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Synthesis of a quaternarized poly(vinylimidazole-co-trifluoroethylmethacrylate-co-divinylbenzene) anion-exchange membrane for nitrate removal
Journal of Environmental Chemical Engineering 2 (2014) 2162–2169
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Synthesis of a quaternarized poly(vinylimidazole-cotrifluoroethylmethacrylate-co-divinylbenzene) anion-exchange membrane for nitrate removal Chang-Min Oh, Chi-Won Hwang, Taek-Sung Hwang * Department of Applied Chemistry and Biological Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 July 2014 Received in revised form 2 September 2014 Accepted 15 September 2014
The effect of functional agents on the ion permselectivity between an anion exchange membrane and solutions of electrolytes containing the most dominant anions of natural waters (Cl, NO3 and SO42) was studied. The vinylimidazole(VI)-co-trifluoroethylmethacrylate(TFEMA)-co-divinylbenzene(DVB) copolymer was synthesized by solution polymerization. The copolymer was functionalized with compounds such as 1-bromo-2-methylpropane, 1-bromobutane, 1-bromo-3-methylbutane and 1-bromopentane. Quaternarized poly(vinylimidazole-co-trifluoroethylmethacrylate-co-divinylbenzene) (QPVTD) anion exchange membranes were prepared using the casting method. The molecular structure of the membrane was confirmed by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy. The membrane properties, such as water uptake, ion exchange capacity, electrical resistance and ion permselectivity for nitrate, were measured. The maximum water uptake, electrical resistance and ion exchange capacity were 60.8%, 3.7 V/cm2 and 1.25 meq/g dry, respectively. ã 2014 Published by Elsevier Ltd.
Keywords: Quaternization Nitrate removal Ion permselectivity Anion exchange membrane Ion selectivity
Introduction Recently, nitrate concentrations have increased in groundwater used for drinking water and industrial water in agricultural and fishing villages due to industrial development and population growth [1,2]. Among all water resources on earth, groundwater, one of the few drinking water resources, accounts for just 0.6% of total water resources globally. The quality as well as the dwindling quantity of drinking water resources have become serious issues as industry, animal husbandry and agricultural activities have increased. In particular, nitrate (NO3) concentrations in groundwater are increasing due to the increased use of nitrogenous fertilizers. Furthermore, increased nitrate concentration in groundwater is caused by increases in the release of domestic wastewater, industrial wastewater and animal waste release to the environment [3–5]. If a high-concentration of the nitrate contained in groundwater is ingested, nitrosamine, a carcinogen, is formed in the body, and for children under 6 years of age, NO2 created from NO3 is combined with hemoglobin to form methemoglobin, which causes fatal blue baby syndrome [6,7]. Therefore, to solve this problem, many countries, including advanced countries such as the U.S., have
* Corresponding author. Tel.: +82 42 821 5687; fax: +82 42 822 8995. E-mail address: [email protected] (T.-S. Hwang). http://dx.doi.org/10.1016/j.jece.2014.09.014 2213-3437/ ã 2014 Published by Elsevier Ltd.
limited the acceptable concentration of nitrate in drinking water. Europe limits nitrate to 25 mg/L or less [8], and the World Health Organization (WHO) limits nitrate to 50 mg/L or less [9]. Established methods for removing nitrate from drinking water include reverse osmosis, adsorption, select biological and chemical methods and ion exchange. Among those, reverse osmosis has a high removal efficiency but requires high-concentration total dissolved solids (TDS) treatment and corrosion prevention treatments, which are costly processes. In addition, adsorption is sensitive to pH and temperature. Furthermore, chemical methods create by-products that must be treated and often have a low treatment efficiency, and biological methods are sensitive to temperature and produce biomass waste. Therefore, ion exchange, which is able to avoid those unfortunate process weaknesses, has been studied actively because it is not affected by pH and has a high processing efficiency [10,11]. In particular, nitrate removal using an ion exchange membrane is easily applied to a process, has a high removal efficiency, has a greater processing capacity, and can be used in various fields, meaning that related studies can be actively conducted [12,13]. Nitrate removal from water using an ion exchange membrane has been studied extensively, but nitrate ions in water are very stable and highly water-soluble. Therefore, an ion exchange membrane with an ion permselective ligand must be made to remove nitrate ions from the water completely [14]. In addition,
C.-M. Oh et al. / Journal of Environmental Chemical Engineering 2 (2014) 2162–2169
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(TFEMA) with a purity of 99% (Esstech, Essington, USA). We used N,N-dimethylformamide (DMF; 99%), a reactive solvent (Samchun, Seoul, South Korea) and a,a0 -azobis (isobutyronitrile) (AIBN; 98%) (Junsei, Tokyo, Japan) as an initiator. For the quaternization of a synthesized copolymer, we used 1-bromo-2-methylpropane (purity 99%), 1-bromobutane (99%), 1-bromopentane (98%) (Aldrich, NY, USA), and 1-bromo-3-methylbutane (96%) (TCI, Tokyo, Japan). For all other reagents, we used first class reagents without purification.
there are ions that compete with the nitrate ions for removal by the exchange membrane, such as chloride (Cl) and sulfate ions (SO42). Further research is needed to develop more ion selective separation membranes that efficiently select nitrate [1,15–18]. Among water nitrate permselective ligand compounds studied to date, a quaternized amine group has been shown to be the most effective. Most studies that have been conducted combine a macromolecular matrix with a quaternized amine group. Studies of how nitrate permselectivity depends on the bulkiness and the length of a ligand chain have not been reported to date. Therefore, in this study, we synthesized an anion exchange membrane to separate nitrate ions selectively from water and identified the extent of nitrate permselectivity depending upon alkyl halide functional agents with different quaternized amine ligand structures. To confirm the structure of an anion exchange membrane produced, Fourier transform infrared (FT-IR) and nuclear magnetic resonance (1H-NMR) spectra were analyzed. Furthermore, we investigated the effect of a functional group structure of the quaternarized poly(vinylimidazole-co-trifluoroethylmethacrylateco-divinylbenzene) (QPVTD) anion exchange membrane on nitrate permselectivity by measuring the ion exchange capacity, water uptake, equilibrium ratio, electrical resistance, electrical conductivity and ion permselectivity of the membrane.
Synthesis of a poly(vinylimidazole-co-trifluoroethylmethacrylate-codivinylbenzene) anion exchange membrane A poly(VI-co-TFEMA-co-DVB) (PVTD below) copolymer was synthesized using a stirrer, a cooler and a 1-L four-neck flask reactor with a nitrogen gas inlet and a sample inlet, adhering to the synthesis mechanism shown in Fig. 1 and under the conditions presented in Table 1. For copolymer synthesis, VI, TFEMA and DVB were put into a reactor in a nitrogen atmosphere at 50 C for 24 h to synthesize a PVTD copolymer [19]. To synthesize an anion exchange solution with the introduction of a quaternized amine group, the functionalization of the synthesized copolymer was performed according to the synthesis mechanism shown in Fig. 1 and with the functional agents presented in Table 2. A synthesized anion exchange solution was cast on a glass plate by a doctor blade and dried in an oven at 50–130 C to produce the QPVTD anion exchange membrane.
Experimental Materials
Membrane characterization For poly(vinylimidazole-co-trifluoroethylmethacrylate-codivinylbenzene) (PVTD) copolymer synthesis, we purchased vinylimidazole (VI) with a purity of 99% and divinylbenzene (DVB) (Aldrich, NY, USA) and used 2,2,2-trifluoroethylmethacrylate
Molecular weight and structural analysis Gel permeation chromatography (GPC; Waters 2690) was used to identify the molecular weight of the synthesized PVTD copolymer.
F
F
F
N N
O N
O
+ N
N
n
F
2,2,2-Trif luoroethyl methacrylate(TFEMA)
F
F
1,4-Divinylbenzene(DVB)
n
Br
F R
R N
OH
OH
O
100 oC 24 hr
O
R
N
O
OH F
m
OH
n F
N
O
N Br
F
F Br
Br
m
F
m
+
N
O
DMF F
N O
O
AIBN 50o C, 24 hr
F
Vinylimidazole(VI)
O
N
N N
+
F
n
Fig. 1. Preparation mechanism of anion exchange membrane.
R
m
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Table 1 Synthesis conditions of PVTD copolymer by solution polymerization. Monomer (mole/L)
AIBN
DMF
Temp.
Time
VI
TFEMA
DVB
(wt%)
(wt%)
( C)
(h)
2.50
5.00
0.15
1.00
275
50
24
Columns used to measure the molecular weight included m-Styragel HR-1, HR-2, and HR-3 columns. Polystyrene was used as the GPC reference material, and tetrahydrofuran (THF) was used as the solvent. A flow rate of 1 mL/min was used for molecular weight measurement. To identify the anion exchange membrane structure before and after functionalization, a Shimatzu Fourier transform infrared (FT-IR) spectrometer was used to analyze functional groups according to the attenuated total reflection absorption spectroscopy (ATR) method between 4000 and 600 cm1 in wavelength with a scan number of 20 and a resolution of 4 cm1. In addition, for 1H-NMR spectrum analysis, a Fourier-transform nuclear magnetic resonance spectrometry (FT-NMR) spectrometer (JNM-AL400) from JEOL was used with deuterated methanol as the solvent for NMR spectrum analysis of the copolymer. Water uptake of the QPVTD membrane To measure water uptake of the synthesized QPVTD membrane, the membrane was cut into specifically sized pieces (3 3 cm) and immersed in distilled water for 24 h to fully equilibrate. The membrane pieces were then removed, the surface was dried, and the membrane weight was measured. Membrane water uptake was calculated according to the following formula: WaterUptakeð% Þ ¼
W wet W dry 100 W dry
where Wdry and Wwet are the membrane weight before wetting and after reaching equilibrium in the distilled water, respectively. The water uptakes were recorded with an accuracy of 1.0%. Ion exchange capacity (IEC) of QVTD membrane The ion exchange capacity of the synthesized QPVTD membrane was measured by titration. The membrane was cut into 3 3-cm pieces and immersed in a 1 M HCl solution for 24 h. Then, the supernatant was removed, an indicator was added, and the solution was titrated by a 0.1 M NaOH standard solution. The ion exchange capacity of the membrane was calculated according to the following formula: meq ðV C HCl Þ ðV NaOH C NaOH Þ dry ¼ HCl IECvalue g W dry whereVHCl and VNaOH are the volume of HCl and NaOH, respectively, CHCl and CNaOH are the concentration of HCl and NaOH, respectively, and Wdry is the dry weight of the membrane. The ion exchange capacities were recorded with an accuracy of 0.1 meq/g.
The samples were then immersed in a 0.5 M NaCl solution for 24 h, and the membrane electrical resistance was measured using a 2-compartment cell. A 0.5 M NaCl solution was used as the electrolyte, and the electrical resistance was measured by applying a voltage of 0.8 V. The membrane electrical resistance was calculated according to the following formula: ERðV cm2 Þ ¼ ðR1 R2 Þ A where R1 is the electrical resistance value after installing a membrane, and R2 is the electrical resistance value before installing a membrane. A is the membrane effective area. In addition, the ion conductivity of the QVTD anion exchange membrane was calculated according to the following formula: S L s ¼ cm ER A where ER is the membrane electrical resistance (V), A is the membrane effective area (cm2), and L is the membrane thickness (cm). The electrical resistances were recorded with an accuracy of 0.3 V cm2. Properties of ion selective separation To confirm nitrate ion permselectivity of a membrane, an ion exchange membrane, synthesized using a 2-compartment unit cell as shown in Fig. 2, was installed. The unit cell consists of two cylindrical compartments that were separated by the membrane. A NaCl/NaNO3/Na2SO4 mixed solution was put to the cathode, and a NaCl solution, an electrolyte solution, was put to the anode, and then, a voltage of 12 V was applied. Sampling was conducted at 5,10, 30, 60, 120 and 240 min. Then, Cl, NO3 and SO42 ion concentrations of the membrane were measured using ion chromatography (881 Compact IC pro, Metrohm Co., Switzerland). Results and discussion Molecular weight and structural analysis As a result of measuring the molecular weight of the PVTD copolymer synthesized to remove nitrate, the number-average molecular weight (Mn) of the PVTD copolymer was 46,000, the weight-average molecular weight (Mw) was 59,000, and the
Electric resistance and ion conductivity of QVTD membrane The membrane was cut into 1.5 1.5-cm pieces to measure the membrane electrical resistance by an LCR meter (HIOKI Co., Japan). Table 2 Synthesis conditions of quaternarized PVTD polyelectrolyte solution. Code no.
PVTD (wt%)
Functional agents (wt%)
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
85.7 84.1 85.7 84.1
1-Bromobutane 1-Bromo-2-methylpropane 1-Bromo-3-methylbutane 1-Bromopentane
14.3 14.3 15.9 15.9
Temp ( C)
Time (h)
100 100 100 100
24 24 24 24
Fig. 2. Schematic diagram of apparatus for measuring capacity of ion permselectivity.
C.-M. Oh et al. / Journal of Environmental Chemical Engineering 2 (2014) 2162–2169
before functionalization are marked by a–e. Characteristic peaks of each functional group after functionalization are marked by f–j. An N–C–H characteristic peak appeared at d = 3.3 ppm in all QPVTD membrane samples after functionalization. In addition, peaks appearing at d = 1.4 ppm, d = 2.0 ppm, d = 1.6 ppm and d = 0.9 ppm were characteristic peaks of the QPVTD-1, QPVTD-2, QPVTD-3 and QPVTD-4 membrane functional groups, respectively. This peak movement depended upon the chain length of the functional agent. Therefore, in the result of FT-IR and 1H-NMR analysis, it was confirmed that a macromolecule was definitely synthesized and could play a role as an anion exchange membrane due to the appropriate location of the functional group after functionalization.
PVTD QPVTD-1 QPVTD-2
QPVTD-4
2860 2926
1089
1504
Water uptake and ion exchange capacity
1439 1255
1666
1163
657
1384
3500
3000
2500
2000
1500
1000
-1
Wavenumbers (cm ) Fig. 3. FT-IR spectra of PVTD, QPVTD-1, QPVTD-2, QPVTD-3 and QPVTD-4.
polydispersity index (PDI) was 1.29. Therefore, it was determined that the physical properties of the synthesized copolymer were equal because the molecular weight distribution was small. The results of the FT-IR spectrum analysis for membrane structure identification before and after functionalization is shown in Fig. 3. As shown in Fig. 3, C—H stretching vibration peaks at 2926 and 2860 cm1, a CN characteristic peak at 1666 cm1, a C¼C peak characteristic of imidazole rings at 1504 cm1[20], a C—C stretching peak of aromatic rings at 1439 cm1[21], a bending vibration peak of an a-methyl group at 1385 cm1[19], trifluoromethyl C–F characteristic peaks at 1256 and 1165 cm1[19], a C—N stretching peak at 1089 cm1 and a C–F3 characteristic peak at 657 cm1 appeared. Therefore, it was confirmed that a PVTD copolymer was synthesized. The 1H-NMR spectrum of the QVTD membrane is shown in Fig. 4. As shown in Fig. 4, characteristic peaks presenting a PVTD structure
PVTD
g
QPVTD-3
QPVTD-2
QPVTD-1
f
ff
i
N
N N
QPVTD-4 j
f
g
Electrical resistance and conductivity Electrical resistance of an ion permselective membrane is a key factor for battery performance and water treatment. Therefore, in this study, electrical resistance of the membranes produced with different functional agents was measured to identify the effect. The result is shown in Fig. 6. As shown in Fig. 6, the electrical resistance of the membranes produced with different functional agents was
f N
N
N
N
N
The result of measuring water uptake and the ion exchange capacity of the QVTD anion exchange membrane synthesized with different functional agents is shown in Fig. 5. As shown in Fig. 5, membrane water uptake decreased from 60.8% to 30.9%, and the swelling ratio decreased from 44.0% to 23.5%, likely because the chain combined with a ligand and became more bulky. The cause of this result is the membrane permeability reduction of water molecules due to the increased bulkiness of the functional group. Furthermore, water uptake is closely related to functional agent solubility. As shown in Table 3, water solubility decreased [22], and hydrophobicity increased as functional agent solubility (log Sw) increased. Therefore, the lower the solubility of the functional agent was confirmed the membrane water uptake was decreased. The ion exchange capacity decreased from 1.25 meq/g dry to 0.84 meq/g dry, a similar trend to water uptake. The ion exchange capacity may decrease because steric hindrance interrupts ion movement as the bulkiness of a functional group structure is increased.
80 Water Uptake Ion exchange capacity
h
70
1.4 1.2
f
i
QPVTD-3 h
QPVTD-2
10
0.8
50
0.6 40 0.4
g
QPVTD-1 PVTD
Water Uptake (%)
g
QPVTD-4
1.0
60
j
30 d
c
8
6
a
b
4
0.2
Ion exchange capacity (meq/g dry)
Transmittance
QPVTD-3
4000
2165
e
2
0
ppm Fig. 4. 1H-NMR spectra of PVTD, QPVTD-1, QPVTD-2, QPVTD-3 and QPVTD-4.
0.0
20 QPVTD-1
QPVTD-2
QPVTD-3
QPVTD-4
Fig. 5. Water uptake and ion exchange capacity of QPVTD-1, QPVTD-2, QPVTD3 and QPVTD-4.
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Table 3 Basic properties of QPVTD membranes. Code no.
Solubility (log Sw)
Water uptake (%)
Swelling ratio (%)
Ion exchange capacity (meq/g dry)
Electrical resistance (V)
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
2.20 2.43 2.88 3.08
60.85 49.39 37.93 30.92
44.00 33.10 28.15 23.50
1.25 1.02 0.94 0.84
3.71 5.79 6.68 8.01
3.7 V cm1, 5.8 V cm1, 6.7 V cm1 and 8.0 V cm1. Electrical resistance increased as the chain length of the ligand compound, a functional agent, and the bulkiness of the molecular structure increased. It is thought that increased electrical resistance was caused by a lowered ion exchange capacity due to the steric hindrance effect as the molecular structure of the functional group became more bulky. In addition, electric conductivity tended to decrease as the bulkiness of the functional agent was increased. The result was caused by ion permselectivity reduction due to ion flow interruption between the membrane and the electrode as the functional group structure became more bulky. QPVTD-1 membrane has excellent electrical resistance than AMX (Tokuyama Soda Co., Ltd., Japan) commercial membrane. These values are presented in Table 3.
25
20
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
-
Concentration(Cl ) (ppm)
(a)
15
10
5
0 0
50
100
150
200
250
Time (min) 25
(b) Concentration(NO3) (ppm)
Ion permeability of a single solution
-
The results of the ion permeability test of the synthesized QPVTD membrane in a chloride, sulfate and nitrate single solution are shown in Fig. 7. As shown in Fig. 7, sulfate ion permeability was lower than chloride ion and nitrate ion permeability. This result can be explained by the ionic radius presented in Table 4. A sulfate ion, which has a bigger radius, is harder to pass through a membrane compared to a chloride ion and a nitrate ion. Furthermore, ion permeability of the QVTD-3,4 membrane was lower than the QPVTD-1,2 membrane because steric hindrance affected ion movement due to increased bulkiness as the molecular weight of the functional agent introduced into a membrane was increased.
20
15
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
10
5
0 0
50
100
150
200
250
Time (min) 25
(c) QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
20
2-
Concentration(SO4 ) (ppm)
Ion permeability of a mixed solution The results of the nitrate permselectivity test for a nitrate, sulfate and chlorine mixed solution in the QPVTD membranes synthesized with different functional agents are shown in Fig. 8.
15
10
5
0 0
50
100
150
200
250
Time (min)
Fig. 7. Ion permeability of QPVTD membrane in the single solution. (a) Chloride ion solution, (b) nitrate ion solution, (c) sulfate ion solution.
The result of a nitrate ion permselectivity test for a mixed solution in the membrane synthesized using 1-bromobutane as a functional agent is shown in Fig. 8. As shown in Fig. 8, the ion permeability of the membrane synthesized using 1-bromobutane as a functional agent increased over time. In the result of the 4-h Table 4 Gibbs hydration energy and Ionic radius of anion [26,27].
Fig. 6. Electrical resistance and conductivity of QPVTD membranes.
Anion species
DGh(kJ/mol)
Ionic radius (Å)
NO3 Cl SO42
270 317 1000
1.89 1.81 2.40
C.-M. Oh et al. / Journal of Environmental Chemical Engineering 2 (2014) 2162–2169
16
16 -
-
Cl NO3
14
2-
SO4
12 10 8 6
14
Cl NO3
12
SO4
2-
Concentration (ppm)
Concentration (ppm)
2167
10 8 6
4
4
2
2
0
0 0
50
100
150
200
250
0
50
100
Time (min)
150
200
250
Time (min)
Fig. 8. Nitrate ion permselectivity of QPVTD-1 membrane in the mixture.
Fig. 10. Nitrate ion permselectivity of QPVTD-3 membrane in the mixture.
permselectivity test for a 20-ppm mixed solution with equal concentrations of NO3, Cl, SO42, the ion permeability of nitrate, chlorine and sulfate ions was 11.2, 10.4 and 9.5 ppm, respectively; therefore, nitrate ion permselectivity was higher than other competitive ions. The result of the nitrate ion permselectivity test under the same conditions in a mixed solution of the QPVTD-2 membrane using 1-bromo-2-methylpropane as a functional agent, which has the same molecular weight as 1-bromobutane but a different structure, is shown in Fig. 9. In the result of the 4-h permselectivity test for a 20-ppm mixed solution with equal concentrations of NO3, Cl, SO42, the ion permeability of nitrate, chlorine and sulfate ions was 10.0, 8.8 and 7.9 ppm, respectively; therefore, the nitrate removal rate was lower than that of the QPVTD-1 membrane. Thus, nitrate permselectivity decreased because the molecular structure of the functional group introduced into the membrane changed; therefore, a channel where nitrate ions passed became narrow, and less ions could pass through. However, the nitrate removal rate was higher than that of other ions, confirming nitrate ion permselectivity of the QPVTD-2 membrane. The result of the nitrate ion permselectivity test of the QPVTD3 membrane with a 1-bromo-3-methylbutane functional group introduced is shown in Fig. 10. As shown in Fig. 10, nitrate, chlorine
and sulfate ion permeability of the QPVTD-3 membrane with 1bromo-3-methylbutane introduced were 8.9, 7.5 and 6.5 ppm, respectively; therefore, nitrate ion permeability was lower than that of QPVTD-1,2 because of steric hindrance, likely due to increased bulkiness as the chain length of the functional agent increased. Furthermore, the result of the nitrate ion permselectivity test for the QPVTD-4 membrane using 1-bromopentane as a functional agent is shown in Fig. 11. Ion permeability of nitrate, chlorine and sulfate ions was 7.4, 5.0 and 4.8 ppm, respectively; therefore, the removal rate of QPVTD-4 was the lowest. It was determined that the anion permeability of a mixed solution became lower because hydrophobicity increased due to steric hindrance as bulkiness increased. The anion removal rates of the synthesized QPVTD membranes are presented in Table 5. It was concluded that nitrate ion permselectivity of the QPVTD-1 membrane, whose chain length was short and molecular structure was simple, was most appropriate among mixed solutions based on the results of nitrate ion permselectivity tests of the membranes with different functional agents introduced and with different molecular structures and chain lengths, as shown in Figs. 8–11.
16
16 -
14
2-
SO4
12
Concentration (ppm)
Concentration (ppm)
-
Cl NO3
10 8 6
14
Cl NO3
12
SO4
2-
10 8 6
4
4
2
2
0
0 0
50
100
150
200
250
Time (min) Fig. 9. Nitrate ion permselectivity of QPVTD-2 membrane in the mixture.
0
50
100
150
200
250
Time (min) Fig. 11. Nitrate ion permselectivity of QPVTD-4 membrane in the mixture.
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Table 5 Anion removal rate of QPVTD membranes. Code no.
Cl (%)
NO3 (%)
SO4 (%)
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
52.43 43.97 37.72 30.15
56.20 50.13 44.73 36.74
47.42 39.47 32.45 24.35
Conclusions
Ion-selective constant of QPVTD membranes The ion-selective constant of a QPVTD membrane is an important factor for permselectivity. Therefore, in this study, the NO ion selectivity coefficient (ki 3 ) of the synthesized membranes with different functional agents was calculated to identify the effect according to the following formula [23]. The result is shown in Fig. 12. NO kCl 3 NO
kSO43
C NO3 =C Cl ¼ C RNO3 =C RCl C NO3 =C SO4 ¼ C RNO3 =C RSO4
where C NO3 ,C Cl , C SO4 are the concentration of nitrate ions, sulfate ions and chloride ions removed, respectively, and C RNO3 , C RCl , C RSO4 are solution concentrations before removing nitrate ions, sulfate ions and chloride ions, respectively. As the selectivity coefficient increases, membrane affinity for the ion being removed is higher than for competing ions. As shown in Fig. 12, the chloride ion and nitrate ion selectivity coefficient, NO NO kCl 3 , and the sulfate ion and nitrate ion selectivity coefficient, kSO43 , NO3 tended to increase from QPVTD-1 to QPVTD-4. kCl was higher NO than kSO43 , confirming that nitrate permselectivity for a sulfate ion rather than a chloride ion in a mixed solution was higher. This conclusion is confirmed by the results in Figs. 8–11. Permselectivity increased as the ion selectivity coefficient increased, which can be explained by the Gibbs hydration energy, which resulted in ion hydrophilicity. The Gibbs hydration energy (DGh) of each ion is presented in Table 4. The NO3 ion, which has the lowest energy among these three ions, is relatively hydrophobic, indicating that NO3 ions are excluded as the hydrophobicity of a functional group increases. Therefore, if a functional group of a synthesized membrane is hydrophobic, the permselectivity of a hydrophilic ion decreases and permselectivity of NO3, a hydrophobic ion, increases. Based on the results reported by Guter, the nitrate ion permselectivity of a membrane increased as the number of carbon
NO3
1.5
k Cl
NO
k SO3 4
A
Selectivity (kB)
1.4
1.3
1.2
1.1
QPVTD-1
QPVTD-2
QPVTD-3
Fig. 12. Ion selectivity of QPVTD membrane.
atoms combined with quaternized amine increased [24]. In addition, Clifford and Webber reported that nitrate permselectivity increased as the nonpolarity and hydrophobicity of a functional group increased [25]. As presented in Table 5, the results presented herein agree well with the results from these other studies. Therefore, it was confirmed that the synthesized QPVTD membrane exhibited nitrate permselectivity.
QPVTD-4
This study addressed the synthesis of a partially fluorinated anion exchange membrane and its nitrate permselectivity properties depending upon the properties of added functional agents to synthesize a nitrate ion permselective membrane. The conclusions are as follows: 1 The number-average molecular weight (Mn) of a partially
2
3
4
5
fluorinated PVTD copolymer was 46,000, and the weightaverage molecular weight (Mw) was 59,000. Molecular weight distribution (PDI) was 1.29, indicating that the physical properties of the membrane were equal and, therefore, great. Membrane water uptake decreased from 60.8% to 30.9%, and the swelling rate decreased from 44% to 23.5% depending upon the type of functional group ligand used. Membrane electrical resistance for each functional agent type was 3.7 V cm1, 5.8 V cm1, 6.7 V cm1 and 8.0 V cm1; therefore, electrical resistance increases depending on the bulkiness of a ligand compound, a functional agent. As membrane bulkiness increases, the nitrate selectivity coefficient increases, thus the nitrate ion permselectivity compared to that of chloride and sulfate ions also increased. Nitrate permselectivity of QPVDT membranes was highest for the QPVTD-1 membrane, whose chain length was short and molecular structure was simple. In addition, the nitrate and sulfate ion selectivity coefficient was higher than the chloride and nitrate ion selectivity coefficient. Based upon this result, it was concluded that nitrate ion permselectivity is great among mixed solutions of QPVTD membranes. We can prepare nitrate removal membrane easily than commercial membrane (e.g., AMX; Tokuyama Soda).
References [1] K. Kesore, F. Janowski, V.A. Shaposhnik, Highly effective electrodialysis for selective elimination of nitrates from drinking water, J. Membr. Sci. 127 (1997) 17–24, doi:http://dx.doi.org/10.1016/S0376-7388(96)00282-7. [2] K. Kimura, M. Nakamura, Y. Watanabe, Nitrate removal by a combination of elemental sulfur-based denitrification and membrane filtration, Water Res. 36 (2002) 1758–1766, doi:http://dx.doi.org/10.1016/S0043-1354(01)00376-1. 12044075. [3] S. Ghafari, M. Hasan, M.K. Aroua, Bio-electrochemical removal of nitrate from water and wastewater – a review, Bioresour. Technol. 99 (2008) 3965–3974, doi:http://dx.doi.org/10.1016/j.biortech.2007.05.026. 17600700. [4] O. Altintas, A. Tor, Y. Cengeloglu, M. Ersoz, Removal of nitrate from the aqueous phase by Donnan dialysis, Desalination 239 (2009) 276–282, doi:http://dx.doi. org/10.1016/j.desal.2008.03.024. [5] P.C. Mishra, R.K. Patel, Use of agricultural waste for the removal of nitratenitrogen from aqueous medium, J. Environ. Manage. 90 (2009) 519–522, doi: http://dx.doi.org/10.1016/j.jenvman.2007.12.003. 18294755. [6] A. Kapoor, T. Viraraghavan, Nitrate removal from drinking water—review, J. Environ. Eng. 123 (1997) 371–380, doi:http://dx.doi.org/10.1061/(ASCE)07339372(1997)123:4(371). [7] K. Mizuta, T. Matsumoto, Y. Hatate, K. Nishihara, T. Nakanishi, Removal of nitrate-nitrogen from drinking water using bamboo powder charcoal, Bioresour. Technol. 95 (2004) 255–257, doi:http://dx.doi.org/10.1016/j.biortech.2004.02.015. 15288267. [8] M. Alikhani, M.R. Moghbeli, Ion-exchange polyHIPE type membrane for removing nitrate ions: preparation, characterization, kinetics and adsorption studies, Chem. Eng. J. 239 (2014) 93–104, doi:http://dx.doi.org/10.1016/j. cej.2013.11.013.
C.-M. Oh et al. / Journal of Environmental Chemical Engineering 2 (2014) 2162–2169 [9] S. Ben Hamouda, K. Touati, M. Ben Amor, Donnan dialysis as membrane process for nitrate removal from drinking water: membrane structure effect, Arab. J. Chem. (2012) 1–6. [10] A. Bhatnagar, M. Sillanpää, A review of emerging adsorbents for nitrate removal from water, Chem. Eng. J. 168 (2011) 493–504, doi:http://dx.doi.org/ 10.1016/j.cej.2011.01.103. [11] S. Shams, Assessing Innovative Technologies for Nitrate Removal from Drinking Water, University of Waterloo, Ontario, Canada, 2010. [12] T.W. Xu, Y. Li, L. Wu, W.H. Yang, A simple evaluation of microstructure and transport parameters of ion-exchange membranes from conductivity measurements, Sep. Purif. Technol. 60 (2008) 73–80, doi:http://dx.doi.org/10.1016/ j.seppur.2007.07.049. [13] T. Xu, Ion exchange membranes: state of their development and perspective, J. Membr. Sci. 263 (2005) 1–29, doi:http://dx.doi.org/10.1016/j.memsci.2005.05.002. [14] A.A. Hekmatzadeh, A. Karimi-Jashani, N. Talebbeydokhti, B. Kløve, Modeling of nitrate removal for ion exchange resin in batch and fixed bed experiments, Desalination 284 (2012) 22–31, doi:http://dx.doi.org/10.1016/j. desal.2011.08.033. [15] F. Guesmi, C. Hannachi, B. Hamrouni, Modification of the AMX membrane surface: temperature dependence of anion exchange equilibrium, Can. J. Chem. Eng. 91 (2013) 1465–1473, doi:http://dx.doi.org/10.1002/cjce.21744. [16] T. Sata, K. Teshima, T. Yamaguchi, Permselectivity between two anions in anion exchange membranes crosslinked with various diamines in electrodialysis, J. Polym. Sci. Part A Polym. Chem. 34 (1996) 1475–1482. [17] M.J. Ariza, T.F. Otero, Nitrate and chloride transport through a smart membrane, J. Membr. Sci. 290 (2007) 241–249, doi:http://dx.doi.org/ 10.1016/j.memsci.2006.12.040. [18] Y. Berbar, M. Amara, H. Kerdjoudj, Anion exchange resin applied to a separation between nitrate and chloride ions in the presence of aqueous soluble polyelectrolyte, Desalination 223 (2008) 238–242, doi:http://dx.doi.org/ 10.1016/j.desal.2007.01.218.
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[19] C.W. Hwang, H.-M. Park, C.M. Oh, T.S. Hwang, J. Shim, C.-S. Jin, Synthesis and characterization of vinylimidazole-co-trifluoroethylmethacrylate-co-divinylbenzene anion-exchange membrane for all-vanadium redox flow battery, J. Membr. Sci. (2014) 98–106. [20] A. Bobrowski, B. Grabowska, The impact of temperature on furan resin and binders structure, Metall. Foundry Eng. 38 (1) (2012) 73–80, doi:http://dx.doi. org/10.7494/mafe.2012.38.1.73. [21] A. Shoaib, N. Akhtar, N. Aqsa Aftab, Fourier transform-infrared spectroscopy to monitor modifications in canola biochemistry caused by alternaria destruens, Pak. J. Phytopathol. 25 (2) (2013) 105–109. [22] P.D.T. Huibers, A.R. Katritzky, Correlation of the aqueous solubility of hydrocarbons and halogenated hydrocarbons with molecular structure, J. Chem. Inf. Model. 38 (1998) 283–292, doi:http://dx.doi.org/10.1021/ ci9700438. [23] T. Sata, Studies on anion exchange membranes having permselectivity for specific anions in electrodialysis – effect of hydrophilicity of anion exchange membranes on permselectivity of anions, J. Membr. Sci. 167 (2000) 1–31, doi: http://dx.doi.org/10.1016/S0376-7388(99)00277-X. [24] G. Guter, Removal of Nitrate from Contaminated Water Supplies for Public Use, United States Environ. Prot. Agency. EPA-600/S2, 1981. [25] D.A. Clifford, W.J. Weber, Nitrate Removal from Water Supplies by Ion Exchange, United States Environ. Prot. Agency. EPA-600/2, 1978. [26] T. Sata, T. Yamaguchi, K. Matsusaki, Effect of hydrophobicity of ion exchange groups of anion exchange membranes on permselectivity between two anions, J. Phys. Chem. 99 (1995) 12875–12882, doi:http://dx.doi.org/10.1021/ j100034a028. [27] F. Guesmi, C. Hannachi, B. Hamrouni, Effect of temperature on ion exchange equilibrium between AMX membrane and binary systems of Cl, NO3 and SO42 ions, Desalin. Water Treat. 23 (2010) 32–38, doi:http://dx.doi.org/ 10.5004/dwt.2010.1837.
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Synthesis of a quaternarized poly(vinylimidazole-cotrifluoroethylmethacrylate-co-divinylbenzene) anion-exchange membrane for nitrate removal Chang-Min Oh, Chi-Won Hwang, Taek-Sung Hwang * Department of Applied Chemistry and Biological Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 July 2014 Received in revised form 2 September 2014 Accepted 15 September 2014
The effect of functional agents on the ion permselectivity between an anion exchange membrane and solutions of electrolytes containing the most dominant anions of natural waters (Cl, NO3 and SO42) was studied. The vinylimidazole(VI)-co-trifluoroethylmethacrylate(TFEMA)-co-divinylbenzene(DVB) copolymer was synthesized by solution polymerization. The copolymer was functionalized with compounds such as 1-bromo-2-methylpropane, 1-bromobutane, 1-bromo-3-methylbutane and 1-bromopentane. Quaternarized poly(vinylimidazole-co-trifluoroethylmethacrylate-co-divinylbenzene) (QPVTD) anion exchange membranes were prepared using the casting method. The molecular structure of the membrane was confirmed by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy. The membrane properties, such as water uptake, ion exchange capacity, electrical resistance and ion permselectivity for nitrate, were measured. The maximum water uptake, electrical resistance and ion exchange capacity were 60.8%, 3.7 V/cm2 and 1.25 meq/g dry, respectively. ã 2014 Published by Elsevier Ltd.
Keywords: Quaternization Nitrate removal Ion permselectivity Anion exchange membrane Ion selectivity
Introduction Recently, nitrate concentrations have increased in groundwater used for drinking water and industrial water in agricultural and fishing villages due to industrial development and population growth [1,2]. Among all water resources on earth, groundwater, one of the few drinking water resources, accounts for just 0.6% of total water resources globally. The quality as well as the dwindling quantity of drinking water resources have become serious issues as industry, animal husbandry and agricultural activities have increased. In particular, nitrate (NO3) concentrations in groundwater are increasing due to the increased use of nitrogenous fertilizers. Furthermore, increased nitrate concentration in groundwater is caused by increases in the release of domestic wastewater, industrial wastewater and animal waste release to the environment [3–5]. If a high-concentration of the nitrate contained in groundwater is ingested, nitrosamine, a carcinogen, is formed in the body, and for children under 6 years of age, NO2 created from NO3 is combined with hemoglobin to form methemoglobin, which causes fatal blue baby syndrome [6,7]. Therefore, to solve this problem, many countries, including advanced countries such as the U.S., have
* Corresponding author. Tel.: +82 42 821 5687; fax: +82 42 822 8995. E-mail address: [email protected] (T.-S. Hwang). http://dx.doi.org/10.1016/j.jece.2014.09.014 2213-3437/ ã 2014 Published by Elsevier Ltd.
limited the acceptable concentration of nitrate in drinking water. Europe limits nitrate to 25 mg/L or less [8], and the World Health Organization (WHO) limits nitrate to 50 mg/L or less [9]. Established methods for removing nitrate from drinking water include reverse osmosis, adsorption, select biological and chemical methods and ion exchange. Among those, reverse osmosis has a high removal efficiency but requires high-concentration total dissolved solids (TDS) treatment and corrosion prevention treatments, which are costly processes. In addition, adsorption is sensitive to pH and temperature. Furthermore, chemical methods create by-products that must be treated and often have a low treatment efficiency, and biological methods are sensitive to temperature and produce biomass waste. Therefore, ion exchange, which is able to avoid those unfortunate process weaknesses, has been studied actively because it is not affected by pH and has a high processing efficiency [10,11]. In particular, nitrate removal using an ion exchange membrane is easily applied to a process, has a high removal efficiency, has a greater processing capacity, and can be used in various fields, meaning that related studies can be actively conducted [12,13]. Nitrate removal from water using an ion exchange membrane has been studied extensively, but nitrate ions in water are very stable and highly water-soluble. Therefore, an ion exchange membrane with an ion permselective ligand must be made to remove nitrate ions from the water completely [14]. In addition,
C.-M. Oh et al. / Journal of Environmental Chemical Engineering 2 (2014) 2162–2169
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(TFEMA) with a purity of 99% (Esstech, Essington, USA). We used N,N-dimethylformamide (DMF; 99%), a reactive solvent (Samchun, Seoul, South Korea) and a,a0 -azobis (isobutyronitrile) (AIBN; 98%) (Junsei, Tokyo, Japan) as an initiator. For the quaternization of a synthesized copolymer, we used 1-bromo-2-methylpropane (purity 99%), 1-bromobutane (99%), 1-bromopentane (98%) (Aldrich, NY, USA), and 1-bromo-3-methylbutane (96%) (TCI, Tokyo, Japan). For all other reagents, we used first class reagents without purification.
there are ions that compete with the nitrate ions for removal by the exchange membrane, such as chloride (Cl) and sulfate ions (SO42). Further research is needed to develop more ion selective separation membranes that efficiently select nitrate [1,15–18]. Among water nitrate permselective ligand compounds studied to date, a quaternized amine group has been shown to be the most effective. Most studies that have been conducted combine a macromolecular matrix with a quaternized amine group. Studies of how nitrate permselectivity depends on the bulkiness and the length of a ligand chain have not been reported to date. Therefore, in this study, we synthesized an anion exchange membrane to separate nitrate ions selectively from water and identified the extent of nitrate permselectivity depending upon alkyl halide functional agents with different quaternized amine ligand structures. To confirm the structure of an anion exchange membrane produced, Fourier transform infrared (FT-IR) and nuclear magnetic resonance (1H-NMR) spectra were analyzed. Furthermore, we investigated the effect of a functional group structure of the quaternarized poly(vinylimidazole-co-trifluoroethylmethacrylateco-divinylbenzene) (QPVTD) anion exchange membrane on nitrate permselectivity by measuring the ion exchange capacity, water uptake, equilibrium ratio, electrical resistance, electrical conductivity and ion permselectivity of the membrane.
Synthesis of a poly(vinylimidazole-co-trifluoroethylmethacrylate-codivinylbenzene) anion exchange membrane A poly(VI-co-TFEMA-co-DVB) (PVTD below) copolymer was synthesized using a stirrer, a cooler and a 1-L four-neck flask reactor with a nitrogen gas inlet and a sample inlet, adhering to the synthesis mechanism shown in Fig. 1 and under the conditions presented in Table 1. For copolymer synthesis, VI, TFEMA and DVB were put into a reactor in a nitrogen atmosphere at 50 C for 24 h to synthesize a PVTD copolymer [19]. To synthesize an anion exchange solution with the introduction of a quaternized amine group, the functionalization of the synthesized copolymer was performed according to the synthesis mechanism shown in Fig. 1 and with the functional agents presented in Table 2. A synthesized anion exchange solution was cast on a glass plate by a doctor blade and dried in an oven at 50–130 C to produce the QPVTD anion exchange membrane.
Experimental Materials
Membrane characterization For poly(vinylimidazole-co-trifluoroethylmethacrylate-codivinylbenzene) (PVTD) copolymer synthesis, we purchased vinylimidazole (VI) with a purity of 99% and divinylbenzene (DVB) (Aldrich, NY, USA) and used 2,2,2-trifluoroethylmethacrylate
Molecular weight and structural analysis Gel permeation chromatography (GPC; Waters 2690) was used to identify the molecular weight of the synthesized PVTD copolymer.
F
F
F
N N
O N
O
+ N
N
n
F
2,2,2-Trif luoroethyl methacrylate(TFEMA)
F
F
1,4-Divinylbenzene(DVB)
n
Br
F R
R N
OH
OH
O
100 oC 24 hr
O
R
N
O
OH F
m
OH
n F
N
O
N Br
F
F Br
Br
m
F
m
+
N
O
DMF F
N O
O
AIBN 50o C, 24 hr
F
Vinylimidazole(VI)
O
N
N N
+
F
n
Fig. 1. Preparation mechanism of anion exchange membrane.
R
m
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Table 1 Synthesis conditions of PVTD copolymer by solution polymerization. Monomer (mole/L)
AIBN
DMF
Temp.
Time
VI
TFEMA
DVB
(wt%)
(wt%)
( C)
(h)
2.50
5.00
0.15
1.00
275
50
24
Columns used to measure the molecular weight included m-Styragel HR-1, HR-2, and HR-3 columns. Polystyrene was used as the GPC reference material, and tetrahydrofuran (THF) was used as the solvent. A flow rate of 1 mL/min was used for molecular weight measurement. To identify the anion exchange membrane structure before and after functionalization, a Shimatzu Fourier transform infrared (FT-IR) spectrometer was used to analyze functional groups according to the attenuated total reflection absorption spectroscopy (ATR) method between 4000 and 600 cm1 in wavelength with a scan number of 20 and a resolution of 4 cm1. In addition, for 1H-NMR spectrum analysis, a Fourier-transform nuclear magnetic resonance spectrometry (FT-NMR) spectrometer (JNM-AL400) from JEOL was used with deuterated methanol as the solvent for NMR spectrum analysis of the copolymer. Water uptake of the QPVTD membrane To measure water uptake of the synthesized QPVTD membrane, the membrane was cut into specifically sized pieces (3 3 cm) and immersed in distilled water for 24 h to fully equilibrate. The membrane pieces were then removed, the surface was dried, and the membrane weight was measured. Membrane water uptake was calculated according to the following formula: WaterUptakeð% Þ ¼
W wet W dry 100 W dry
where Wdry and Wwet are the membrane weight before wetting and after reaching equilibrium in the distilled water, respectively. The water uptakes were recorded with an accuracy of 1.0%. Ion exchange capacity (IEC) of QVTD membrane The ion exchange capacity of the synthesized QPVTD membrane was measured by titration. The membrane was cut into 3 3-cm pieces and immersed in a 1 M HCl solution for 24 h. Then, the supernatant was removed, an indicator was added, and the solution was titrated by a 0.1 M NaOH standard solution. The ion exchange capacity of the membrane was calculated according to the following formula: meq ðV C HCl Þ ðV NaOH C NaOH Þ dry ¼ HCl IECvalue g W dry whereVHCl and VNaOH are the volume of HCl and NaOH, respectively, CHCl and CNaOH are the concentration of HCl and NaOH, respectively, and Wdry is the dry weight of the membrane. The ion exchange capacities were recorded with an accuracy of 0.1 meq/g.
The samples were then immersed in a 0.5 M NaCl solution for 24 h, and the membrane electrical resistance was measured using a 2-compartment cell. A 0.5 M NaCl solution was used as the electrolyte, and the electrical resistance was measured by applying a voltage of 0.8 V. The membrane electrical resistance was calculated according to the following formula: ERðV cm2 Þ ¼ ðR1 R2 Þ A where R1 is the electrical resistance value after installing a membrane, and R2 is the electrical resistance value before installing a membrane. A is the membrane effective area. In addition, the ion conductivity of the QVTD anion exchange membrane was calculated according to the following formula: S L s ¼ cm ER A where ER is the membrane electrical resistance (V), A is the membrane effective area (cm2), and L is the membrane thickness (cm). The electrical resistances were recorded with an accuracy of 0.3 V cm2. Properties of ion selective separation To confirm nitrate ion permselectivity of a membrane, an ion exchange membrane, synthesized using a 2-compartment unit cell as shown in Fig. 2, was installed. The unit cell consists of two cylindrical compartments that were separated by the membrane. A NaCl/NaNO3/Na2SO4 mixed solution was put to the cathode, and a NaCl solution, an electrolyte solution, was put to the anode, and then, a voltage of 12 V was applied. Sampling was conducted at 5,10, 30, 60, 120 and 240 min. Then, Cl, NO3 and SO42 ion concentrations of the membrane were measured using ion chromatography (881 Compact IC pro, Metrohm Co., Switzerland). Results and discussion Molecular weight and structural analysis As a result of measuring the molecular weight of the PVTD copolymer synthesized to remove nitrate, the number-average molecular weight (Mn) of the PVTD copolymer was 46,000, the weight-average molecular weight (Mw) was 59,000, and the
Electric resistance and ion conductivity of QVTD membrane The membrane was cut into 1.5 1.5-cm pieces to measure the membrane electrical resistance by an LCR meter (HIOKI Co., Japan). Table 2 Synthesis conditions of quaternarized PVTD polyelectrolyte solution. Code no.
PVTD (wt%)
Functional agents (wt%)
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
85.7 84.1 85.7 84.1
1-Bromobutane 1-Bromo-2-methylpropane 1-Bromo-3-methylbutane 1-Bromopentane
14.3 14.3 15.9 15.9
Temp ( C)
Time (h)
100 100 100 100
24 24 24 24
Fig. 2. Schematic diagram of apparatus for measuring capacity of ion permselectivity.
C.-M. Oh et al. / Journal of Environmental Chemical Engineering 2 (2014) 2162–2169
before functionalization are marked by a–e. Characteristic peaks of each functional group after functionalization are marked by f–j. An N–C–H characteristic peak appeared at d = 3.3 ppm in all QPVTD membrane samples after functionalization. In addition, peaks appearing at d = 1.4 ppm, d = 2.0 ppm, d = 1.6 ppm and d = 0.9 ppm were characteristic peaks of the QPVTD-1, QPVTD-2, QPVTD-3 and QPVTD-4 membrane functional groups, respectively. This peak movement depended upon the chain length of the functional agent. Therefore, in the result of FT-IR and 1H-NMR analysis, it was confirmed that a macromolecule was definitely synthesized and could play a role as an anion exchange membrane due to the appropriate location of the functional group after functionalization.
PVTD QPVTD-1 QPVTD-2
QPVTD-4
2860 2926
1089
1504
Water uptake and ion exchange capacity
1439 1255
1666
1163
657
1384
3500
3000
2500
2000
1500
1000
-1
Wavenumbers (cm ) Fig. 3. FT-IR spectra of PVTD, QPVTD-1, QPVTD-2, QPVTD-3 and QPVTD-4.
polydispersity index (PDI) was 1.29. Therefore, it was determined that the physical properties of the synthesized copolymer were equal because the molecular weight distribution was small. The results of the FT-IR spectrum analysis for membrane structure identification before and after functionalization is shown in Fig. 3. As shown in Fig. 3, C—H stretching vibration peaks at 2926 and 2860 cm1, a CN characteristic peak at 1666 cm1, a C¼C peak characteristic of imidazole rings at 1504 cm1[20], a C—C stretching peak of aromatic rings at 1439 cm1[21], a bending vibration peak of an a-methyl group at 1385 cm1[19], trifluoromethyl C–F characteristic peaks at 1256 and 1165 cm1[19], a C—N stretching peak at 1089 cm1 and a C–F3 characteristic peak at 657 cm1 appeared. Therefore, it was confirmed that a PVTD copolymer was synthesized. The 1H-NMR spectrum of the QVTD membrane is shown in Fig. 4. As shown in Fig. 4, characteristic peaks presenting a PVTD structure
PVTD
g
QPVTD-3
QPVTD-2
QPVTD-1
f
ff
i
N
N N
QPVTD-4 j
f
g
Electrical resistance and conductivity Electrical resistance of an ion permselective membrane is a key factor for battery performance and water treatment. Therefore, in this study, electrical resistance of the membranes produced with different functional agents was measured to identify the effect. The result is shown in Fig. 6. As shown in Fig. 6, the electrical resistance of the membranes produced with different functional agents was
f N
N
N
N
N
The result of measuring water uptake and the ion exchange capacity of the QVTD anion exchange membrane synthesized with different functional agents is shown in Fig. 5. As shown in Fig. 5, membrane water uptake decreased from 60.8% to 30.9%, and the swelling ratio decreased from 44.0% to 23.5%, likely because the chain combined with a ligand and became more bulky. The cause of this result is the membrane permeability reduction of water molecules due to the increased bulkiness of the functional group. Furthermore, water uptake is closely related to functional agent solubility. As shown in Table 3, water solubility decreased [22], and hydrophobicity increased as functional agent solubility (log Sw) increased. Therefore, the lower the solubility of the functional agent was confirmed the membrane water uptake was decreased. The ion exchange capacity decreased from 1.25 meq/g dry to 0.84 meq/g dry, a similar trend to water uptake. The ion exchange capacity may decrease because steric hindrance interrupts ion movement as the bulkiness of a functional group structure is increased.
80 Water Uptake Ion exchange capacity
h
70
1.4 1.2
f
i
QPVTD-3 h
QPVTD-2
10
0.8
50
0.6 40 0.4
g
QPVTD-1 PVTD
Water Uptake (%)
g
QPVTD-4
1.0
60
j
30 d
c
8
6
a
b
4
0.2
Ion exchange capacity (meq/g dry)
Transmittance
QPVTD-3
4000
2165
e
2
0
ppm Fig. 4. 1H-NMR spectra of PVTD, QPVTD-1, QPVTD-2, QPVTD-3 and QPVTD-4.
0.0
20 QPVTD-1
QPVTD-2
QPVTD-3
QPVTD-4
Fig. 5. Water uptake and ion exchange capacity of QPVTD-1, QPVTD-2, QPVTD3 and QPVTD-4.
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Table 3 Basic properties of QPVTD membranes. Code no.
Solubility (log Sw)
Water uptake (%)
Swelling ratio (%)
Ion exchange capacity (meq/g dry)
Electrical resistance (V)
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
2.20 2.43 2.88 3.08
60.85 49.39 37.93 30.92
44.00 33.10 28.15 23.50
1.25 1.02 0.94 0.84
3.71 5.79 6.68 8.01
3.7 V cm1, 5.8 V cm1, 6.7 V cm1 and 8.0 V cm1. Electrical resistance increased as the chain length of the ligand compound, a functional agent, and the bulkiness of the molecular structure increased. It is thought that increased electrical resistance was caused by a lowered ion exchange capacity due to the steric hindrance effect as the molecular structure of the functional group became more bulky. In addition, electric conductivity tended to decrease as the bulkiness of the functional agent was increased. The result was caused by ion permselectivity reduction due to ion flow interruption between the membrane and the electrode as the functional group structure became more bulky. QPVTD-1 membrane has excellent electrical resistance than AMX (Tokuyama Soda Co., Ltd., Japan) commercial membrane. These values are presented in Table 3.
25
20
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
-
Concentration(Cl ) (ppm)
(a)
15
10
5
0 0
50
100
150
200
250
Time (min) 25
(b) Concentration(NO3) (ppm)
Ion permeability of a single solution
-
The results of the ion permeability test of the synthesized QPVTD membrane in a chloride, sulfate and nitrate single solution are shown in Fig. 7. As shown in Fig. 7, sulfate ion permeability was lower than chloride ion and nitrate ion permeability. This result can be explained by the ionic radius presented in Table 4. A sulfate ion, which has a bigger radius, is harder to pass through a membrane compared to a chloride ion and a nitrate ion. Furthermore, ion permeability of the QVTD-3,4 membrane was lower than the QPVTD-1,2 membrane because steric hindrance affected ion movement due to increased bulkiness as the molecular weight of the functional agent introduced into a membrane was increased.
20
15
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
10
5
0 0
50
100
150
200
250
Time (min) 25
(c) QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
20
2-
Concentration(SO4 ) (ppm)
Ion permeability of a mixed solution The results of the nitrate permselectivity test for a nitrate, sulfate and chlorine mixed solution in the QPVTD membranes synthesized with different functional agents are shown in Fig. 8.
15
10
5
0 0
50
100
150
200
250
Time (min)
Fig. 7. Ion permeability of QPVTD membrane in the single solution. (a) Chloride ion solution, (b) nitrate ion solution, (c) sulfate ion solution.
The result of a nitrate ion permselectivity test for a mixed solution in the membrane synthesized using 1-bromobutane as a functional agent is shown in Fig. 8. As shown in Fig. 8, the ion permeability of the membrane synthesized using 1-bromobutane as a functional agent increased over time. In the result of the 4-h Table 4 Gibbs hydration energy and Ionic radius of anion [26,27].
Fig. 6. Electrical resistance and conductivity of QPVTD membranes.
Anion species
DGh(kJ/mol)
Ionic radius (Å)
NO3 Cl SO42
270 317 1000
1.89 1.81 2.40
C.-M. Oh et al. / Journal of Environmental Chemical Engineering 2 (2014) 2162–2169
16
16 -
-
Cl NO3
14
2-
SO4
12 10 8 6
14
Cl NO3
12
SO4
2-
Concentration (ppm)
Concentration (ppm)
2167
10 8 6
4
4
2
2
0
0 0
50
100
150
200
250
0
50
100
Time (min)
150
200
250
Time (min)
Fig. 8. Nitrate ion permselectivity of QPVTD-1 membrane in the mixture.
Fig. 10. Nitrate ion permselectivity of QPVTD-3 membrane in the mixture.
permselectivity test for a 20-ppm mixed solution with equal concentrations of NO3, Cl, SO42, the ion permeability of nitrate, chlorine and sulfate ions was 11.2, 10.4 and 9.5 ppm, respectively; therefore, nitrate ion permselectivity was higher than other competitive ions. The result of the nitrate ion permselectivity test under the same conditions in a mixed solution of the QPVTD-2 membrane using 1-bromo-2-methylpropane as a functional agent, which has the same molecular weight as 1-bromobutane but a different structure, is shown in Fig. 9. In the result of the 4-h permselectivity test for a 20-ppm mixed solution with equal concentrations of NO3, Cl, SO42, the ion permeability of nitrate, chlorine and sulfate ions was 10.0, 8.8 and 7.9 ppm, respectively; therefore, the nitrate removal rate was lower than that of the QPVTD-1 membrane. Thus, nitrate permselectivity decreased because the molecular structure of the functional group introduced into the membrane changed; therefore, a channel where nitrate ions passed became narrow, and less ions could pass through. However, the nitrate removal rate was higher than that of other ions, confirming nitrate ion permselectivity of the QPVTD-2 membrane. The result of the nitrate ion permselectivity test of the QPVTD3 membrane with a 1-bromo-3-methylbutane functional group introduced is shown in Fig. 10. As shown in Fig. 10, nitrate, chlorine
and sulfate ion permeability of the QPVTD-3 membrane with 1bromo-3-methylbutane introduced were 8.9, 7.5 and 6.5 ppm, respectively; therefore, nitrate ion permeability was lower than that of QPVTD-1,2 because of steric hindrance, likely due to increased bulkiness as the chain length of the functional agent increased. Furthermore, the result of the nitrate ion permselectivity test for the QPVTD-4 membrane using 1-bromopentane as a functional agent is shown in Fig. 11. Ion permeability of nitrate, chlorine and sulfate ions was 7.4, 5.0 and 4.8 ppm, respectively; therefore, the removal rate of QPVTD-4 was the lowest. It was determined that the anion permeability of a mixed solution became lower because hydrophobicity increased due to steric hindrance as bulkiness increased. The anion removal rates of the synthesized QPVTD membranes are presented in Table 5. It was concluded that nitrate ion permselectivity of the QPVTD-1 membrane, whose chain length was short and molecular structure was simple, was most appropriate among mixed solutions based on the results of nitrate ion permselectivity tests of the membranes with different functional agents introduced and with different molecular structures and chain lengths, as shown in Figs. 8–11.
16
16 -
14
2-
SO4
12
Concentration (ppm)
Concentration (ppm)
-
Cl NO3
10 8 6
14
Cl NO3
12
SO4
2-
10 8 6
4
4
2
2
0
0 0
50
100
150
200
250
Time (min) Fig. 9. Nitrate ion permselectivity of QPVTD-2 membrane in the mixture.
0
50
100
150
200
250
Time (min) Fig. 11. Nitrate ion permselectivity of QPVTD-4 membrane in the mixture.
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Table 5 Anion removal rate of QPVTD membranes. Code no.
Cl (%)
NO3 (%)
SO4 (%)
QPVTD-1 QPVTD-2 QPVTD-3 QPVTD-4
52.43 43.97 37.72 30.15
56.20 50.13 44.73 36.74
47.42 39.47 32.45 24.35
Conclusions
Ion-selective constant of QPVTD membranes The ion-selective constant of a QPVTD membrane is an important factor for permselectivity. Therefore, in this study, the NO ion selectivity coefficient (ki 3 ) of the synthesized membranes with different functional agents was calculated to identify the effect according to the following formula [23]. The result is shown in Fig. 12. NO kCl 3 NO
kSO43
C NO3 =C Cl ¼ C RNO3 =C RCl C NO3 =C SO4 ¼ C RNO3 =C RSO4
where C NO3 ,C Cl , C SO4 are the concentration of nitrate ions, sulfate ions and chloride ions removed, respectively, and C RNO3 , C RCl , C RSO4 are solution concentrations before removing nitrate ions, sulfate ions and chloride ions, respectively. As the selectivity coefficient increases, membrane affinity for the ion being removed is higher than for competing ions. As shown in Fig. 12, the chloride ion and nitrate ion selectivity coefficient, NO NO kCl 3 , and the sulfate ion and nitrate ion selectivity coefficient, kSO43 , NO3 tended to increase from QPVTD-1 to QPVTD-4. kCl was higher NO than kSO43 , confirming that nitrate permselectivity for a sulfate ion rather than a chloride ion in a mixed solution was higher. This conclusion is confirmed by the results in Figs. 8–11. Permselectivity increased as the ion selectivity coefficient increased, which can be explained by the Gibbs hydration energy, which resulted in ion hydrophilicity. The Gibbs hydration energy (DGh) of each ion is presented in Table 4. The NO3 ion, which has the lowest energy among these three ions, is relatively hydrophobic, indicating that NO3 ions are excluded as the hydrophobicity of a functional group increases. Therefore, if a functional group of a synthesized membrane is hydrophobic, the permselectivity of a hydrophilic ion decreases and permselectivity of NO3, a hydrophobic ion, increases. Based on the results reported by Guter, the nitrate ion permselectivity of a membrane increased as the number of carbon
NO3
1.5
k Cl
NO
k SO3 4
A
Selectivity (kB)
1.4
1.3
1.2
1.1
QPVTD-1
QPVTD-2
QPVTD-3
Fig. 12. Ion selectivity of QPVTD membrane.
atoms combined with quaternized amine increased [24]. In addition, Clifford and Webber reported that nitrate permselectivity increased as the nonpolarity and hydrophobicity of a functional group increased [25]. As presented in Table 5, the results presented herein agree well with the results from these other studies. Therefore, it was confirmed that the synthesized QPVTD membrane exhibited nitrate permselectivity.
QPVTD-4
This study addressed the synthesis of a partially fluorinated anion exchange membrane and its nitrate permselectivity properties depending upon the properties of added functional agents to synthesize a nitrate ion permselective membrane. The conclusions are as follows: 1 The number-average molecular weight (Mn) of a partially
2
3
4
5
fluorinated PVTD copolymer was 46,000, and the weightaverage molecular weight (Mw) was 59,000. Molecular weight distribution (PDI) was 1.29, indicating that the physical properties of the membrane were equal and, therefore, great. Membrane water uptake decreased from 60.8% to 30.9%, and the swelling rate decreased from 44% to 23.5% depending upon the type of functional group ligand used. Membrane electrical resistance for each functional agent type was 3.7 V cm1, 5.8 V cm1, 6.7 V cm1 and 8.0 V cm1; therefore, electrical resistance increases depending on the bulkiness of a ligand compound, a functional agent. As membrane bulkiness increases, the nitrate selectivity coefficient increases, thus the nitrate ion permselectivity compared to that of chloride and sulfate ions also increased. Nitrate permselectivity of QPVDT membranes was highest for the QPVTD-1 membrane, whose chain length was short and molecular structure was simple. In addition, the nitrate and sulfate ion selectivity coefficient was higher than the chloride and nitrate ion selectivity coefficient. Based upon this result, it was concluded that nitrate ion permselectivity is great among mixed solutions of QPVTD membranes. We can prepare nitrate removal membrane easily than commercial membrane (e.g., AMX; Tokuyama Soda).
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