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Comparative Efficacy Study of Different Types of Ion Exchange Membranes for Production of Ultrapure Water via Electrodeionization vaibhavee bhadja, Babubhai S Makwana, Subarna Maiti, Saroj Sharma, and Uma Chatterjee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03043 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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Comparative Efficacy Study of Different Types of Ion Exchange Membranes for Production of Ultrapure Water via Electrodeionization a,c

Vaibhavee Bhadja, aBabubhai S Makwana, bSubarna Maiti, Chatterjee*

a

a,c

Saroj Sharma,* and

a,c

Uma

Electromembrane Processes Division, bProcess Design and Engineering Division, cAcSIR-

Central Salt and Marine Chemical Research Institute, Bhavnagar, Gujarat, 364002, India ABSTRACT: High purity ultrapure water is required in electronics and pharmaceutical industry. Herein, we report production of high purity ultrapure water (18.5 L/h) by Electrodeionization (EDI) technique in a laboratory scale EDI unit (effective membrane area 30 cm X 5 cm) using polyethylene interpolymer based ion-exchange membranes. The ultrapure water also prepared by EDI process in the same EDI unit using two different types of commercial membranes (Ionsep and Fujifilm type II). The resistance of ultrapure water was 18.2 MΩ cm with the interpolymer based membranes at 25 volt/cell pair applied potential, whereas

with commercial Ionsep and Fujifilm type II membranes the final

resistance of ultrapure water was 15 MΩ cm and 17.1 MΩ cm respectively at 30 volt/cell pair applied potential. The power consumption (W) and current efficiency (CE) values were 0.324 KWhKg-1 and 58.86% respectively for interpolymer based ion-exchange membranes, whereas for Ionsep and Fujifilm type II membranes, W and CE values were 0.658 KWhKg-1 0.43 KWhKg-1, and 34.83%, 53.88% respectively. The better quality of ultrapure water produced with interpolymer membrane and the better efficacy of the EDI process with the interpolymer membrane compared to other two membranes is ascribed to the lower water uptake, high transport number and lower ion back diffusion through the former membrane.

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1.

INTRODUCTION

Ultrapure water is widely used in semiconductor manufacturing industry.1-3 Large volume of ultrapure water containing minimum total dissolved solid (TDS) along with high resistance is required for the preparation of electronic devices of computer.4,5 Earlier, ultrapure water was prepared by passing water through a column filled with mixed bed ion-exchange resins. The main drawback of this process is that after certain time interval the resins get exhausted. The regeneration of resin requires post treatment by chemical followed by washing with large volume of water. This process was not suitable for removing trace amount of weak acid and base present in the water. Another approach for the production of ultrapure water is the combination of reverse osmosis (RO) and mixed bed ion-exchange resins. A polishing unit comprised of mixed bed resin is attached at the end of the RO permeate for the removal of trace amount of salts. Electrodialysis (ED) is an electrodriven separation process. This process has been widely applied for brackish water desalination as well as concentration of value added chemicals present in effluent.6-9 ED process is not an economical when the concentration of ions is low. The electrical resistance of diluate compartment becomes very high at low ion concentration. Therefore current density of membranes becomes very less. Hence, in order to run ED at low ion concentration large membrane area is required to achieve required current density. Therefore, operation cost increases. This difficulty led to the development of a promising alternative process, electrodeionization (EDI)10,11 which is a process based on the combination of ED and ion exchange resins. Cation exchange membrane (CEM), anion exchange membrane (AEM) and mixed bed resins (cation and anion exchange resins together) are used in EDI unit. Similar to ED, EDI cell also consists of three compartments e.g. diluate, concentrate, and electrode wash compartment. Mostly, in EDI unit the electrode wash compartment is combined with concentrate compartment. Under the influence of an applied potential, ions from the diluate compartment exchanged with mixed-bed resins and simultaneously transferred across the membranes (AEM and CEM). The ion-exchange resin helps to move the ions in a faster rate. Therefore, the resistance of diluate compartment decreases. The key step in EDI process is the transfer of ions from diluate compartment to the surface of the ion exchange resins. The ion exchange resins are continuously regenerated electrochemically by H+ ions and OH– ions which are produced by water splitting at an applied field above the limiting current density of CEM and AEM.12 The performance of EDI depends on the properties of CEM and AEM. The 2 ACS Paragon Plus Environment


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AEM and CEM should possess low to moderate water uptake, high ionic conductivity and high transport number. A voltage gradient is applied as a driving force for the separation of ions. EDI is used for the removal of Na+, K+, SO4 2−, Cl− and NO3− ions from water.13,14 EDI has also been used for different applications such as removal of boron and silica from the RO permeate of geothermal water,15 removal of Chromium,16 Cobalt,17,18 Copper,19 and Nickel20 from waste water. EDI is mainly used for ultrapure water production.21,22 The influence of applied voltage and feed concentration on the performance of EDI was also reported.23,24 Market cost of commercially available EDI unit (Milli-Q), is around 8500 USD. The flow and final resistance of produced ultrapure water by this EDI unit is 10L/h and 18.2 MΩ cm respectively. Preparation of ultrapure water using bipolar membrane by EDI process was reported.25,26 Preparation of demineralized water from geothermal water by EDI process was reported.27 Geothermal water was first passed through RO module and the purified water having TDS 7.5 mg/L was passed through EDI unit, where ion exchange resins were filled in the layered bed configuration. The diluted compartment was filled with ion-exchange resins. The membrane used in the EDI unit was commercial Selemion membranes.27 Preparation of ultrapure water from geothermal water by EDI technology using mixed bed resin in both diluted and concentrated compartment was also reported.28 The dependence of the quality of ultrapure water on the membrane thickness as well as on applied voltage was reported.28 Preparation of polyethylene-polystyrene interpolymer based CEM and polyethylene poly(p-methylstyrene) interpolymer based AEM for water desalination

has been

reported.29,30 Both the membranes exhibited moderate water uptake, high ion exchange capacity, high transport number and high ionic conductivity. The mechanical properties of the prepared membranes are sufficient for ED applications. Although, the prepared CEM and AEM showed excellent water desalination performance by ED which is comparable with other reported commercial membranes, the membranes were not used for ultrapure water production by EDI process. Herein, our main objective is to produce high purity ultrapure water by EDI process using interpolymer based ion exchange membranes. The process parameters such as flow rate, applied voltage have been optimised. The purity of ultrapure water in terms of resistance, pH and TDS value has been compared with the results obtained by replacing the interpolymer based membranes with two different types of commercial membranes (Ionsep and Fujifilm Type II) under similar experimental conditions. The ionic resistance of 3 ACS Paragon Plus Environment

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ultrapure water obtained with interpolymer based membranes is 18.2 MΩ cm at 25 volt/cell pair applied potential. In case of Ionsep membrane the ionic resistance of ultrapure water is 15 MΩ cm at 30 volt/cell pair applied potential. The ionic resistance of ultrapure water obtained with Fujifilm type II membranes is 17.1 MΩ cm at 30 volt/cell pair applied potential. Therefore it is observed that the best purity ultrapure water is produced by interpolymer based ion exchange membranes. The production cost of interpolymer membranes per 5 Kg batch is around 30-35 USD for 1 m2 of AEM and 10-15 USD for 1 m2 CEM which is lower than the market cost of commercial ion exchange membranes. Therefore overall cost of EDI unit using our interpolymer based ion exchange membrane will be lower than the market cost of commercial EDI unit.

2. EXPERIMENTAL SECTION 2.1. Materials. IONSEP-HC-C and IONSEP-HC-A are commercial AEM and CEM were purchased from IONSEP, Hanzhou, China. Commercial AEM-II and CEM-II were purchased from Fujifilm Membrane Technology, Netherland. High density polyethylene (F-46) and linear low density polyethylene (F-19) were purchased from Reliance Industries, India. Styrene, benzoyl peroxide, toluene, xylene, sodium chloride and methanol all AR grade chemicals were purchased from S.D. Fine Chemicals India. 4-methyl styrene were purchased from Aldrich Chemicals. AC- DC rectifier was supplied by M/s. Aplab India. Ltd., Mumbai, India. The cation exchange resin (CER), cross-linked sulfonated polystyrene bead with ion exchange capacity (IEC) 1.9 meq/ml and anion exchange resin (AER), cross-linked quaternary ammonium polystyrene beads with IEC 1.0 meq/ml were purchased from Ion Exchange India. The CER to AER ratio was 2:1 (w/w) in EDI unit. 2.2. Preparation of Indigenous CEM and AEM. The interpolymer based ion-exchange membranes used in this work were prepared by reported procedure.29,30 The CEM was prepared by sulfonation of polyethylene-polystyrene interpolymer film by chlorosulfonic acid and is designated as CEM-1.29 AEM-1 is prepared from polyethylene-poly4-methyl styrene based interpolymer film followed by benzylic bromination using solar concentrator as an energy source as reported earlier.30

The membranes were treated with 1 M HCl and 1 M

NaOH followed by washing with distilled water and equilibrated in 1 M NaCl solution before use in EDI unit.



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2.3. Characterizations. 2.3.1. FT-IR Spectra of CEM-1 and AEM-1. FT-IR spectra of the CEM-1, AEM-1 were recorded in a Perkin Elmer FTIR Instrument at room temperature. Before analysis the pellets were made by mixing samples with KBr. 2.3.2. Water Uptake of Different Membranes. The water uptake of all the membranes were determined by keeping a known weight of dry samples (md) in distilled water for 24 h. The weight of wet sample (mw) was measured by removing the surface water with tissue paper. The water uptake was determined using the following equation % water uptake = (mw-md/md) X 100

(1)

2.3.3. IEC, Ionic Conductivity and Transport Number of the Membranes. The IEC of all the membranes was determined by the classical titration method as reported earlier.7-9,29,30 The membrane resistance (Rm) and membrane conductivity (Km) of all the membranes were determined in an in house made cell, composed of two black graphite electrodes fixed on acrylic plates as reported earlier.7-9,30 The Rm and Km was estimated from the following equation: Km= ∆x/ARm

(2)

where ∆x is the thickness of the membrane, A is the effective membrane area. The membrane potential (Em) of all the membranes was measured in a two compartment cell made of acrylic sheet of effective membrane area 10.0 cm2 using reported procedure.7-9,30 The concentrations of NaCl in the two compartments were 0.1 M and 0.01 M respectively. The transport number (t) was calculated from the following equation:7-9,30

where R is the gas constant, F is the Faraday constant, T is the absolute temperature (298 0K), C1 and C2 are the concentration of NaCl solutions in the testing cell. 2.3.4. Description of EDI Unit and Production of Ultrapure Water in the EDI Unit. The ultrapure water was prepared in an in-house fabricated EDI stack. Rigid polyvinylchloride sheet (PVC) of thickness 10 mm was used as electrode housings. Platinum coated expanded metal of titanium was used as anode and a 2 mm thick stainless steel (SS-316) was used as cathode. PVC sheets of thickness 4-5 mm were used as spacer gaskets. The holes and slits on the membrane and gaskets were done for proper flow arrangement. The EDI unit was 5 ACS Paragon Plus Environment

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operated in continuous mode. Concentrate compartment was attached with the electrode wash compartment. Hence, no extra electrolyte solution was required to flush the electrodes. Electrical potential was applied in parallel mode to the compartments and water was passed in series mode throughout the compartments. The final flow of product water was 18.5 L/h in diluate compartment whereas in concentrate compartment the flow was 7.5 L/h. Figure 1 shows the membrane arrangement in the EDI unit. Five pieces of CEM-1 and AEM-1 of effective membrane area (30 cm X 5 cm) were used in the stack. Mixed bed ion-exchange resin beads were placed in between the CEM and AEM. The RO permeate water of TDS 35 mg/L with total organic carbon content (TOC) 2 ppb was fed in the EDI unit and the resistance of ultrapure water were measured. Similar experiments were carried out using commercial Ionsep and Fujifilm type II membranes.

Figure 1. Schematic diagram of membrane arrangement in EDI unit 2.3.5. Determination of TDS, Resistance, Conductivity and TOC of Ultrapure Water. The TDS, resistance and conductivity of prepared ultrapure water was measured in an EDI unit attached online to resistivity controller, TDS meter and conductivity meter all are purchased from HANNA Instruments. TOC was determined in an A10 TOC monitor purchased from Millipore. The values of different parameters were noted time to time.



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2.3.6. Power Consumption and Current Efficiency during EDI Process. Power consumption (W in KWh/Kg) during EDI process is defined as the amount of energy needed to transport one Kg of NaCl from diluate compartment to concentrate compartment. W has been calculated using the following equation:31

W= / (4) where V is the applied voltage; I is the current (amp); dt is the time (h) allowed for the EDI process; and w is the weight of salt (Kg) removed. The current efficiency (CE) defined as the fraction of the current transported by the specific ion and has been calculated using the following equation:31

where F is the Faraday constant (26.8 amp.h mole-1); M is the molecular weight of NaCl (58.5 g.mol-1), N is the number of cell pairs used in the ED unit (5 pairs), Q is the amount of electricity passed throughout the system (amp.h). 3.

RESULTS AND DISCUSSION

3.1.

Preparation and Characterization of Ion-Exchange Membranes. Polyethylene-

polystyrene and polyethylene-polypmethylstyrene interpolymer pellets were prepared by free radical polymerization of styrene and divinyl benzene and 4- methyl styrene and divinyl benzene in presence of polyethylene.29,30 The pellets were converted to interpolymer film by blow film extrusion process. The interpolymer film was converted to CEM-1 by treatment with chlorosulfonic acid.29 AEM-1 was prepared by benzylic bromination of polyethylenepolypmethylstyrene interpolymer film in presence of NBS and BPO, using concentrated solar radiation as energy source followed by quaternization with trimethylamine.30 Figure 2 shows the FT-IR spectra of CEM-1 and AEM-1 respectively. In the spectrum of CEM-1, the absorption bands appeared at 3406 cm-1 and 2964 cm-1 is assigned to the stretching vibrations of OH of SO3H, C-H bonds due to aromatic protons of PStSO3H part. The bands appeared at 1244–1177 cm-1 and 1043 cm-1 are assigned to the stretching vibrations for S=O and O=S=O, respectively.7 In the spectrum of AEM-1 absorption band appeared at 1642 cm-1 and 1458 cm-1 were assigned to the aromatic ring stretching vibration. The absorption band appeared at 3385 cm-1 was due to quaternary ammonium moiety.8,9,30


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Figure 2. FT-IR spectra of CEM-1 and AEM-1. The acid-base stability of the CEM-1 and AEM-1 is excellent due to chemical inertness of polyethylene backbone. The membranes have been used for water desalination via ED.29,30 3.2.

Physical and Electro-Chemical Properties of Ion Exchange Membranes and

Resins. Table 1 shows physical characterization e.g. water uptake and electrochemical characterizations e.g. IEC, Km and t of different ion exchange membranes. These properties evaluate the suitability of an ion exchange membrane for ED applications. A good ion exchange membrane should possess moderate water uptake, high IEC, Km and t values for use in electro driven separation application. CEM-1 and AEM-1 exhibited 29% and 20% water uptake respectively. Commercial membranes viz. IONSEP-HC-C, IONSEP-HC-A, CEM-II, and AEM-II show 40%, 40%, 30% and 25% water uptake respectively. An increase in ionic group concentration i.e. IEC is responsible for higher ionic conductivity of membrane.30-32 All the CEMs and AEMs showed required electrochemical properties for use in EDI applications. Hence these membranes were employed here for the production of ultrapure water by EDI. A comparative analysis of electrochemical properties, specifically, average t value of CEM-1 and AEM-1 pair is highest among the membranes listed in Table 1. This is mainly attributed to the comparatively low degree of average water uptake by CEM-1 and AEM-1 pair among the membranes. The low degree of water uptake lowers the back diffusion of ions with water which in turn enhances the average t of the CEM-1 and AEM-1 pair. The hydrophobic polyethylene base of interpolymer membranes is responsible for



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maintaining appropriate water uptake by the membranes. As a result, the superior performance of CEM-1 and AEM-1 pair is expected in EDI. As far as resin quality is concerned, CER and AER should exhibit moderate to high water uptake and high IEC for use in EDI applications. This is because the resin should exchange the H+ and OH- ions reasonably faster rate for efficient EDI process. Table 1. Physical and electrochemical properties of ion-exchange resins (CER and AER) and different membranes Samples

Thickness /diameter (mm)

Water IEC uptake (%) (meqg1/meqmL-1)

Km (mSc m-1)

t

a

CER

0.80

55

1.9

-

-

a

AER

0.80

60

1.0

-

-

b

0.20

29

2.45

2.86

0.94

b

0.20

20

1.10

1.12

0.92

b

0.40

40

2.20

3.2

0.85

b

0.40

40

2.0

3.7

0.86

b

0.14

30

2.07

2.17

0.88

b

0.13

29

2.12

2.69

0.90

CEM-1 AEM-1 IONSEP-HC-C IONSEP-HC-A CEM-II (Fujifilm type II) AEM-II (Fujifilm type II)

a-IEC is expressed in meqmL-1; b-IEC is expressed in meqg1 3.3. Current-Voltage (i-v) Curves in EDI Unit. The I vs V curve of different membranes combinations were determined in an EDI unit containing 5 pieces of each kind of membranes in continuous mode. Permeate of RO water of salt concentration 35 mg/L with TOC 2 ppb was fed in the EDI unit. Voltage was varied from 1-40 volts/cell pair. In the I vs V curves two main regions appeared. Figure 3 shows the I vs V plots of three diffeent types of membranes. At the applied potential 1-20 volt/cell pair (Region I) current density increased linearly with applied potential. At the applied potential 21-40 volt/cell pair high current density is observed (Region II).32,33 In region I, salts are transported by the respective membranes and a linear increase in current density with applied potential is observed. In region II water dissociation starts and H+ and OH- ions are generated. A drastic increase in current density was observed due to transport of H+ and OH− to the central compartment and the resins are


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regenerated by absorbing H+ and OH-. After increasing the applied potential to 34 volt/cell pair, the current density increases but the obtained ultrapure water becomes acidic (pH 6.56.7) and the resistance of water also decreases when Ionsep and Fujifilm membranes were used. Therefore, regeneration of resin does not take place fully at 34 volt/cell pair applied potential for these membranes. The pH of ultrapure water remained almost neutral (7.2-7.5) at applied potential 34 volt/cell pair when interpolymer based membranes were used. The most suitable applied potential value obtained from Figure 3 for the water splitting phenomena as well as for regeneration of resin for interpolymer based membrane, Ionsep and Fujifilm type II membranes were 25, 30 and 30 volt/cell pair respectively.

Figure 3. Current density vs applied volt/cell pair plot obtained with different membrane combinations 3.4.

EDI Experiments. EDI experiment was carried out using permeate of RO water of

TDS 35 mg/L. The water was passed through EDI unit packed with interpolymer based membranes. Separate experiments were conducted by packing the EDI unit with commercial membranes (Ionsep and Fujifilm type II). Voltage and flow was optimized in all the experiments. 3.5.

Effect of Applied Potential on Resistance of Ultrapure Water. The effect of

applied potential on the resistance of ultrapure water was studied by varying the applied potential without changing the flow rate using three different types of membranes. The flow was kept constant at 18.5 L/h. Figure 4 (a) shows the variation of current density with time and Figure 4 (b) shows the variation of resistance with time at different applied potential for 10 ACS Paragon Plus Environment


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interpolymer based membranes. The current density increases with applied potential due to increased dissociation of water molecule at higher applied potential. The value of current density at any applied potential increases initially and then remains constant throughout the experiment. The initial increase of current density may be due to contact of surface water with the mixed bed resin which is present in larger concentration, having conductivity higher than diluted feed water. The pH (ca. 7.2) of ultrapure remained constant through the experiment at all the applied potential. It is observed from Fig.4 (b) that the resistance of product water increases with the increase of applied potential. At higher applied potential dissociation of water into H+ and OH- becomes faster which helps to regenerate the ion exchange resin. As a result, the final resistance of water increases. The EDI unit was also operated at 34 volt/cell pair applied potential. The current density increased and resistance (18.3 MΩ cm) and pH (7.2) of produced ultrapure water was also remained constant. Therefore, the EDI unit can be operated successfully up to applied potential 34 volt/cell pair. Only difference is that W value at higher potential (34 volt/cell pair) will increase.

Figure 4. (a) Current density vs time plots and (b) Resistance vs time plots of interpolymer based membranes in EDI unit at four different applied potentials. The current density at different applied potential was calculated for commercial Ionsep and Fujifilm Type II membranes. Figure 5 (a) shows the variation of current density with time and Figure 5(b) shows the variation of resistance with time at different applied potential for commercial Ionsep membranes. For this membrane pair, current density 11 ACS Paragon Plus Environment

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increases up to applied potential 30 volt/cell pair. The resistance of produced ultrapure water also increases. The pH value remains constant (7.0-7.2). After increasing the applied potential to 34 volt/cell pair, current density, resistance (13.8 MΩ cm) and pH (6.0-6.5) of produced ultrapure water decreased. The ultrapure water was acidic in nature at higher applied potential (34 volt/cell pair). Therefore, 30 volt/cell pair is the operating voltage of EDI unit packed with commercial Ionsep membranes.

Figure 5. (a) Current density vs time plots and (b) Resistance vs time plots of commercial Ionsep membranes in EDI unit at four different applied potentials. The current density (mA cm-2) value at a fixed 30 volt/cell pair applied potential obtained with commercial Ionsep membrane is higher than the current density value obtained with interpolymer based membranes at 25 volt/cell pair applied potential. The Ionsep membranes are heterogeneous fabric supported membranes. The presence of charged moieties in the Ionsep membranes is higher than interpolymer based membranes. Hence, IEC and Km values of Ionsep membranes are higher than the interpolymer based membranes. However, the final resistance of ultrapure water obtained with Ionsep membrane (15 MΩ cm) is lower than the value obtained with the interpolymer based membranes (18.2 MΩ cm). The average water uptake (40%) of Ionsep is higher than the water uptake (24.5%) of interpolymer based membranes. The average t value (t=0.855) of Ionsep membranes is lower



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than that of interpolymer based membranes (t= 0.93). Therefore, back diffusion of ions with water molecule from concentrate compartment to diluate compartment occurs which lowers the final resistance of ultrapure water obtained with Ionsep membranes.7,8 Similarly, Figure 6 (a) shows the variation of current density with time and Figure 6 (b) shows the variation of resistance with time at four different applied potential for commercial Fujifilm type-II membranes.

Figure 6. (a) shows the variation of current density with time and (b) shows the variation of resistance with time at four different applied potential for commercial Fujifilm type II membranes. With commercial Fujifilm type-II membranes current density increases with applied potential upto 30 volt/cell pair. The resistance of produced ultrapure water also increases upto this applied potential. The pH value remains constant (7.0-7.2). After increasing the applied potential to 34 volt/cell pair for these membranes, current density value was increased but the resistance (16.5 MΩ cm) and pH (6.5-6.7) of produced ultrapure water decreased. Therefore, 30 volt/cell pair is the operating voltage of EDI unit packed with commercial Fujifilm membranes. The current density (mA cm-2) value at 30 volt/cell pair applied potential obtained with commercial Fujifilm type II membranes is close to the current density value obtained with interpolymer based membranes at 25 volt/cell pair applied potential. The IEC Km values 13 ACS Paragon Plus Environment

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of Fujifilm type II membranes are close to interpolymer based membranes. The % of water uptake of Fujifilm type II membranes is slightly higher and transport number values are slightly lower than interpolymer based membrane. Therefore, back diffusion of ion also takes place with this membrane but relatively lower rate compared to Ionsep membranes. Therefore, the final resistance of ultrapure water obtained with Fujifilm type II membranes (17.1 MΩ cm) is slightly lower than the value obtained with the interpolymer based membranes (18.2 MΩ cm) but higher than Ionsep membranes (15 MΩ cm). 3.6. Effect of Flow on the Resistance of Ultrapure Water using Different Membranes. The effect of flow (L/h) on the resistance of ultrapure water was studied separately at four different flow rate (15.5, 16.5, 18.5 and 20 L/h) using three different types of membranes. The operating applied potential was 25, 30 and 30 volt/cell pair for interpolymer based, commercial Ionsep and commercial Fujifilm type II membranes respectively. Figure 7 (a) shows current density vs flow rate and Figure 7 (b) shows the resistance vs flow rate of three different types of membranes.

Figure 7. (a) shows the variation of current density with flow and (b) variation of resistance with flow for different membranes. The current density as well as resistance of ultrapure water increases with the increase of flow rate from 15.5-18.5 L/h for all three type of membranes. At higher flow rate, the concentration of both migrating ions as well as quantity of water increases to diluate compartments which enhances the current density value. The migrated ions move from diluate compartment to concentrate compartment in relatively faster rate with increasing flow rate of EDI unit. As a result, the moving ions exchange with the resins rapidly and the 14 ACS Paragon Plus Environment


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resistance of produced ultrapure water increases. The resins are also regenerate in a faster rate. It is noted here that after increasing the flow rate from 18.5 to 20 L/h the current density value increased but the resistance of ultrapure water decreased. The flow of the concentrate compartment (7.5 L/h) was much lower than diluate compartment (20 L/h). Therefore, volume thickness of diluate compartment becomes much higher than concentrate compartment. In this condition, the movement of ions from diluate compartment to concentrate compartment was not enough faster at higher flow rate since the applied potential is kept constant. Polarization may also take place at higher flow rate which results in decrease of resistance of ultrapure water.34 Hence, in order to increase the resistance of ultrapure water at higher flow rate (20 L/h), higher applied potential/pressure is required for faster transportation of ions. Therefore, there is an optimum flow rate at optimum applied potential in which the resistance of ultrapure water become maximum for all the three different types of membranes. This depends on the design of the EDI unit. The final resistance value of 18.2 MΩ cm was achieved with interpolymer based membrane. 3.7.

Performance Comparison of Three Different Types of Membranes.

3.7.1. Resistance Comparison. Figure 8 shows the resistance vs applied potential plots of three different types of membranes. It is observed from Fig.8, that the final resistance of ultrapure water (18.2 MΩ cm) was achieved with the interpolymer based membrane whereas for commercial Ionsep and Fujifilm type II membranes the resistance values were 15 and 17.1 MΩ cm respectively.

Figure 8. Resistance vs applied potential plots of three different types of membranes.


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3.7.2. Quantity of Produced Ultrapure Water. The EDI units were continuously run for seven hours/day for upto thirty days to compare the performance of all three different types of membranes in a continuous mode of operation. Without considering the quality (resistance) of produced ultrapure water, all three different EDI units (flow rate 18.5 L/h) produced ca. 3500 L ultrapure water. The final average resistance values of ultrapure water upto one month were 17.5 ± 0.5, 13 ± 0.5 and 10 ± 0.5 MΩ cm for interpolymer, Fujifilm and Ionsep membranes respectively. Therefore, the purity of ultrapure water was highest with interpolymer based membranes when the collected volume of ultrapure water was similar. 3.7.3.

W and CE (%) Comparison. The W and CE (%) value are the crucial parameters of

EDI process. For efficient EDI process the W value should be lower and CE value will be higher. Final TDS, pH, resistance, W and CE values have been calculated for all the membranes at different applied potential during EDI operation. The flow of product water was kept constant at 18.5 L/h. These results are summarized in Table 2. Table 2. Comparative value of final TDS, pH, ionic resistance, W and CE of different membranes during EDI process. The data were collected after 12 h run of units.

Types of membrane

Applied potential (volt/cell pair)

Final TDS (ppm)

pH

Ionic resistance ( MΩ cm)

W (KWhKg-1)

CE (%)

Interpolymer

21

0.050

6.5

11.6

0.189

84.63

Interpolymer

22

0.070

6.7

15.1

0.226

74.00

Interpolymer

25

0.027

7.0

18.2

0.324

58.86

Ionsep

20

0.050

7.0

10.0

0.309

49.37

Ionsep

26

0.078

7.2

13.0

0.469

42.28

Ionsep

30

0.070

7.4

15.0

0.658

34.83

Fujifilm type-II

20

0.052

6.5

10.0

0.155

98.73

Fujifilm type-II

25

0.077

6.8

13.0

0.258

73.99

Fujifilm type-II

30

0.032

7.2

17.1

0.430

53.88



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It is observed from Table 2, that as the applied potential increased, purity of ultrapure water in terms of lower TDS, higher resistance increases for all the three different types of membranes. The W value increases and CE value decreases with the increase of applied potential for all three different types of membranes. At higher applied potential, as the TDS of final ultrapure water decreases, W value increases, which subsequently lowers the CE value. The W values follow the trend for the membranes,

Ionsep>Fujifilm type

II>interpolymer based membrane. Hence, W value was higher and CE value was lower in case of Ionsep membrane, whereas obtained W value was lower and CE value was higher for interpolymer based membranes. The lower performance of Ionsep membrane is ascribed to higher water uptake and lower transport number of this membrane than the other two membranes which favours the back diffusion of ions from concentrated compartment to diluted compartment, which in turn increases the W value. 3.7.4. Cost, Final Conductivity and TOC Comparison. Table 3 summarizes the market cost of Ionsep and Fujifilm-type II membranes. The estimated production cost of our interpolymer membranes is also included in the Table. It is noted that the market cost of commercial membranes may not be their production cost. The final conductivity and TOC content of produced ultrapure water by use of these membranes are also presented in Table 3. Table 3. Cost, TOC and final conductivity of ultrapure water obtained with different membranes Types of membrane

TOC (ppb)

Final conductivity (µScm-1)

Cost of membrane CEM+AEM (2 m 2)

Interpolymer baseda

1.0

0.054

40-50 USD

Ionsepb

1.0

0.067

80 USD

Fujifilm- type II b

1.0

0.062

130 EURO

a-production cost at 5 Kg batch scale; b-market cost It is clear from Table 3 that the final conductivity produced ultrapure water was lowest for interpolymer based membranes which proves that the concentration of ion present in the ultrapure water was lowest when interpolymer based membrane was used in the EDI unit. The lower TOC content once again supports the higher purity of ultrapure water 17 ACS Paragon Plus Environment

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produced using interpolymer based membrane. The production cost in a 5 Kg batch scale for interpolymer based membrane is also less compared to market cost of two commercial membranes. 4. CONCLUSION Ultrapure water of high resistance has been produced by electrodeionization process using three different types of ion exchange membranes. The applied potential and flow were optimised to obtain ultrapure water of high resistance. The resistance of ultrapure water increases with increasing the applied potential as well as with increasing water flow rate. Standardization experiments revealed that ultrapure water of resistance 18.2 MΩ cm with product rate 18.5 L/h has been achieved with interpolymer based ion-exchange membrane at 25 volt/cell pair applied potential. The power consumption (W) and current efficiency (CE) values were 0.324 KWhKg-1 and 58.86% respectively. In case of commercial Ionsep and Fujifilm type II membranes, resistance of ultrapure water was 15 and 17.1 MΩ cm at 30 volt/cell pair applied potential. The W and CE values were 0.658 KWhKg-1 and 34.83% respectively for Ionsep membranes whereas for Fujifilm type II membranes the values were 0.430 KWhKg-1 and 53.88% respectively. The lower resistance of produced ultrapure water obtained with Ionsep membranes is ascribed to high water uptake and low transport number of this membrane pair than the interpolymer based membranes. The higher water uptake of the Ionsep membrane allows the diffusion of ions with water from concentrate compartment to the diluate compartment which decreases the resistance of the ultrapure water. The advantage of using interpolymer based ion exchange membrane are (i) low cost, (ii) preparation method is by melt extrusion process, (iii) minimum usage of solvent, (iv) large quantity of blown film can be prepared in a very short time and (v) high mechanical as well as acid-base stability due to chemically inert polyethylene base. Therefore, overall cost of EDI unit can be reduced using interpolymer based ion exchange membrane. This approach can be used to set up moderate capacity EDI plant (size 250 L/h) using interpolymer based ion-exchange membrane. AUTHOR INFORMATION Corresponding author Email: [email protected] (Saroj Sharma), [email protected] (Uma Chatterjee)



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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS CSIR-CSMCRI registration number 109/2015. Centralized analytical facility and in house laboratory project (OLP-0078) of CSIR-CSMCRI is acknowledged. Uma Chatterjee acknowledges DST, Govt of India (YSS/2015/000653) for financial support under Young Scientist scheme. Dr. Suresh K Jewrajka is acknowledged for giving valuable inputs, which has increased the quality of the work. Dr. V. Kulshrestha, Mr. P. D. Maru and Mr. M. N. Parmar is acknowledged for helping in interpolymer membrane preparation.V. Bhadja acknowledges CSIR network project (CSC 0104) for providing fellowship. REFERENCES 1. Goffin, C.; Calay, J. C. Use of continuous electrodeionization to reduce ammonia concentration in steam generators blow-down of PWR nuclear power plants, Desalination 2000, 132, 249-253. 2. Ganzi, G. C.; Jha, A. D.; DiMascio, F.; Wood, J. H. Electrodeionization-theory and practice of continuous electrodeionization, Ultrapure Water 1997, 14, 64–69. 3. Neumeister, H.; F¨urst, L.; Flucht, R.; Nguyen, Y. D.; Verbeek, H. M. Theory and experiments involving an electrodeionization process for high-purity water production,

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