Application of electrodeionization (EDI) for removal of boron and silica from reverse osmosis (RO) permeate of geothermal water

Desalination 310 (2013) 25–33

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Application of electrodeionization (EDI) for removal of boron and silica from reverse osmosis (RO) permeate of geothermal water Özgür Arar a, Ümran Yüksel a, Nalan Kabay b,⁎, Mithat Yüksel b a b

Ege University, Department of Chemistry, Faculty of Science, 35100 Izmir, Turkey Ege University, Department of Chemical Engineering, Faculty of Engineering, 35100 Izmir, Turkey

H I G H L I G H T S ► Removal of boron and silica from RO permeate of geothermal water by EDI ► The effect of process parameters of EDI on product water quality ► Effect of central compartment configuration on water quality

a r t i c l e

i n f o

Article history: Received 31 July 2012 Received in revised form 29 September 2012 Accepted 1 October 2012 Available online 22 October 2012 Keywords: Boron Electrodeionization Geothermal water Ion exchange membrane Reverse osmosis Silica

a b s t r a c t In this study, a hybrid process coupling reverse osmosis (RO) with electrodeionization (EDI) was investigated to remove boron and silica from geothermal water. The effect of applied voltage, feed flow rate, Na2SO4 concentration in the electrode compartments, membrane type and ion exchange resin bed configuration on the removal of boron and silica has been investigated. Geothermal water was obtained from the discharge lines in the geothermal plant of Izmir Geothermal Co. The RO system contained two parallel mounted brackish reverse osmosis membranes (BW-30-2540, Dow FilmTech). The RO permeate was collected using a single membrane configuration of operation by applying a 12 bar of pressure. The concentration of boron remaining in the permeate was 5.9 mg/L. It was obtained that the concentration of boron decreased from 5.9 mg/L to 0.4 mg/L and silica from 0.3 mg Si/L to 0.1 mg Si/L with a layered bed configuration of EDI system when a 40 V of voltage was applied to the EDI system. The feed flow rate did not have any significant effect on the removal of boron and silica from the RO permeate of geothermal water by EDI. On the other hand, the transport of boron to the anode compartment of EDI system enhanced with increasing the feed flow rate. In a mixed bed EDI system, when the applied potential was 40 V, boron and silica concentrations in the product water were 1.60 mg B/L and 0.2 mg Si/L, respectively. The type of the membrane was found to be another important parameter on the removal of boron and silica from the RO permeate. © 2012 Elsevier B.V. All rights reserved.

1. Introduction In many regions, water resources frequently suffer from severe salinity problems that endanger present and future developments. One of the species that is difficult to remove is boron [1]. Boron is a widely distributed element throughout lithosphere and usually appears in the form of either neutral boric acid or anionic borate species. Generally, boron has been found to have a dual effect on the living systems on Earth. In the case of plants, the effect of boron has the unambiguous meaning. It was shown that there is a small range difference between boron deficiency and boron toxicity levels. Boron plays a critical role in the metabolism of carbohydrate, sugar translocation, hormone action, normal growth and functioning of the apical meristem, nucleic acid synthesis, and biological membrane structure and function. Symptoms of ⁎ Corresponding author. Tel.: +90 232 3112290; fax: +90 232 3887776. E-mail address: [email protected] (N. Kabay). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.10.001

boron deficiency are visible in root and leaf growth, bark splitting, retardation of enzyme reactions, and can cause even a death of plants [2]. Boron concentration in the irrigation water which is only slightly higher than the permissible level will cause a negative effect for plant growth and will exhibit signs of “boron poisoning” as yellowish spots on the leaves and the fruit, accelerated decay, and ultimately plant expiration. Also, the World Health Organization (WHO) defined boron level of 0.5 mg/L as the non-observed effect level (NOEL) for drinking water until 2011. This level was increased very recently to 2.4 mg/L for drinking water although the toxic effect of boron for some plants should be still kept in mind for irrigation water [3]. Thus, the removal of boron from water and wastewater is still important from the environmental viewpoint. The treatment methods used for the removal of boron from water can be divided into several categories. The first is coagulation and electro-coagulation processes [4,5]. The next two categories are adsorption [6,7] and ion-exchange processes [8–14]. The last category

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Ö. Arar et al. / Desalination 310 (2013) 25–33

consists of membrane processes such as Donnan dialysis [15], electrodialysis [16–18], reverse osmosis (RO) [19–22] and electrodeionization (EDI) [23]. Excess of silica in water is also undesirable for several industries but it is difficult to remove silica and silicate scales completely in various equipments. It is deposited on the high-pressure steam-turbine blades. The silica content is usually 1–20 mg/L for the surface waters and 7–45 mg/L for the ground waters. Silica concentrations exceeding 1000 mg/L are found in the brackish waters and brines. Considerable efforts and expenses have been made over the years to inhibit silica scale deposition from the geothermal brines during energy extraction [24]. In semiconductor processing, the high silica levels can lead to silica deposits on wafers which in turn lead to expensive chip failures. Silica levels are usually maintained at or below what can be detected by on-line instrumentation to prevent silica from affecting the wafer yield of a semiconductor plant. EDI is an excellent process for achieving low levels of silica and boron in water [25]. EDI is a process that removes the ionic species from the liquids using electrically active media and an electrical potential to affect the ion transport. The electrically active media in EDI devices may function to alternately collect and discharge ionic species or to facilitate the transport of ions continuously by ionic or electronic substitution mechanisms. EDI devices may comprise media of permanent or temporary charge and may be operated batchwise, intermittently or continuously [26]. In an EDI system, cation exchange and anion exchange membranes are placed between the electrodes like in an electrodialysis (ED) system. The dilute compartment is filled with the cation exchange and anion exchange resins that enhance the transport of cations and anions under the driving force of a direct current. Water dissociation reaction during an EDI operation produces H+ ions and OH− ions. These ions continuously regenerate the ion exchange resins electrochemically without using any chemicals. Due to the fact that there is no any chemical consumption, EDI is known as an environmentally friendly process which is alternative to the ion exchange (IX) process for water purification [27]. In this study, the removal of boron and silica from the RO permeate of geothermal water with EDI method was investigated. 2. Experimental Geothermal water was obtained from the discharge lines in the geothermal plant of Izmir Geothermal Co., Turkey. A mini-pilot scale RO system was installed in the plant and was operated with a single membrane configuration employing the BWRO-30-2540 membrane and applying a 12 bar of operating pressure. The properties of geothermal water and its RO permeate were summarized in Table 1. The permeate collected from the RO operation was further treated with the EDI process in the laboratory using a micro flow cell EDI device (Electrocell Co.) for 3 h of operation period. The experimental set-up of Table 1 The characteristics of the geothermal water and its RO permeate.

Characteristics +

Na K+ Ca2+ Cl− HCO3− SO42− Mg2+ B Si pH EC (μS/cm) TDS Salinity (‰)

RO feed

RO permeate

Concentration (mg/L)

Concentration (mg/L)

343.0 40.0 19.0 212.0 620.0 157.9 2.4 10.6 68.0 7.30 1685 843 0.7

1.62 0.17 0.18 3.15 19.7 0.33 0 5.9 0.3 6.71 15.0 7.5 0

the EDI system used was described before [28]. It consists of a three compartment-cell with a dimensionally stable anode (titanium with Ir/Ru coating) and a stainless steel cathode along with three separate stream lines (Fig. 1). The electrical potential was controlled with a voltmeter. The current was monitored with an amperometer through a power supply. The cathode and anode compartments of the cell were separated from the central compartment with the ion exchange membranes. The central compartment was filled with the strong acid cation exchange (Purolite CT 175) and strong base anion exchange (Purolite A 500) resins. For the layered bed configuration, Selemion AME anion exchange and Selemion CME cation exchange membranes were used. In order to find the effect of membrane type on the removal performance of boron and silica from the RO permeate of geothermal water, Selemion (-AHT and -CMD) and Neosepta (-AMX and -CMX) membranes were also employed. The properties of the ion exchange resins used were listed in Table 2 and ion exchange membranes in Table 3. For EDI tests, 1 L of Na2SO4 solution with a 500 μS/cm of conductivity was feed as an electrode solution through the electrode compartments and the RO permeate of geothermal water (1 L) through the central compartment. The samples (10 mL) were collected from the streams of anode, cathode and central compartments by regular time intervals and were analyzed. The concentration of silica was determined by a standard colorimetric method using a Merck Nova 60 model colorimeter. The analysis of boron was performed by Azomethine-H method using a Jasco V-530 model spectrophotometer. 2.1. Removal of boron and silica by layered bed EDI system In a layered bed configuration, cation exchange and anion exchange resins were placed into the central compartment in a way that the cation exchange resins are in the bottom part while the anion exchange resins are in the upper part. In such configuration, feed water was first contacted with the cation exchange resins then with the anion exchange resins. The effect of applied voltage, feed flow rate and concentration of electrode solution (Na2SO4 solution) on the removal of boron and silica was studied. In addition, the influence of the applied potential on the product water quality was monitored in an applied potential range of 20–40 V. In this experiment, the RO permeate of geothermal water (15 μs/cm) was used as a feed solution and NaSO4 solution with a 500 μS/cm of conductivity was circulated through the electrode compartments. The EDI stack was operated with a flow rate of 0.6 L/h under a constant voltage mode. To investigate the effect of feed flow rate, the RO permeate of geothermal water was fed through the central compartment at a flow rate of 0.4–1 L/h. The Na2SO4 solution with a 500 μS/cm of conductivity was circulated through the electrode compartments with a flow rate of 0.6 L/h. A voltage of 40 V was applied to the system. Finally, the effect of Na2SO4 concentration on the product water quality was checked. For this, the conductivity of Na2SO4 solution was varied from 250 to 750 μS/cm with a 0.6 L/h of flow rate while permeate flow rate was kept constant as 1.0 L/h. A voltage of 40 V was applied to the system. Selemion-AME anion exchange and Selemion-CME cation exchange membranes were used in all experiments. 2.2. Removal of boron and silica by mixed bed EDI system In these series of experiments, anion exchange and cation exchange resins were first mixed in a way balancing their ion exchange capacities and then were placed into to the central compartment. In the mixed bed configuration, two parameters such as membrane type and applied potential were studied. Various potentials in the range of 25–40 V were applied to the system while the membranes Selemion-AME anion exchange and Selemion-CME cation exchange membranes were employed. The RO permeate of geothermal water with a conductivity of 15 μs/cm was used as a feed solution

Ö. Arar et al. / Desalination 310 (2013) 25–33

27

Fig. 1. a) Layered bed and b) mixed bed EDI system. (1. Central compartment filled with ion exchange resin, 2. Cathode compartment, 3. Anode compartment, 4. Anion exchange membrane, 5. Cation exchange membrane, 6. Cathode, 7. Anode, 8. Magnetic pumps, 9. Power supply, and 10. Reservoir).

with a feed flow rate of 1.0 L/h and the solution of Na2SO4 having a conductivity of 500 μS/cm was circulated through the electrode compartments with a flow rate of 0.6 L/h. In order to see the effect of membrane type, Selemion I (-AHT anion exchange and -CMD cation exchange), Selemion II (-AME anion exchange and -CME cation exchange), Neosepta (-AMX anion exchange and -CMX cation exchange) membranes were used for EDI tests. For this, the RO permeate of geothermal water (15 μs/cm) was used as a feed solution and EDI stack was operated with a feed flow rate of 1.0 L/h under a constant voltage mode. A voltage of 25 V was applied to the system. A Na2SO4 solution with a 500 μS/cm of conductivity was circulated in the electrode compartments.

3. Results and discussions 3.1. Removal of boron and silica by layered bed EDI system 3.1.1. Effect of applied potential The analytical data of the product water after the RO operation were given in Table 1. The majority of contaminants in the feed water were removed by the RO process to yield a good quality of the product water. In the first step of the desalination process, the BW30-2540 membrane removed more than 95% of all ionic contaminants and silica from the geothermal water at the end of the process. Since the rejection of boron was only 44%, it was considered that the excess amount of boron was still in the product water and its concentration was higher than the permissible level for irrigation water. Therefore, it is necessary to remove boron from the product water at a high percentage before going to consider this water to utilize as irrigation water and/or to discharge it to the environment. For this, an EDI process following the RO process was considered as the

Table 2 Physicochemical properties of ion exchange resins [29].

Polymer structure Functional group Capacity (eq/L)

Purolite CT 175

Purolite A 500

Macroporous polystyrene crosslinked with divinylbenzene Sulfonic acid

Macroporous polystyrene crosslinked with divinylbenzene Type 1 Quaternary Ammonium

1.8 (H+ form)

1.15 (Cl− form)

secondary stage of the treatment process to eliminate excess boron and silica from the product water. First, the effect of applied potential on the quality of the product water obtained by EDI method was investigated in a potential range of 20–40 V. In this experiment, the RO permeate of the geothermal water (15 μs/cm) was used as a feed solution and a Na2SO4 solution with a 500 μS/cm of conductivity was circulated through the electrode compartments. The EDI stack was operated with a feed flow rate of 0.6 L/h under a constant voltage mode. Selemion-AME anion exchange and Selemion-CME cation exchange membranes were used in all experiments. According to the obtained results, the concentrations of boron vs. time plots for the central and anode compartments were shown in Figs. 2 and 3. Ionic depletions in the electro-regeneration region resulted in the development of high voltage gradients. Under the conditions, the voltage gradient on the resin surfaces exceeded the thermodynamic potential of 0.83 V. Such high potential caused catalytic reactions for water splitting or decomposition to form hydrogen and hydroxide ions, which may be used for regeneration of ion exchange resins [32]. As seen in Fig. 2, when the applied potential was increased, the boron concentration in the central compartment declined. On the other hand, the concentration of boron increased in the anode compartment (Fig. 3). This is because of the fact that the concentration of OH − ions increased at a high potential applied [33]. Thus, both SiO2 and H3BO3 gave reactions to produce their ionic forms as shown in the following equations [23]: −

−

H3 BO3 þ OH ⇆BðOHÞ4

−

−

SiO2 þ OH ⇆HSiO3

−

−

2−

HSiO3 þ OH ⇆SiO3 þ H2 O:

ð1Þ

ð2Þ

ð3Þ

When the applied potential is high, the current for transporting the ions increased [28]. Therefore, the concentration of boron started to increase in the anode compartment. As shown in Fig. 3, when the applied potential was increased from 20 V to 40 V, the boron concentration in the anode compartment elevated from 0.30 mg B/L (at 20 V) to 1.80 mg B/L (at 40 V). The concentration of silica in the anode compartment was 0.2 mg Si/L at all potential values. At the

28

Ö. Arar et al. / Desalination 310 (2013) 25–33

Table 3 Physicochemical properties of ion exchange membranes [30,31]. Grade

Neosepta AMX

Neosepta CMX

Selemion AHT

Selemion CMD

Selemion AME

Selemion CME

Type

Strongly basic anion exchange 2.0–3.5

Strongly acidic cation exchange 1.8–3.8

Strongly basic anion exchange 20

Strongly acidic cation exchange 17

Strongly basic anion exchange 18–25

Strongly acidic cation exchange 15–22

0.12–0.18

0.14–0.20

0.3

0.4

0.55–0.65

0.55–0.65

Electrical resistance (Ω-cm2) Thickness (mm)

end of the experiment of 3 h, the concentration of silica in the central compartment declined from 0.3 to 0.1 mg Si/L while the concentration of boron decreased to 0.3 mg B/L from 5.9 mg B/L. 3.1.2. Effect of feed flow rate In these series of experiments, the flow rate of the RO permeate of geothermal water was varied from 0.4 to 1 L/h. The Na2SO4 solution with a 500 μS/cm of conductivity was circulated through the electrode compartments with a flow rate of 0.6 L/h. A voltage of 40 V was applied to the EDI system. The obtained results were depicted in Figs. 4 and 5. As shown in Fig. 4, the feed flow rate did not have a big influence on the boron removal. On the other hand, the transport of boron from the central compartment to the anode compartment was facilitated when the feed flow rate was increased (Fig. 5). Thus, the contact time for ions with the ion exchange resin particles decreased [34]. This may lead to the transport of the ions from the central compartment to the anode compartment. The concentrations of boron in the anode compartment at the end of 3 h of operation were 0.4 and 2.0 mg B/L for the feed flow rates of 0.4 and 1.0 L/h, respectively. In the central compartment, however, the minimum concentration of boron was around 0.5 mg B/ L at both flow rates. In the case of silica, when the feed flow rate is high, the silica concentration in the central compartment decreased, so the percent removal of silica increased also. The concentration of silica in the central compartment was 0.2 mg Si/L with a 0.4 L/h of feed flow rate and it decreased to 0.1 mg Si/L with a 1.0 L/h of feed flow rate. In the anode compartment, the silica concentration was 0.1 mg Si/L with 0.4 L/h of feed flow rate and it increased to 0.2 mg Si/L with a 1.0 L/h of feed flow rate. 3.1.3. Effect of Na2SO4 concentration in electrode compartment In order to investigate the influence of Na2SO4 concentration on the quality of product water, the solutions of Na2SO4 with a conductivity of

250–750 μS/cm were employed by circulating them through the electrode compartments at a flow rate of 0.6 L/h while the permeate flow rate was kept constant as 1.0 L/h. A voltage of 40 V was applied to the system. As shown in Fig. 6, changing Na2SO4 concentration affected boron removal slightly. But, silica removal was not influenced by the change in the concentration of Na2SO4. When the Na2SO4 concentration is low, the removal of boron declined. This is due to the fact the resistivity of the system increased and this leads to a decrease of pH in the central compartment when the Na2SO4 concentration is low. When the pH in the central compartment decreased, the ionization of boric acid was low [23]. As a result of this, a decline in boron removal was observed. When the Na2SO4 concentration was high, pH in the central compartment increased accordingly. As the conductivity of the electrode compartment was increased, the electrical current in EDI cell became high. This leads to the dissociation of water greatly and also the generation of OH − ions accordingly. These OH − ions react with boric acid and silica as shown in Eqs. (1)–(3) and thus the ions produced move to the anode compartment. The minimum boron concentration in the central compartment was 0.5 mg B/L at a Na2SO4 conductivity of 750 μS/cm. The concentrations of boron in the anode compartment increased with an increase in the conductivity of the concentrate streams (Fig. 7) [23]. Indeed, the concentration of boron in the anode compartment was 1.2 mg/L with a Na2SO4 solution of 250 μS/cm, but it became 2.5 mg/L when the conductivity of Na2SO4 solution was increased to 750 μS/cm. Also, the concentration of silica in the anode compartment was 0.1 mg Si/L when the Na2SO4 conductivity was 250 μS/cm. The respective value for silica became 0.2 mg Si/L with a Na2SO4 conductivity of 750 μS/cm. 3.2. Removal of boron and silica by mixed bed EDI system 3.2.1. Effect of applied voltage In this section, a mixed bed EDI system was employed. Cation exchange and anion exchange resins were mixed first in a way using the

7

2.0

30 V

5

Boron concentration (mg/L)

Boron concentration (mg/L)

20 V

6

40 V

4 3 2 1 0

1.8

20 V

1.6

30 V

1.4

40 V

1.2 1.0 0.8 0.6 0.4 0.2 0.0

0

50

100

150

200

Time (min) Fig. 2. Boron concentration in the central compartment versus time as a function of applied potential. (Central compartment configuration: layered bed, membranes: Selemion-AME and -CME, feed flow rate: 0.6 L/h, Na2SO4 conductivity: 500 μS/cm, and flow rate of electrolyte solution: 0.6 L/h).

0

50

100

150

200

Time (min) Fig. 3. Boron concentration in the anode compartment versus time as a function of applied potential. (Central compartment configuration: layered bed, membranes: Selemion- AME and -CME, feed flow rate: 0.6 L/h, Na2SO4 conductivity: 500 μS/cm, and flow rate of electrolyte solution: 0.6 L/h).

Ö. Arar et al. / Desalination 310 (2013) 25–33

7

7

6

Boron concentration (mg/L)

0.4 L/h

Boron concentration (mg/L)

29

0.6 L/h

5

1.0 L/h

4 3

2 1

250 µS/cm 500 µS/cm 750 µS/cm

6 5 4 3 2 1 0 0

50

0 0

50

100

150

150

200

200

Time (min) Fig. 4. Changes of boron concentration in the product water versus time as a function of feed flow rate. (Central compartment configuration: layered bed, Membranes: Selemion-AME and -CME, applied potential: 40 V, Na2SO4 conductivity: 500 μS/cm, and flow rate of electrolyte solution: 0.6 L/h).

resins at equivalent ion exchange capacities and then placed into the central compartment. The RO permeate of geothermal water with a conductivity of 15 μs/cm was used as a feed solution passing through the central compartment at a flow rate of 1.0 L/h under a constant voltage mode. A Na2SO4 solution with a conductivity of 500 μS/cm was circulated through the electrode compartments with a 0.6 L/h of flow rate. Selemion-AME anion exchange and Selemion-CME cation exchange membranes were used in these tests. The applied voltage was 25–40 V. Figs. 8 and 9 show the boron concentration vs. time plots in the central and the anode compartments, respectively. According to Fig. 8, when the applied potential was high, the boron concentration in the central compartment increased. As explained in Section 3.1.1 when a potential was applied to the EDI cell, water molecules start to split on the anion exchange resin (membrane)/cation exchange resin (membrane) contact regions. In a mixed bed configuration, number of these regions is much more than in a layered bed system. Thus, in a mixed bed EDI system, more water molecules split and more OH − ions are generated. These OH − ions react with boric acid and silica to form their ionic species. Under driving force of the applied potential, borate and silicate ions formed move to the anode compartment. On the other hand, when their concentrations

2.5

Fig. 6. Changes of boron concentration in the central compartment versus time as a function of Na2SO4 conductivity. (Central compartment configuration: layered bed, membranes: Selemion-AME and -CME, applied potential: 40 V, feed flow rate: 1.0 L/h, and flow rate of electrolyte solution: 0.6 L/h).

increased at the anode compartment, these ions diffused back from the anode compartment to the central compartment. Thus, the boron concentration at the central compartment increased again [23]. As can be seen from Fig. 9, when the applied potential was high, there is an apparent increase in the boron concentration at the anode compartment. This is also because of the generations of the excess OH − ions as explained above. When a potential of 25 V was applied to EDI system, boron concentration in the central compartment was 0.90 mg B/L and it increased to 1.60 mg B/L at 40 V. At the same voltage, the concentration of silica was 0.03 mg Si/L in the central compartment and it increased to 0.2 mg Si/L at 40 V. The concentrations of silica in the anode compartment were 0.1 mg Si/L at 25 V and 0.2 mg Si/L at 40 V. The respective values for boron were 1.10 mg B/L at 25 V and 1.80 mg B/L at 40 V. 3.2.2. Effect of membrane type In order to investigate the effect of membrane type, series of experiments were carried out using the RO permeate of geothermal water with a conductivity of 15 μs/cm as a feed solution, passing through the central compartment at a flow rate of 1.0 L/h under a constant voltage mode. A Na2SO4 solution with a conductivity of 500 μS/cm was circulated in the electrode compartments at a flow rate of 0.6 L/h. Various ion exchange membrane pairs (Selemion-AME anion exchange membrane/Selemion-CME cation exchange membrane, Selemion-AHT anion exchange membrane/Selemion-CMD cation exchange membrane

0.4 L/h 3.0

0.6 L/h

2.0

Boron concentration (mg/L)

Boron concentration (mg/L)

100

Time (min)

1.0 L/h 1.5 1.0 0.5 0.0

250 µS/cm 500 µS/cm 750 µS/cm

2.5 2.0 1.5 1.0 0.5 0.0

0

50

100

150

200

Time (min) Fig. 5. Changes of boron concentration in the anode compartment versus time as a function of feed flow rate. (Central compartment configuration: layered bed, membranes: Selemion-AME and -CME, applied potential: 40 V, Na2SO4 conductivity: 500 μS/cm, and flow rate of electrolyte solution: 0.6 L/h).

0

50

100

150

200

Time (min) Fig. 7. Changes of boron concentration in the anode compartment versus time as a function of Na2SO4 conductivity. (Central compartment configuration: layered bed, membranes: Selemion-AME and -CME, applied potential: 40 V, feed flow rate: 1.0 L/h, and flow rate of electrolyte solution: 0.6 L/h).

30

Ö. Arar et al. / Desalination 310 (2013) 25–33

7

25 V 30 V 40 V

6 5

Selemion AME and CME

Boron Concentration (mg/L)

Boron Concentration (mg/L)

7

4

3 2 1 0 0

50

100

150

6

Selemion AHT and CMD Neosepta AMT and CMX

5 4 3 2 1

200

Time (min)

0 0

Fig. 8. Changes of boron concentration in central compartment versus time as a function of applied potential. (Central compartment configuration: mixed bed, membranes: Selemion-AME and -CME, feed flow rate: 1.0 L/h, Na2SO4 conductivity: 500 μS/cm, and flow rate of electrolyte solution: 0.6 L/h).

and Neosepta-AMX anion exchange membrane/Neosepta-CMX cation exchange membrane) were used in these tests. A potential of 25 V was applied to the EDI cell. Figs. 10 and 11 show the boron concentrations vs. time plots in the central and the anode compartments, respectively. As can be seen in Fig. 10, at the end of the process the concentration of boron in the central compartment was slightly lower with a thin membrane pair of Selemion-AHT/Selemion-CMD than that obtained by a thick membrane pair of Selemion-AME/Selemion-CME. When the thickness of the membrane decreased, the resistivity of the membrane becomes low [35,36]. With a thin membrane, the transport of the ions through the membrane increased, thus the conductivity of water decreased much faster. As a result, an increase in the resistivity between resin and membrane interface was obtained. When the resistivity increased, the splitting of water molecules occurs greatly. As explained in Section 3.2.1, OH − ions formed leads to the increase in the concentration of boron in the anode compartment (Fig. 11). But later on, boron diffused back from the anode compartment to the central compartment, thus boron concentration in the product water increased. Similar results were obtained for silica also. The resistivity of membrane is another important parameter on EDI performance. When the resistivity of membrane decreased, the

50

100

150

200

Time (min) Fig. 10. Changes of boron concentration in the central compartment versus time as a function of membrane type. (Central compartment configuration: mixed bed, applied potential: 25 V, feed flow rate: 1.0 L/h, Na2SO4 conductivity: 500 μS/cm, and flow rate of electrolyte solution: 0.6 L/h).

permeability of an ion-exchange membrane for the counter-ions under the driving force of an electrical potential gradient increased [37]. By using a membrane having low resistivity, the ion transport increased and thus the decrease in solution conductivity becomes much faster. This leads to the increase in the resistivity between resin and membrane interface. When the resistivity increased, splitting of water molecules occurs. The OH− ions formed react with boric acid and silica to form their ionic forms. Later, these ions transport to the anode compartment. In the case of a thick membrane pair (Selemion-AME and SelemionCME), the minimum concentrations of boron were 0.8 mg B/L in the central compartment and 1.1 mg B/L in the anode compartment. The silica concentration in the product water decreased to 0.03 mg Si/L. On the other hand, the silica concentration in the anode compartment was 0.10 mg Si/L. When the membrane pair of Neosepta-AMX and Neosepta-CMX was employed, the concentration of boron in the central compartment became 0.90 mg B/L. In the anode compartment, however, the boron concentration was 1.3 mg B/L. The respective values for Si were 0.20 mg Si/L in the central compartment and 0.10 mg Si/L in the anode compartment.

1.8

25 V

1.6

30 V

1.4

40 V

1.4 Selemion AME and CME

Boron Concentration (mg/L)

Boron Concentration (mg/L)

2.0

1.2 1.0 0.8 0.6 0.4 0.2 0.0

1.2

Selemion AHT and CMD 1.0 Neosepta AMX and CMX

0.8 0.6 0.4 0.2 0.0

0

50

100

150

200

Time (min) Fig. 9. Changes of boron concentration in the anode compartment versus time as a function of applied potential. (Central compartment configuration: mixed bed, membranes: Selemion-AME and -CME, feed flow rate: 1.0 L/h, Na2SO4 conductivity: 500 μS/cm, and flow rate of electrolyte solution: 0.6 L/h).

0

50

100

150

200

Time (min) Fig. 11. Changes of boron concentration in the anode compartment versus time as a function of membrane type. (Central compartment configuration: mixed bed, applied potential: 25 V, feed flow rate: 1.0 L/h, Na2SO4 conductivity: 500 μS/cm, and flow rate of electrolyte solution: 0.6 L/h).

Ö. Arar et al. / Desalination 310 (2013) 25–33

31

Table 4 Calculated values for boron and silica at layered bed EDI system. Layered bed Parameter

Applied potential (V)

ReB (%) ReSi (%) Selectivity CeB (%) CeSi (%) JB (mol m−2 s−1) JSi (mol m−2 s−1) kB (ms−1) kSi (ms−1) SPC (Wh/L)

20 92 67 1.4 16.0 0.2 8.1E-10 1.2E-11 1.5E-06 1.1E-06 0.42

30 93 67 1.4 8.1 0.1 8.3E-10 1.2E-11 1.5E-06 1.1E-06 1.21

Feed flow rate (L/h) 40 95 67 1.4 6.7 0.1 8.4E-10 1.2E-11 1.6E-06 1.1E-06 1.97

0.4 92 33 2.7 4.4 0.0 5.4E-10 3.9E-12 1.0E-06 3.6E-07 1.96

3.3. Performance of EDI system The EDI performance was evaluated in terms of removal efficiency (Re), current efficiency (Ce), selectivity, flux (J), mass transfer coefficient (k) and specific power consumption (SPC), respectively, by using Eqs. (1)–(6) [17,38–41]. In these equations, Ci represents the initial concentration; Cf, the final concentration in dilute compartment; F, Faraday constant; Q, flow rate; and I, current; z, ion charge; E, applied potential and VD, the volume of the solution. Reð% Þ ¼

Ci −Cf 100 Ci

Ceð% Þ ¼ z  F  Q  Selectivity ¼

Ci −Cf  100 I

ð2Þ

ðCBoron−i −CBoron−f Þ  CSilica−i ðCSilica−i −CSilica−f Þ  CBoron−i

ð3Þ

Q nA

ð4Þ

ðCi −Cf Þ Q Ci nA

ð5Þ

J ¼ ðCi −Cf Þ k¼

ð1Þ

t

E∫ Idt SPCðWh=LÞ ¼

0

ð6Þ

VD

In the layered bed configuration of EDI, the process was run with a flow rate of 1.0 L/h using Na2SO4 in the electrode compartment with a 500 μS/cm of conductivity at a voltage of 40 V. The calculated values were as follows: the removal efficiency of boron 93%, current efficiency of boron 10.3%, removal efficiency of silica 67%, and current efficiency of

0.6 95 67 1.4 6.7 0.1 8.4E-10 1.2E-11 1.6E-06 1.1E-06 1.97

Na2SO4 conductivity (μS/cm) 1.0 93 67 1.4 10.3 0.1 1.4E-09 1.9E-11 2.5E-06 1.8E-06 2.11

250 93 67 1.4 13.2 0.2 1.4E-09 1.9E-11 2.5E-06 1.8E-06 1.67

500 93 67 1.4 10.3 0.1 1.4E-09 1.9E-11 2.5E-06 1.8E-06 2.11

750 92 67 1.4 9.7 0.1 1.4E-09 1.9E-11 2.5E-06 1.8E-06 2.20

silica 0.1%. When the EDI system was run in a mixed bed configuration of EDI at a voltage of 25 V with a flow rate of 1.0 L/h using Na2SO4 as electrode solution (500 μS/cm) by employing Selemion AME and Selemion CME membranes, the removal efficiency of boron was calculated as 86%, the current efficiency of boron as 19%, the removal efficiency of silica as 90% and the current efficiency of silica as 0.4%. When the EDI system with a layered bed configuration was compared with mixed bed configuration, it could be seen that the values of flux, mass transfer coefficient, and current efficiency of boron are higher for the EDI system with mixed bed configuration. This is because of the fact that water molecules dissociated more in the mixed bed configuration than in the layered bed one. Therefore, ionization and transport of boron increased. The obtained k and J values for silica are also higher for a mixed bed configuration of EDI system than that obtained by a layered bed configuration of EDI system. The calculated values were summarized in Table 4 for layered bed configuration and in Table 5 for mixed bed configuration. When the applied voltage was increased for the layered bed configuration, the calculated values of Re, J and k for boron increased. In the case of silica, when the applied voltage was higher, the values of Re, J and k for silica did not change. When the feed flow rate was increased, the calculated Ce, J and K values of boron and silica increased also but Re values of boron and silica did not change much. When the concentration of Na2SO4 was increased, no change was obtained for Ce, Re, J and k values of boron and silica. In the mixed bed configuration, the calculated values of Ce, Re, J and k for boron and silica decreased with an increase in the applied voltage. On the other hand, when the membrane thickness increased, Re, Ce and k values of boron and silica increased also. Selectivity of boron over silica in EDI system was also calculated. In the layered bed configuration of EDI, the selectivity of boron over silica did not change when the applied potential and Na2SO4 concentration were increased. On the other hand, when the feed flow rate was decreased, the selectivity value increased to 2.7.

Table 5 Calculated values for boron and silica at mixed bed EDI system. Mixed bed Parameter

Applied potential (V)

Membrane type

25

30

40

Selemion AME and CME

Selemion AHT and CMD

Neosepta AMX and CMX

ReB (%) ReSi (%) Selectivity CeB (%) CeSi (%) JB (mol m−2 s−1) JSi (mol m−2 s−1) kB (ms−1) kSi (ms−1) SPC (Wh/L)

85 90 0.9 19.0 0.4 1.3E-09 2.6E-11 2.3E-06 2.5E-06 0.66

80 33 2.4 15.5 0.1 1.2E-09 9.7E-12 2.2E-06 9.1E-07 0.91

73 33 2.2 11.5 0.1 1.1E-09 9.7E-12 2.0E-06 9.1E-07 1.49

86 90 1.0 13.7 0.3 1.3E-09 2.6E-11 2.4E-06 2.5E-06 0.66

90 33 2.7 14.2 0.1 1.3E-09 9.7E-12 2.4E-06 9.1E-07 0.39

85 33 2.5 13.4 0.1 1.3E-09 9.7E-12 2.3E-06 9.1E-07 0.90

32

Ö. Arar et al. / Desalination 310 (2013) 25–33

In the mixed bed configuration when the applied potential was increased from 25 V to 40 V, the selectivity value for boron over silica increased from 0.9 to 2.2. Membrane thickness had an apparent effect on the selectivity. When a thin membrane was used, the selectivity of boron over silica increased from 1.0 with a membrane pair of Selemion-AME/Selemion-CME to 2.5 with a membrane pair of Neosepta-AMX/Neosepta-CMX. 4. Conclusions The removal of boron and silica from the RO permeate of geothermal water has been investigated by employing EDI method. The obtained results showed that it was not possible to lower the concentration of boron in the product water to the permissible level for irrigation water by means of RO process at the natural pH of geothermal water. When the RO permeate collected was passed through an EDI cell, it was possible to decrease the concentration of boron to below 0.5 mg B/L at the end of the EDI operation when the optimum conditions were employed. The process conditions such as the central compartment configuration in an EDI cell, the type of ion exchange membranes employed, the potential applied, the concentration of electrode solution were found to be important for a complete removal of boron and silica from the RO permeate of geothermal water. The best results for the removal of boron were achieved with a layered bed configuration of EDI cell, by using a feed flow rate of 1.0 L/h, voltage of 40 V and Na2SO4 solution of 500 μS/cm of conductivity. The highest J and k values for boron were found as 1.4× 10−9 mol m −2 s −1 and 2.5 × 10−6 ms −1, respectively, with a layered bed configuration, 1.0 L/h of feed flow rate and a 750 μS/cm of Na2SO4 solution at 40 V. For the case of silica, the highest J and k values were 2.6 × 10−11 mol m −2 s −1 and 2.5 × 10−6 ms−1, respectively. These values for silica was obtained in a mixed bed configuration of EDI system by employing 1.0 L/h of feed flow rate and Na2SO4 electrode solution of 500 μS/cm of conductivity at 25 V. In layered bed and mixed bed configurations, J values of boron was higher than that of silica due to the fact that J is linearly proportional with initial concentration. Indeed, initial concentration of boron in RO permeate is much higher than that of silica. Nomenclature Re removal efficiency (%) Ci initial concentration (mol/L) Cf final concentration (mol/L) Ce current efficiency (%) z ion charge F Faraday constant (96,453 As/mol) Q flow rate (m 3 s –1) I current (A) J flux (mol m −2 s −1) n number of cell pair A membrane area (m 2) k mass transfer coefficient (ms −1) SPC specific power consumption (Wh/L) E applied potential (V) VD volume of solution (L)

Acknowledgment This study has been partially supported by Ege University Scientific Research Project (EU-2009 FEN 081). We would like to acknowledge Izmir Geothermal Co. for the support to perform RO tests in the geothermal field. The authors thank E. Yavuz for his support during RO tests in the field. We are especially grateful to Asahi Glass Co. (Japan), and Astom Co. (Japan) for sending us ion exchange membranes and Purolite Int. Co. for ion exchange resins.

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