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Electrochemical removal of bisphenol A using a CNT-covered polyester yarn electrode
Accepted Manuscript Short communication Electrochemical removal of bisphenol A using a CNT-covered polyester yarn electrode Jianjun He, Hongwen Yu, Bunshi Fugetsu, Shunitz Tanaka, Ling Sun PII: DOI: Reference:
S1383-5866(13)00149-4 http://dx.doi.org/10.1016/j.seppur.2013.03.011 SEPPUR 11095
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
21 January 2013 8 March 2013 8 March 2013
Please cite this article as: J. He, H. Yu, B. Fugetsu, S. Tanaka, L. Sun, Electrochemical removal of bisphenol A using a CNT-covered polyester yarn electrode, Separation and Purification Technology (2013), doi: http:// dx.doi.org/10.1016/j.seppur.2013.03.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical removal of bisphenol A using a CNT-covered polyester yarn electrode Jianjun He a*, Hongwen Yu b, Bunshi Fugetsu a, Shunitz Tanaka a, and Ling Sun a a
Laboratory of Environmental Remediation, Graduate School of Environmental
Science, Hokkaido University, Sapporo 060-0810, Japan b
Northeast Institute of Geography and Agricultural Ecology, Chinese Academy of
Sciences, Changchun 130102, China
*
Corresponding author: Tel/fax: [email protected] (J. He).
+81-11-7069294 1
E-mail
address:
Abstract A CNT-based electrode was prepared using dispersed multi-walled CNTs and anionic polyurethane binder as the dyestuff for coating CNTs on polyester yarns, followed by washing with ethanol to remove the binder. The resulting CNT-based electrode with a CNT network on surface, has a high special surface area and a high electrochemical activity, making it useful for the removal of phenolic compounds from the contaminated water. Through electrochemical polymerization and degradation on this electrode, high removal efficiency of bisphenol A was obtained, at levels as high as 8.1×10-5 mol/g with 0.75 V applied potential.
Keywords:
CNT-covered
polyester
yarn,
Degradation, Bisphenol A. 2
Electrochemical
polymerization,
1. Introduction Phenolic compounds are common pollutants from oil refineries, chemical and plastic plants, and they are toxic to human health even in low concentrations. For example, 2,2-bis(4′-hydroxyphenyl)propane (bisphenol A; BPA) is a normal phenolic compound，and a well-known as endocrine disruptor [1, 2]. It is widely used as a monomer
in
the
production
of
polycarbonate,
epoxy,
and
unsaturated
polyester-styrene, as well as in flame antioxidants and rubber chemicals, and over six billion pounds are produced worldwide each year. Consequently, there is a high risk of releasing BPA to the environment. In recent years, numerous treatment technologies have been developed including chemical [3], physical [4, 5], biological [6, 7], electrochemical [8, 9], and photochemical [10, 11] methods to remove BPA from
the
environment.
However,
conventional
biological
methods
are
time-consuming for organic removal in wastewater, and chemical methods tend to be expensive due to the need for a large amount of reactive chemical reagents. Also, photochemical method has some limitations such as photocatalysts activity and high cost of UV light source. In contrast, electrochemical methods have several advantages for environmental remediation, such as low cost, easy to control, no need for toxic compounds, and the ability to work well at room temperature. The electrochemical degradation of BPA has been investigated on different electrodes [8, 9]. Also, the electrochemical polymerization of BPA was reported on a carbon fiber electrode to remove BPA from the contaminated water [12]. In the last two decades, the development of nanotechnology has resulted in many materials with superior properties which are excellent candidates for a wide range of applications. Specially, one-dimensional structural carbon nanotubes (CNTs) are key nanomaterials that have applications in energy storage, environmental science, and life science, due to their unique structure, strength, and electrical, optical, and thermal properties [13, 14]. The applications of CNTs to environmental remediation have been primarily focused on their use in three categories such as novel sorbents [15, 16], a part of photocatalysts [17, 18], and novel working electrode materials [19]. The 3
unique electrical properties and chemical stability of CNTs allow them to be used as working electrodes with large surface area. Many studies describe the application of CNTs or CNTs and metal/metal oxide in the energy field. For example use for fuel cell, capacitor, and Li-ion battery electrode materials. A few studies [20, 21] have examined their use in environmental remediation since the electrodes are environmentally friendly and large scale preparation is easy. Recently, Yang et al. [20] reported an electrochemical wastewater treatment using CNTs that packed two pieces of activated carbon fiber felts for both anode and cathode. The results revealed that the current efficiency of the electrode increased 340~519% compared with an electrode from conventional activated carbon fiber felts. Normally, commercially available of CNTs are aggregates, because the high surface area to volume ratio of nanomaterials easily causes uncontrolled aggregations by van der Waals forces; this dramatically reduces the performance of nanomaterial-based electrodes. Therefore, researchers have focused on the development of high surface area and electrochemical active electrodes using dispersed CNTs. Vecitis et al. [21] reported the preparation of a randomly oriented and aligned porous CNT filter electrode through the vacuum filtration of dispersed CNTs on a PTFE membrane, based on the CNT properties of high aspect ratio and mechanical stability. The results revealed that the pore volume in the filter is ~85%. The adsorption and oxidation of aqueous dyes were performed efficiently on this electrochemical CNT filter at 2 V applied potential. Until now, there is still remaining the challenge of how to design a CNT-based electrode which can take advantage of the high specific surface area and electrochemical activity of nanomaterials, which both meet the needs of environmental protection. Here we report on the preparation of a CNT-covered polyester yarn electrode using dispersed CNT-based dyestuff, which does not contain binders and stably covered on polyester yarn surface via CNT network. Furthermore, BPA was employed as the target to elevate the electrochemical activity of the CNT-covered polyester yarn electrode for environmental remediation. A high removal efficiency of BPA resulted from the electrochemical polymerization and degradation of BPA on this electrode. 2. Experimental 4
2.1.
Reagents
Bisphenol A (BPA) with a purity of 95% was purchased from Kanto Chem. Co. Inc. (Tokyo, Japan). A sodium sulfate solution of 0.1 M was prepared with Na2SO4, and the pH of the solution was 6.3. 2.2.
Preparation of CNT-covered polyester yarn electrode
The CNT-dyed polyester yarn preparation was reported in detail in our previous work [22]. Briefly, dyestuff containing a 3% multi-walled CNT (Baytube C 150P) suspension and a certain amount of anionic polyurethane binder were put into a bath. Then, commercially available polyester yarns were passed through the dye bath at 40 °C under microwave irradiation. Finally, the CNT-dyed polyester yarns were obtained after heating at 170 °C for 30 s. To obtain the CNT-covered polyester yarn electrode, a bundle of 100 strings of CNT-dyed polyester yarn (5 cm length) was immersed in a test tube containing 50 mL of a 95% ethanol solution, which was then shaken using a vortex shaker for 24 h. After washing with distilled water, the CNT-covered polyester yarn bundle was placed in an oven at 70 °C for 24 h to completely remove the ethanol and water. 2.3.
Electrochemical treatment of BPA
The measurement of cyclic voltammograms and the bulk degradation with coulometry was performed using a Model 600C electrochemical analyzer (BAS Inc., Japan) with a conventional three-electrode cell. A bundle of 100 strings (around 60 mg) of CNT-dyed polyester yarn (about 70 μm in diameter and 5 cm in length) was used as a working electrode. An Ag/AgCl (3.0 M NaCl) electrode and a Pt mesh were used as a reference electrode and a counter electrode, respectively. The electrodes were inserted in a glass beaker (50 ml) containing 0.1 M Na2SO4 and a certain concentration of BPA. After treatment for BPA, the electrode was regenerated by 2 V anodic potential for 2 hours in 0.1 M Na2SO4. Another set as control, a bundle of 100 strings (around 290 mg) of carbon fiber (Hokuto Denko, about 500 μm in diameter and 5 cm in length) was also used as a working electrode. The concentration of BPA was determined with high performance liquid 5
chromatography (HPLC; HITACHI, Japan) using an L-6000 pump and L-4200 UV-VIS detector (225 nm, HITACHI, Japan) with an inertsil PH column (150×4.6 mm2, GL science). The mobile phase was methanol/water (6/4) and the flow rate was 1.0 ml/min. The value of total organic carbon (TOC) was determined by a TOC-5000 TOC analyzer (Shimazu Co., Japan). Scanning electron microscopy (SEM; JSM-6390, JEOL Co., Japan) was used for morphological observations of the CNT-covered polyester yarns. 3. Results and discussion 3.1.
Characterizations of CNT-covered polyester yarns
CNT-dyed polyester yarns (Fig. 1a) with around 1700 ohm/cm electric resistivity were produced by a dye-print approach with the dyestuff including single dispersing multi-walled CNTs and a small amount of anionic polyurethanes binder. A SEM image clearly shows that the CNTs were coated on the polyester yarn surfaces and were wrapped in polymer (Fig. 1b, c). To produce a desired electrode which exposes CNTs to the environment, the procedure have to remove the binder and allow the CNTs to remain on the surface of the polyester yarns. We found that a washing procedure with an organic solution meets these two requirements, and ethanol as the washing liquid provides superior conductivity of the CNT-covered polyester yarns than other solvents such as methanol or acetone. While the binder was slowly removed in the ethanol solution, random network formation of high aspect ratio CNTs was carried. As a result, CNTs stably covered on the surface of the polyester yarns, and exposed it to environment (Fig. 1d). The electric conductivity of the CNT-covered polyester yarns was improved to 87~120 ohm/cm and the specific surface area was increased to 11.3 m2/g (measured by the Brunauer–Emmett–Teller (BET) method).
3.2.
Electrochemical removal of BPA
The pH dependence of BPA electrochemical polymerization was investigated in detail on a glassy carbon electrode surface in our previous work [12]. The redox marker (potassium ferrocyanide) revealed that the electrochemical polymerization of BPA was more readily achieved in neutral media than in alkaline media. Therefore, 0.1 M 6
Na2SO4 solutions with pH 6.3 were used as the electrolytes in this study. Fig. 2 shows progressive cyclic voltammograms (CVs) of 0.1 mM BPA on a CNT-covered polyester yarn electrode for 5 cycles. The CVs exhibited irreversible behavior, and the anodic peak height shows a gradual decrease as the scan number increased. This resulted since no electrochemical active layer formed on the electrode surface. As described in previous reports, the phenolic compounds can be electrochemically oxidized to produce phenoxyl radicals, and then form some types of dimmers. The dimmers can then be further oxidized to form new radicals which couple with other radicals to produce polymers [23]. BPA is a special phenolic compound which can be oxidized to generate some types of diradicals (Fig. 3). These diradicals continuously and randomly couple and easily form polymers. The electrochemical polymerization of phenolic compounds is what allows the removal of aromatic pollutants from the polluted environment. A series of applied potentials was employed to evaluate the optimal potential for the efficient removal of BPA by a CNT-covered polyester yarn electrode. Fig. 4 shows the results of BPA removal in which the BPA concentrations were monitored by HPLC. The CNT-covered polyester yarns showed good adsorptive capacity even without applied potential; 28.1% BPA was removed as the initial concentration of BPA was 0.1 mM. At a low anodic potential (0.5 V), there appeared to be no effect on the removal of BPA comparing to the result from the case with no applied potential. The best removal efficiency was observed when the anodic potential was increased to 0.75 V. At higher values of the applied potential such as 1.0 or 1.25 V, the removal efficiencies decreased from the maximum point at 0.75 V, which may have contributed to the occurrence of water degradation on the CNT-covered polyester yarn electrode at high potential. The formed O2 bubbles prevented the generation of BPA polymerized film on the electrode surface. Fig. 5 shows a cyclic voltammogram which reveals water oxide became stronger on this electrode at higher applied potential in a blank solution. Fig. 6 shows the amount of organic carbon except BPA in the residual electrolyte, as calculated from the residual of BPA based on an HPLC analysis and the total 7
carbon determined by TOC measurement. In addition, some intermediate products of the electrochemical degradation of BPA were observed by GC-MS, including for example isopropenylphenol, aliphatic acids and aliphatic alcohols [8]. These results reveal that degradation as well as polymerization of BPA occurred on the CNT-covered polyester yarn electrode. The lowest concentration of BPA was found at 0.75 V, and high levels of total carbon excepting BPA were observed at high applied potential in Fig. 6. The electrochemical oxidative degradation of organic pollutants normally was performed on metal or metal oxide electrodes [8, 24]. Recently, Yang et al. [20] achieved degradation of brilliant red X-3B by generating free radicals on CNT-packed activated carbon fiber felt electrode at anodic potentials of 8 V and 10 V. Table 1 summarizes the data change of the BPA polymerization and degradation under a series of applied potentials. The effect of BPA degradation showed an increasing tendency as the anode potential increased. In this work, the polymerization and degradation of BPA play important roles by providing good removal efficiency while the applied potential is 0.75 V. A commercially available carbon fiber electrode was employed to compare with the CNT-covered polyester yarn electrode as an electrochemical treatment for BPA. Fig. 7 shows the results of the removal efficiency with an initial concentration of 0.1 mM BPA containing 0.1 M Na2SO4 at pH 6.3. Although the carbon fiber has a higher specific surface area (14.1 m2/g) than the CNT-covered polyester yarns (11.3 m2/g), the adsorption capacities of the carbon fiber and the CNT-covered polyester yarns were 1.2×10-6 mol/g and 2.3×10-5 mol/g without an applied potential, respectively. Furthermore, the electrochemical removal efficiency was 4.7×10-6 mol/g and 8.1× 10-5 mol/g, respectively, under the 0.75 V applied potential in the BPA solution with an initial concentration of 0.1 mM. Obviously, the removal efficiency for the CNT-covered polyester yarn electrode was 17.2 times that of the carbon fiber electrode. The close packed adsorption between the curved surface of CNTs and the butterfly-shaped BPA [25] may has contributed much to the high efficiency. It should be noted that a subsequently study found that the CNT-covered polyester yarn electrode can be reutilized after the BPA polymer layer was stripped by a 2 V applied 8
potential in a 0.1 M Na2SO4 electrolyte; this is a benefit as it reduces the cost of the electrode material.
In this study, a CNT-based electrode was developed using a dispersed CNT-based dyestuff. After removal of the polymer binder and dispersant, CNTs would cover the surface of the polyester yarns and result in an increased specific surface area and electrochemical activity of the working electrode. Therefore, the electrochemical polymerization and degradation of BPA efficiently performed together on the CNT-covered polyester yarn electrode at the low applied potential of 0.75 V. Furthermore, high electrochemical removal efficiency of BPA was obtained (8.1× 10-5 mol/g), which was 17.2 times greater of that using a carbon fiber electrode.
9
References [1] A.M. Calafat, X. Ye, L.Y. Wong, J.A. Reidy, L.L. Needham, Exposure of the US population to bisphenol A and 4-tertiary-octylphenol: 2003-2004, Environ. Health Perspect, 116 (2008) 39-44. [2] N. Benachour, A. Aris, Toxic effects of low doses of Bisphenol-A on human placental cells, Toxicol. Appl. Pharmacol. 241 (2009) 322–328. [3] P. Zhang, G. Zhang, J. Dong, M. Fan, G. Zeng, Bisphenol A oxidative removal by ferrate (Fe(VI)) under a weak acidic condition, Sep. Purif. Technol. 84 (2012) 46-51. [4]
B. Pan, D. Lin, H. Mashayeskhi, B. Xing, Adsorption and hysteresis of bisphenol A and 17α-ethinyl estradiol on carbon nanomaterials, Environ. Sci. Technol. 42 (2008) 5480-5485.
[5]
J. Heo, J. R.V. Flora, N. Her, Y. Park, J. Cho, A. Son, Y. Yoon, Removal of bisphenol A and 17α-estradiol in single walled carbon nanotubes-ultrafiltration (SWNTs-UF) membrane systems, Sep. Purif. Technol. 90 (2012) 39-52.
[6]
B. Seyhi, P. Drogui, G. Buelna, J.F. Blais, Removal of bisphenol-A from spiked synthetic effluents using an immersed membrane activated sludge process, Sep. Purif. Technol. 87 (2012) 101-109.
[7] S. Okazaki, J. Michizoe, M. Goto, S. Furusaki, H. Wariishi, H. Tanaka, Isolation of a Pseudomonas lipase produced in pure hydrocarbon substrate and its application in the synthesis of isoamyl acetate using membrane-immobilised lipase, Enzyme Microb. Technol. 31 (2002) 725-735. [8] Y. Cui, X. Li, G. Chen, Y. Cui, X. Li, G. Chen, Electrochemical degradation of bisphenol A on different anodes, Water Res. 43 (2009) 1968-1976. [9] S. Tanaka, Y. Nakata, T. Kimura, S. Yustiawati, M. Kawasaki, H. Kuramitz, Electrochemical decomposition of Bisphenol A using Pt/Ti and SnO2/Ti anodes, J. Appl. Electrochem. 32 (2002) 197-201. 10
[10] Y. Ohko, I. Ando, C. Niwa, T. Tatsuma, T. Yamamura, T. Nakashima, Y. Kubota, A. Fujishima, Degradation of bisphenol-A in water by TiO2 photo-catalyst, Environ. Sci. Technol. 35 (2001) 2365-2368. [11] I. Gultekin, N.H. Ince, Synthetic endocrine disruptors in the environment and water remediation by advanced oxidation processes, J. Environ. Manag. 85 (2007) 816-832. [12] H. Kuramitz, Y. Nakata, M. Kawasaki, S. Tanaka, Electrochemical oxidation of Bisphenol A. Application to the removal of bisphenol A using a carbon fiber electrode, Chemosphere 45 (2001) 37-43. [13] A. Jorio, G. Dresselhaus, M.S. Dresselhaus, Carbon Nanotubes: Advanced Topics in the Synthesis, Structure, Properties and Applications, Spring Berlin Heidelberg, New York, 2008, pp. 165-250. [14] P.J.F. Harris. Carbon Nanotube Science: Synthesis, Properties and Applications, Cambridge University Press, New York, 2009, pp. 146-226. [15] X. Ren, C. Chen, M. Nagatsu, X. Wang, Carbon nanotubes as adsorbents in environmental pollution management: A review, Chem. Eng. J. 170 (2011) 395-410. [16] H. Yu, B. Fugetsu, A novel adsorbent obtained by inserting carbon nanotubes into cavities of diatomite and applications for organic dye elimination from contaminated water, J. Hazard. Mater. 170 (2010) 138-145. [17] A.D. Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, J. Hazard. Mater. 211-2012 (2012) 3-29. [18] Y. Yu, J.C. Yu, C. Chan, Y. Che, J. Zhao, L. Ding, W. Ge, P. Wong, Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye, Appl. Catal. B: Environ. 61 (2005) 1-11. [19] J. Garcia, H.T. Gomes, Ph Serp, Ph Kalck, J.L. Figuriredo, J.L. Faria, Carbon nanotube supported ruthenium catalysts for the treatment of high strength wastewater with aniline using wet air oxidation, Carbon 44 (2006) 2384-2391. [20] J. Yang, J. Wang, J. Jia, Improvement of electrochemical wastewater treatment 11
through mass transfer in a seepage carbon nanotube electrode reactor, Environ. Sci. Technol. 43 (2009) 3796-3802. [21] C.D. Vecitis, G. Gao, H. Liu, Electrochemical carbon nanotube filter for adsorption, desorption, and oxidation of aqueous dyes and anions, J. Phys. Chem. C 115 (2011) 3621-3629. [22] B. Fugetsu, E. Akira, M. Hachiya, M. Endo, The production of soft, durable, and electrically conductive polyester multifilament yarns by dye-printing them with carbon nanotubes, Carbon 47 (2009) 527-544. [23] N.B. Tahar, A. Savall, Electrochemical removal of phenol in alkaline solution. Contribution of the anodic polymerization on different electrode materials, Electrochimica Acta 54 (2009) 4809-4816. [24] C. Comninellis, Electrocatalysis in the electrochemical conversion/combustion of organic pollutions for waste water treatment, Electrochimica Acta 39 (1994) 1857-1862. [25] B. Pan, D. Lin, H. Mashayekhi, B. Xing, Adsorption and hysteresis of bisphenol A and 17α-ethinyl estradiol on carbon nanomaterials, Environ. Sci. Technol . 42 (2008) 5480-5485.
12
Figures and table Fig. 1 Morphology of CNT-dyed polyester yarns (a) photograph of CNT-dyed polyester yarns, (b) SEM image of CNT-dyed polyester yarns, (c) high-magnification SEM image of CNT-dyed polyester yarns before removing binder, (d) surface image of CNT-dyed polyester yarns after removing binder.
a
b
d
c
13
Fig. 2 Progressive cyclic voltammograms for 0.1 mM BPA in 0.1 M Na2SO4 solution at CNT-covered polyester yarn electrode. Scan rate is 50 mV/s.
14
Fig. 3 Schematic diagram of electrochemical polymerization of BPA.
HO
OH
bisphenol a
O
O
O
O
-2e , -2H
Polymer O
O
other type diradicals
15
Fig. 4 Relationships between the residual BPA in solution and treatment time with a series of applied potentials in 0.1 mM BPA solution containing 0.1 M Na2SO4 on CNT-covered polyester yarn electrode.
16
Fig. 5 Cyclic voltammogram was obtained on CNT-covered polyester yarn electrode in 0.1 M Na2SO4 solution. Scan rate is 50 mV/s.
17
Fig. 6 The amount of organic carbon except BPA in residual electrolyte which was calculated from the residual of BPA based on HPLC analysis and total carbon by TOC measurement in BPA solution with initiate concentration of 0.1 mM containing 0.1 M Na2SO4 after 2 h electrochemical treatment with a series of applied potentials.
18
Table 1 The data of removal efficiencies of BPA by electrochemical polymerization and degradation with a series of applied potentials in 50 ml BPA solution with initiate concentration of 0.1 mM (total weight of BPA is 1.14 mg) containing 0.1 M Na2SO4 after 2 h. Applied potential (V) without 0.5 0.7 0.75 1.0 1.25
Removal amount of BPA (mg) a 0.29 0.31 0.81 1.13 0.98 0.67
a
Polymerization (mg) b 0.3 0.3 0.5 0.6 0.5 0.2
Degradation (mg) c -0.01d 0.01 0.31 0.53 0.48 0.47
The data was calculated from the residual of BPA based on HPLC analysis The data were obtained from weight increase of electrode, and data correct to one decimal place due to detection limit. c Weight of degradation was calculated from the removal amount of BPA deduced the weight increase of electrode. d Negative value is due to detection limit of weight increase of electrode. b
19
Fig. 7 Treatment time dependent removal of BPA without applied potential (dotted line) and with applied potential of 0.75 V (solid line) in BPA solution with initiate concentration of 0.1 mM at CNT-covered polyester yarn electrode (red) and carbon fiber electrode (blue).
20
Graphical Abstract (for review)
Research Highlights
A novel CNT-covered polyester yarn electrode was developed.
Electrochemical polymerization and degradation of BPA occurred on this electrode
A high removal efficiency of BPA was obtained at 0.75 V applied potential.
The removal efficiency of BPA was promoted 17.2 times than carbon fiber electrode.
S1383-5866(13)00149-4 http://dx.doi.org/10.1016/j.seppur.2013.03.011 SEPPUR 11095
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
21 January 2013 8 March 2013 8 March 2013
Please cite this article as: J. He, H. Yu, B. Fugetsu, S. Tanaka, L. Sun, Electrochemical removal of bisphenol A using a CNT-covered polyester yarn electrode, Separation and Purification Technology (2013), doi: http:// dx.doi.org/10.1016/j.seppur.2013.03.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical removal of bisphenol A using a CNT-covered polyester yarn electrode Jianjun He a*, Hongwen Yu b, Bunshi Fugetsu a, Shunitz Tanaka a, and Ling Sun a a
Laboratory of Environmental Remediation, Graduate School of Environmental
Science, Hokkaido University, Sapporo 060-0810, Japan b
Northeast Institute of Geography and Agricultural Ecology, Chinese Academy of
Sciences, Changchun 130102, China
*
Corresponding author: Tel/fax: [email protected] (J. He).
+81-11-7069294 1
address:
Abstract A CNT-based electrode was prepared using dispersed multi-walled CNTs and anionic polyurethane binder as the dyestuff for coating CNTs on polyester yarns, followed by washing with ethanol to remove the binder. The resulting CNT-based electrode with a CNT network on surface, has a high special surface area and a high electrochemical activity, making it useful for the removal of phenolic compounds from the contaminated water. Through electrochemical polymerization and degradation on this electrode, high removal efficiency of bisphenol A was obtained, at levels as high as 8.1×10-5 mol/g with 0.75 V applied potential.
Keywords:
CNT-covered
polyester
yarn,
Degradation, Bisphenol A. 2
Electrochemical
polymerization,
1. Introduction Phenolic compounds are common pollutants from oil refineries, chemical and plastic plants, and they are toxic to human health even in low concentrations. For example, 2,2-bis(4′-hydroxyphenyl)propane (bisphenol A; BPA) is a normal phenolic compound，and a well-known as endocrine disruptor [1, 2]. It is widely used as a monomer
in
the
production
of
polycarbonate,
epoxy,
and
unsaturated
polyester-styrene, as well as in flame antioxidants and rubber chemicals, and over six billion pounds are produced worldwide each year. Consequently, there is a high risk of releasing BPA to the environment. In recent years, numerous treatment technologies have been developed including chemical [3], physical [4, 5], biological [6, 7], electrochemical [8, 9], and photochemical [10, 11] methods to remove BPA from
the
environment.
However,
conventional
biological
methods
are
time-consuming for organic removal in wastewater, and chemical methods tend to be expensive due to the need for a large amount of reactive chemical reagents. Also, photochemical method has some limitations such as photocatalysts activity and high cost of UV light source. In contrast, electrochemical methods have several advantages for environmental remediation, such as low cost, easy to control, no need for toxic compounds, and the ability to work well at room temperature. The electrochemical degradation of BPA has been investigated on different electrodes [8, 9]. Also, the electrochemical polymerization of BPA was reported on a carbon fiber electrode to remove BPA from the contaminated water [12]. In the last two decades, the development of nanotechnology has resulted in many materials with superior properties which are excellent candidates for a wide range of applications. Specially, one-dimensional structural carbon nanotubes (CNTs) are key nanomaterials that have applications in energy storage, environmental science, and life science, due to their unique structure, strength, and electrical, optical, and thermal properties [13, 14]. The applications of CNTs to environmental remediation have been primarily focused on their use in three categories such as novel sorbents [15, 16], a part of photocatalysts [17, 18], and novel working electrode materials [19]. The 3
unique electrical properties and chemical stability of CNTs allow them to be used as working electrodes with large surface area. Many studies describe the application of CNTs or CNTs and metal/metal oxide in the energy field. For example use for fuel cell, capacitor, and Li-ion battery electrode materials. A few studies [20, 21] have examined their use in environmental remediation since the electrodes are environmentally friendly and large scale preparation is easy. Recently, Yang et al. [20] reported an electrochemical wastewater treatment using CNTs that packed two pieces of activated carbon fiber felts for both anode and cathode. The results revealed that the current efficiency of the electrode increased 340~519% compared with an electrode from conventional activated carbon fiber felts. Normally, commercially available of CNTs are aggregates, because the high surface area to volume ratio of nanomaterials easily causes uncontrolled aggregations by van der Waals forces; this dramatically reduces the performance of nanomaterial-based electrodes. Therefore, researchers have focused on the development of high surface area and electrochemical active electrodes using dispersed CNTs. Vecitis et al. [21] reported the preparation of a randomly oriented and aligned porous CNT filter electrode through the vacuum filtration of dispersed CNTs on a PTFE membrane, based on the CNT properties of high aspect ratio and mechanical stability. The results revealed that the pore volume in the filter is ~85%. The adsorption and oxidation of aqueous dyes were performed efficiently on this electrochemical CNT filter at 2 V applied potential. Until now, there is still remaining the challenge of how to design a CNT-based electrode which can take advantage of the high specific surface area and electrochemical activity of nanomaterials, which both meet the needs of environmental protection. Here we report on the preparation of a CNT-covered polyester yarn electrode using dispersed CNT-based dyestuff, which does not contain binders and stably covered on polyester yarn surface via CNT network. Furthermore, BPA was employed as the target to elevate the electrochemical activity of the CNT-covered polyester yarn electrode for environmental remediation. A high removal efficiency of BPA resulted from the electrochemical polymerization and degradation of BPA on this electrode. 2. Experimental 4
2.1.
Reagents
Bisphenol A (BPA) with a purity of 95% was purchased from Kanto Chem. Co. Inc. (Tokyo, Japan). A sodium sulfate solution of 0.1 M was prepared with Na2SO4, and the pH of the solution was 6.3. 2.2.
Preparation of CNT-covered polyester yarn electrode
The CNT-dyed polyester yarn preparation was reported in detail in our previous work [22]. Briefly, dyestuff containing a 3% multi-walled CNT (Baytube C 150P) suspension and a certain amount of anionic polyurethane binder were put into a bath. Then, commercially available polyester yarns were passed through the dye bath at 40 °C under microwave irradiation. Finally, the CNT-dyed polyester yarns were obtained after heating at 170 °C for 30 s. To obtain the CNT-covered polyester yarn electrode, a bundle of 100 strings of CNT-dyed polyester yarn (5 cm length) was immersed in a test tube containing 50 mL of a 95% ethanol solution, which was then shaken using a vortex shaker for 24 h. After washing with distilled water, the CNT-covered polyester yarn bundle was placed in an oven at 70 °C for 24 h to completely remove the ethanol and water. 2.3.
Electrochemical treatment of BPA
The measurement of cyclic voltammograms and the bulk degradation with coulometry was performed using a Model 600C electrochemical analyzer (BAS Inc., Japan) with a conventional three-electrode cell. A bundle of 100 strings (around 60 mg) of CNT-dyed polyester yarn (about 70 μm in diameter and 5 cm in length) was used as a working electrode. An Ag/AgCl (3.0 M NaCl) electrode and a Pt mesh were used as a reference electrode and a counter electrode, respectively. The electrodes were inserted in a glass beaker (50 ml) containing 0.1 M Na2SO4 and a certain concentration of BPA. After treatment for BPA, the electrode was regenerated by 2 V anodic potential for 2 hours in 0.1 M Na2SO4. Another set as control, a bundle of 100 strings (around 290 mg) of carbon fiber (Hokuto Denko, about 500 μm in diameter and 5 cm in length) was also used as a working electrode. The concentration of BPA was determined with high performance liquid 5
chromatography (HPLC; HITACHI, Japan) using an L-6000 pump and L-4200 UV-VIS detector (225 nm, HITACHI, Japan) with an inertsil PH column (150×4.6 mm2, GL science). The mobile phase was methanol/water (6/4) and the flow rate was 1.0 ml/min. The value of total organic carbon (TOC) was determined by a TOC-5000 TOC analyzer (Shimazu Co., Japan). Scanning electron microscopy (SEM; JSM-6390, JEOL Co., Japan) was used for morphological observations of the CNT-covered polyester yarns. 3. Results and discussion 3.1.
Characterizations of CNT-covered polyester yarns
CNT-dyed polyester yarns (Fig. 1a) with around 1700 ohm/cm electric resistivity were produced by a dye-print approach with the dyestuff including single dispersing multi-walled CNTs and a small amount of anionic polyurethanes binder. A SEM image clearly shows that the CNTs were coated on the polyester yarn surfaces and were wrapped in polymer (Fig. 1b, c). To produce a desired electrode which exposes CNTs to the environment, the procedure have to remove the binder and allow the CNTs to remain on the surface of the polyester yarns. We found that a washing procedure with an organic solution meets these two requirements, and ethanol as the washing liquid provides superior conductivity of the CNT-covered polyester yarns than other solvents such as methanol or acetone. While the binder was slowly removed in the ethanol solution, random network formation of high aspect ratio CNTs was carried. As a result, CNTs stably covered on the surface of the polyester yarns, and exposed it to environment (Fig. 1d). The electric conductivity of the CNT-covered polyester yarns was improved to 87~120 ohm/cm and the specific surface area was increased to 11.3 m2/g (measured by the Brunauer–Emmett–Teller (BET) method).
3.2.
Electrochemical removal of BPA
The pH dependence of BPA electrochemical polymerization was investigated in detail on a glassy carbon electrode surface in our previous work [12]. The redox marker (potassium ferrocyanide) revealed that the electrochemical polymerization of BPA was more readily achieved in neutral media than in alkaline media. Therefore, 0.1 M 6
Na2SO4 solutions with pH 6.3 were used as the electrolytes in this study. Fig. 2 shows progressive cyclic voltammograms (CVs) of 0.1 mM BPA on a CNT-covered polyester yarn electrode for 5 cycles. The CVs exhibited irreversible behavior, and the anodic peak height shows a gradual decrease as the scan number increased. This resulted since no electrochemical active layer formed on the electrode surface. As described in previous reports, the phenolic compounds can be electrochemically oxidized to produce phenoxyl radicals, and then form some types of dimmers. The dimmers can then be further oxidized to form new radicals which couple with other radicals to produce polymers [23]. BPA is a special phenolic compound which can be oxidized to generate some types of diradicals (Fig. 3). These diradicals continuously and randomly couple and easily form polymers. The electrochemical polymerization of phenolic compounds is what allows the removal of aromatic pollutants from the polluted environment. A series of applied potentials was employed to evaluate the optimal potential for the efficient removal of BPA by a CNT-covered polyester yarn electrode. Fig. 4 shows the results of BPA removal in which the BPA concentrations were monitored by HPLC. The CNT-covered polyester yarns showed good adsorptive capacity even without applied potential; 28.1% BPA was removed as the initial concentration of BPA was 0.1 mM. At a low anodic potential (0.5 V), there appeared to be no effect on the removal of BPA comparing to the result from the case with no applied potential. The best removal efficiency was observed when the anodic potential was increased to 0.75 V. At higher values of the applied potential such as 1.0 or 1.25 V, the removal efficiencies decreased from the maximum point at 0.75 V, which may have contributed to the occurrence of water degradation on the CNT-covered polyester yarn electrode at high potential. The formed O2 bubbles prevented the generation of BPA polymerized film on the electrode surface. Fig. 5 shows a cyclic voltammogram which reveals water oxide became stronger on this electrode at higher applied potential in a blank solution. Fig. 6 shows the amount of organic carbon except BPA in the residual electrolyte, as calculated from the residual of BPA based on an HPLC analysis and the total 7
carbon determined by TOC measurement. In addition, some intermediate products of the electrochemical degradation of BPA were observed by GC-MS, including for example isopropenylphenol, aliphatic acids and aliphatic alcohols [8]. These results reveal that degradation as well as polymerization of BPA occurred on the CNT-covered polyester yarn electrode. The lowest concentration of BPA was found at 0.75 V, and high levels of total carbon excepting BPA were observed at high applied potential in Fig. 6. The electrochemical oxidative degradation of organic pollutants normally was performed on metal or metal oxide electrodes [8, 24]. Recently, Yang et al. [20] achieved degradation of brilliant red X-3B by generating free radicals on CNT-packed activated carbon fiber felt electrode at anodic potentials of 8 V and 10 V. Table 1 summarizes the data change of the BPA polymerization and degradation under a series of applied potentials. The effect of BPA degradation showed an increasing tendency as the anode potential increased. In this work, the polymerization and degradation of BPA play important roles by providing good removal efficiency while the applied potential is 0.75 V. A commercially available carbon fiber electrode was employed to compare with the CNT-covered polyester yarn electrode as an electrochemical treatment for BPA. Fig. 7 shows the results of the removal efficiency with an initial concentration of 0.1 mM BPA containing 0.1 M Na2SO4 at pH 6.3. Although the carbon fiber has a higher specific surface area (14.1 m2/g) than the CNT-covered polyester yarns (11.3 m2/g), the adsorption capacities of the carbon fiber and the CNT-covered polyester yarns were 1.2×10-6 mol/g and 2.3×10-5 mol/g without an applied potential, respectively. Furthermore, the electrochemical removal efficiency was 4.7×10-6 mol/g and 8.1× 10-5 mol/g, respectively, under the 0.75 V applied potential in the BPA solution with an initial concentration of 0.1 mM. Obviously, the removal efficiency for the CNT-covered polyester yarn electrode was 17.2 times that of the carbon fiber electrode. The close packed adsorption between the curved surface of CNTs and the butterfly-shaped BPA [25] may has contributed much to the high efficiency. It should be noted that a subsequently study found that the CNT-covered polyester yarn electrode can be reutilized after the BPA polymer layer was stripped by a 2 V applied 8
potential in a 0.1 M Na2SO4 electrolyte; this is a benefit as it reduces the cost of the electrode material.
In this study, a CNT-based electrode was developed using a dispersed CNT-based dyestuff. After removal of the polymer binder and dispersant, CNTs would cover the surface of the polyester yarns and result in an increased specific surface area and electrochemical activity of the working electrode. Therefore, the electrochemical polymerization and degradation of BPA efficiently performed together on the CNT-covered polyester yarn electrode at the low applied potential of 0.75 V. Furthermore, high electrochemical removal efficiency of BPA was obtained (8.1× 10-5 mol/g), which was 17.2 times greater of that using a carbon fiber electrode.
9
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[10] Y. Ohko, I. Ando, C. Niwa, T. Tatsuma, T. Yamamura, T. Nakashima, Y. Kubota, A. Fujishima, Degradation of bisphenol-A in water by TiO2 photo-catalyst, Environ. Sci. Technol. 35 (2001) 2365-2368. [11] I. Gultekin, N.H. Ince, Synthetic endocrine disruptors in the environment and water remediation by advanced oxidation processes, J. Environ. Manag. 85 (2007) 816-832. [12] H. Kuramitz, Y. Nakata, M. Kawasaki, S. Tanaka, Electrochemical oxidation of Bisphenol A. Application to the removal of bisphenol A using a carbon fiber electrode, Chemosphere 45 (2001) 37-43. [13] A. Jorio, G. Dresselhaus, M.S. Dresselhaus, Carbon Nanotubes: Advanced Topics in the Synthesis, Structure, Properties and Applications, Spring Berlin Heidelberg, New York, 2008, pp. 165-250. [14] P.J.F. Harris. Carbon Nanotube Science: Synthesis, Properties and Applications, Cambridge University Press, New York, 2009, pp. 146-226. [15] X. Ren, C. Chen, M. Nagatsu, X. Wang, Carbon nanotubes as adsorbents in environmental pollution management: A review, Chem. Eng. J. 170 (2011) 395-410. [16] H. Yu, B. Fugetsu, A novel adsorbent obtained by inserting carbon nanotubes into cavities of diatomite and applications for organic dye elimination from contaminated water, J. Hazard. Mater. 170 (2010) 138-145. [17] A.D. Paola, E. García-López, G. Marcì, L. Palmisano, A survey of photocatalytic materials for environmental remediation, J. Hazard. Mater. 211-2012 (2012) 3-29. [18] Y. Yu, J.C. Yu, C. Chan, Y. Che, J. Zhao, L. Ding, W. Ge, P. Wong, Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye, Appl. Catal. B: Environ. 61 (2005) 1-11. [19] J. Garcia, H.T. Gomes, Ph Serp, Ph Kalck, J.L. Figuriredo, J.L. Faria, Carbon nanotube supported ruthenium catalysts for the treatment of high strength wastewater with aniline using wet air oxidation, Carbon 44 (2006) 2384-2391. [20] J. Yang, J. Wang, J. Jia, Improvement of electrochemical wastewater treatment 11
through mass transfer in a seepage carbon nanotube electrode reactor, Environ. Sci. Technol. 43 (2009) 3796-3802. [21] C.D. Vecitis, G. Gao, H. Liu, Electrochemical carbon nanotube filter for adsorption, desorption, and oxidation of aqueous dyes and anions, J. Phys. Chem. C 115 (2011) 3621-3629. [22] B. Fugetsu, E. Akira, M. Hachiya, M. Endo, The production of soft, durable, and electrically conductive polyester multifilament yarns by dye-printing them with carbon nanotubes, Carbon 47 (2009) 527-544. [23] N.B. Tahar, A. Savall, Electrochemical removal of phenol in alkaline solution. Contribution of the anodic polymerization on different electrode materials, Electrochimica Acta 54 (2009) 4809-4816. [24] C. Comninellis, Electrocatalysis in the electrochemical conversion/combustion of organic pollutions for waste water treatment, Electrochimica Acta 39 (1994) 1857-1862. [25] B. Pan, D. Lin, H. Mashayekhi, B. Xing, Adsorption and hysteresis of bisphenol A and 17α-ethinyl estradiol on carbon nanomaterials, Environ. Sci. Technol . 42 (2008) 5480-5485.
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Figures and table Fig. 1 Morphology of CNT-dyed polyester yarns (a) photograph of CNT-dyed polyester yarns, (b) SEM image of CNT-dyed polyester yarns, (c) high-magnification SEM image of CNT-dyed polyester yarns before removing binder, (d) surface image of CNT-dyed polyester yarns after removing binder.
a
b
d
c
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Fig. 2 Progressive cyclic voltammograms for 0.1 mM BPA in 0.1 M Na2SO4 solution at CNT-covered polyester yarn electrode. Scan rate is 50 mV/s.
14
Fig. 3 Schematic diagram of electrochemical polymerization of BPA.
HO
OH
bisphenol a
O
O
O
O
-2e , -2H
Polymer O
O
other type diradicals
15
Fig. 4 Relationships between the residual BPA in solution and treatment time with a series of applied potentials in 0.1 mM BPA solution containing 0.1 M Na2SO4 on CNT-covered polyester yarn electrode.
16
Fig. 5 Cyclic voltammogram was obtained on CNT-covered polyester yarn electrode in 0.1 M Na2SO4 solution. Scan rate is 50 mV/s.
17
Fig. 6 The amount of organic carbon except BPA in residual electrolyte which was calculated from the residual of BPA based on HPLC analysis and total carbon by TOC measurement in BPA solution with initiate concentration of 0.1 mM containing 0.1 M Na2SO4 after 2 h electrochemical treatment with a series of applied potentials.
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Table 1 The data of removal efficiencies of BPA by electrochemical polymerization and degradation with a series of applied potentials in 50 ml BPA solution with initiate concentration of 0.1 mM (total weight of BPA is 1.14 mg) containing 0.1 M Na2SO4 after 2 h. Applied potential (V) without 0.5 0.7 0.75 1.0 1.25
Removal amount of BPA (mg) a 0.29 0.31 0.81 1.13 0.98 0.67
a
Polymerization (mg) b 0.3 0.3 0.5 0.6 0.5 0.2
Degradation (mg) c -0.01d 0.01 0.31 0.53 0.48 0.47
The data was calculated from the residual of BPA based on HPLC analysis The data were obtained from weight increase of electrode, and data correct to one decimal place due to detection limit. c Weight of degradation was calculated from the removal amount of BPA deduced the weight increase of electrode. d Negative value is due to detection limit of weight increase of electrode. b
19
Fig. 7 Treatment time dependent removal of BPA without applied potential (dotted line) and with applied potential of 0.75 V (solid line) in BPA solution with initiate concentration of 0.1 mM at CNT-covered polyester yarn electrode (red) and carbon fiber electrode (blue).
20
Graphical Abstract (for review)
Research Highlights
A novel CNT-covered polyester yarn electrode was developed.
Electrochemical polymerization and degradation of BPA occurred on this electrode
A high removal efficiency of BPA was obtained at 0.75 V applied potential.
The removal efficiency of BPA was promoted 17.2 times than carbon fiber electrode.