Closed-Loop Electrochemical Recycling of Spent Copper(II) from

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Sustainability Engineering and Green Chemistry

Closed-Loop Electrochemical Recycling of Spent Copper(II) from Etchant Wastewater Using a Carbon Nanotube Modified Graphite Felt Anode Yan Chang, LIN DENG, Xiaoyang Meng, Wen Zhang, Chunzhen Wang, Yuxin Wang, Song Zhao, Li Lin, and John C. Crittenden Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06298 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Environmental Science & Technology

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Closed-Loop Electrochemical Recycling of Spent

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Copper(II) from Etchant Wastewater Using a Carbon

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Nanotube Modified Graphite Felt Anode

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Yan Chang1, Lin Deng2,3, Xiaoyang Meng2, Wen Zhang1, 2*, Chunzhen Wang1, Yuxin Wang1,

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Song Zhao1, Li Lin2,4, John C. Crittenden2

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1 State Key Laboratory of Chemical Engineering, Co-Innovation Center of Chemical Science &

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Engineering, Tianjin Key Laboratory of Membrane Science & Desalination Technology and

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School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China.

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2 Brook Byer Institute for Sustainable Systems and School of Civil and Environmental

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Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.

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3 Key Laboratory of Building Safety & Energy Efficiency and Department of Water Engineering

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and Science, College of Civil Engineering, Hunan University, Changsha 410082, China.

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4 Basin Water Environmental Research Department, Changjiang River Scientific Research

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Institute, Wuhan, 430010, China.

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* To whom correspondence should be addressed (W. Z.)

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E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT

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Developing effective technologies for treatment of spent etchant in printed circuit boards

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industries is of paramount for sustainable copper reuse and reducing copper discharge. We

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developed a novel closed-loop electrochemical cell for on-site regeneration of spent acidic cupric

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chloride etchant. It does not have any emissions and recycles all the copper using a three-

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dimensional graphite felt anode decorated with carbon nanotube (CNT/GF). The CNT/GF anode

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oxidizes Cu(I) to Cu(II) so that the spent cuprous chloride can be converted to cupric chloride

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and reused. The decorated CNT layer with abundant oxygen-containing functional groups

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significantly enhanced the electrocatalytic activity for Cu(II)/Cu(I) redox. The CuCl 32 - is

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oxidized to CuCl + at the anode and the CuCl + is reduced to Cu(0) at the cathode. The closed-

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loop cycle system converts the catholyte into the anolyte. On average, the energy consumption of

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Cu(I) oxidation by CNT/GF is decreased by 12%, comparing to that by untreated graphite felt.

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The oxidation rate of Cu(I) is determined by the current density, and there is no delay for the

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mass transport of Cu(I). This study highlights the outstanding electrocatalytic performance, the

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rapid mass-transfer kinetics and the excellent stability of the CNT/GF electrode, and provides an

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energy-efficient and zero-emission strategy for the regeneration of etchant waste.


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INTRODUCTION

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Because of the rapid increasing market demand for electronic and electrical equipment, the

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printed circuit boards (PCB) industry has expanded sharply in the past decade.1 Current PCB

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industries involve diverse toxic chemicals during different manufacturing processes including

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board preparation, circuit pattern transfer, etching and plating processes. These processes pose a

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serious potential hazard to the environmental and human health.2-4 In particular, about 70% of

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copper is removed from copper covered PCB in the etching step, and it generates a spent etchant

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with a high concentration of copper ion. The quantity of spent etchant produced by 1 m2 of PCB

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is between 1.5 to 3.5 liters, and the total spent etchant yield is about 1 billion cubic meters in the

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world annually.5 Accordingly, developing an efficient regeneration process for spent etchant is a

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crucial issue to minimize copper wastes originating from PCB industries and recovery of copper.

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Since acidic cupric chloride (CuCl2) solution has the advantages of high etch rate, high

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dissolved copper capacity and excellent operating controllability, and it is the dominant etchant

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in PCB industries.6 When removing of the exposed copper to create desired circuit patterns using

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acidic CuCl2 etchant, a comproportionation takes place, as shown in Equation (1).7

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Cu(II)Cl 2 + Cu(0) → 2Cu(I)Cl

(1)

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In this reaction, the cupric ion (Cu(II)) responsible for etching is consumed, and cuprous ion

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(Cu(I)) is produced. With the accumulation of dissolved Cu(I) in CuCl2 etchant, a sparingly

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insoluble passivation CuCl film will be formed on the etching surface, and the rate of etching

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will decrease dramatically.8, 9 Therefore, developing effective technologies for removal of Cu(I)

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is of paramount for recycling of spent CuCl2 etchant.


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Several methods have been used to treat spent etchant, including chemical precipitation,10-12

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flocculation,13 ultrasonic treatment,14 solvent extraction,15,

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electrochemical regeneration17-21

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and membrane technology.22 Among these methods, the electrochemical regeneration has

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advantages of good reuse, no secondary contamination, high removal rate and purified recovery,

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and the environment values of electrochemical method is greater than the others.5 In the

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electrolytic process, Cu(I) is oxidized at the anode and some Cu(0) is deposited at the cathode.

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However, chloride gas (Cl2) evolves from the anode at high anode potential and causes safety

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issues.18 To avoid Cl2 evolution, Oxley et al.23 developed a complicated electrochemical system

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with a flow-through carbon anode and two electrolytic cells. However, the two cells may raise the

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equipment cost and complicate the recovery operation. In our previous study, a simple anion

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exchange separated electrolytic cell was designed using a graphite felt (GF) anode.17 Overall,

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these flow-through electrode materials and electrolytic regeneration processes are encouraging,

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but they still suffer from low energy efficiency for Cu(I) oxidation and secondary waste.

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Therefore, efforts are still required to seek out more efficient electrodes and electrolytic

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strategies to reduce energy use and discharge of secondary waste.

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Heretofore, because of the excellent electrical conductivity, distinguished stability and low

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cost, GF electrode has been successfully applied in microbial fuel cells,24-27 vanadium flow redox

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cells28, 29 and electrochemical decontamination.30 However, it still suffers from hydrophilicity

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and poor electrocatalytic activity. To enhance its electrolyte accessibility and active sites, these

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methods have been used: (1) noble metals deposition,25, 30, 31 (2) acid/alkali/plasma treatment28, 32,

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33

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electrode has not been reported as electrocatalysts for the Cu(II)/Cu(I) redox couple before. As

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the strongest carbon nanomaterial, CNT is not altered by strong acidic etchant, due to its high

and (3) carbon nanomaterials deposition34-36. To our knowledge, the use of composite GF


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chemical/mechanical stability.37 Compared with other modified layers, CNT also has the

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advantages of low synthesis cost, high electrical conductivity, large surface area for

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accommodating more active sites.38, 39 Therefore, a composite GF decorated with CNT is an

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ideal electrode for electro-oxidation of Cu(I) in PCB etchant.

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In this study, we developed a novel electrolytic regeneration technology for acidic spent CuCl2

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etchant to recycle copper, eliminate waste water production and save energy, using a composite

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GF electrode in a closed-loop electrolytic cell. For one thing, we developed a CNT modified GF

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anode (CNT/GF) for enhancing Cu(I) electro-oxidation performance. For another, we elaborated

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an electrochemical strategy converting the catholyte into the anolyte to create a zero-emission

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regeneration process. The CNT/GF electrodes were characterized by their structure, composition,

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wettability, and the electroactivity for the Cu(II)/Cu(I) redox couple. The reaction mechanism

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was studied by thermodynamic analysis and DFT calculation. The mass transfer of reactants was

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evaluated to clarify the kinetic impact from the decoration of CNT on the GF surface.40-42 The

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operating parameters were optimized on the basis of thermodynamics and kinetics analyses. The

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energy consumption of Cu(I) oxidation and Cu(0) deposition are estimated. The long-term

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stability of the composite anode was also investigated. We believe these results will be useful for

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the further engineering application of this regenerative electrolytic cell, as well as the similar

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electrochemical systems using composite GF electrodes.

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MATERIALS AND METHODS

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Growth of carbon nanotubes on graphite felt. The graphite felt (GF, Beijing Sanye Carbon

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Co.), was tailored into cylinders with the diameter of 14 mm and height of 5 mm, and subjected

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to ultrasonic treatment in alcohol to remove microfiber fragments. Then they were immersed in


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Pluronic F127 (2.5 wt. %, EO106PO70EO106 , Sigma-Aldrich) aqueous solution for 2 h and dried

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at 50 °C for 12 h. The nickel was deposited onto these GF cylinders using incipient impregnation

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with a 0.1 mol/L Ni(NO3)2 ethanol solution. After drying at 50 °C for 3 h, they were decorated

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by growing of carbon nanotubes (CNT) using chemical vapor deposition (CVD).43

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The Ni deposited GF cylinders were calcined under a N2 atmosphere (300 cm3/min flow rate)

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at 450 °C for 1.5 h. Then they were reduced in a mixture of H2 (60 cm3/min)/N2 (100 cm3/min)

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at 500 °C for 1 h. Then, the furnace was heated to 900 °C, 1000 °C, or 1100 °C under N2

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atmosphere (100 cm3/min), respectively. Then a stream of toluene gas (heated to 80 °C) was

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introduced for 40 min to grow CNT. Finally, the furnace was allowed to cool to room

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temperature. As-grown CNT modified GF (CNT/GF) samples were refluxed in 2 mol/L HNO3 at

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90 °C for 1 h to remove metal impurities and introduce the surface oxygen-containing groups.

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After that, the samples were washed with distilled water and dried at 100 °C in air for 12 h. For a

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clear description, CNT/GF grown in 900 °C, 1000 °C and 1100 °C were denoted as CNT900/GF,

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CNT1000/GF and CNT1100/GF, respectively.

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Electrode characterization. The surface morphology of the CNT on GF was examined by

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scanning electron microscopy (SEM, Nova Nanosem 430), transmission electron microscopy

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(TEM, JEOL JEM2100) and Raman spectroscopy (DXR Microscope, Excitation wavelength 532

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nm). The functional groups of the CNT on GF were analyzed via an X-ray photoelectron

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spectroscope (XPS, Thermo Electron PHI-5000 II, C1s at 285 eV).

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The cyclic voltammogram (CV) was measured by an electrochemical workstation

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(PARSTAT4000) with a three-electrode arrangement at 25 °C. The GF working electrode was

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connected to platinum wire with a Ag/AgCl reference electrode and a Pt plate auxiliary electrode


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(30 mm

╳ 30

mm). The test was performed in 0.1 mol/L CuCl + 2 mol/L HCl + 2 mol/L NaCl

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solution at different scan rates range from 1 to 10 mV/s under N2 atmosphere.

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Regeneration of acidic CuCl2 etchant. A small-scale flow electrolytic system was

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constructed as shown in Figure 1. The two electrode compartments, with the same size of 2 cm

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length

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AMI-7001, Membranes International Inc.). This AEM was pretreated by immersion in the

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etchant for 24 hours before use. UGF or CNT1100/GF was used as the anode and copper foil (99

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wt. %) was the cathode. The composition of spent CuCl2 etchant was provided by Tianjin

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Printronics Circuit Corp., and it was 1.70 mol/L CuCl2 + 0.10 mol/L CuCl + 2 mol/L HCl + 2

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mol/L NaCl (50 °C). This etchant, was fed to the anode compartment directly as the anolyte. The

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catholyte (0.50 mol/L CuCl2 + 0.03 mol/L CuCl + 2 mol/L HCl + 2 mol/L NaCl, 50 °C) was a

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diluted etchant obtained by mixing the pristine etchant with 2 mol/L HCl + 2 mol/L NaCl

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solution. Peristaltic pumps and thermostats were used to keep the electrolytes at constant rates

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and temperatures (50 ± 1 °C), respectively. The flow rates were calibrated by weighing the

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anolyte or catholyte. After 1 hour’s regeneration test, the concentration of Cu(I) in the

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electrolytes was determined via redox titrimetry,17 while the Cu(II) was measured by a

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spectrophotometer at 630 nm.44

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RESULTS AND DISCUSSION

╳

1 cm width

╳

2 cm height, were segregated by an anion exchange membrane (AEM,

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Characterization of CNT/GF Composite Anode. Figure 2a and 2b exhibit the SEM images

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of the UCF with a diameter of 15 µm. The slightly uneven fine-grained surface of UCF can

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provide abundant impregnation sites for development of Ni catalyst, which can initiate CNT

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growth. From the representative TEM images shown in Figure S1, the diameter and the wall


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thickness of CNT increase as the temperature increases. The morphology of CNT is directly

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relevant to the nucleation rate and growth rate of Ni catalyst, which are mainly controlled by

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temperature.45,

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nucleation rate of Ni catalyst is fast and the growth rate of it is slow. As a result, the slim CNT

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fiber is agglomerated into three-dimensional nanotube fibrous ‘silk flower’ architectures (Figure

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2c and 2d), and the growth stops due interference between the nanotubes. CNT900/GF has a

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specific surface area of 22.2 m2/g (Figure S2) and abundant 2-5 nm mesoporous structures

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(Figure S3). For CNT1000/GF, SEM images (Figure 2e and 2f) show that a dense CNT fibers,

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with the diameter of 100-300 nm and length of 10-50 µm covers the GF surface. As the

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temperature rises to 1100 °C, the CNT fibers become thicker and straighter, and interwoven into

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a three-dimensional network microstructure (Figure 2g and 2h). The N2 adsorption and

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desorption curves for CNF1000/CF and CNF1100/CF are almost overlapping (Figure S2),

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suggesting a similar pore structure.47 The specific surface area of CNF1000/CF and

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CNF1100/CF are 16.4 m2/g and 7.6 m2/g, respectively, which are much larger than that of

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untreated GF (UGF) (0.4 m2/g). Compared with UCF, the weights of CNT900/GF, CNT1000/GF

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and CNT1100/GF increase by 6.1 %, 22.9 % and 61.4 %, respectively.

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At a low temperature (900 °C), a fine Ni cluster is developed because the

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XPS Analyses. From XPS survey scan spectra (Figure S4), the oxygen content (atom.%) in

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UGF, CNT900/GF, CNT1000/GF and CNT1100/GF samples are 4.04%, 5.79%, 5.2% and

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11.23%, respectively. Figure 3a and 3b shows the C1s and O1s narrow scan spectra of the four

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samples. The deconvolution of the C1s main peak (284.5 eV) suggests five carbon bonding

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configurations, namely, ―COOH (C1, 289.2 eV), C=O (C2, 287.0 eV), C―O (C3, 285.9 eV), C

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― C (C4, 285.2 eV) and C=C (C5, 284.4 eV). Compared with CNT, the percentages of all

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oxygen functional groups on the surfaces of CNT/GF are increased (Figure 3c). In particular, the


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amount of C=O, COOH and C ― O of CNT1100/GF are increased by106%. Consistent with

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previous studies,48, 49 there are three types of oxygen bonding configurations on the surfaces of

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samples, including C―O―C (O1, 533.5 eV), C―OH (O2, 532.6 eV) and C=O (O3, 531.4 eV)

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functional groups. Compared with UGF, the CNT/GF samples have more C=O oxygen, which is

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contributed by carbonyl/lactone groups (C=O) and carboxyl groups (O=C―OH) on the CNT

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surfaces (Figure 3d).

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Raman characterization. The amount of defects of UGF and CNT/GF samples was

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characterized by Raman spectroscopy, as shown in Figure 4. The Raman spectra are fitted by

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four peaks, G band (1580 cm-1, ideal graphitic lattice), D band (1350 cm-1, disordered graphitic

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lattice (edges)), A band (1520 cm-1, amorphous carbon) and I band (1200 cm-1, polyenes and

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ionic impurities).50 The intensity ratio of the G and D bands (IG/ID) can be used as an indicator of

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the amount of defects sites presented in the graphite fibers of GF. The IG/ID decreases

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remarkably in the sequence of UGF < CNT1000/GF < CNT900/GF < CNT1100/GF, which is in

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good agreement of oxygen content measured by XPS.

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Electrochemical measurement. Before the electrochemical experiments, all the samples

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were pretreated by immersion in the etchant for 48 hours to eliminate the interfacial gas layer

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between the electrode and etchant electrolyte (Text SI4). The SI videos also show that GF and

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CNT/GF materials have an excellent wettability after immersion.

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Cyclic voltammetry (CV) was conducted to evaluate the electrochemical activity of UGF and

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CNT/GF electrodes toward the Cu(II)/Cu(I) redox reaction. Figure 5a―5d illustrate CV results

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for UGF and CNT/GF electrodes at different scan rates. For all the electrodes, the peak

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separation (∆Ep) between anodic peak potential (Ep,a) and cathodic peak potential (Ep,c) increase


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with increasing scan rates, while the average peak potential ((Ep,a + Ep,c)/2) remains constant.

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These results indicate that the Cu(II)/Cu(I) redox reaction on UGF and CNT/GF electrodes are

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quasi-reversible, and this is controlled by both diffusion and charge transfer processes.51 As

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shown in Figure 5e, for all electrodes, the anodic peak currents (Ip,a) are proportional to the

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square root of the scan rates (v). The increasing of the slopes follow this order: UGF (2.91)

Closed-Loop Electrochemical Recycling of Spent Copper(II) from

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