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Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes
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Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes Chen-Chia Huang∗, Shu-Fang Siao Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, 123, Sec. 3, University Road Douliu, Yunlin, Taiwan
a r t i c l e
i n f o
Article history: Received 26 September 2017 Revised 23 January 2018 Accepted 4 February 2018 Available online xxx Keywords: Electrosorption Mesoporous carbon Copper ions Chelating agent
a b s t r a c t The ordered mesoporous carbon (OMC) synthesized by the template method was prepared as an electrosorption electrode in this study to discuss the electrosorption capacity of copper ions from an aqueous solution containing a chelating agent when the potential was applied. The physicochemical properties of OMC were analyzed by TEM, a nitrogen adsorption/desorption apparatus, an X-ray photoelectron spectrometer, and a Zeta potential meter. The concentration change of copper ions in the aqueous solution was measured by atomic absorption spectrometry, so as to calculate the electrosorption capacity. When the chelating agent was added in, the Cu-chelated complexes were adsorbed onto the anode, and the free copper ions were adsorbed by the cathode. When 0.8 V voltages were applied, the electrosorption capacity for copper ions from the aqueous solution with a slight amount of chelating agent was larger than that in the solution without chelating agent. For aqueous solution without chelating agent, the equilibrium electrosorption capacity for copper ions was 35.90 mg/g at pH = 4. For solutions with 10% citric acid or EDTA chelating agent, the copper ion electrosorption capacities increased to 70.18 mg/g and 59.26 mg/g, respectively. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Industrial effluent often contains heavy metal ions, such as those for electroplating operations, the semiconductor industry, metal finishing, printed circuit board production, the metallurgical industry, leather processing, and the catalyst industry. Most heavy metal ions are highly toxic, and if they are discharged without proper treatment, then there will be a heavy impact on the human body and natural environment. The conventional methods for treating wastewater containing heavy metal ions are approximately divided into physico-chemical and biological processes. Awaleh and Soubaneh [1] reviewed some chemical treatment processes of industrial effluent, in which the heavy metal ions were removed effectively, thus meeting the effluent standard. However, these methods have some defects, such as aiming at only one object of treatment, having a high equipment cost, incurring a high energy loss, and exhibiting a high consumption of the chemical reagent, thus leading to secondary pollution and so on. In metal finishing and printed circuit board industries, with the extensive application of chelating agents, heavy metals in wastewaters often exist in chelated form. The heavy metal chelated
∗
Corresponding author. E-mail address: [email protected] (C.-C. Huang).
complexes are very stable increasing the solubility of heavy metal ions greatly, which are unlikely to be removed by using a conventional coagulation process or chemical precipitation [2]. Due to forming chelated heavy metal complexes, concentration of heavy metal ion in wastewater to meet the increasingly strict effluent discharge standard has become a challenging issue. In the past few years, many methods for the removal of metal-chelated ions complexes have been reported such as membrane filtration [3], photocatalytic oxidation [4], electrochemical oxidation [5], electrochemical coagulation [6], ion exchange [7,8], adsorption [9-11], etc. Some researchers have even recently used zero valent nano-iron which has a high stability constant with EDTA to replace Cu of EDTA–Cu complexes and employed other methods to remove free copper ions [12,13]. However, the application of these methods is often limited because of technical or economic constraint. Electrosorption is a separation technique that is combined with adsorption and electrochemistry. It is a non-Faradic process unrelated to electron gain and loss [14]. The required current is only used to form an electrical double layer for the electrode and solution interface. The used electrode can be regenerated in situ by power-off or reverse current, to avoid using the powerwasting thermal regeneration, or with solvent wash or chemical agent regeneration. Therefore, electrosorption technique is a low power consuming and energy-saving cleaning process. Quite a few recent studies have discussed the removal of heavy metal ions
https://doi.org/10.1016/j.jtice.2018.02.005 1876-1070/© 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: C.-C. Huang, S.-F. Siao, Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.02.005
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from wastewater by electrosorption [15-22]. The researchers have fabricated electrodes for electrosorption by using carbon materials, including activated carbon cloths [15], carbon nanotube [16,17], ordered mesoporous carbon (OMC) [18], and activated carbon [20]. In recent, some composite electrode materials such as manganese dioxide/carbon fiber [21] and poly(m-phenylenediamine)/reduced graphene oxide [22] have been performed. While electrosorption of copper ions on different adsorbents has been studied [15-22], little effort has been focused on studying the effect of organic complexing agents on copper removal. In this paper the synthesized OMC was used to prepare electrodes for electrosorption of Cu-EDTA or Cu-citrate ions from aqueous solutions. The effects of experimental conditions such as pH, different chelating agents, and different ratios of chelating agent to metal on electrosorption capacity were investigated intensively. The regeneration of the used OMC electrode by open circuit was also examined. 2. Materials and methods 2.1. Materials Pluronic 123 (P123), sucrose (99%), sulfuric acid, Poly(vinylidene difluoride) (PVDF) with M.W. 53,400 were obtained from Aldrich. Copper nitrate trihydrate, nitric acid, NaOH, and citric acid were purchased from Merck. Tetraethyl orthosilicate (TEOS) was obtained from Fluka. Ethylenediaminetetraacetic acid (EDTA) was obtained from Katayama Chemical Ind. Co. LTD. Carbon black (XC-72) was obtained from Cabot Co.. Sodium hydroxide, nitric acid, and all other chemicals used in this study were of analytical reagent grade and were used without any further purification. 2.2. Synthesis of ordered mesoporous carbon Ordered mesoporous silica (SBA-15) was synthesized using P123 as the structure directing agent and TEOS (98%) as the silica source following a procedure described in the literature [23]. Briefly, 3.3 g P123 was dissolved in 100 mL of 1.7 M HCl solution at 323 K and 6.67 g TEOS was then added. The solution was stirred at 323 K for 2 h and was subsequently transferred to a Teflon-lined autoclave which was kept at 381 K for 24 h. The resulting material was recovered by filtration followed by washing with de-ionized water. The organic template P123 was removed by calcination at 773 K in air for 6 h. The ordered mesoporous carbon (OMC) was synthesized using calcined SBA-15 as the template and sucrose as the carbon source. Typically, 1.0 g of SBA-15 was mixed with 5 mL of aqueous solution containing 1.25 g of sucrose and 0.14 g of concentrated sulfuric acid. The mixture was heated at 373 K for 6 h and subsequently at 433 K for a further 6 h. The resultant material was ground into powder and again mixed with 5 mL of aqueous solution containing 0.8 g of sucrose and 0.09 g of concentrated sulfuric acid by the thermal treatment described above. The composite was then carbonized at 1173 K for 6 h under nitrogen flow. The resultant black sample was treated with 2 M NaOH solution to remove the silica template, washed with distilled water, and dried at 378 K for 4 h [18,24].
from the amount of nitrogen adsorbed at a relative pressure of 0.95. The micropore volume was calculated by the D–R (Dubinin– Radushkevich) method [26]. The OMC surface potential was measured by using an interface potential analyzer (Nano Plus). The surface element contents of the OMC before and after electrosorption were determined by an X-ray photoelectron spectrometer (Axis Ultra DLD). 2.4. Fabrication of carbon electrode and electrosorption apparatus The paste was prepared by using OMC powder: carbon black (XC-72R): PVDF at a weight ratio of 85:5:10. The prepared paste was coated on a titanium plate uniformly by a blade coater, and the area was controlled at 5 cm × 5 cm. The thickness of coating layer was about 0.4 mm. It was placed in a 333 K oven for preliminary drying and then placed in a 378 K oven to remove the solvent and moisture. The active material of each electrode weighs about 25 mg. The conductivity of the OMC electrode was 9.83 × 10−4 S/cm measured by a Keithley 2400 SMU instrument. Electrosorption experiments were conducted in a batch-mode recycling system which comprised an electrosorption tank, a DC power supply, a solution storage tank, and a peristaltic pump. The schematic diagram of the electrosorption system is shown in Fig. S1. The electrosorption tank was a cuboid made by Plexiglas plates and the space between anode and cathode was about 0.5 cm. 2.5. Electrosorption experiment The aqueous solutions were prepared by dissolving cupric nitrate and chelating agent (citric acid or EDTA) under a molar ratio (10:1, 20:1, or 30:1) in deionized water. The initial copper concentration varied from 50 to 200 ppm. The aqueous solution was adjusted by 0.1 M nitric acid to the required pH value. A 100 mL of the prepared solution was put in a 250 mL beaker, the power supply was adjusted to the required voltage for the experiment, the peristaltic pump was actuated for circulation, and the flow rate was about 10 mL/min. The electrosorption operation procedure is detailed in [18]. The sample was taken at the predetermined time intervals, whereby the concentration of copper ions in the sample solution was analyzed by an atomic absorption spectrometry (Perkin Elmer AAnalyst 400). The system volume decreased due to repeated sampling, and the electrode adsorption capacity was calculated according to Eq. (1).
qn =
C0Vo − Cn (Vo − nv ) − v m
n
1 Ci
(1)
where qn : accumulative electrode adsorption capacity up to No. nth sampling (mg/g); C0 : initial concentration of solution (mg/L); Cn : concentration of No. nth sample solution (mg/L); Ci : concentration of No. ith sample solution (mg/L); V0 : initial solution volume (L); v: sample solution volume (L); n: number of samplings; and m: weight of electrode material (g). 3. Results and discussion 3.1. Characterization of OMC
2.3. Characterization of OMC The structure of the synthesized OMC was observed through a Transmission Electron Microscope (JEOL TEM-1400). The nitrogen adsorption and desorption isotherms at 77 K were measured by a gas adsorption apparatus (Micromeritics ASAP 2020). Specific surface area was calculated according to the BET equation. The pore size distribution was calculated by the BJH (Barrett–Joyner– Halenda) method [25]. The total pore volume was calculated
Fig. 1(a) and (b) shows the TEM images of the OMC prepared by using the template method. It is observed that the structure of the OMC is an ordered hexagonal arrangement of cylindrical mesoporous channels [18,27]. The ordered mesoporous channels will facilitate the mass transport of Cu-chelated complex ions into the OMC. Chemical analysis (by EDS) of the OMC showed that the surface elements consist of about 85.50 wt% C, 13.55 wt% O, and 0.95 wt% Si. As the silicon template was not removed completely,
Please cite this article as: C.-C. Huang, S.-F. Siao, Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.02.005
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Fig. 1. TEM images of the OMC for (a) pore structure, (b) ordered channels. Table 1 Texture characteristics of the OMC and OMC electrode. Sample
SBET (m2 /g)
Vtotal (cm3 /g)
Vmic (cm3 /g)
Vmeso (cm3 /g)
Vmeso /Vtotal (%)
Average pore size (nm)
OMC OMC electrode
1053 487
1.14 0.44
0.14 0.15
1.00 0.29
88 65
4.34 3.59
SBET : BET specific surface area. Vtotal : the total pore volume calculated from N2 isotherm at P/P0 = 0.995. Vmic : the micropore volume was determined by the Dubinin-Radushkevithch equation [26]. Vmeso = Vtotal -Vmic.
the carbon material still contained a slight amount of Si element. The surface morphology of the obtained OMC was observed by SEM, as shown in Fig. S2. The image reveals that the calcined OMC presents morphology with uniform micrometer rod-like particles in concordance with other ones reported previously [28]. Fig. 2a presents the nitrogen adsorption–desorption isotherms of the OMC and OMC electrode. According to IUPAC classification, the nitrogen adsorption–desorption curve of the OMC is of Type IV. When the relative pressure is higher than 0.4, as the capillary condensation results in a H1 hysteresis loop, a typical adsorption– desorption isotherm curve of mesopore arises [29]. It is clear from Fig. 2a that the OMC electrode has a lower nitrogen adsorption capacity than that of the OMC. This is expected because 10 wt% of PVDF was blended in the electrode. The pore size distributions of OMC and OMC calculated by using the BJH method are shown in Fig. 2b. The vertical axis is the differential value of the pore volume to pore size, and the horizontal axis is the pore size (nm). It is observed that the pore size of the OMC is mostly 3.37–5.91 nm, approximately to 3.81–4.41 in the literature [30]. The texture characteristics of the OMC and OMC electrode are listed in Table 1. The BET specific surface area of the OMC and OMC electrode are 1053 and 487 m2 /g, respectively. Liu et al. [30] synthesized the CMK-3 and obtained the specific surface area of 1074 m2 /g. The BET surface area and pore volume of the OMC electrode are dramatically decreased to less than 40% of those of the OMC. This result can be attributed by pore blockage or partial pore filling by the presence of PVDF binder. The chemical bonding of the OMC was investigated by XPS characterization. Fig. 3 exhibits the full XPS spectrum of the OMC. Two peaks corresponding to carbon and oxygen were the most abundant elements. Trace amount of Si (0.58%) was also determined from XPS analysis. This result is confirmed with the observation by EDS. As shown in inset plot of Fig. 3, the XPS C1s spectrum had been de-convoluted into four peaks with binding energies centered at 284.6, 286.0, 287.3, and 288.9 eV, were attributed to graphitic sites (C–C, 56%), phenols or alcohol (C–O–C, 16%), carbonyl or quinone (C=O, 14%), and carboxylic groups or esters (COO, 14%), respectively [31].
Fig. 4 shows the surface potential of the OMC in solutions of different pH values measured by a zeta potential meter. The zeta potential measured on the colloid of the OMC varied from 6.8 to −32.5 mV in the pH range of 3–8. It is observed that the isoelectric point of OMC is at pH = 3.8. The literature [32] indicates that the zero potential point of activated carbon varies with raw materials, at pH values of 1.4–7.1. The surface potential of the OMC increases gradually as the pH value decreases, and the surface potential tends to be positively charged when the pH value is smaller than 3.8. There is an electrostatic repulsion to Cu2+ , which is unfavorable for adsorption. When the pH value is greater than 3.8, as the functional groups on the carbon surface is negatively charged, there is an increasing electrostatic attraction for cations. 3.2. Electrosorption 3.2.1. Electrosorption of copper ion from an aqueous solution It is known that the copper ion exists as different valence states in solutions of different pH values, and it is mainly the Cu2+ valence state that is below pH 5. When the pH value is larger than 6, the valence state of Cu2+ decreases, and the forms CuOH+ and Cu2 (OH)2 2+ occur. When the pH value is greater than 8, the valence states of copper are mainly the form of Cu(OH)3 − and the precipitate of Cu(OH)2 [33]. When the pH value is high, the precipitation of copper ions might result in adsorption measurement errors. Therefore, this study uses pH = 3–5 as the scope of the adsorption experiment. Typical electrosorption curves are shown in Fig. S.3. It is found electrosorption equilibrium was reached after 5 h for 50 ppm at 0.8 V electrical potential. Fig. 5 depicts the effect of different pH values on electrosorption capacity for copper ions from an aqueous solution when 0.8 V voltages are applied. It was found that the electrosorption capacity for copper ions increased obviously with increasing pH value of the solution. When the pH value is low, there are considerable hydrogen ions in the aqueous solution, the hydrogen ions and copper ions have a competitive adsorption effect, and the active site on the OMC surface is occupied by hydrogen ions, so that the copper ion adsorption capacity decreases [10]. The aforesaid zeta potential analysis indicated that the isoelectric point of the OMC
Please cite this article as: C.-C. Huang, S.-F. Siao, Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.02.005
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Fig. 2. Nitrogen adsorption–desorption isotherms (a) and pore size distributions (b) of the OMC and OMC electrode.
is at pH = 3.8. Therefore, when pH = 3, the OMC surface groups were protonated and positive charged, leading to a relatively poor copper ion adsorption, which was only 11.72 mg/g. When pH = 5, the OMC surface was negatively charged, the electrode material had high copper ion electrosorption capacity, and the equilibrium electrosorption capacity was 44.48 mg/g. Huang et al. [20] reported the equilibrium electrosorption capacity of Cu2+ at 0.8 V was 24.57 mg/g on the electrode of commercial activated carbon. Huang and He [18] noted that the electrosorption capacity of copper ion onto the OMC electrode was 56.62 mg/g at a polarization of 0.9 V and pH = 5. Adsorption/electrosorption isotherms of copper ions on the OMC electrodes at pH = 5 are shown in Fig. 6. The equilibrium
adsorption capacity at open circuit was 9.5 mg/g. Lee et al [34] synthesized mesoporous carbon CMK-1 and found copper ion adsorption capacity of 3.72 mg/g at pH = 5. The adsorption capacity of copper ions on the OMC electrode is higher than that of CMK-1. The equilibrium electrosorption capacity of the OMC electrode at polarization of 0.8 V was 44.84 mg/g, which was 4.7 times that at open circuit. This result indicates that electrical polarization showed significant enhancement of copper ion adsorption on the OMC electrode. Similar results were reported by Huang et al. [20] and Zhan et al. [16]. The experimental isotherm data were well correlated by the Langmuir model as shown in Fig. 6. The parameters of the Langmuir model, maximum amount of adsorbed copper ions corresponding to monolayer coverage (qm ) and the
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Fig. 3. Full XPS spectrum and C1s XPS spectrum (inset) of the OMC.
Fig. 4. Zeta potential of the OMC under different pH values.
Langmuir constant related to binding energy (KL ), were regressed as 49.0 (mg/g) and 0.151 (L/mg) for electrosorption and 13.5 (mg/g) and 0.034 (L/mg) for adsorption, respectively. 3.2.2. Effect of different copper ion-chelating agent ratios The chelating agent and metal ions in the solution form different forms of complexes. The addition level of chelating agent influences the adsorption effect. Maketon et al. [7] indicated that EDTA was mostly [H2 EDTA]2− and [H3 EDTA]− forms at pH 3–5; the copper ions and EDTA were mixed in a molar ratio of 1:1 in an acetic acid buffer solution at pH 5.5, and the copper ions existed as [CuEDTA]2− complex. If the copper ions and EDTA are mixed in a molar ratio of 1:0.5 (Cu: 3.78 mM in sodium acetate
buffer) at pH 5.5, then the solution contains 50% [CuEDTA]2− , 26% [CuAcetate2 ], and 21% [CuAcetate]+ complexes, and a few Cu2+ ions [7]. In the case of more copper excess, much more free copper ions exist in the Cu–EDTA solution. This laboratory used an activated carbon fiber cloth for electrosorption of copper ions in the Cu-chelated solution and found that the electrosorption capacity was the highest when the copper ion: chelating agent ratio was 10:1 (outcome not published). Therefore, we selected 10:1 for the experiment, and the chelating agent content was reduced to 20:1 and 30:1 respectively. Fig. 7 shows the electrosorption capacity of the OMC electrode for copper ions in different chelated (citric acid or EDTA) solutions. The operating conditions are voltage 0.8 V, pH = 4, and the
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Fig. 5. Effect of pH on electrosorption capacity of copper ions from an aqueous solution under 0.8 V voltages.
Fig. 6. Adsorption/electrosorption isotherms of copper ions on the OMC electrode with Langmuir model correlation.
concentration of copper ions is 50 ppm. According to Fig. 7, the equilibrium electrosorption capacity of copper ions from either EDTA or citric acid chelated solution decreases as the Cu: chelating agent ratio is 10:1 > 20:1 > 30:1. The difference in the electrosorption capacity for copper ions can be explained by the schematic diagram in Fig. 8. When Cu: EDTA = 1:1, all of the copper ions in the solution exist as the [CuEDTA]2− complex; there is no free copper ion; and the Cu-EDTA complex is adsorbed to the anode. As the Cu-chelated complex ion is larger than the free copper ion, the electrodes of the same surface area adsorb relatively less for larger complex ions. When Cu: EDTA = 10:1, a majority of unchelated free copper ions (Cu2+ ) exist in aqueous solution, and only about 10% of copper ions are combined with EDTA and
formed [CuEDTA]2-complexes. Most free Cu2+ are adsorbed to the cathode, and the [CuEDTA]2− is adsorbed to the anode or attracted to the outer layer of the anode electrode under the electrical double layer effect. Therefore, the quantity of copper ions adsorbed from the Cu-chelated solution is larger than that from the solution without chelating agent. When the copper: chelating agent = 20:1 or 30:1, as the number of chelated copper ions is very small, the [CuEDTA]2− concentration in the solution is relatively low, the adsorption is similar to the case without chelating agent, and the adsorbed ions are mainly free Cu2+ migrating toward the cathode. Therefore, the electrosorption capacity for copper ions is highest when the ratio of chelating agent is 10:1.
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Fig. 7. Electrosorption capacity from Cu-chelated solutions in different Cu:chelating agent ratios: (a) EDTA, (b) citric acid (applied voltage 0.8 V, pH = 4, C0 = 50 ppm).
3.2.3. Effect of pH for a solution with chelating agent and copper ions Effect of pH on equilibrium electrosorption capacity of copper ions from aqueous solution containing chelating agent is shown in Fig. 9. The solution with a little chelating agent (copper: chelating agent is 10:1) was applied with 0.8 V voltages. It was found the order of electrosorption capacity of copper ions was pH 4 > pH 5 > pH 3 from solutions containing either citric acid or EDTA. In comparison to Fig. 5, it is found that a small amount of chelating agent is obviously helpful to the removal of copper ions. When pH = 3, which was lower than the isoelectric point of the OMC (pH 3.8), the carbon electrode surface had positive potential that was unfavorable for copper ion adsorption, and the electrosorption capacity for copper ions was not high no matter whether the chelating agent was added in or not. When pH = 4 and the
chelating agent was not added in, the equilibrium electrosorption capacity for copper ions is 35.90 mg/g. When the citric acid added, the electrosorption capacity for copper ions increased to 70.18 mg/g; and when the EDTA added, the copper electrosorption capacity also increased to 59.26 mg/g. Piispanen and Lajunen [38] indicated that when the pH value was 4, the copper and citric acid mainly existed as [CuCit]− and [Cu2 H-2 Cit2 ]4− complexes. Maketon et al. [7] indicated that when the pH value was 4, the EDTA and copper ions mainly formed [CuEDTA]2− and [CuHEDTA]− . As the chelating agent added was only 10% of copper ions, the free Cu2+ is still predominant in the solution, and there was a small amount of [CuEDTA]2− or [CuCit]− . When pH = 4, thus approximating the zero potential point of carbon material, the carbon electrode surface only had a
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Fig. 8. Schematic diagram of electrosorption of different ions on electrodes from Cu-EDTA solutions.
few negative charges, which was favorable for adsorbing the positively charged free Cu2+ to the cathode. Moreover, the negatively charged [CuEDTA]2− or [CuCit]− was adsorbed to the anode, and the overall electrosorption capacity for copper ions was higher than the case without chelating agent. When pH = 5, the carbon electrode surface had more negative charges, and the CuEDTA2− or CuCit− negatively charged complexes in the solution formed electrostatic repulsion; thus, the overall copper ion electrosorption capacity was reduced slightly. The citric acid mainly forms a
bidentate or tridentate coordination complex in a cupric nitrate aqueous solution, and EDTA and copper ions can form a tridentate or quadridentate coordination complex. This may cause that the citric acid has smaller steric hindrance than EDTA, therefore, the adsorption capacity of copper ions from containing citric acid is larger than from solution containing EDTA. In Table 2 the results of this work are compared to other results obtained by other authors studying copper sorption at pH 3–5.5 using different adsorbents. As seen in the Table 2, the sorption
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Table 2 Comparison of copper sorption capacities of different sorbents from Cu-chelated solutions. Sorbent
Co (mg/L)
pH
Chelating agent(Cu/chelator)
Electro-polarization (V)
qm (mg/g)
Reference
Grape stalk Exhausted coffee waste Chitosan Chitosan Chitosan NNN-SBA-15 OMC OMC
25.4 25.4 30 635 1143 63.5 50 50
5.2 5.2 4 3 4 5.5 4 4
EDTA (1/0.5) EDTA (1/0.5) EDTA (1/1) EDTA (1/2) Citrate (1/1) EDTA (1/1) EDTA (10/1) Citrate (10/1)
– – – – – – 0.8 0.8
23.86 10.52 44.45 28.57 289.5 26.3 59.26 70.18
[11] [11] [35] [36] [37] [8] This study This study
Fig. 9. Effect of pH on electrosorption capacity of copper ions from solutions containing chelating agents under 0.8 V voltages.
capacity obtained from Cu-EDTA solution by electrosorption is greater than that reported in literature. The electrosorption capacity from Cu-citrate solution is lower than sorption capacity of high concentration copper ions on chitosan reported by Lu et al. [37].
3.2.4. X-ray photoelectron spectrograph analysis Fig. 10 shows comparison of full XPS spectra of the anode carbon electrodes after electrosorption copper from solution in presence and in absence of chelating agent. The electrosorption conditions were set as applied potential 0.8 V, aqueous solution pH = 4, concentration of copper ions 50 ppm, and copper ionchelating agent ratio 10:1. It is observed that the signals of binding energy 285 eV, 532 eV, and 688 eV are the element characteristic peaks of C 1 s, O 1 s, and F 1 s, respectively. The peak of the fluorine element may result from the PVDF adhesive added for producing the electrode plate. When there is no chelating agent added in the solution, the XPS of the anode carbon electrode does not have the characteristic peak of copper, meaning the copper ions have not been deposited on the anode. When the chelating agent is added in, the XPS analysis of the anode carbon electrode shows a slight Cu2p peak at 933 eV, meaning quite a low content of copper has been deposited on the anode electrode. It is observed that the atomic ratio of Cu is only 0.43 and 0.52% for citrate and EDTA in presence, respectively. This implies that most Cu-chelated ions are physically adsorbed on the electrode surface by the formed electrical double layer in the electrosorption process. This is also confirmed Cu-chelated complexes such as [CuEDTA]2− or [CuCit]− electrosorbed on the anode. Huang et al. [20] applied 1.2 V potential to a commercial activated carbon electrode for electrosorption of copper ions, found the copper was reduced to cuprous oxide (Cu2 O) and/or copper
Fig. 10. Comparison of XPS spectra of the OMC anode electrodes after electrosorption.
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Fig. 11. Two electrosorption–desorption cycles of 50 mg/L Cu-citric acid (10:1) solution on the OMC electrode upon the polarization and depolarization.
metal electrodeposited on the cathode electrode, and suggested 0–0.8 V for electrosorption operation. Zhan et al. [17] applied 1.2 V potential to a nanotube-chitosan composite electrode for electrosorption of copper ions and found 0.29 at.% copper on the cathode electrode. Huang and He [18] applied 0.9 V potential to an OMC electrode for electrosorption and found 0.17 at.% copper on the cathode electrode. As the free Cu2+ or Cu-chelated ions are physically adsorbed to the carbon material or electrical double layer, there is only a slight amount of electrodeposited copper on the electrode. When the power supply is cut off, the copper ions are desorbed from the carbon surface, and the electrode can be regenerated in situ easily. Fig. 11 shows the electrosorption/desorption cycles of the OMC electrodes, which was conducted by repeating two charging (0.8 V) and discharging (0.0 V) processes in a 50 ppm Cu: citric acid (10:1) solution at pH = 4. As can be seen, the polarization of the OMC electrodes at 0.8 V leads to a decrease of copper concentration. Furthermore, the regeneration can be achieved upon the electrodes depolarization at 0.0 V. As shown in Fig. 11, the same pattern can be found in two electrosorption–desorption cycles. It demonstrates that the electrosorbed copper ions can be desorbed by removing an electric field, and then the OMC electrodes can be reused. Therefore, the Cu-chelated electrosorption process of the solution containing chelating agent is a reversible green separation process that is applicable to chemical plating wastewater treatment.
4. Conclusions This study used the OMC to prepare an electrosorption electrode and discussed the adsorptive capacity of a carbon electrode for copper ions in an aqueous solution containing a slight amount of chelating agent (EDTA, citric acid). The experimental results showed that the addition of the chelating agent contributes to the electrosorption capacity of the OMC electrode for copper ions. The electrosorption capacity for copper ions was highest when the addition level of the chelating agent (copper ion: chelating agent) was 10:1. The pH value of the solution can influence the electrosorption capacity for copper ions; the maximum equilibrium electrosorption capacity was reached when pH = 4; and
the equilibrium electrosorption capacity of citric acid and EDTA was 70.18 mg/g and 59.68 mg/g, respectively. The XPS analysis of electrode after electrosorption proved that the electrosorption behavior is of physical adsorption rather than redox reaction, and the electrode material can be regenerated by cutting off the power in situ after electrosorption. The prepared electrode material can be used for treatment of chemical plating effluent by means of an electrosorption process. Acknowledgments This work was financially supported by the Ministry of Science and Technology, R.O.C. under Grant no. MOST 103-2221-E-224-072. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2018.02.005. References [1] Awaleh MO, Soubaneh YD. Waste water treatment in chemical industries: the concept and current technologies. J Waste Water Treatment Anal 2014;5(1):1–13. [2] Ju F, Hu Y. Removal of EDTA-chelated copper from aqueous solution by interior microelectrolysis. Sep Purif Technol 2011;78:33–41. [3] Juang RS, Chen MN. Removal of copper(II) chelates of EDTA and NTA from dilute aqueous solutions by membrane filtration. Ind Eng Chem Res 1997;36:179–86. [4] Cho H, Shin IS, Yang JK, Lee SM, Shin WT. Removal of Cu(II)-EDTA complex using TiO2 solar light the effect of operational parameters and feasibility of solar light application. J Environ Sci Health A 2006;41:1027–41. [5] Chang JH, Ellis AV, Yan CT, Tung CH. The electrochemical phenomena and kinetics of EDTA-copper wastewater reclamation by electrodeposition and ultrasound. Sep Purif Technol 2009;68:216–21. [6] Yeh RS, Wang YY, Wan CC. Removal of Cu-EDTA compounds via electrochemical process with coagulation. Water Res 1995;29:597–9. [7] Maketon W, Zenner CZ, Ogden KL. Removal efficiency and binding mechanisms of copper and copper-EDTA complexes using polyethyleneimine. Environ Sci Technol 2008;42(6):2124–9. [8] Wu L, Wang H, Lan H, Liu H. Adsorption of Cu(II)-EDTA chelates on tri-ammonium-functionalized mesoporous silica from aqueous solution. Sep Purif Technol 2013;117:118–23. [9] Chang C, Ku Y. The adsorption and desorption characteristics of EDTA-chelated copper ion by activated carbon. Sep Sci Technol 1995;30(6):899–915.
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Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes Chen-Chia Huang∗, Shu-Fang Siao Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, 123, Sec. 3, University Road Douliu, Yunlin, Taiwan
a r t i c l e
i n f o
Article history: Received 26 September 2017 Revised 23 January 2018 Accepted 4 February 2018 Available online xxx Keywords: Electrosorption Mesoporous carbon Copper ions Chelating agent
a b s t r a c t The ordered mesoporous carbon (OMC) synthesized by the template method was prepared as an electrosorption electrode in this study to discuss the electrosorption capacity of copper ions from an aqueous solution containing a chelating agent when the potential was applied. The physicochemical properties of OMC were analyzed by TEM, a nitrogen adsorption/desorption apparatus, an X-ray photoelectron spectrometer, and a Zeta potential meter. The concentration change of copper ions in the aqueous solution was measured by atomic absorption spectrometry, so as to calculate the electrosorption capacity. When the chelating agent was added in, the Cu-chelated complexes were adsorbed onto the anode, and the free copper ions were adsorbed by the cathode. When 0.8 V voltages were applied, the electrosorption capacity for copper ions from the aqueous solution with a slight amount of chelating agent was larger than that in the solution without chelating agent. For aqueous solution without chelating agent, the equilibrium electrosorption capacity for copper ions was 35.90 mg/g at pH = 4. For solutions with 10% citric acid or EDTA chelating agent, the copper ion electrosorption capacities increased to 70.18 mg/g and 59.26 mg/g, respectively. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Industrial effluent often contains heavy metal ions, such as those for electroplating operations, the semiconductor industry, metal finishing, printed circuit board production, the metallurgical industry, leather processing, and the catalyst industry. Most heavy metal ions are highly toxic, and if they are discharged without proper treatment, then there will be a heavy impact on the human body and natural environment. The conventional methods for treating wastewater containing heavy metal ions are approximately divided into physico-chemical and biological processes. Awaleh and Soubaneh [1] reviewed some chemical treatment processes of industrial effluent, in which the heavy metal ions were removed effectively, thus meeting the effluent standard. However, these methods have some defects, such as aiming at only one object of treatment, having a high equipment cost, incurring a high energy loss, and exhibiting a high consumption of the chemical reagent, thus leading to secondary pollution and so on. In metal finishing and printed circuit board industries, with the extensive application of chelating agents, heavy metals in wastewaters often exist in chelated form. The heavy metal chelated
∗
Corresponding author. E-mail address: [email protected] (C.-C. Huang).
complexes are very stable increasing the solubility of heavy metal ions greatly, which are unlikely to be removed by using a conventional coagulation process or chemical precipitation [2]. Due to forming chelated heavy metal complexes, concentration of heavy metal ion in wastewater to meet the increasingly strict effluent discharge standard has become a challenging issue. In the past few years, many methods for the removal of metal-chelated ions complexes have been reported such as membrane filtration [3], photocatalytic oxidation [4], electrochemical oxidation [5], electrochemical coagulation [6], ion exchange [7,8], adsorption [9-11], etc. Some researchers have even recently used zero valent nano-iron which has a high stability constant with EDTA to replace Cu of EDTA–Cu complexes and employed other methods to remove free copper ions [12,13]. However, the application of these methods is often limited because of technical or economic constraint. Electrosorption is a separation technique that is combined with adsorption and electrochemistry. It is a non-Faradic process unrelated to electron gain and loss [14]. The required current is only used to form an electrical double layer for the electrode and solution interface. The used electrode can be regenerated in situ by power-off or reverse current, to avoid using the powerwasting thermal regeneration, or with solvent wash or chemical agent regeneration. Therefore, electrosorption technique is a low power consuming and energy-saving cleaning process. Quite a few recent studies have discussed the removal of heavy metal ions
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from wastewater by electrosorption [15-22]. The researchers have fabricated electrodes for electrosorption by using carbon materials, including activated carbon cloths [15], carbon nanotube [16,17], ordered mesoporous carbon (OMC) [18], and activated carbon [20]. In recent, some composite electrode materials such as manganese dioxide/carbon fiber [21] and poly(m-phenylenediamine)/reduced graphene oxide [22] have been performed. While electrosorption of copper ions on different adsorbents has been studied [15-22], little effort has been focused on studying the effect of organic complexing agents on copper removal. In this paper the synthesized OMC was used to prepare electrodes for electrosorption of Cu-EDTA or Cu-citrate ions from aqueous solutions. The effects of experimental conditions such as pH, different chelating agents, and different ratios of chelating agent to metal on electrosorption capacity were investigated intensively. The regeneration of the used OMC electrode by open circuit was also examined. 2. Materials and methods 2.1. Materials Pluronic 123 (P123), sucrose (99%), sulfuric acid, Poly(vinylidene difluoride) (PVDF) with M.W. 53,400 were obtained from Aldrich. Copper nitrate trihydrate, nitric acid, NaOH, and citric acid were purchased from Merck. Tetraethyl orthosilicate (TEOS) was obtained from Fluka. Ethylenediaminetetraacetic acid (EDTA) was obtained from Katayama Chemical Ind. Co. LTD. Carbon black (XC-72) was obtained from Cabot Co.. Sodium hydroxide, nitric acid, and all other chemicals used in this study were of analytical reagent grade and were used without any further purification. 2.2. Synthesis of ordered mesoporous carbon Ordered mesoporous silica (SBA-15) was synthesized using P123 as the structure directing agent and TEOS (98%) as the silica source following a procedure described in the literature [23]. Briefly, 3.3 g P123 was dissolved in 100 mL of 1.7 M HCl solution at 323 K and 6.67 g TEOS was then added. The solution was stirred at 323 K for 2 h and was subsequently transferred to a Teflon-lined autoclave which was kept at 381 K for 24 h. The resulting material was recovered by filtration followed by washing with de-ionized water. The organic template P123 was removed by calcination at 773 K in air for 6 h. The ordered mesoporous carbon (OMC) was synthesized using calcined SBA-15 as the template and sucrose as the carbon source. Typically, 1.0 g of SBA-15 was mixed with 5 mL of aqueous solution containing 1.25 g of sucrose and 0.14 g of concentrated sulfuric acid. The mixture was heated at 373 K for 6 h and subsequently at 433 K for a further 6 h. The resultant material was ground into powder and again mixed with 5 mL of aqueous solution containing 0.8 g of sucrose and 0.09 g of concentrated sulfuric acid by the thermal treatment described above. The composite was then carbonized at 1173 K for 6 h under nitrogen flow. The resultant black sample was treated with 2 M NaOH solution to remove the silica template, washed with distilled water, and dried at 378 K for 4 h [18,24].
from the amount of nitrogen adsorbed at a relative pressure of 0.95. The micropore volume was calculated by the D–R (Dubinin– Radushkevich) method [26]. The OMC surface potential was measured by using an interface potential analyzer (Nano Plus). The surface element contents of the OMC before and after electrosorption were determined by an X-ray photoelectron spectrometer (Axis Ultra DLD). 2.4. Fabrication of carbon electrode and electrosorption apparatus The paste was prepared by using OMC powder: carbon black (XC-72R): PVDF at a weight ratio of 85:5:10. The prepared paste was coated on a titanium plate uniformly by a blade coater, and the area was controlled at 5 cm × 5 cm. The thickness of coating layer was about 0.4 mm. It was placed in a 333 K oven for preliminary drying and then placed in a 378 K oven to remove the solvent and moisture. The active material of each electrode weighs about 25 mg. The conductivity of the OMC electrode was 9.83 × 10−4 S/cm measured by a Keithley 2400 SMU instrument. Electrosorption experiments were conducted in a batch-mode recycling system which comprised an electrosorption tank, a DC power supply, a solution storage tank, and a peristaltic pump. The schematic diagram of the electrosorption system is shown in Fig. S1. The electrosorption tank was a cuboid made by Plexiglas plates and the space between anode and cathode was about 0.5 cm. 2.5. Electrosorption experiment The aqueous solutions were prepared by dissolving cupric nitrate and chelating agent (citric acid or EDTA) under a molar ratio (10:1, 20:1, or 30:1) in deionized water. The initial copper concentration varied from 50 to 200 ppm. The aqueous solution was adjusted by 0.1 M nitric acid to the required pH value. A 100 mL of the prepared solution was put in a 250 mL beaker, the power supply was adjusted to the required voltage for the experiment, the peristaltic pump was actuated for circulation, and the flow rate was about 10 mL/min. The electrosorption operation procedure is detailed in [18]. The sample was taken at the predetermined time intervals, whereby the concentration of copper ions in the sample solution was analyzed by an atomic absorption spectrometry (Perkin Elmer AAnalyst 400). The system volume decreased due to repeated sampling, and the electrode adsorption capacity was calculated according to Eq. (1).
qn =
C0Vo − Cn (Vo − nv ) − v m
n
1 Ci
(1)
where qn : accumulative electrode adsorption capacity up to No. nth sampling (mg/g); C0 : initial concentration of solution (mg/L); Cn : concentration of No. nth sample solution (mg/L); Ci : concentration of No. ith sample solution (mg/L); V0 : initial solution volume (L); v: sample solution volume (L); n: number of samplings; and m: weight of electrode material (g). 3. Results and discussion 3.1. Characterization of OMC
2.3. Characterization of OMC The structure of the synthesized OMC was observed through a Transmission Electron Microscope (JEOL TEM-1400). The nitrogen adsorption and desorption isotherms at 77 K were measured by a gas adsorption apparatus (Micromeritics ASAP 2020). Specific surface area was calculated according to the BET equation. The pore size distribution was calculated by the BJH (Barrett–Joyner– Halenda) method [25]. The total pore volume was calculated
Fig. 1(a) and (b) shows the TEM images of the OMC prepared by using the template method. It is observed that the structure of the OMC is an ordered hexagonal arrangement of cylindrical mesoporous channels [18,27]. The ordered mesoporous channels will facilitate the mass transport of Cu-chelated complex ions into the OMC. Chemical analysis (by EDS) of the OMC showed that the surface elements consist of about 85.50 wt% C, 13.55 wt% O, and 0.95 wt% Si. As the silicon template was not removed completely,
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Fig. 1. TEM images of the OMC for (a) pore structure, (b) ordered channels. Table 1 Texture characteristics of the OMC and OMC electrode. Sample
SBET (m2 /g)
Vtotal (cm3 /g)
Vmic (cm3 /g)
Vmeso (cm3 /g)
Vmeso /Vtotal (%)
Average pore size (nm)
OMC OMC electrode
1053 487
1.14 0.44
0.14 0.15
1.00 0.29
88 65
4.34 3.59
SBET : BET specific surface area. Vtotal : the total pore volume calculated from N2 isotherm at P/P0 = 0.995. Vmic : the micropore volume was determined by the Dubinin-Radushkevithch equation [26]. Vmeso = Vtotal -Vmic.
the carbon material still contained a slight amount of Si element. The surface morphology of the obtained OMC was observed by SEM, as shown in Fig. S2. The image reveals that the calcined OMC presents morphology with uniform micrometer rod-like particles in concordance with other ones reported previously [28]. Fig. 2a presents the nitrogen adsorption–desorption isotherms of the OMC and OMC electrode. According to IUPAC classification, the nitrogen adsorption–desorption curve of the OMC is of Type IV. When the relative pressure is higher than 0.4, as the capillary condensation results in a H1 hysteresis loop, a typical adsorption– desorption isotherm curve of mesopore arises [29]. It is clear from Fig. 2a that the OMC electrode has a lower nitrogen adsorption capacity than that of the OMC. This is expected because 10 wt% of PVDF was blended in the electrode. The pore size distributions of OMC and OMC calculated by using the BJH method are shown in Fig. 2b. The vertical axis is the differential value of the pore volume to pore size, and the horizontal axis is the pore size (nm). It is observed that the pore size of the OMC is mostly 3.37–5.91 nm, approximately to 3.81–4.41 in the literature [30]. The texture characteristics of the OMC and OMC electrode are listed in Table 1. The BET specific surface area of the OMC and OMC electrode are 1053 and 487 m2 /g, respectively. Liu et al. [30] synthesized the CMK-3 and obtained the specific surface area of 1074 m2 /g. The BET surface area and pore volume of the OMC electrode are dramatically decreased to less than 40% of those of the OMC. This result can be attributed by pore blockage or partial pore filling by the presence of PVDF binder. The chemical bonding of the OMC was investigated by XPS characterization. Fig. 3 exhibits the full XPS spectrum of the OMC. Two peaks corresponding to carbon and oxygen were the most abundant elements. Trace amount of Si (0.58%) was also determined from XPS analysis. This result is confirmed with the observation by EDS. As shown in inset plot of Fig. 3, the XPS C1s spectrum had been de-convoluted into four peaks with binding energies centered at 284.6, 286.0, 287.3, and 288.9 eV, were attributed to graphitic sites (C–C, 56%), phenols or alcohol (C–O–C, 16%), carbonyl or quinone (C=O, 14%), and carboxylic groups or esters (COO, 14%), respectively [31].
Fig. 4 shows the surface potential of the OMC in solutions of different pH values measured by a zeta potential meter. The zeta potential measured on the colloid of the OMC varied from 6.8 to −32.5 mV in the pH range of 3–8. It is observed that the isoelectric point of OMC is at pH = 3.8. The literature [32] indicates that the zero potential point of activated carbon varies with raw materials, at pH values of 1.4–7.1. The surface potential of the OMC increases gradually as the pH value decreases, and the surface potential tends to be positively charged when the pH value is smaller than 3.8. There is an electrostatic repulsion to Cu2+ , which is unfavorable for adsorption. When the pH value is greater than 3.8, as the functional groups on the carbon surface is negatively charged, there is an increasing electrostatic attraction for cations. 3.2. Electrosorption 3.2.1. Electrosorption of copper ion from an aqueous solution It is known that the copper ion exists as different valence states in solutions of different pH values, and it is mainly the Cu2+ valence state that is below pH 5. When the pH value is larger than 6, the valence state of Cu2+ decreases, and the forms CuOH+ and Cu2 (OH)2 2+ occur. When the pH value is greater than 8, the valence states of copper are mainly the form of Cu(OH)3 − and the precipitate of Cu(OH)2 [33]. When the pH value is high, the precipitation of copper ions might result in adsorption measurement errors. Therefore, this study uses pH = 3–5 as the scope of the adsorption experiment. Typical electrosorption curves are shown in Fig. S.3. It is found electrosorption equilibrium was reached after 5 h for 50 ppm at 0.8 V electrical potential. Fig. 5 depicts the effect of different pH values on electrosorption capacity for copper ions from an aqueous solution when 0.8 V voltages are applied. It was found that the electrosorption capacity for copper ions increased obviously with increasing pH value of the solution. When the pH value is low, there are considerable hydrogen ions in the aqueous solution, the hydrogen ions and copper ions have a competitive adsorption effect, and the active site on the OMC surface is occupied by hydrogen ions, so that the copper ion adsorption capacity decreases [10]. The aforesaid zeta potential analysis indicated that the isoelectric point of the OMC
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Fig. 2. Nitrogen adsorption–desorption isotherms (a) and pore size distributions (b) of the OMC and OMC electrode.
is at pH = 3.8. Therefore, when pH = 3, the OMC surface groups were protonated and positive charged, leading to a relatively poor copper ion adsorption, which was only 11.72 mg/g. When pH = 5, the OMC surface was negatively charged, the electrode material had high copper ion electrosorption capacity, and the equilibrium electrosorption capacity was 44.48 mg/g. Huang et al. [20] reported the equilibrium electrosorption capacity of Cu2+ at 0.8 V was 24.57 mg/g on the electrode of commercial activated carbon. Huang and He [18] noted that the electrosorption capacity of copper ion onto the OMC electrode was 56.62 mg/g at a polarization of 0.9 V and pH = 5. Adsorption/electrosorption isotherms of copper ions on the OMC electrodes at pH = 5 are shown in Fig. 6. The equilibrium
adsorption capacity at open circuit was 9.5 mg/g. Lee et al [34] synthesized mesoporous carbon CMK-1 and found copper ion adsorption capacity of 3.72 mg/g at pH = 5. The adsorption capacity of copper ions on the OMC electrode is higher than that of CMK-1. The equilibrium electrosorption capacity of the OMC electrode at polarization of 0.8 V was 44.84 mg/g, which was 4.7 times that at open circuit. This result indicates that electrical polarization showed significant enhancement of copper ion adsorption on the OMC electrode. Similar results were reported by Huang et al. [20] and Zhan et al. [16]. The experimental isotherm data were well correlated by the Langmuir model as shown in Fig. 6. The parameters of the Langmuir model, maximum amount of adsorbed copper ions corresponding to monolayer coverage (qm ) and the
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Fig. 3. Full XPS spectrum and C1s XPS spectrum (inset) of the OMC.
Fig. 4. Zeta potential of the OMC under different pH values.
Langmuir constant related to binding energy (KL ), were regressed as 49.0 (mg/g) and 0.151 (L/mg) for electrosorption and 13.5 (mg/g) and 0.034 (L/mg) for adsorption, respectively. 3.2.2. Effect of different copper ion-chelating agent ratios The chelating agent and metal ions in the solution form different forms of complexes. The addition level of chelating agent influences the adsorption effect. Maketon et al. [7] indicated that EDTA was mostly [H2 EDTA]2− and [H3 EDTA]− forms at pH 3–5; the copper ions and EDTA were mixed in a molar ratio of 1:1 in an acetic acid buffer solution at pH 5.5, and the copper ions existed as [CuEDTA]2− complex. If the copper ions and EDTA are mixed in a molar ratio of 1:0.5 (Cu: 3.78 mM in sodium acetate
buffer) at pH 5.5, then the solution contains 50% [CuEDTA]2− , 26% [CuAcetate2 ], and 21% [CuAcetate]+ complexes, and a few Cu2+ ions [7]. In the case of more copper excess, much more free copper ions exist in the Cu–EDTA solution. This laboratory used an activated carbon fiber cloth for electrosorption of copper ions in the Cu-chelated solution and found that the electrosorption capacity was the highest when the copper ion: chelating agent ratio was 10:1 (outcome not published). Therefore, we selected 10:1 for the experiment, and the chelating agent content was reduced to 20:1 and 30:1 respectively. Fig. 7 shows the electrosorption capacity of the OMC electrode for copper ions in different chelated (citric acid or EDTA) solutions. The operating conditions are voltage 0.8 V, pH = 4, and the
Please cite this article as: C.-C. Huang, S.-F. Siao, Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.02.005
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Fig. 5. Effect of pH on electrosorption capacity of copper ions from an aqueous solution under 0.8 V voltages.
Fig. 6. Adsorption/electrosorption isotherms of copper ions on the OMC electrode with Langmuir model correlation.
concentration of copper ions is 50 ppm. According to Fig. 7, the equilibrium electrosorption capacity of copper ions from either EDTA or citric acid chelated solution decreases as the Cu: chelating agent ratio is 10:1 > 20:1 > 30:1. The difference in the electrosorption capacity for copper ions can be explained by the schematic diagram in Fig. 8. When Cu: EDTA = 1:1, all of the copper ions in the solution exist as the [CuEDTA]2− complex; there is no free copper ion; and the Cu-EDTA complex is adsorbed to the anode. As the Cu-chelated complex ion is larger than the free copper ion, the electrodes of the same surface area adsorb relatively less for larger complex ions. When Cu: EDTA = 10:1, a majority of unchelated free copper ions (Cu2+ ) exist in aqueous solution, and only about 10% of copper ions are combined with EDTA and
formed [CuEDTA]2-complexes. Most free Cu2+ are adsorbed to the cathode, and the [CuEDTA]2− is adsorbed to the anode or attracted to the outer layer of the anode electrode under the electrical double layer effect. Therefore, the quantity of copper ions adsorbed from the Cu-chelated solution is larger than that from the solution without chelating agent. When the copper: chelating agent = 20:1 or 30:1, as the number of chelated copper ions is very small, the [CuEDTA]2− concentration in the solution is relatively low, the adsorption is similar to the case without chelating agent, and the adsorbed ions are mainly free Cu2+ migrating toward the cathode. Therefore, the electrosorption capacity for copper ions is highest when the ratio of chelating agent is 10:1.
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Fig. 7. Electrosorption capacity from Cu-chelated solutions in different Cu:chelating agent ratios: (a) EDTA, (b) citric acid (applied voltage 0.8 V, pH = 4, C0 = 50 ppm).
3.2.3. Effect of pH for a solution with chelating agent and copper ions Effect of pH on equilibrium electrosorption capacity of copper ions from aqueous solution containing chelating agent is shown in Fig. 9. The solution with a little chelating agent (copper: chelating agent is 10:1) was applied with 0.8 V voltages. It was found the order of electrosorption capacity of copper ions was pH 4 > pH 5 > pH 3 from solutions containing either citric acid or EDTA. In comparison to Fig. 5, it is found that a small amount of chelating agent is obviously helpful to the removal of copper ions. When pH = 3, which was lower than the isoelectric point of the OMC (pH 3.8), the carbon electrode surface had positive potential that was unfavorable for copper ion adsorption, and the electrosorption capacity for copper ions was not high no matter whether the chelating agent was added in or not. When pH = 4 and the
chelating agent was not added in, the equilibrium electrosorption capacity for copper ions is 35.90 mg/g. When the citric acid added, the electrosorption capacity for copper ions increased to 70.18 mg/g; and when the EDTA added, the copper electrosorption capacity also increased to 59.26 mg/g. Piispanen and Lajunen [38] indicated that when the pH value was 4, the copper and citric acid mainly existed as [CuCit]− and [Cu2 H-2 Cit2 ]4− complexes. Maketon et al. [7] indicated that when the pH value was 4, the EDTA and copper ions mainly formed [CuEDTA]2− and [CuHEDTA]− . As the chelating agent added was only 10% of copper ions, the free Cu2+ is still predominant in the solution, and there was a small amount of [CuEDTA]2− or [CuCit]− . When pH = 4, thus approximating the zero potential point of carbon material, the carbon electrode surface only had a
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Fig. 8. Schematic diagram of electrosorption of different ions on electrodes from Cu-EDTA solutions.
few negative charges, which was favorable for adsorbing the positively charged free Cu2+ to the cathode. Moreover, the negatively charged [CuEDTA]2− or [CuCit]− was adsorbed to the anode, and the overall electrosorption capacity for copper ions was higher than the case without chelating agent. When pH = 5, the carbon electrode surface had more negative charges, and the CuEDTA2− or CuCit− negatively charged complexes in the solution formed electrostatic repulsion; thus, the overall copper ion electrosorption capacity was reduced slightly. The citric acid mainly forms a
bidentate or tridentate coordination complex in a cupric nitrate aqueous solution, and EDTA and copper ions can form a tridentate or quadridentate coordination complex. This may cause that the citric acid has smaller steric hindrance than EDTA, therefore, the adsorption capacity of copper ions from containing citric acid is larger than from solution containing EDTA. In Table 2 the results of this work are compared to other results obtained by other authors studying copper sorption at pH 3–5.5 using different adsorbents. As seen in the Table 2, the sorption
Please cite this article as: C.-C. Huang, S.-F. Siao, Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.02.005
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Table 2 Comparison of copper sorption capacities of different sorbents from Cu-chelated solutions. Sorbent
Co (mg/L)
pH
Chelating agent(Cu/chelator)
Electro-polarization (V)
qm (mg/g)
Reference
Grape stalk Exhausted coffee waste Chitosan Chitosan Chitosan NNN-SBA-15 OMC OMC
25.4 25.4 30 635 1143 63.5 50 50
5.2 5.2 4 3 4 5.5 4 4
EDTA (1/0.5) EDTA (1/0.5) EDTA (1/1) EDTA (1/2) Citrate (1/1) EDTA (1/1) EDTA (10/1) Citrate (10/1)
– – – – – – 0.8 0.8
23.86 10.52 44.45 28.57 289.5 26.3 59.26 70.18
[11] [11] [35] [36] [37] [8] This study This study
Fig. 9. Effect of pH on electrosorption capacity of copper ions from solutions containing chelating agents under 0.8 V voltages.
capacity obtained from Cu-EDTA solution by electrosorption is greater than that reported in literature. The electrosorption capacity from Cu-citrate solution is lower than sorption capacity of high concentration copper ions on chitosan reported by Lu et al. [37].
3.2.4. X-ray photoelectron spectrograph analysis Fig. 10 shows comparison of full XPS spectra of the anode carbon electrodes after electrosorption copper from solution in presence and in absence of chelating agent. The electrosorption conditions were set as applied potential 0.8 V, aqueous solution pH = 4, concentration of copper ions 50 ppm, and copper ionchelating agent ratio 10:1. It is observed that the signals of binding energy 285 eV, 532 eV, and 688 eV are the element characteristic peaks of C 1 s, O 1 s, and F 1 s, respectively. The peak of the fluorine element may result from the PVDF adhesive added for producing the electrode plate. When there is no chelating agent added in the solution, the XPS of the anode carbon electrode does not have the characteristic peak of copper, meaning the copper ions have not been deposited on the anode. When the chelating agent is added in, the XPS analysis of the anode carbon electrode shows a slight Cu2p peak at 933 eV, meaning quite a low content of copper has been deposited on the anode electrode. It is observed that the atomic ratio of Cu is only 0.43 and 0.52% for citrate and EDTA in presence, respectively. This implies that most Cu-chelated ions are physically adsorbed on the electrode surface by the formed electrical double layer in the electrosorption process. This is also confirmed Cu-chelated complexes such as [CuEDTA]2− or [CuCit]− electrosorbed on the anode. Huang et al. [20] applied 1.2 V potential to a commercial activated carbon electrode for electrosorption of copper ions, found the copper was reduced to cuprous oxide (Cu2 O) and/or copper
Fig. 10. Comparison of XPS spectra of the OMC anode electrodes after electrosorption.
Please cite this article as: C.-C. Huang, S.-F. Siao, Removal of copper ions from an aqueous solution containing a chelating agent by electrosorption on mesoporous carbon electrodes, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.02.005
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Fig. 11. Two electrosorption–desorption cycles of 50 mg/L Cu-citric acid (10:1) solution on the OMC electrode upon the polarization and depolarization.
metal electrodeposited on the cathode electrode, and suggested 0–0.8 V for electrosorption operation. Zhan et al. [17] applied 1.2 V potential to a nanotube-chitosan composite electrode for electrosorption of copper ions and found 0.29 at.% copper on the cathode electrode. Huang and He [18] applied 0.9 V potential to an OMC electrode for electrosorption and found 0.17 at.% copper on the cathode electrode. As the free Cu2+ or Cu-chelated ions are physically adsorbed to the carbon material or electrical double layer, there is only a slight amount of electrodeposited copper on the electrode. When the power supply is cut off, the copper ions are desorbed from the carbon surface, and the electrode can be regenerated in situ easily. Fig. 11 shows the electrosorption/desorption cycles of the OMC electrodes, which was conducted by repeating two charging (0.8 V) and discharging (0.0 V) processes in a 50 ppm Cu: citric acid (10:1) solution at pH = 4. As can be seen, the polarization of the OMC electrodes at 0.8 V leads to a decrease of copper concentration. Furthermore, the regeneration can be achieved upon the electrodes depolarization at 0.0 V. As shown in Fig. 11, the same pattern can be found in two electrosorption–desorption cycles. It demonstrates that the electrosorbed copper ions can be desorbed by removing an electric field, and then the OMC electrodes can be reused. Therefore, the Cu-chelated electrosorption process of the solution containing chelating agent is a reversible green separation process that is applicable to chemical plating wastewater treatment.
4. Conclusions This study used the OMC to prepare an electrosorption electrode and discussed the adsorptive capacity of a carbon electrode for copper ions in an aqueous solution containing a slight amount of chelating agent (EDTA, citric acid). The experimental results showed that the addition of the chelating agent contributes to the electrosorption capacity of the OMC electrode for copper ions. The electrosorption capacity for copper ions was highest when the addition level of the chelating agent (copper ion: chelating agent) was 10:1. The pH value of the solution can influence the electrosorption capacity for copper ions; the maximum equilibrium electrosorption capacity was reached when pH = 4; and
the equilibrium electrosorption capacity of citric acid and EDTA was 70.18 mg/g and 59.68 mg/g, respectively. The XPS analysis of electrode after electrosorption proved that the electrosorption behavior is of physical adsorption rather than redox reaction, and the electrode material can be regenerated by cutting off the power in situ after electrosorption. The prepared electrode material can be used for treatment of chemical plating effluent by means of an electrosorption process. Acknowledgments This work was financially supported by the Ministry of Science and Technology, R.O.C. under Grant no. MOST 103-2221-E-224-072. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2018.02.005. References [1] Awaleh MO, Soubaneh YD. Waste water treatment in chemical industries: the concept and current technologies. J Waste Water Treatment Anal 2014;5(1):1–13. [2] Ju F, Hu Y. Removal of EDTA-chelated copper from aqueous solution by interior microelectrolysis. Sep Purif Technol 2011;78:33–41. [3] Juang RS, Chen MN. Removal of copper(II) chelates of EDTA and NTA from dilute aqueous solutions by membrane filtration. Ind Eng Chem Res 1997;36:179–86. [4] Cho H, Shin IS, Yang JK, Lee SM, Shin WT. Removal of Cu(II)-EDTA complex using TiO2 solar light the effect of operational parameters and feasibility of solar light application. J Environ Sci Health A 2006;41:1027–41. [5] Chang JH, Ellis AV, Yan CT, Tung CH. The electrochemical phenomena and kinetics of EDTA-copper wastewater reclamation by electrodeposition and ultrasound. Sep Purif Technol 2009;68:216–21. [6] Yeh RS, Wang YY, Wan CC. Removal of Cu-EDTA compounds via electrochemical process with coagulation. Water Res 1995;29:597–9. [7] Maketon W, Zenner CZ, Ogden KL. Removal efficiency and binding mechanisms of copper and copper-EDTA complexes using polyethyleneimine. Environ Sci Technol 2008;42(6):2124–9. [8] Wu L, Wang H, Lan H, Liu H. Adsorption of Cu(II)-EDTA chelates on tri-ammonium-functionalized mesoporous silica from aqueous solution. Sep Purif Technol 2013;117:118–23. [9] Chang C, Ku Y. The adsorption and desorption characteristics of EDTA-chelated copper ion by activated carbon. Sep Sci Technol 1995;30(6):899–915.
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