Synthesis and properties of an anion-exchange membrane based on vinylbenzyl chloride–styrene–ethyl methacrylate copolymers

Journal of Membrane Science 423–424 (2012) 293–301

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Synthesis and properties of an anion-exchange membrane based on vinylbenzyl chloride–styrene–ethyl methacrylate copolymers Jin Sun Koo, Noh-Seok Kwak, Taek Sung Hwang n Department of Applied Chemistry & Biological Engineering, Chungnam National University, 79 Daehangno, Yuseong-gu, Daejeon 305-764, Republic of Korea

a r t i c l e i n f o

abstract

Article history: Received 5 June 2012 Received in revised form 30 July 2012 Accepted 12 August 2012 Available online 23 August 2012

Novel synthetic ion-exchange membranes composed of 4-vinylbenzyl chloride, styrene, and ethylmethacrylate were synthesized by radical polymerization using various monomer ratios. Copolymers with the desired molecular weights were successfully synthesized (Mn 4 39.5  103 g/mol, Mw 4 52.8  103 g/mol). After the radical polymerization, the ion-exchange membranes were prepared through amination and heat curing. The chemical structures of the membranes were characterized using Fourier-transform infrared spectroscopy and 1H and 13C nuclear magnetic resonance spectroscopy. The thermal, electrical, and other basic properties of the membranes, including water uptake and ion-exchange capacities, were measured. The glass temperature values of the polymers ranged from 67.5 to 81.2 1C. The nitrate permeation properties were also investigated by applying the membranes in a capacitive deionization system. Compared to conventional membranes, the nitrate concentrations in the cyclical charge and discharge currents indicated that the synthesized membranes were more efficient. & 2012 Elsevier B.V. All rights reserved.

Keywords: Radical polymerization Electrical properties Nitrate Anion-exchange membrane Capacitive deionization

1. Introduction Ion-exchange membranes have been used extensively in industrial applications because of their novel selectivities for specific ions and their various applications, including the recovery of the valuable metals in effluent streams from diffusion dialysis (DD), fuel cells, water treatment, such as the desalination of seawater, capacitive deionization processes, the production of ultra-pure water, and various other purposes [1–8]. Membrane capacitive deionization (MCDI) [9–11] is a water treatment process in which an electrical potential difference is generated between two porous electrodes placed on either side of an ion-exchange membrane. MCDI is an improvement on classical CDI to increase ion removal [12–14]. In MCDI, a spacer compartment separates the two oppositely charged membrane-electrode assemblies. Ions are removed from the aqueous solution that flows through the spacer compartments and are stored on the internal surfaces of the porous electrodes, resulting in an effluent product stream with a reduced ion concentration. Ions can be released back into the solution by reducing or reversing the applied voltage to generate a product stream with a high ion concentration.

n

Corresponding author. Tel.: þ82 42 821 5687. E-mail addresses: [email protected], [email protected], [email protected] (T.S. Hwang). 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.08.024

The removal of ions from water can be achieved using the MCDI process. Currently, there are several harmful ions that are potentially present in drinking water. The nitrate ion, in particular, is one of the most toxic ions to infants, adults, and marine life. Infants are especially vulnerable to methemoglobinemia, or blue baby syndrome, because nitrate metabolizing triglycerides are present at higher concentrations in the body than they are at other stages of development [15–22]. In recent years, the concentrations of nitrate ions in groundwater have been rapidly increasing in Japan, Europe, and Korea due to the massive use of artificial fertilizers. Although the nitrate concentration of drinking water is less than 20 ppm in most of the country, the concentration of nitrate ions in groundwater exceeds 50 ppm in certain locations. To address this problem, numerous methods for removing nitrate ions from groundwater have been suggested and employed [23–29]. Although there are various other anions that can be removed using anion-exchange membranes, nitrate and sulfate ions are the most abundant ions in groundwater, and therefore, these are appropriate representative compounds for the study of anion removal. Because concentrations of sulfate ions are much higher than concentrations of nitrate ions in groundwater, and normal ion-exchange membranes have good selectivities for sulfate ions, the efficiency of nitrate removal using the ion-exchange method has been substandard [30,31]. To improve the selectivity of anion-exchange membranes for nitrate, novel copolymer ion-exchange membranes were synthesized from monomers using 4-vinylbenzyl chloride (VBC), styrene

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obtained, and the solution was then purged with nitrogen for at least 10 min. The mixture was reacted at 75 1C for 12 h to complete the radical polymerization. Fig. 1 presents the schematic diagram of the synthetic procedure for producing the VBC/St/EMA copolymer. To introduce tripropylamine into the synthesized VBC/St/EMA polymers, amination reactions were performed using various reaction times (1, 3, 5, 7, and 9 h). The VBC/St/EMA copolymers were put into a 1 L four-necked flask fitted with a reflux condenser and heated to 50 1C for a fixed amount of time (1, 3, 5, 7, and 9 h), after which 50 mL of TPA and 100 mL of acetone were added through a dropping funnel. After the amination reactions, the unreacted materials and solvent were removed through a quenching method using acetone. To remove any impurities, the solutions were placed in a 40 1C vacuum oven for 3 h until the polymer was dry.

(St), and ethylmethacrylate (EMA). An amination reaction was conducted using tripropylamine as an amination agent. Sulfate ion permeability is hindered by the stereostructure of tripropylamine, which has bulkier alkyl groups than trimethylamine or triethylamine, thus improving the selectivity for nitrate. The chemical structure and basic properties of the membrane were investigated and compared with those of AMX, a commercial membrane from Tokuyama Soda. The nitrate permeation properties of the membrane were also confirmed using the CDI process.

2. Experimental 2.1. Materials Ethyl methacrylate (EMA; 99%), 4-vinylbenzyl chloride (VBC; 90%), and styrene (St; 99%) were purchased from Sigma Aldrich (New York, USA) and used without further purification. Benzoyl peroxide (BPO; 75%) was obtained from Lancaster Synthesis Ltd. (Morecambe, England). Tripropylamine (TPA; 98%) and N,Ndimethylformamide (DMF) were obtained from Sigma Aldrich (New York, USA) and used as an amination agent and a solvent, respectively, without further purification.

2.3. Preparation of ion-exchange membranes To prepare the aminated ion-exchange membranes, 60 wt% of the above product was diluted with dimethylformamide (DMF) and cast at a thickness of 100 mm on a glass plate using a doctor blade. After rinsing, the anion-exchange membranes were heated from 20 to 130 1C by increasing the temperature by 10 1C every hour. To prepare the anion-exchange membrane in the OH  form, the membranes were immersed in 1 M KOH for at least 24 h and rinsed with distilled water until the rinse water had a neutral pH value.

2.2. Synthesis and functionalization of VBC/St/EMA copolymers To synthesize the tertiary copolymers, mixtures of the monomers VBC, EMA, and styrene were prepared as shown in Table 1. The monomers and an initiator were added to a 500 mL three-necked flask fitted with a reflux condenser and a mechanical stirrer. The mixture was stirred at 130 rpm until a homogeneous solution was

2.4. Membrane characterization 2.4.1. Structural analysis A structural analysis was conducted based on 1H NMR and 13C NMR spectra obtained with a 400 MHz FT-NMR spectrometer (NMR, Jeol JNM-AL 400 NMR spectrometer) using CDCl3 as a solvent and tetramethylsilane (TMS) as the internal reference. The chemical structures of the ion-exchange membranes were also confirmed by Fourier transform infrared spectroscopy (FT-IR, Shimatzu Model IRPrestige-21 ATR). The sample membranes were prepared with a size of 1  1 cm, and infrared spectra were recorded in the transmittance mode over a range of 4000 to 600 cm  1. The resolution and the number of cycles in each of the spectra were 4 cm  1 and 20, respectively.

Table 1 Conditions for the synthesis of the 4-vinylbenzyl chloride/styrene/ethylmethacrylate copolymers. Code no.

VSE-1 VSE-2 VSE-3

Molar ratio St

VBC

EMA

0.45 0.45 0.45

0.10 0.20 0.30

0.45 0.35 0.25

Initiator (g)

Reaction time (h)

Temp. (1C)

0.159 0.161 0.163

12 12 12

75 75 75

l

Radical Polymerization O

O

m O

n

O

BPO, 75°C, 12h

CH2Cl

CH2Cl

l Amination

m O

n

O

TPA, 65°C

H7 C3

+

N H 7C3

Cl

-

C 3H7

Fig. 1. Scheme for the synthesis of the 4-vinylbenzyl chloride/styrene/ethylmethacrylate anion-exchange membranes.

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295

2.4.2. Molecular weight of the copolymer To measure the molecular weights and polydispersities of the synthesized copolymers, gel permeation chromatography (GPC; waters 2690) was used. The columns used were m-styragel HR-1, HR-2, and HR-3 (Waters), and polystyrene was used as the standard. Tetrahydrofuran was used as a solvent, and the flow rate was 1 mL/min.

were then inserted into the platinum clip cell, and a current was applied. The electrolyte resistance was measured after removing the membrane. The temperature was maintained at 20–25 1C while the electrical properties were being measured. The electrical resistance was calculated according to the following equations [33,34]:

2.4.3. Thermal properties The thermal properties of the synthesized copolymers were investigated using a differential scanning calorimeter (DSC; TA Instruments DSC Q1000). The copolymers were heated from 20 1C to 250 1C and then cooled at a rate of 2 1C/min. Subsequently, the heat flow was measured while the copolymers were heated at a rate of 10 1C/min.

where R1 is the resistance of the membrane and the electrolyte, R2 is the resistance of the electrolyte alone, and A is the effective membrane area. In addition, the membrane conductivity was determined according to the following equation:   S L ¼ ð4Þ Conductivity cm RA

2.4.4. Water uptake The water uptake of the membranes was determined by weighing both vacuum-dried membranes and membranes that were fully equilibrated with water. The membranes were first weighed under wet conditions after being equilibrated in distilled water for 24 h at room temperature. The surfaces of the membranes were then carefully wiped with filter paper (or wiping paper), and the membranes were immediately weighed. The samples were then dried in a vacuum oven at 50 1C for more than 2 days. The water uptake was calculated using the following equation [32]: Water uptakeð%Þ ¼

W wet W dry  100 W dry

Electrical resistance ðO cm2 Þ ¼ ðR1 R2 Þ  A

ð3Þ

where R is the obtained membrane resistance, and L is the membrane thickness. 2.4.7. Mechanical properties The mechanical properties of the membranes were investigated with universal testing machine (AGS-10kNGm, Shimadzu, UTM). The crosssectional area of the sample of known width and thickness was calculated. The films were then placed between the grips of the testing machine. The grip length was 5 cm and the speed of testing was set at the rate of 10 mm/min. Tensile strength was calculated using the equation:   N maxload Tensile strength ð5Þ ¼ crosssectional area mm2

ð1Þ

where Wwet is the mass of the water-swollen membranes, and Wdry is the mass of the dry membranes. 2.4.5. Ion-exchange capacity The ion-exchange capacities (IECs) of the membranes were measured by elemental analysis and titration. After rinsing with deionized water, membrane samples were soaked in a large volume of a 2 N KOH solution for more than 1 day to obtain the membrane in the OH þ form. The excess KOH in the membranes was removed by repeated washing with distilled water. The membranes in the OH þ form were then soaked in a 1 N NaCl solution and equilibrated for at least 1 day to replace the hydroxyl ions with chloride ions. The membranes were removed, and the NaCl solution was titrated with a 0.1 N AgNO3 standard solution using potassium-dichromate (5% in distilled water) as an indicator. A blank run was conducted in which the titration was performed against the indicator alone, and the samples were then measured using 5 replicates for each sample. After the titration, the samples were washed with deionized water, dried for 1 day at 50 1C under vacuum, and weighed. The IEC was calculated according to the following equation:   ðV NaCl  N NaCl ÞðV AgNO3  N AgNO3 Þ meq Ionexchange capacity ¼ Weight of sample g ð2Þ where NNaCl and N AgNO3 are the concentrations of the NaCl and AgNO3 solutions, respectively, and VNaCl and V AgNO3 are the volumes of the NaCl and AgNO3 solutions, respectively. 2.4.6. Electrical properties The electrical properties of the membranes were measured using a clip cell and an LCR meter (HIOKI 3522-50 LCR HiTESTER, Japan) with a frequency of 100 kHz at a constant voltage of 0.8 V. Prior to the measurement, membrane samples were immersed in a 0.5 M NaCl solution for 1 day at room temperature. The samples

2.5. SEM morphology analysis To investigate the morphology of the VSE ion-exchange membrane, a scanning electron microscope (JSM-5600, Jeol Co., Japan, SEM) was used. The samples were coated with a thin layer of gold by ion sputtering prior to microscopic examination. The provided images of the anion-exchange membranes were transferred to a computer and evaluated with an image analysis system. 2.6. Nitrate removal via the CDI process and unit cell test Compared with conventional membranes, the pattern of cyclic charge and discharge for the synthetic membranes indicated more efficient electrosorption and desorption properties. A 3-electrode potentio-galvanostat system (WEIS 500, Wona Tech Co. Ltd.) was used to evaluate the electrosorption and desorption of the VBC/St/EMA ion-exchange membranes using a CDI module, as illustrated in Fig. 2. CMX membrane (Tokuyama, Japan) was fixed between negative electrode and spacer. Carbon electrode (Siontech, Korea) was made of activated carbon P-60 (Kurary chemical, Japan) and PVDF (Shanghai chemical, China). The thickness of the carbon layer is 200 mm. In the CDI system, cathode was connected to working and sensor electrode, and anode was connected to reference and counter electrode. The electrolyte was 70 mg/L NO3 . To observe the adsorption and desorption of nitrate, the current was measured over time while a constant potential was applied. A potential of 1.5 V was applied for 3 min followed by a potential of 1.5 V. The change in current was measured, and the potentiostat cycle was repeated 10 times [35]. The total volume of the water treated over all of the cycles was 2500 mL. The concentration of nitrate ions was detected with nitrate ion selective electrode (Metrohm) by time. To investigate the nitrate selectivities of the membranes, batch form unit cells were prepared using 1 cm  1 cm ion-exchange membranes, and a clean graphite electrode was fixed to each side.

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attributed to the NH bond from the quaternary amine formed in the amination reaction. The intensity of these bands increased with increasing amounts of 4-vinylbenzyl chloride, which has a chloromethyl group that can react with the tertiary amine to produce the quaternary amine functional group. The peak at 1700 cm  1 is ascribed to the CQO vibrations of ethyl methacrylate, and the size of this peak decreased with increasing intensities of the peak characteristic of the NH bond. The bands at 1600 cm  1, 1511 cm  1, and 1446 cm  1 were assigned to the vibrations of the aromatic groups in 4-vinylbenzyl chloride and styrene. Successful polymerization and amination reactions were confirmed by the observation of all characteristic peaks for each of the monomers. 3.2. Molecular weight of the copolymer

Fig. 2. Schematic diagram of the capacitive deionization unit module cell.

On the positive side, 10 mg/L of NaCl was added as an electrolyte, whereas 50 mg/L each of NaNO3 and Na2SO4 were added to the negative side. A potential of 12 V was applied for 1 h, after which the nitrate concentration in the positive side was measured. The concentrations of nitrate and sulfate ions were investigated with ion chromatography measurement (881 Compact IC pro, 844 UV/ VIS Compact IC, Metrohm, IC) to evaluate the nitrate selectivity. The nitrate selectivity of the membrane was calculated using the following equation: S¼

C p ðNO3 Þ  C f ðSO4 Þ C f ðNO3 Þ  C p ðSO4 Þ

ð6Þ

where Cp and Cf are the permeation and feed concentrations, respectively.

3. Results and discussion 3.1. Structural analysis The synthesis of aminated anion-exchange membranes was successfully performed via the bulk polymerization of 4-vinylbenzyl chloride, styrene, and ethylmethacrylate followed by an amination reaction using tripropylamine. The chemical structure of the VBC/St/EMA copolymer was investigated using 13 C NMR and 1H NMR spectroscopy, as shown in Fig. 3(I) and (II), respectively. In the 13C NMR spectrum of the VBC/St/EMA copolymer (Fig. 3(I)), the peaks at 110–140 ppm are attributed to the aromatic C–H peaks of styrene and 4-vinylbenzyl chloride. Peak (a) at 46 ppm is the C–Cl peak of 4-vinylbenzyl chloride. Peaks (b), (c), and (d) are the CH3 and CH2 peaks of ethylmethacrylate at 12, 78, and 17 ppm, respectively. In Fig. 3(II), the aromatic peaks of styrene and 4-vinylbenzyl chloride occur at 5.2–7.5 ppm. Peak (a) at 4.6 ppm is attributed to the CH2 peak of 4-vinylbenzyl chloride. Peaks (b), (c), and (d) are the CH3 and CH2 peaks of ethylmethacrylate at 1.9, 4.2, and 1.2 ppm, respectively. The integrated values for peaks associated with 4-vinylbenzyl chloride changed according to its relative amounts used in the synthesis process, as did the characteristic peaks of styrene and ethyl methacrylate. Fig. 4 shows the FT-IR spectra of the VBC/St/EMA copolymers synthesized with various concentrations of the monomers. The broad bands at approximately 3500 cm  1 and 800 cm  1 were

The molecular weights and polydispersities of the VBC/St/EMA copolymers were investigated using gel permeation chromatography. Fig. 5 shows the chromatogram of the membranes at various mole ratios. As shown in Fig. 5, the peaks of mV value were shifted right side as increasing amount of VBC contents. From these data, molecular weights of the membrane were calculated. Copolymers with the desired molecular weights were successfully synthesized by radical polymerization (Mn439.5  103 g/mol, Mw452.8  103 g/mol). The number average molecular weights of VSE-1, VSE-2, and VSE-3 were 39.5  103, 46.3  103, and 69.1  103 g/mol, respectively, and the weight average molecular weights of VSE-1, VSE-2, and VSE-3 were 52.8  103, 60.4  103, and 83.0  103 g/mol, respectively. The polydispersities of VSE-1, VSE-2, and VSE-3 were 1.34, 1.31, and 1.20, respectively. The molecular weights of the copolymers increased with increasing amounts of 4-vinylbenzyl chloride. As the growth reaction progressed, the formation of long chains of the copolymer was aided by increasing amounts of activated radicals on the vinyl groups of VBC. 3.3. Thermal properties The thermal properties of the synthesized VSE polymers were determined by DSC and are displayed in Fig. 6. The glass temperature (Tg) values of the polymers ranged from 67.5 to 81.2 1C, and the values for VSE-1, VSE-2, and VSE-3 were 67.5, 74.1, and 81.2 1C, respectively. The Tg values increased with increasing amounts of VBC because VBC has the highest Tg value of the monomers used. The thermal properties of the aminated membranes were also investigated and were found to roughly correspond to those of other membranes. The remarkable difference of approximately 10 1C between the Tg values was attributed to the bulkiness of the amination agent tripropylamine. 3.4. Characterization of ion-exchange membrane properties The water uptake, IEC values, electrical properties, and tensile strength of the VBC/St/EMA ion-exchange membranes that were crosslinked using various amination times were measured, and the results are presented in Table 2. 3.4.1. Water uptake Water uptake is regarded as one of the essential properties for anion-exchange membrane applications and is considered to be related to the concentration of aminated groups. Fig. 7 shows the water uptake of the VBC/St/EMA anion-exchange membranes. As the amination time increased, the water uptake and hydrophilicity of the membranes both increased due to the larger numbers of aminated groups introduced into the membranes. The amounts

J. Sun Koo et al. / Journal of Membrane Science 423–424 (2012) 293–301

I

297

b

*

∗

n

l

m O O c

a d Cl

(iii)

(ii) Aromatics

c

(i) 160

140

120

100

d b

a

80

60

40

0 ppm

20

II

(iii) (ii) Aromatics

b

a c 8 13

6

4

2

d

(i ) 0 ppm

1

Fig. 3. C NMR spectra (I) and H NMR spectra (II) (400 MHz FT-NMR; CDCl3 as a solvent) of 4-vinylbenzyl chloride/styrene/ethylmethacrylate anion-exchange membranes with various molar ratios; (i) VSE-1, (ii) VSE-2, and (iii) VSE-3.

80 (a)

70

C=O

50

-NH

(b)

40 mV

Transmittance

60 -NH

(c)

30

(c)

20

(b) (a)

10 0 -10 4000

3500

3000

2500

2000

Wave number

1500

1000

(cm-1)

Fig. 4. FT-IR spectra of 4-vinylbenzyl chloride/styrene/ethylmethacrylate anionexchange membranes with various molar ratios; (a) VSE-1, (b) VSE-2, and (c) VSE-3.

-20 500

1000 Time (s)

1500

Fig. 5. GPC curves for 4-vinylbenzyl chloride/styrene/ethylmethacrylate anionexchange membranes with various molar ratios; (a) VSE-1, (b) VSE-2, and (c) VSE-3.

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using various amination times are presented in Fig. 8. The ionexchange capacities increased with increasing amination times due to the increased amine contents of the membranes. The IEC values for all of the membranes were in the range of 0.37–2.1 meq/g. The spacing between the polymer chains was different for each sample due to the varying amounts of amine groups in the polymers. IEC values increased with increasing amounts of VBC content and amination times, and the highest IEC value of 2.1 meq/g was exhibited by the membrane synthesized with a 0.3 M ratio of VBC and a 9 h amination time. As the ion-exchange capacity values increased, water uptake values increased due to higher hydrophilicities and ion selectivities caused by the quaternary amine groups.

-2.1 -2.2

Heat flow (mW)

-2.3 -2.4 -2.5

(a)

-2.6 -2.7

(b)

-2.8 -2.9 -3.0

(c)

-3.1 -3.2 50

60

70 80 Temperature (°C)

90

100

Fig. 6. DSC curves for 4-vinylbenzyl chloride/styrene/ethylmethacrylate anionexchange membranes with various molar ratios; (a) VSE-1, (b) VSE-2, and (c) VSE-3.

Table 2 Basic properties of the anion-exchange membrane.

4-vinylbenzyl

chloride/styrene/ethylmethacrylate

Conversion (%)

Water uptake (%)

Ion-exchange capacity (meq/g)

Electrical resistance (O cm2)

Tensile strength (MPa)

VSE-1 VSE-2 VSE-3 AMX

86.4 91.2 93.8 –

21.89 29.33 37.36 30.0

0.98 1.47 1.75 1.40

6.22 3.14 1.54 3.50

21.3843 18.4135 14.9513 15.1128

Ion exchange capacity (meq/g)

Code no.

60 50 Water uptake (%)

3.4.3. Electrical properties The electrical resistance and conductivity of an ion-exchange membrane are important properties for electro-separation processes, such as electro-dialysis, electro-deionization, continuous deionization, and capacitive deionization. Hydrophilic ionic groups enable the transport of protons and raise the conductivity of the membrane. Figs. 9 and 10 present the electrical resistance and conductivity values for the VBC/St/EMA anion-exchange membranes. The electrical resistance and conductivity values were largely dependent on the quaternary amine contents.

40 (c)

AMX

2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

(c) (b)

AMX

(a)

0

2

30 (b)

4 6 Amination time (hr)

8

10

Fig. 8. The effect of the amination time on the ion-exchange capacities of 4-vinylbenzyl chloride/styrene/ethylmethacrylate anion-exchange membranes with various molar ratios; (a) VSE-1, (b) VSE-2, and (c) VSE-3.

20 (a) 10 2

4 6 Amination time (hr)

8

10

Fig. 7. The effect of the amination time on the water uptake values of 4-vinylbenzyl chloride/styrene/ethylmethacrylate anion-exchange membranes with various molar ratios; (a) VSE-1, (b) VSE-2, and (c) VSE-3.

of styrene and ethylmethacrylate were important factors in lowering the extent of water uptake. Although styrene provides a rigid structure for the membrane, it also contributes to the brittleness of the material. Therefore, ethylmethacrylate was used because it acts as a softener.

Electrical resistance (ohm*cm2)

16

0

14

10

(b)

8 (c)

6 4

AMX 2 0

3.4.2. Ion-exchange capacity Ion-exchange capacity (IEC) provides an indication of the amine content of the membranes. High IEC values indicate that higher levels of amine groups are present [36]. The ion-exchange capacities of the VBC/St/EMA ion-exchange membranes synthesized

(a)

12

0

2

4 6 Amination time (hr)

8

10

Fig. 9. The effect of the amination time on the electrical resistances of 4-vinylbenzyl chloride/styrene/ethylmethacrylate anion-exchange membranes with various molar ratios; (a) VSE-1, (b) VSE-2, and (c) VSE-3.

J. Sun Koo et al. / Journal of Membrane Science 423–424 (2012) 293–301

A membrane with a low electrical resistance value is desirable because a potential difference is the driving force for mass transfer in ion-exchange membranes. The electrical resistance of the ion-exchange membrane should be at the minimum value that allows for sufficient selectivity. The electrical resistance values of the VBC/St/EMA anion-exchange membranes are presented in Fig. 9. Electrical resistance values were measured in a 0.5 M NaCl solution at room temperature. The minimum and maximum values were 0.603 and 12.29 O cm2, respectively. An optimum value of 0.603 O cm2 was achieved at a 0.3 M ratio of 4-vinylbenzyl chloride using a 9 h amination time. The electrical resistance decreased with increasing amination times and increasing amounts of 4-vinylbenzyl chloride because the ionic mobility and selectivity improve as the space between the polymer chains increases. These results were caused by changing the hydrophilicity of the membrane through varying the amount of 4-vinylbenzyl chloride that is used. The conductivities of the anion-exchange membranes with various concentrations of 4-vinylbenzyl chloride are shown in Fig. 10. The conductivities were in the range of 0.87 to 19.83 mS/cm at room temperature. Conductivities increased with increasing amination times and increasing 4-vinylbenzyl chloride contents because in the same manner as the electrical resistance, the ionic mobility and selectivity improve as the space between the polymer chains increases. The conductivity values for the membranes were directly related to their IEC values. Because the ionexchange capacities of the membranes increased with larger amounts of amine functional groups, the ion conductivity values of the membranes increased as well. The optimum conductivity value of 19.83 mS/cm was achieved at 0.3 M ratio of 4-vinylbenzyl chloride using 9 h amination time.

22

Conductivity (mS/cm)

20 18

(c)

16 14 12 10 8

(b)

6

AMX

4

(a)

2 0

0

2

4 6 Amination time (hr)

8

10

Fig. 10. The effect of the amination time on the conductivities of 4-vinylbenzyl chloride/styrene/ethylmethacrylate anion-exchange membranes with various molar ratios; (a) VSE-1, (b) VSE-2, and (c) VSE-3.

299

3.4.4. Mechanical properties The mechanical properties of an ion-exchange membrane are given in Table 2. From the results, the values of the tensile strength of the membrane were uniformed in principle. However it can be observed that increase in the VBC concentration in the membrane causes slightly increase in the tensile strength. This enhancement may be attributed to the chain movement. Amination causes the polymer matrix to expand since amination reagent, tripropylamine, is more bulky than chloride as expected. This expansion applies an increase in the free volume thereby increasing the movements in the polymer chain which makes the plastic material flexible and softer. This increase of chain movement means that the material changes from glassy state (hard and brittle) to rubbery state (flexible and soft) partially, thereby causing a decrease in the tensile strength at break [37]. 3.5. SEM morphology analysis For morphological evaluations, SEM images of the VSE anionexchange membranes prepared with various mole ratios were obtained; Fig. 11 shows micrographs of the surfaces of the membranes. Fig. 11(a)–(c) indicate the VSE membrane with various mole ratios. Although the surface of the membranes was almost uniform and smooth in principle, the membrane became slightly rougher than the membrane containing low contents of VBC. As shown in Fig. 11, the surface of the membrane was smooth with decreasing amount of VBC because the roughness of the membrane was slightly affected by amination reaction. 3.6. Nitrate removal via CDI process and unit cell test To investigate the permeation of nitrate, CDI cell and unit cell tests were conducted. Cyclic charge–discharge currents were measured to confirm adsorption and desorption at the electric layer. The results are shown in Fig. 12. The adsorption and desorption properties of nitrate ions on the synthesized membranes were investigated at a constant voltage of 1.5 V for 3 min and then at 1.5 V for 3 min to produce a charge– discharge system [35]. Because an electrical double layer was formed on the surface of the electrode to act as an electric condenser at 1.5 V, the nitrate ion concentrations decreased over time. The nitrate concentration increased rapidly during the desorption process, and the adsorption and desorption rates of the VSE-3 membrane were higher than those of the other membranes. The maximum adsorption and desorption capacities of each ionexchange membrane were all greater than 95%, as shown in Fig. 12. The nitrate selectivities of the membranes were measured using unit cells and calculated using Eq. (5). The results are presented in Fig. 13. The maximum and minimum selectivities were 2.59 and 1.08, respectively. The selectivities of the membranes increased with increasing amounts of VBC. When amination reaction was conducted with VBC and tripropylamine, the numerous positive charges

Fig. 11. SEM images for 4-vinylbenzyl chloride/styrene/ethylmethacrylate anion-exchange membranes with various molar ratios; (a) VSE-1, (b) VSE-2, and (c) VSE-3.

300

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500 Nitrate concentration (mg/L)

Nitrate concentration (mg/L)

500 400 300 200 100

400 300 200 100

0

0 0

500

1000

1500 Time (s)

2000

2500

3000

500

1000

1500 Time (s)

2000

2500

3000

0

500

1000

1500 Time (s)

2000

2500

3000

500 Nitrate concentration (mg/L)

Nitrate concentration (mg/L)

500

0

400 300 200 100

400 300 200 100

0

0 0

500

1000

1500 Time (s)

2000

2500

3000

Fig. 12. Cyclic nitrate ion permeation concentrations in a capacitive deionization system for anion-exchange membranes with various molar ratios synthesized with a 5 h amination time; (a) AMX, (b) VSE-1, (c) VSE-2, and (d) VSE-3.

4. Conclusion This study described the preparation of 4-vinylbenzyl chloride/ styrene/ethylmethacrylate anion-exchange membranes through

3.0 (c)

2.5

(b) Selectivity

were applied in ion-exchange membrane but its polymer chain also became bulky. Degree of amination increased with increasing amount of VBC contents. Therefore, as increasing VBC contents, polymer chains of ion-exchange membrane become more bulky and nitrate selectivities of the membrane also increases over sulfate ions. The selectivities also increased with increasing voltage except at 12 V. The selectivities at 12 V were lower than those at 9 V because all of the anions present were subject to the driving force under the influence of the excessive voltage. These results confirm that the membranes can be used for nitrate removal at potentials of 7 or 9 V. As expected, the selectivities of the membranes increased with increasing amination times. Compared with VSE membrane, AMX anion-exchange membrane shows different tendency. As increasing voltage, there is no significant change in the nitrate selectivities of the AMX membrane. However, slight decrease of the nitrate selectivity was confirmed. Since AMX membrane has low selectivities for nitrate ions, the higher voltage was applied, more anions including sulfate ions can easily pass through AMX membrane. From these results, we confirmed that VSE membrane is more suitable than AMX membrane for separation of nitrate ions.

2.0 (a)

1.5

(d) 1.0 0.5

2

4

6 8 Voltage (V)

10

12

Fig. 13. The effect of voltage on the selectivities of 4-vinylbenzyl chloride/styrene/ ethylmethacrylate anion-exchange membranes with various molar ratios at 3, 5, 7, 9 and 11 V; (a) VSE-1, (b) VSE-2, (c) VSE-3, and (d) AMX.

radical polymerization, amination, and heat curing. The chemical structures of the membranes synthesized with various molar ratios of monomers were characterized by FT-IR and NMR, and the successful polymerization and amination was confirmed.

J. Sun Koo et al. / Journal of Membrane Science 423–424 (2012) 293–301

The prepared anion-exchange membranes were investigated in terms of their Tg values, water uptake values, ion-exchange capacities, and electrical properties. The notable increase of approximately 10 1C in the Tg value was ascribed to the bulkiness of the amination agent tripropylamine. The water uptake values and ion-exchange capacities of the membranes increased with increasing amounts of VBC and increasing amination times due to the increase in hydrophilicity attributed to the amine functional groups. The minimum water uptake and maximum ion-exchange capacity and tensile strength values were 12.14%, 2.1 meq/g, and 21.3843 MPa, respectively. Electrical resistance values decreased with increasing amounts of VBC and increasing amination times, whereas conductivity values increased. According to basic properties, each optimum values of electrical resistance and conductivity of 3.136 O cm2 and 4.24 mS/cm, respectively were achieved at 0.2 M ratio of 4-vinylbenzyl chloride using 5 h amination time. The synthesized membrane is more economically effective and has properties that are competitive with a commercial membrane. The surface of the membranes was almost uniform and smooth in principle but the membrane became slightly rougher than the membrane containing low contents of VBC. The nitrate adsorption and desorption properties observed using electrodes confirm that the VBC/St/EMA anion-exchange membranes can be used for nitrate removal processes. Based on these results, we confirmed that the VBC/St/EMA anion-exchange membrane can be applied to nitrate removal in groundwater using a CDI process.

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