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A study of electrosorption selectivity of anions by activated carbon electrodes in capacitive deionization
Desalination 369 (2015) 46–50
Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
A study of electrosorption selectivity of anions by activated carbon electrodes in capacitive deionization Zhaolin Chen a,b, Hongtao Zhang a,⁎, Chunxu Wu a, Yushuang Wang a,b, Wei Li a a b
School of Environment, Tsinghua University, Beijing 100084, PR China Beijing Guohuan Tsinghua Environmental Engineering Design & Research Institute, Beijing 100084, PR China
H I G H L I G H T S • • • •
The electrosorption capacity: trivalent anion > divalent anion > monovalent anion. The electrosorption capacity was dependent on the ionic charge and hydrated radius. Electrosorption selectivity was studied in a competitive mixed solution. Cl− and NO− 3 show obvious competitive electrosorption in mixed solution.
a r t i c l e
i n f o
Article history: Received 23 December 2014 Received in revised form 2 April 2015 Accepted 20 April 2015 Available online Keywords: Activated carbon Capacitive deionization Competitive adsorption Electrosorption
a b s t r a c t The capacitive deionization (CDI) performance of activated carbon electrodes with various anions and competitive electrosorption between two anions in mixed solution were studied. It was found that the electrosorption capacity was strongly dependent on the ionic charge and hydrated radius. With the same concentration, smaller monovalent anions showed their size-affinity to access the pores. The order of normalized equivalent capacity − was as follows: trivalent anion N divalent anion N monovalent anion. In mixed Cl− and NO− 3 solution, Cl is preferably electrosorped over NO− 3 in early period of the electrosorption process. However, during the end of the − electrosorption process, part of Cl− would be exchanged by NO− 3 due to the stronger competitiveness of NO3 2− − − than Cl . There was no obvious selectivity between Cl and SO4 . © 2015 Published by Elsevier B.V.
1. Introduction Water scarcity has become one of the major problems in China [1]. From a technical point of view, saline water desalination is a very attractive solution to this problem. There are many desalination processes which can remove ions from salty water. Conventional desalination technologies such as thermal distillation, reverse osmosis, ion exchange, and electric dialysis [2–5], have issues in maintenance, complex pretreatment, high temperature and high energy-consumption [2]. Capacitive deionization (CDI) is a novel process for brackish water desalination and water purification, which has attracted growing attention from researchers worldwide [6–11]. CDI is defined as potentialinduced adsorption of ions onto the surface of charged electrodes. When an electrical potential is applied to electrodes, charged ions migrate to the electrodes and are held in the electrical double layers. Once the external field is removed, the ions are quickly released back to bulk solution [12]. Compared with the conventional desalination ⁎ Corresponding author. E-mail address: [email protected] (H. Zhang).
http://dx.doi.org/10.1016/j.desal.2015.04.022 0011-9164/© 2015 Published by Elsevier B.V.
technologies, electrosorption is an energy-efficient desalination process due to the fact that it operates at lower electrode potential (about 1–1.2 V) at which no electrolysis reactions occur [13]. In addition, this process is environmentally friendly because it requires no chemicals for regeneration [14–16]. The materials, such as activated carbon, carbon aerogel, carbon nanofibers and carbon nanotubes can be used as electrosorption electrode [17–19]. Although carbon aerogel, carbon nanofibers and carbon nanotubes have excellent electrosorption performance, the manufacturing process of these materials is complicated and mass production is expensive. Therefore, these materials are difficult for industrial application. Activated carbon is the most competitive electrode material for large-scale application at present due to its excellent electrical conductivity and large specific surface area [20]. In this study, a systematic investigation of anions of various charges and ionic radiuses was carried out to have an accurate understanding of ion selectivity in electrosorption. Anions selected for the evaluation − 2− were chloride (Cl−), nitrate (NO− 3 ), fluoride (F ), sulfate (SO4 ) and phosphate (PO3− 4 ). Experiments of electrosorption were conducted to identify the ion selectivity in both single and mixed electrolyte solutions with the CDI equipment.
Z. Chen et al. / Desalination 369 (2015) 46–50
47
Fig. 1. Schematic diagram of CDI experiments.
2. Materials and methods AC powder and graphite paper were purchased from Chengde Beifang Activated Carbon Co. Ltd. (Chengde, China). Acetylene black and phenol formaldehyde resin were obtained from Sinopharm Co. Ltd. (Beijing China). AC powder, acetylene black as conductive material and phenol formaldehyde resin (10 g phenol formaldehyde resin dissolved in 100 mL ethanol) as binder were mixed and used to fabricate the electrode and their percentages in the final electrode were 70%, 10% and 20%, respectively. The mixed slurry was cast on graphite paper with thickness of 1.0 mm, and then dried in an air drier at 353 K for 24 h. The AC electrodes were calcined at 1173 K for 2 h in argon atmosphere with a flow rate of 100 mL/min and a low heating rate of 2.5 K/min till the desired temperature was reached. It was then cooled down to room temperature. The AC electrodes continued to be treated by 1% hydrochloric acid for 3 h, and then washed with deionized water till conductivity of effluent was lower than 2 μS/cm, in order to remove ash inside the AC electrode pores. Each electrode was 60 mm wide × 100 mm long × 1.0 mm thick. Finally, the electrodes were assembled into a capacitive deionization module. The distance between two electrodes is 4 mm. The weight of a pair of the electrodes was about 5 g. The morphology of the AC electrodes was observed by SEM (MIRA3 LMH/LMU). The BET specific surface area of AC electrodes was determined by nitrogen (99.99% purity) adsorption/desorption at 77 K using a surface analyzer (Micromeritics ASAP 2020). Prior to the nitrogen adsorption, the adsorbents were degassed under vacuum at 423 K for 6 h. Surface functional groups were determined by acid–base titration method proposed by Boehm, which had been detailedly described in our previous works [1,21]. Anion concentration in the solution was analyzed by an ion chromatograph (Dionex ICS-2000). Electrosorption experiments were carried out in a flow-through system depicted in Fig. 1. The system consisted of a water tank, a peristaltic pump, a CDI module and a rectifier. The CDI module consisted of two electrodes separated by nylon spacer. The effluent was either recycled or discharged. Ions were removed from the solution by applying a direct voltage of 1.2 V between the opposite electrodes. Solution was kept at a constant temperature of 298 K.
If the effluent was recycled, the amount of ion adsorption per unit mass of AC electrodes was calculated according to Eq. (1): m¼
ðC o −C e ÞV M
ð1Þ
where Co and Ce represented the initial and equilibrium concentrations of ions (mg/L), respectively; V was the volume of the solution (L), and M was the mass of the AC electrodes (g). The electrosorption capacity was similarly defined as the amount of ions adsorbed on the AC electrodes under polarization, which could also be calculated by Eq. (1), but Co and Ce here were the concentrations at the initial and electrosorption equilibrium state. If the effluent was discharged, the amount of ion adsorption per unit mass of AC electrodes was calculated according to Eq. (2): m¼
C i Qt M
ð2Þ
where Ci represented the effluent concentration of the ions (mg/L); Q was the flow rate of the solution (L/min); t was the electrosorption time (min), and M was the mass of the AC electrodes (g). 3. Results and discussion 3.1. Characteristics of AC electrodes Fig. 2 shows the SEM image of the AC electrodes. As shown in Fig. 2, the AC electrodes were a porous material with many nano-pores. The nitrogen adsorption/desorption isotherm and pore size distribution of the AC electrodes are shown in Fig. 3. The BET surface area was 1117 m2/g. The micropore area was 592 m2/g, and the average pore size was 22.91 Å. Fifty three percent of the total surface area was contributed by the presence of micropores (b2 nm). Such pore structure might result in electric double layers overlapping during the electrosorption process. The surface functional groups of the AC electrodes are listed in Table 1. Functional groups are beneficial to physical adsorption of ions, but not to electrosorption.
48
Z. Chen et al. / Desalination 369 (2015) 46–50 Table 1 Surface functional groups of AC electrode. Carboxyl groups (mmol/g)
Hydroxyl groups (mmol/g)
Lactonic groups (mmol/g)
Basic groups (mmol/g)
All groups (mmol/g)
0.025
0.033
0.005
0.083
0.146
capacity was as follows: trivalent anion (PO34 −, 222.9 μmol/ g) N divalent anion (SO2− 4 , 175.4 μmol/g) N monovalent anion. 3.3. Competitive electrosorption between Cl− and NO− 3
Fig. 2. SEM image of AC electrode.
3.2. Electrosorption capacity of different anions To investigate the effects of ion properties on the capacitive charac− teristics, typical monovalent ions (Cl−, NO− 3 , and F ), divalent ion 3− ) and trivalent ion (PO ) were tested in this study. Their hydrat(SO2− 4 4 (3.00 Å) b Cl− ed radii are sorted in ascending order as follows: SO2− 4 3− (3.31 Å) b NO− (3.39 Å) b F− (3.52 Å) [22–24]. All 3 (3.35 Å) b PO4 the electrolyte solutions had a common cation (Na+) with a hydrated radius of 3.58 Å. Electrosorption experiments were conducted with a series of singleelectrolyte solutions to compare the different anions with respect to their removal capacity in deionization. The electrolyte solutions were NaCl, NaNO3, NaF, Na2SO4 and Na3PO4, respectively. The initial concentrations of all the electrolyte solutions were 5 mM with a volume of 700 mL. The effluent was recycled at a flow rate of 50 mL/min. The applied voltage was 1.2 V. Fig. 4 shows the electrosorption capacities of different anions on the AC electrodes. The molar saturation capacity of − 2− and PO34 − were 164.8, 159.2, 138.5, 87.7 and Cl−, NO− 3 , F , SO4 74.3 μmol/g, respectively. With the same charge, ions with smaller hydrated radius resulted in higher electrosorption capacity. It was noted that the molar saturation capacities of divalent and trivalent anions were considerably lower than that of monovalent anions. The molar saturation capacity of SO24 − and PO34 − were 52.2% and 39.0% that of Cl−. However, considering the normalized equivalent capacity (valence × μmol/g), the order of the normalized equivalent
Fig. 5 presents the results of competitive electrosorption between Cl− and NO− 3 under a voltage of 1.2 V. The mixed solution contained + two monovalent anions (Cl− and NO− 3 ) and one cation (Na ). The ini− tial concentration of Cl was fixed at 5 mM, while the initial concentration of NO− 3 was 0.5, 1.0, 2.0, 4.0, and 5.0 mM, respectively. The flow rate of peristaltic pump was 20 mL/min, and the total volume of the solution was 700 mL. A stirrer had been used in the water tank during the electrosorption experiments. The effluent was recycled backed to the influent tank. As shown in Fig. 5, with the increase of NO− 3 concentration, the electrosorption capacity of Cl− decreased, which were 150.5, 127.4, 104.2, 76.8 and 66.4 μmol/g, respectively. When both the Cl− concentration and NO− 3 concentration were 5 mM in the mixed solution, the electrosorption capacities of Cl− and NO− 3 were 66.4 μmol/g and 107.9 μmol/g, respectively. The electrosorption capacity of NO− 3 was nearly 1.6 times of Cl−, while the electrosorption capacity of NO− 3 (with 159.2 μmol/g) was lower than that of Cl− (with 167.8 μmol/g) when these two anions were electrosorped alone. Therefore, it can be − concluded that the ion selectivity for NO− 3 is over Cl when their concentrations are the same. It does not present a size-affinity to be electrosorbed by activated carbon electrodes in mixed solutions. Others, the total capacity of Cl− and NO− 3 in mixed solution was 189.3 μmol/g, which was higher than the capacity of two anions been electrosorped alone. The effluent anions' concentrations are presented in Fig. 6, when the mixed solution was continuously fed at a flow rate of 5 mL/min with the same initial concentration of 5 mM for Cl− and NO− 3 and without effluent recirculation. As shown in Fig. 6, when the electrodes were charged, the concentration of Cl− in the effluent decreased quickly, while the − concentration of NO− 3 in the effluent decreased slower than Cl . The . The lowest concentration of Cl− reached its lowest point prior to NO− 3 point of Cl− was 3.84 mM, and it was 4.10 mM for NO− 3 . Therefore, it can be concluded that Cl− was preferably electrosorped during the early electrosorption period. Since the charge of the ions was the same, it was indicated that ions with smaller hydrated radius would
Electrosorption capacity /µmol/g
400
200
Adsorption Desorption 0.05
3
Volume adsorped (cm /g)
500
300 3
dV/dD (cm /g)
0.04
200 100 0 0.0
0.03 0.02 0.01 0.00 1
0.2
0.4
10 100 Pore diameter (nm)
0.6
0.8
Relative pressure (P/P0) Fig. 3. Pore size distribution of AC electrode.
1.0
160 120 80 40 0
-
Cl
-
NO3
-
F
2-
SO4
3-
PO4
Fig. 4. Electrosorption capacity of different anions using the activated carbon electrodes at applied voltage of 1.2 V.
Z. Chen et al. / Desalination 369 (2015) 46–50
200
200 -
-
Cl
-
NO3
2-
Cl
Electrosorption capacity/µmol/g
Electrosorption capacity/µmol/g
49
160
120
80
40
0
SO4
160
120
80
40
0 0.5
2.0 4.0 1.0 Concentration of NO3 /mM
5.0
0.5
5.0 2.0 4.0 1.0 2Concentration of SO4 in mixed solution/mM
− − Fig. 5. Effect of initial NO− 3 concentration on electrosorption capacity of Cl and NO3 in mixed solutions under voltage of 1.2 V.
result in higher electrosorption rate. However, the effluent Cl− increased to the same concentration of the raw water at 90 min, and exceeded that value thereafter. This phenomenon indicated that Cl− was desorbed from the electrodes during the end of electrosorption process. On contrary, NO− 3 was electrosorped throughout the whole electrosorption process. When the electrodes were saturated at 210 min, no further desorption of Cl− and electrosorption of NO− 3 happened anymore. By calculation, 81.4 μmol/g Cl− was electrosorped in early electrosorption period, and 13.5 μmol/g Cl− was desorbed during the end of the electrosorption process. Therefore, the net electrosorption capacity of Cl− was 67.9 μmol/g. And the net electrosorption capacity of NO− 3 was 108.7 μmol/g. It was in accordance with the rule of the adsorption selec− tivity for NO− 3 over Cl in the mixed solution. The desorption of Cl− from electrodes during the end of electrosorption process is a very interesting finding, which has not been reported previously by any literature in the CDI process. The mechanism behind this phenomenon is similar to the selectivity for NO− 3 over Cl− in ion exchange process [25]. In ion exchange, Cl− could be substituted by NO− 3 according to Eq. (3). When this reaction reaches equi-
Fig. 7. Effect of initial SO2− concentration on electrosorption capacity of Cl− and SO2− in 4 4 mixed solution under voltage of 1.2 V.
NO
‐
K Cl‐ 3 ¼
½R‐NO3 ½Cl− ½RCl½NO3 −
ð4Þ
where [RNO3] represents the concentration of nitrate in the ion exchange resin; [RCl] is the concentration of chloride in the ion exchange − resin; [NO− 3 ] is concentration of nitrate in solution; and [Cl ] is concentration of chloride in solution. Due to the surface functional groups on the AC electrodes, AC electrodes can also be regarded as an ion exchange resin, except that the exchange capacity of the AC electrodes is much lower than the normal ion exchange resin. Along with the oxidization of the electrodes (the increase of charge and discharge cycle), the surface functional groups increase [26–28], the AC electrodes act more like an ion exchange resin, so − that the competition between NO− 3 and Cl becomes significant. 3.4. Competitive electrosorption between Cl− and SO2− 4
−
3 N1, the librium, ion selectivity coefficient is determined by Eq. (4). If K NO − Cl − is stronger than Cl , and then desorption of Cl− competitiveness of NO− 3 occurs.
−
−
R‐Cl þ NO3 ⇄R‐NO3 þ Cl
ð3Þ
where R represents the adsorption site.
Fig. 7 presents the results of competitive electrosorption between under a voltage of 1.2 V. The mixed solution contained Cl− and SO2− 4 + two anions (Cl− and SO2− 4 ) and one cation (Na ). The initial concentration of Cl− was fixed at 5 mM, while the initial concentration of SO2− 4 was 0.5, 1.0, 2.0, 4.0, and 5.0 mM, respectively. The flow rate of peristaltic pump was 20 mL/min, and the total volume of the mixed solution was 700 mL. The desalted solution was recycled back to the raw water tank.
5.6
Concentration of raw solution
5.2
Concentration of raw solution
4.8 4.4 4.0
-
Cl NO3
3.6 3.2
0
40
80 120 Time (min)
160
200
Fig. 6. Electrosorption effluent curves of Cl− and NO− 3 in the mixed solutions at 1.2 V.
Concentrations of anions/ mM
Concentrations of anions/ mM
5.6
5.2 4.8 4.4 -
Cl 2SO4
4.0 3.6 3.2
0
30
60
90 Time/min
120
150
180
Fig. 8. Electrosorption effluent curves of Cl− and SO2− in mixed solutions at 1.2 V. 4
50
Z. Chen et al. / Desalination 369 (2015) 46–50
As shown in Fig. 7, with the increase of the SO2− 4 concentration, the electrosorption capacity of Cl− decreased, which were 147.3, 132.3, 109.9, 85.4 and 82.0 μmol/g, respectively. And the total electrosorption capacity of Cl− and SO24 − were 161.1, 160.4, 156.9, 152.3 and 150.8 μmol/g, which also decreased with the increase of the initial concentration. And the total electrosorption capacity was lower SO2− 4 than the electrosorption capacity of Cl− (167.8 μmol/g) in single electrolyte solution. However, considering the normalized equivalent capacity (valence × μmol/g), the normalized equivalent total capacity for SO2− 4 and Cl− (219.6 μmol/g) was much higher than the electrosorption capacity of Cl− (167.8 μmol/g) in pure NaCl solution. When the concentration of Cl− and SO2− 4 both were 5 mM in mixed solution, and the effluent not backed to the raw solution tank, the flow rate was 5 mL/min, the concentration of anions in effluent is presented in Fig. 8. The electrosorption curves of Cl− and SO2− 4 are similar, and no ion concentration higher than the original solution concentration in the effluent. The concentration of Cl− reached its lowest point early than SO24 −, the lowest point of Cl− is 3.98 mM, and is 4.20 mM for SO24 −. The electrosorption capacity of Cl− and SO24 − were 82.0 μmol/g and 68.8 μmol/g, respectively. The electrosorption capacity of SO24 − and Cl− were nearly, when initial concentrations were same in mixed solution. It didn't like while the two anions been electrosorped alone, the electrosorption capacity of SO24 − (with 87.7 μmol/g) is much lower than Cl− (with 167.8 μmol/g). Compare Fig. 6 with Fig. 8, when the con2− both were 5 mM in mixed solution, the centration of NO− 3 and SO4 electrosorption capacity of Cl− were 66.4 and 82.0 μmol/g, respectively, with two different mixed solutions. It can conclude that the − competiveness of SO2− 4 is lower than NO3 . Through the above analysis, there is no obvious competitive electrosorption between Cl− and SO2− 4 . 4. Conclusion The electrosorption preference of different anions and competitive electrosorption in mixed solution were investigated. By applying an electric field on AC electrodes, the obtained results show that the electrosorption capacity is strongly dependent on the ionic charge and hydrated radius. With the same concentration, smaller monovalent anions show their size-affinity to be electrosorped. The order of normalized equivalent capacity is as follows: trivalent anion N divalent − anion N monovalent anion. In mixed Cl− and NO− 3 solution, Cl is preferably electrosorped over NO− 3 in early period of the electrosorption process. Due to the same charge, ions associated with smaller hydrated radius result in higher electrosorption rate. However, during the end of the electrosorption process, part of Cl− would be exchanged by NO− 3 − due to the stronger competitiveness of NO− 3 than Cl . There is no obvious selectivity between Cl− and SO24 −. The findings of this study can provide fundamental aspects of competitive electrosorption of anions in the activated carbon-based CDI for its applications in water desalination. Acknowledgments This research was supported partially by the National Key Project Major Science and Technology Program for Water Pollution Control and Treatment (No. 2013ZX07209-001). References [1] Z. Chen, C. Song, X. Sun, H. Guo, G. Zhu, Kinetic and isotherm studies on the electrosorption of NaCl from aqueous solutions by activated carbon electrodes, Desalination 267 (2011) 239–243.
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Contents lists available at ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
A study of electrosorption selectivity of anions by activated carbon electrodes in capacitive deionization Zhaolin Chen a,b, Hongtao Zhang a,⁎, Chunxu Wu a, Yushuang Wang a,b, Wei Li a a b
School of Environment, Tsinghua University, Beijing 100084, PR China Beijing Guohuan Tsinghua Environmental Engineering Design & Research Institute, Beijing 100084, PR China
H I G H L I G H T S • • • •
The electrosorption capacity: trivalent anion > divalent anion > monovalent anion. The electrosorption capacity was dependent on the ionic charge and hydrated radius. Electrosorption selectivity was studied in a competitive mixed solution. Cl− and NO− 3 show obvious competitive electrosorption in mixed solution.
a r t i c l e
i n f o
Article history: Received 23 December 2014 Received in revised form 2 April 2015 Accepted 20 April 2015 Available online Keywords: Activated carbon Capacitive deionization Competitive adsorption Electrosorption
a b s t r a c t The capacitive deionization (CDI) performance of activated carbon electrodes with various anions and competitive electrosorption between two anions in mixed solution were studied. It was found that the electrosorption capacity was strongly dependent on the ionic charge and hydrated radius. With the same concentration, smaller monovalent anions showed their size-affinity to access the pores. The order of normalized equivalent capacity − was as follows: trivalent anion N divalent anion N monovalent anion. In mixed Cl− and NO− 3 solution, Cl is preferably electrosorped over NO− 3 in early period of the electrosorption process. However, during the end of the − electrosorption process, part of Cl− would be exchanged by NO− 3 due to the stronger competitiveness of NO3 2− − − than Cl . There was no obvious selectivity between Cl and SO4 . © 2015 Published by Elsevier B.V.
1. Introduction Water scarcity has become one of the major problems in China [1]. From a technical point of view, saline water desalination is a very attractive solution to this problem. There are many desalination processes which can remove ions from salty water. Conventional desalination technologies such as thermal distillation, reverse osmosis, ion exchange, and electric dialysis [2–5], have issues in maintenance, complex pretreatment, high temperature and high energy-consumption [2]. Capacitive deionization (CDI) is a novel process for brackish water desalination and water purification, which has attracted growing attention from researchers worldwide [6–11]. CDI is defined as potentialinduced adsorption of ions onto the surface of charged electrodes. When an electrical potential is applied to electrodes, charged ions migrate to the electrodes and are held in the electrical double layers. Once the external field is removed, the ions are quickly released back to bulk solution [12]. Compared with the conventional desalination ⁎ Corresponding author. E-mail address: [email protected] (H. Zhang).
http://dx.doi.org/10.1016/j.desal.2015.04.022 0011-9164/© 2015 Published by Elsevier B.V.
technologies, electrosorption is an energy-efficient desalination process due to the fact that it operates at lower electrode potential (about 1–1.2 V) at which no electrolysis reactions occur [13]. In addition, this process is environmentally friendly because it requires no chemicals for regeneration [14–16]. The materials, such as activated carbon, carbon aerogel, carbon nanofibers and carbon nanotubes can be used as electrosorption electrode [17–19]. Although carbon aerogel, carbon nanofibers and carbon nanotubes have excellent electrosorption performance, the manufacturing process of these materials is complicated and mass production is expensive. Therefore, these materials are difficult for industrial application. Activated carbon is the most competitive electrode material for large-scale application at present due to its excellent electrical conductivity and large specific surface area [20]. In this study, a systematic investigation of anions of various charges and ionic radiuses was carried out to have an accurate understanding of ion selectivity in electrosorption. Anions selected for the evaluation − 2− were chloride (Cl−), nitrate (NO− 3 ), fluoride (F ), sulfate (SO4 ) and phosphate (PO3− 4 ). Experiments of electrosorption were conducted to identify the ion selectivity in both single and mixed electrolyte solutions with the CDI equipment.
Z. Chen et al. / Desalination 369 (2015) 46–50
47
Fig. 1. Schematic diagram of CDI experiments.
2. Materials and methods AC powder and graphite paper were purchased from Chengde Beifang Activated Carbon Co. Ltd. (Chengde, China). Acetylene black and phenol formaldehyde resin were obtained from Sinopharm Co. Ltd. (Beijing China). AC powder, acetylene black as conductive material and phenol formaldehyde resin (10 g phenol formaldehyde resin dissolved in 100 mL ethanol) as binder were mixed and used to fabricate the electrode and their percentages in the final electrode were 70%, 10% and 20%, respectively. The mixed slurry was cast on graphite paper with thickness of 1.0 mm, and then dried in an air drier at 353 K for 24 h. The AC electrodes were calcined at 1173 K for 2 h in argon atmosphere with a flow rate of 100 mL/min and a low heating rate of 2.5 K/min till the desired temperature was reached. It was then cooled down to room temperature. The AC electrodes continued to be treated by 1% hydrochloric acid for 3 h, and then washed with deionized water till conductivity of effluent was lower than 2 μS/cm, in order to remove ash inside the AC electrode pores. Each electrode was 60 mm wide × 100 mm long × 1.0 mm thick. Finally, the electrodes were assembled into a capacitive deionization module. The distance between two electrodes is 4 mm. The weight of a pair of the electrodes was about 5 g. The morphology of the AC electrodes was observed by SEM (MIRA3 LMH/LMU). The BET specific surface area of AC electrodes was determined by nitrogen (99.99% purity) adsorption/desorption at 77 K using a surface analyzer (Micromeritics ASAP 2020). Prior to the nitrogen adsorption, the adsorbents were degassed under vacuum at 423 K for 6 h. Surface functional groups were determined by acid–base titration method proposed by Boehm, which had been detailedly described in our previous works [1,21]. Anion concentration in the solution was analyzed by an ion chromatograph (Dionex ICS-2000). Electrosorption experiments were carried out in a flow-through system depicted in Fig. 1. The system consisted of a water tank, a peristaltic pump, a CDI module and a rectifier. The CDI module consisted of two electrodes separated by nylon spacer. The effluent was either recycled or discharged. Ions were removed from the solution by applying a direct voltage of 1.2 V between the opposite electrodes. Solution was kept at a constant temperature of 298 K.
If the effluent was recycled, the amount of ion adsorption per unit mass of AC electrodes was calculated according to Eq. (1): m¼
ðC o −C e ÞV M
ð1Þ
where Co and Ce represented the initial and equilibrium concentrations of ions (mg/L), respectively; V was the volume of the solution (L), and M was the mass of the AC electrodes (g). The electrosorption capacity was similarly defined as the amount of ions adsorbed on the AC electrodes under polarization, which could also be calculated by Eq. (1), but Co and Ce here were the concentrations at the initial and electrosorption equilibrium state. If the effluent was discharged, the amount of ion adsorption per unit mass of AC electrodes was calculated according to Eq. (2): m¼
C i Qt M
ð2Þ
where Ci represented the effluent concentration of the ions (mg/L); Q was the flow rate of the solution (L/min); t was the electrosorption time (min), and M was the mass of the AC electrodes (g). 3. Results and discussion 3.1. Characteristics of AC electrodes Fig. 2 shows the SEM image of the AC electrodes. As shown in Fig. 2, the AC electrodes were a porous material with many nano-pores. The nitrogen adsorption/desorption isotherm and pore size distribution of the AC electrodes are shown in Fig. 3. The BET surface area was 1117 m2/g. The micropore area was 592 m2/g, and the average pore size was 22.91 Å. Fifty three percent of the total surface area was contributed by the presence of micropores (b2 nm). Such pore structure might result in electric double layers overlapping during the electrosorption process. The surface functional groups of the AC electrodes are listed in Table 1. Functional groups are beneficial to physical adsorption of ions, but not to electrosorption.
48
Z. Chen et al. / Desalination 369 (2015) 46–50 Table 1 Surface functional groups of AC electrode. Carboxyl groups (mmol/g)
Hydroxyl groups (mmol/g)
Lactonic groups (mmol/g)
Basic groups (mmol/g)
All groups (mmol/g)
0.025
0.033
0.005
0.083
0.146
capacity was as follows: trivalent anion (PO34 −, 222.9 μmol/ g) N divalent anion (SO2− 4 , 175.4 μmol/g) N monovalent anion. 3.3. Competitive electrosorption between Cl− and NO− 3
Fig. 2. SEM image of AC electrode.
3.2. Electrosorption capacity of different anions To investigate the effects of ion properties on the capacitive charac− teristics, typical monovalent ions (Cl−, NO− 3 , and F ), divalent ion 3− ) and trivalent ion (PO ) were tested in this study. Their hydrat(SO2− 4 4 (3.00 Å) b Cl− ed radii are sorted in ascending order as follows: SO2− 4 3− (3.31 Å) b NO− (3.39 Å) b F− (3.52 Å) [22–24]. All 3 (3.35 Å) b PO4 the electrolyte solutions had a common cation (Na+) with a hydrated radius of 3.58 Å. Electrosorption experiments were conducted with a series of singleelectrolyte solutions to compare the different anions with respect to their removal capacity in deionization. The electrolyte solutions were NaCl, NaNO3, NaF, Na2SO4 and Na3PO4, respectively. The initial concentrations of all the electrolyte solutions were 5 mM with a volume of 700 mL. The effluent was recycled at a flow rate of 50 mL/min. The applied voltage was 1.2 V. Fig. 4 shows the electrosorption capacities of different anions on the AC electrodes. The molar saturation capacity of − 2− and PO34 − were 164.8, 159.2, 138.5, 87.7 and Cl−, NO− 3 , F , SO4 74.3 μmol/g, respectively. With the same charge, ions with smaller hydrated radius resulted in higher electrosorption capacity. It was noted that the molar saturation capacities of divalent and trivalent anions were considerably lower than that of monovalent anions. The molar saturation capacity of SO24 − and PO34 − were 52.2% and 39.0% that of Cl−. However, considering the normalized equivalent capacity (valence × μmol/g), the order of the normalized equivalent
Fig. 5 presents the results of competitive electrosorption between Cl− and NO− 3 under a voltage of 1.2 V. The mixed solution contained + two monovalent anions (Cl− and NO− 3 ) and one cation (Na ). The ini− tial concentration of Cl was fixed at 5 mM, while the initial concentration of NO− 3 was 0.5, 1.0, 2.0, 4.0, and 5.0 mM, respectively. The flow rate of peristaltic pump was 20 mL/min, and the total volume of the solution was 700 mL. A stirrer had been used in the water tank during the electrosorption experiments. The effluent was recycled backed to the influent tank. As shown in Fig. 5, with the increase of NO− 3 concentration, the electrosorption capacity of Cl− decreased, which were 150.5, 127.4, 104.2, 76.8 and 66.4 μmol/g, respectively. When both the Cl− concentration and NO− 3 concentration were 5 mM in the mixed solution, the electrosorption capacities of Cl− and NO− 3 were 66.4 μmol/g and 107.9 μmol/g, respectively. The electrosorption capacity of NO− 3 was nearly 1.6 times of Cl−, while the electrosorption capacity of NO− 3 (with 159.2 μmol/g) was lower than that of Cl− (with 167.8 μmol/g) when these two anions were electrosorped alone. Therefore, it can be − concluded that the ion selectivity for NO− 3 is over Cl when their concentrations are the same. It does not present a size-affinity to be electrosorbed by activated carbon electrodes in mixed solutions. Others, the total capacity of Cl− and NO− 3 in mixed solution was 189.3 μmol/g, which was higher than the capacity of two anions been electrosorped alone. The effluent anions' concentrations are presented in Fig. 6, when the mixed solution was continuously fed at a flow rate of 5 mL/min with the same initial concentration of 5 mM for Cl− and NO− 3 and without effluent recirculation. As shown in Fig. 6, when the electrodes were charged, the concentration of Cl− in the effluent decreased quickly, while the − concentration of NO− 3 in the effluent decreased slower than Cl . The . The lowest concentration of Cl− reached its lowest point prior to NO− 3 point of Cl− was 3.84 mM, and it was 4.10 mM for NO− 3 . Therefore, it can be concluded that Cl− was preferably electrosorped during the early electrosorption period. Since the charge of the ions was the same, it was indicated that ions with smaller hydrated radius would
Electrosorption capacity /µmol/g
400
200
Adsorption Desorption 0.05
3
Volume adsorped (cm /g)
500
300 3
dV/dD (cm /g)
0.04
200 100 0 0.0
0.03 0.02 0.01 0.00 1
0.2
0.4
10 100 Pore diameter (nm)
0.6
0.8
Relative pressure (P/P0) Fig. 3. Pore size distribution of AC electrode.
1.0
160 120 80 40 0
-
Cl
-
NO3
-
F
2-
SO4
3-
PO4
Fig. 4. Electrosorption capacity of different anions using the activated carbon electrodes at applied voltage of 1.2 V.
Z. Chen et al. / Desalination 369 (2015) 46–50
200
200 -
-
Cl
-
NO3
2-
Cl
Electrosorption capacity/µmol/g
Electrosorption capacity/µmol/g
49
160
120
80
40
0
SO4
160
120
80
40
0 0.5
2.0 4.0 1.0 Concentration of NO3 /mM
5.0
0.5
5.0 2.0 4.0 1.0 2Concentration of SO4 in mixed solution/mM
− − Fig. 5. Effect of initial NO− 3 concentration on electrosorption capacity of Cl and NO3 in mixed solutions under voltage of 1.2 V.
result in higher electrosorption rate. However, the effluent Cl− increased to the same concentration of the raw water at 90 min, and exceeded that value thereafter. This phenomenon indicated that Cl− was desorbed from the electrodes during the end of electrosorption process. On contrary, NO− 3 was electrosorped throughout the whole electrosorption process. When the electrodes were saturated at 210 min, no further desorption of Cl− and electrosorption of NO− 3 happened anymore. By calculation, 81.4 μmol/g Cl− was electrosorped in early electrosorption period, and 13.5 μmol/g Cl− was desorbed during the end of the electrosorption process. Therefore, the net electrosorption capacity of Cl− was 67.9 μmol/g. And the net electrosorption capacity of NO− 3 was 108.7 μmol/g. It was in accordance with the rule of the adsorption selec− tivity for NO− 3 over Cl in the mixed solution. The desorption of Cl− from electrodes during the end of electrosorption process is a very interesting finding, which has not been reported previously by any literature in the CDI process. The mechanism behind this phenomenon is similar to the selectivity for NO− 3 over Cl− in ion exchange process [25]. In ion exchange, Cl− could be substituted by NO− 3 according to Eq. (3). When this reaction reaches equi-
Fig. 7. Effect of initial SO2− concentration on electrosorption capacity of Cl− and SO2− in 4 4 mixed solution under voltage of 1.2 V.
NO
‐
K Cl‐ 3 ¼
½R‐NO3 ½Cl− ½RCl½NO3 −
ð4Þ
where [RNO3] represents the concentration of nitrate in the ion exchange resin; [RCl] is the concentration of chloride in the ion exchange − resin; [NO− 3 ] is concentration of nitrate in solution; and [Cl ] is concentration of chloride in solution. Due to the surface functional groups on the AC electrodes, AC electrodes can also be regarded as an ion exchange resin, except that the exchange capacity of the AC electrodes is much lower than the normal ion exchange resin. Along with the oxidization of the electrodes (the increase of charge and discharge cycle), the surface functional groups increase [26–28], the AC electrodes act more like an ion exchange resin, so − that the competition between NO− 3 and Cl becomes significant. 3.4. Competitive electrosorption between Cl− and SO2− 4
−
3 N1, the librium, ion selectivity coefficient is determined by Eq. (4). If K NO − Cl − is stronger than Cl , and then desorption of Cl− competitiveness of NO− 3 occurs.
−
−
R‐Cl þ NO3 ⇄R‐NO3 þ Cl
ð3Þ
where R represents the adsorption site.
Fig. 7 presents the results of competitive electrosorption between under a voltage of 1.2 V. The mixed solution contained Cl− and SO2− 4 + two anions (Cl− and SO2− 4 ) and one cation (Na ). The initial concentration of Cl− was fixed at 5 mM, while the initial concentration of SO2− 4 was 0.5, 1.0, 2.0, 4.0, and 5.0 mM, respectively. The flow rate of peristaltic pump was 20 mL/min, and the total volume of the mixed solution was 700 mL. The desalted solution was recycled back to the raw water tank.
5.6
Concentration of raw solution
5.2
Concentration of raw solution
4.8 4.4 4.0
-
Cl NO3
3.6 3.2
0
40
80 120 Time (min)
160
200
Fig. 6. Electrosorption effluent curves of Cl− and NO− 3 in the mixed solutions at 1.2 V.
Concentrations of anions/ mM
Concentrations of anions/ mM
5.6
5.2 4.8 4.4 -
Cl 2SO4
4.0 3.6 3.2
0
30
60
90 Time/min
120
150
180
Fig. 8. Electrosorption effluent curves of Cl− and SO2− in mixed solutions at 1.2 V. 4
50
Z. Chen et al. / Desalination 369 (2015) 46–50
As shown in Fig. 7, with the increase of the SO2− 4 concentration, the electrosorption capacity of Cl− decreased, which were 147.3, 132.3, 109.9, 85.4 and 82.0 μmol/g, respectively. And the total electrosorption capacity of Cl− and SO24 − were 161.1, 160.4, 156.9, 152.3 and 150.8 μmol/g, which also decreased with the increase of the initial concentration. And the total electrosorption capacity was lower SO2− 4 than the electrosorption capacity of Cl− (167.8 μmol/g) in single electrolyte solution. However, considering the normalized equivalent capacity (valence × μmol/g), the normalized equivalent total capacity for SO2− 4 and Cl− (219.6 μmol/g) was much higher than the electrosorption capacity of Cl− (167.8 μmol/g) in pure NaCl solution. When the concentration of Cl− and SO2− 4 both were 5 mM in mixed solution, and the effluent not backed to the raw solution tank, the flow rate was 5 mL/min, the concentration of anions in effluent is presented in Fig. 8. The electrosorption curves of Cl− and SO2− 4 are similar, and no ion concentration higher than the original solution concentration in the effluent. The concentration of Cl− reached its lowest point early than SO24 −, the lowest point of Cl− is 3.98 mM, and is 4.20 mM for SO24 −. The electrosorption capacity of Cl− and SO24 − were 82.0 μmol/g and 68.8 μmol/g, respectively. The electrosorption capacity of SO24 − and Cl− were nearly, when initial concentrations were same in mixed solution. It didn't like while the two anions been electrosorped alone, the electrosorption capacity of SO24 − (with 87.7 μmol/g) is much lower than Cl− (with 167.8 μmol/g). Compare Fig. 6 with Fig. 8, when the con2− both were 5 mM in mixed solution, the centration of NO− 3 and SO4 electrosorption capacity of Cl− were 66.4 and 82.0 μmol/g, respectively, with two different mixed solutions. It can conclude that the − competiveness of SO2− 4 is lower than NO3 . Through the above analysis, there is no obvious competitive electrosorption between Cl− and SO2− 4 . 4. Conclusion The electrosorption preference of different anions and competitive electrosorption in mixed solution were investigated. By applying an electric field on AC electrodes, the obtained results show that the electrosorption capacity is strongly dependent on the ionic charge and hydrated radius. With the same concentration, smaller monovalent anions show their size-affinity to be electrosorped. The order of normalized equivalent capacity is as follows: trivalent anion N divalent − anion N monovalent anion. In mixed Cl− and NO− 3 solution, Cl is preferably electrosorped over NO− 3 in early period of the electrosorption process. Due to the same charge, ions associated with smaller hydrated radius result in higher electrosorption rate. However, during the end of the electrosorption process, part of Cl− would be exchanged by NO− 3 − due to the stronger competitiveness of NO− 3 than Cl . There is no obvious selectivity between Cl− and SO24 −. The findings of this study can provide fundamental aspects of competitive electrosorption of anions in the activated carbon-based CDI for its applications in water desalination. Acknowledgments This research was supported partially by the National Key Project Major Science and Technology Program for Water Pollution Control and Treatment (No. 2013ZX07209-001). References [1] Z. Chen, C. Song, X. Sun, H. Guo, G. Zhu, Kinetic and isotherm studies on the electrosorption of NaCl from aqueous solutions by activated carbon electrodes, Desalination 267 (2011) 239–243.
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