison of different studies carried out for recovery of copper nanoparticles from waste printed circuit boards.

Particle size

Reference

R. Seif El-Nasr, et al.

3.3. Formation mechanism of Cu NPs The copper nanoparticles were prepared via using ammoniacal/ ammonium salt solution because it produced the complex [Cu (NH3)4]2+. Then, this cuprammine complex dissociated to give copper cations that further reduced by L-ascorbic acid in water giving copper nuclei. As the reaction progressed, copper nanoparticles with different configurations and sizes formed with variable yields based on the solution they are synthesized from (See Table S1 in the supplemental). Lascorbic acid is a carboxylic acid having electrons in the hydroxyl and carbonyl groups on its ring. As such, its structure enables it to convert Cu2+ ions to Cu0 nanoparticles in two stages as proposed in Fig. 12. As can be seen L-ascorbic acid undergoes a reversible two-phase reductionoxidation process involving a free-radical intermediate. The first phase is forming ascorbate intermediate radical and an electron reduces cupric to cuprous ion. Another following one-electron phase leads to the formation of dehydroascorbic acid and reducing Cu(I) to Cu0 as nanoparticles. 3.4. Cost study of the Cu-NPs preparation A plain cost study of the Cu NPs synthesis method relied on cost of the prepared copper nanoparticles, reagents, and electricity used up through the complete laboratory process. Keeping in mind that the Cu proportion in WPCBs is 26 wt%, typical WPCBs contains 20–22% of CuNPs if handled via the proposed technique and one kg of recovered Cu would give 220 g of nanoparticles. Hence, based upon the predictable cost analysis and being ecofriendly compared to conventional techniques, the process seems promising and can be applied industrially for synthesizing nanoparticles from waste. Table S2 shows financial analysis on synthesis of 100 g Cu NPs from wasted printed circuit boards on laboratory scale. A comparison of the current work with various studies performed for recovering copper as nanoparticles from WPCBs is shown in Table 2. 3.5. Copper oxide nanoparticles preparation Fig. 13(A) shows a typical XRD peaks for the prepared single phase CuO nanoparticles confirmed from the standard card JCPDS 48-1548 with no impurity peaks, indicative of the highly pure particles. The broadening of the peaks shows that the crystal size is very small which was predictable from the peak (1 1 1) by using the Scherer formula as ranging to 15–16 nm. The FT-IR vibration spectra of the CuO nanoparticles are also exhibited in Fig. 13(B). The peak at 506 cm−1 is referred to the vibrations of Cu–O. The frequency with no other oxide (Cu2O) impurity [57,58]. The TEM images along with electron diffraction pattern shown in Fig. 14(A, B) established the single crystals nature of the nanoparticles. When ammonium chloride was used with ammonia solution as leachant for copper, Cu2+ ions form complex with anions of the solutions, the 9

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(200) (111)

B)

JCPDS #48-1548

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50

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1455 1373

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506 3421

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1635

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A)

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80

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3000

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Fig. 13. (A) XRD pattern (B) FTIR of CuO nanoparticles synthesized from 0.5 M ammoniacal/ammonium chloride leachant solution for time period 3 h and L/S ratio: 10.

Fig. 14. (A) TEM image, (B) SAED of CuO synthesized by ascorbic acid from ammoniacal/ammonium chloride solution.

bond strength of which adopts the order OH > NH3 [59]. At the pH of synthesis (pH:10), the highest stable species present is the complex [Cu (NH3)4] 2+. A rise in the pH causes lessening the stability of this complex, resulting in Cu(OH)2 precipitation. When the concentration of Cu2+ and OH− ions exceeded certain limit, the Cu(OH)2 converted into CuO and CuO nuclei started to precipitate.

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4. Conclusion A simple, efficient and encouraging method for the preparation of copper nanoparticles from the leach solutions of WPCBs of old computers. Ammonia/ammonium salt leaching process is proposed to selectively extract copper from the WPCBs with high extraction yields over 90% and avoiding the shortcomings of leaching with acids. Cu NPs can be prepared from the leachant solutions using an ecofriendly and low-cost method by reducing copper salts with L-ascorbic acid as reductant and stabilizer. Type of the copper salt and the way or reductant addition were found controlling the morphology and size of the nanoparticles. Since the reagents used are ecofriendly and the synthesis technique is economic, this method can be effortlessly utilized for different applications as electrochemical sensors or electronics inking. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.115860. 10

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