removal of hardness ions

8/3/2019 Removal of Hardness Ions

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Desalination 202 (2006) 18

Presented at the conference on Wastewater Reclamation and Reuse for Sustainability (WWRS2005), November

811, 2005, Jeju, Korea. Organized by the International Water Association (IWA) and the Gwangju Institute of

Science and Technology (GIST).

0011-9164/06/$ See front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.desal.0000.00.000

Removal of hardness ions from tap water usingelectromembrane processes

Ji-Suk Park, Jung-Hoon Song, Kyeong-Ho Yeon,Seung-Hyeon Moon*

Department of Environmental Science and Engineering, Gwangju Institute of Scienceand Technology (GIST), 1 Oryongdong, Bukgu, Gwangju, South Korea

Tel. +82 62 970-2435; Fax +82 62 970-2434; email: [email protected]

Received 31 July 2005; accepted 23 December 2005

Abstract

Performances of electromembrane systems were investigated for removal of the hardness materials in a tapwater. Six cells of ED, EDR, and EDIR systems with an electrode area of 12.5 cm8.0 cm were operated. Theresults showed scaling formation due to water dissociation on the surface of cation exchange membrane, but EDR

and EDIR enabled to avoid the scaling problem. EDIR operation lowered the resistance and power consumptioncompared to EDR due to the conductance of ion exchange resins. This study demonstrated the feasibility of EDRand EDIR processes for water softening.

Keywords: Electrodialysis; Electrodeionization; Water softening; Hardness

1. Introduction

The hardness in water represents the contents

of divalent ions such as iron, manganese, cal-

cium, and magnesium. Among the hardnessions, calcium and magnesium are known as

dominant species in tap water. Hardness ions are

responsible for two harmful effects, formation

of deposits and destruction of soap. The deposits

of hardness ions generally occur due to the reac-

tion with soap anions. Produced soap scum

dulls clothes and drastically reduces soaps

cleaning efficiency [1]. The removal of hardness

for the purpose of avoiding the difficulties is

termed water-softening.

For the water softening, ion exchange (EX)

is broadly used due to ease of operation and

high removal efficiency. However, an EX pro-

cess releases high concentration of monovalent

cations and requires periodical regeneration of*Corresponding author.


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2 J.-S. Park et al. / Desalination 202 (2006) 18

resins when the ion exchange capacity becomes

saturated [36].

Electrodialysis reversal (EDR) and electro-

deionization reversal (EDIR) are electric forcedriven processes to remove ions from electrolyte

solution. An EDR process was developed to

overcome the shortcomings of an ED process

such as membrane fouling, using polarity rever-

sal of electrodes [7,8]. Thus, the EDR process is

applied for the treatment of water containing

fouling species. Likewise an EDR system, an

EDIR was developed to prevent the fouling phe-

nomena observed in electrodeionization (EDI).

It contains ion exchange resin in the diluatecompartment of the EDR stack [9]. The ion

exchange resins can reduce the polarization phe-

nomena in the treatment of dilute feed solution.

Also, high conductance of the ion exchange

resin brings the high removal performances

[10], overcoming the membrane fouling. Thus,

an EDIR system can be applied for the treatment

of dilute water like an EDR system.

Both EDR and EDIR have been of impor-

tance due to its environmentally friendly feature

and good applicability for portable use [11].These processes may provide the efficient meth-

ods for water softening from the practical point

of view. They overcome the difficulties of an

EX process for the removal of hardness ions due

to the different removal mechanism.

In this study, the performances of EDR and

EDIR were investigated for removal of hardness

ions in tap water, and the performances for the

removal of hardness materials were compared

with an ED process. Also the power consump-tion and current efficiency were examined.

2. Experimental

2.1. Materials

Neosepta CMX strong cation exchange

membrane and AMX strong anion exchange

membrane (Tokuyama Co., Japan) were used as

ion exchange membranes in all electromembrane

systems. Amberite IR120 Na cation exchangeresins and Amberite IRA 402 Cl anion exchange

resins (Rohm and Hass Co., France) were used as

ion exchange resins for EDIR process. Tap water

in GIST (Gwangju Institute of Science and

Technology, South Korea) was selected as feed

solution in all electromembrane systems. The feed

solution was prepared in the water tank to avoid

the composition variations during the operation

of each process. The major ionic species of feed

water was analyzed with ion chromatography(DX120, Dionex). The pH and conductivity

was measured with a pH meter (Orion) and a

conductivity meter (Cole Parmer). The feed

composition is presented in Table 1.

2.2. Stack configuration

Electromembrane systems of three hydraulic

stages with 6 cells were used for the treatment

of tap water. Platinum-coated titanium with thearea of 12.5cm 8.0cm was used as electrodematerials. Electrode rinsing solution was

circulated from anodic to cathodic compartment

to prevent precipitation of metal hydroxides.

Fig. 1 shows the schematic illustration of stack

configuration of the electromembrane system.

In the case of the EDIR system, ion exchange

resins were packed in the first and third

hydraulic stages as shown in Fig. 1.

Table 1

Composition of tap water in GIST (November, 2004)

Component Amount

Conductivity (mS/cm) 77

pH 5.95

Na+ (mg/L) 5.9

Ca2+ (mg/L) 5.0

Mg2+ (mg/L) 3.0

SO24 (mg/L) 14.5

Cl(mg/L) 10.5

Total hardness (mg/L as CaCO3) 24.8


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J.-S. Park et al. / Desalination 202 (2006) 18 3

2.3. Operation of electromembrane systemsTap water obtained in GIST was flushed into

the three streams, diluate (D), concentrate (C)

and electrode rinsing (E) compartment of each

electromembrane system. The recovery rate of

each system was determined as 70% by fixing

the flow rate of D as 70mL/min and C and E as

15 mL/min respectively. Pt wire was placed in

each hydraulic stage and the potential drop in the

hydraulic stage was measured using a multimeter

(Model 34404, Agilent technologies, USA). Theeffect of flow rate was also investigated to

understand potential drop in a diluate compart-

ment. The flow rate of diluate compartment was

varied from 30 to 120mL/min at a fixed flow

rate of C and E of 15 mL/min. The frequency of

polarity reversal was determined as 15 min. The

effluent of each system was analyzed by ion

chromatography (DX-120, Dionex, USA).

A conductivity meter (Cole-Parmer, USA) and a pH meter (Orion, USA) were used to measure

conductivity and pH during the operation.

3. Results and discussion

3.1. Operation of ED system for the removal of

hardness ions in a tap water

ED operation was examined to understand

the performance for removal of hardness ions

from a tap water. Fig. 2 shows the variations ofthe cell resistance, pH and conductivity of efflu-

ent solution. The pH of effluent increased with

operating time due to the water dissociation on

the surface of cation exchange membrane. The

rapid increase of pH with operation time was

due to the accelerated water dissociation caused

by the precipitation of metal hydroxide on the

surface of cation exchange membrane. It was

Stage 1 Stage 2

D in

C out

C in

D out

CEM AEM1 CEM AEM2 CEM AEM3 CEM AEM4 CEM AEM5 CEM AEM6 CEM

Stage 3

Anode

Cathode

Din

DoutCout

Cin

Stage 2

2 cell pair

EinEout

Stage 1

2 cell pair

Stage 3

2 cell pair

Fig. 1. Schematic illustration of stack configuration of used electromembrane systems consisting of three hydraulic

stages with 6 cell pairs. (AEM: Anion exchange membrane, CEM: cation exchange membrane, C: Concentrate solution,

D: Dilaute solution, E: Electrode rinsing solution).


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reported that water dissociation on the cationexchange membrane increases rapidly when the

calcium, magnesium, and other transition metals

serve as the electrolytes [12]. Fig. 3 shows the

illustration of the water dissociation and pre-

cipitation of calcium hydroxide on the cation

exchange membrane surface. When calcium

ions approach to the cation exchange membrane

surface, it seems that the bipolar structure is

instantly formed at the membranesolution

interface [13]. As a result, an electrostatic interac-

tion occurs between the fixed ion exchangeablegroups and their hydrated counter ions in the

repulsion zone and water molecules are possibly

polarized between them. Then, water dissocia-

tion, taking place at the interface of ion-exchange

membrane and electrolyte solution, causes theformation of metallic hydroxide such as calcium

hydroxide shown in Fig. 3. Moreover, the precip-

itation of metallic hydroxide induces water

dissociation. Simons reported that the metal

species might react with water molecules under

the influence of strong electric field [14]. It was

presumed that a thin layer of metallic com-

pounds, possibly an oxide or hydroxide, formed

at the interface, decreases the water-dissociation

potential because the interface became morehydrophilic and, consequently, more conducting

due to hydrogen and hydroxyl ions. Thus, from

the previous study, the pH rise in Fig. 2 may be

related to the precipitation of metal hydroxide

on the surface of the cation exchange mem-

brane. This was also confirmed when the ion

exchange membrane was separated from the

stack after 24 h operation. The surface of cation

exchange membrane was covered with white

metal hydroxide. Hence, the polarity reversal of

ED was considered as an effective softeningprocess of a tap water.

Operation time (h)0

Conductivity(S/cm)

0

10

20

30

40

50

pH

5

6

7

8

9

10

Resistance

()

0

100

200

300

400500

600

700

800

ConductivitypHResistance

4 8 12 16 20 24

Fig. 2. Changes of pH, conductivity, and cell resistance

with the operation of ED.

OOH

H

O

HH

+

+

+

+

+

+

+

H

HCa2+

Solution phase CEM

2SO3

SO3

SO3

SO3

SO3

2SO3

2SO3

2SO3

Solution phase CEM

2OH H

2H+2OH

2OH 2H+

Ca(OH)2

(a) (b)

O

Ca2+

Ca2+

Ca2+

Fig. 3. Schematic drawing of scaling mechanism of calcium hydroxide on cation exchange membrane (a) prepolariza-

tion of water molecules (b) precipitation of calcium hydroxide on the cation exchange membrane (CEM indicates cation

exchange membrane).


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3.2. EDR operation for the removal of hardness

materials in tap water

An EDR system was operated to prevent the

membrane scaling by hardness species. Fig. 4

shows the properties of effluent pH, conductiv-

ity, and cell resistance with the operating time of

the EDR process. The effluent of EDR process

shows stable pH and conductivity compared to

the ED process, indicating no scaling on the

membrane surface. These results indicate that

the EDR process was effectively applied to

control the scaling problem in a softening

process of tap water.

Fig. 5 shows the current efficiency (h) andremoval rate of major cations in the EDR process.

Current efficiency between two time points, t1 and

t2 (min) was calculated by the following Eq. (1):

(1)

where F is the Faraday constant (96,490 C for

1 mole of electrons), is the total applied

charge until time t2

(coulomb), is the total

applied charge until t1 (coulomb), is

the total moles of calcium, magnesium and

sodium ions removed from the initial time to

time t2, and is the total moles of

calcium and magnesium and sodium ions

removed from the initial time to t1.

Removal rates of the cation species were

calculated by the following Eq. (2):

(2)

whereR (%) is the removal rate of each ions, Cin(ppm) the feed concentration, andCout (ppm) the

effluent concentration.

Fig. 5 shows the current efficiency and

removal rates of each component in the EDR

system. The current efficiency maintained at

about 80%. The removal rates of calcium and

magnesium were higher than that of sodium ions

during the EDR operation. It can be explained by the difference in ionic mobility in the solu-

tion phase and selectivity of the cation exchange

membrane [7,15]. The mobility and selectivity

of calcium and magnesium ions in solution and

ion exchange media are greater than those of

sodium ions.

The greater ionic mobility of calcium and

magnesium induce the faster transport of cal-

cium and magnesium than sodium ion to the

membrane surface, and the higher selectivityresults in the higher removal rate, as shown in

Fig. 5. Hardness in the feed water was reduced

from 25 to 4 mg/L as CaCO3. Also, the EDR

system for removal of hardness materials

Operation time (h)

0

Conductivity(S/cm)

0

5

10

15

20

25

30

35

40

45

pH

5.5

6.0

6.5

7.0

7.5

8.0

Resistance()

0

100

200

300

400500

600

700Conductivity pH Resistance

4 8 12 16 20 24

Fig. 4. The time profiles of pH, conductivity and cell

resistance with the operation of EDR.

Ni

Ca,Mg,Na Ca,Mg,Na=

-

-

2 2 1

2 1

F n n

Q Q

t t

t t

( )

Qt2

Q

t1

nt

Ca,Mg,Na2

nt

Ca,Mg,Na1

RC C

C=

-

in out

in

100 (%)

Operation time (h)0

Removalrateorcurr

entefficiency(%)

0

20

40

60

80

100

Removal rate of Mg(II)Removal rate of Ca(II)Removal rate of Na(I)

Current efficiency

4 8 12 16 20 24

Fig. 5. The current efficiency of EDR system and

removal rate of major species.


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showed no scale on the membrane surface

inspite of the high recovery rate about 70%.

These experiments demonstrated the feasibility

of EDR system for removal of the hardness ions

in a tap water. However, the high potential drop

in an EDR system was observed in the first

hydraulic stage as shown in Fig. 6. The highpotential drop was caused by the low conductiv-

ity of dilute solution in the first stage. The high

potential drop led to a high power consumption

of an EDR process at a low flow rate. To

increase the conductivity of the cell, ion

exchange resin was placed in the first and third

hydraulic stages.

3.3. EDIR operation for the removal of hardness

materials in tap water

Ion exchange resins were filled within the first

and third hydraulic stages in the EDR system

to reduce the cell resistance. Fig. 7 shows the

resistance variations of stack and each hydraulic

stage of EDR and EDIR systems at different feed

flow rate. As presented in Fig. 7, reduction of

stack resistance was observed by placing ion

exchange resin. The EDIR system was operated

under the same conditions as the EDR system for

the comparison of performance between EDR and

EDIR systems. Fig. 8 showed the effluent conduc-

tivity and pH for the EDR and EDIR processes.

The conductivity and pH of produced water

slightly increased after packing the ion exchange

resin. The reason for the increased pH and con-

ductivity of an EDIR process is explained by

water dissociation in the diluate compartment as

Hydraulic stage

0

V/I

0

100

200

300

400

50030 mL/min60 mL/min90 mL/min

120 mL/min

1 2 3 4

Fig. 6. Resistance of the hydraulic stage in EDR

process. (number 1 is near to outlet of diluate compartment and number 2 indicates the inlet of diluate

compartment).

Fig. 7. Cell resistance of EDR and EDIR with flow rate.

Flow rate (mL/min)0

Resistance()

0

200

400

600

Stack resistane of EDIR

Resistance of 1st stage of EDIR

Resistance of 2nd stage of EDIR

Resistance of 3rd stage of EDIR

Stack resistance of EDR

Resistance of 1st stage of EDR

Resistance of 2nd stage of EDR

Resistance of 3rd stage of EDR

30 60 90 120 150

Flow rate (mL/min)

20

Conductivity(S

/cm)

0

10

20

30

40

50

60

70

pH

5

6

7

8

9

Conductivity of EDIRConductivity of EDR

pH of EDIRpH of EDR

40 60 80 100 120 140

Fig. 8. Conductivity and pH variations with flow rate.


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illustrated in Fig. 9. In the EDIR system, a bipolar

interface was formed on the contacting region

between cation exchange membrane and anion

exchange resins. The water dissociation in anEDIR system even at a low current density

becomes dominant on the cation exchange mem-

brane due to a lower limiting current density

(LCD) than that of anion exchange membrane.

Moreover, a high potential drop in the bipolar

interface dissociate water molecules into hydrogen

and hydroxide ions [11]. The hydroxide ion on the

surface of cation exchange membrane in the dilu-

ate compartment induces increment of pH and

conductivity. The pH rise under the reversal con-dition is also explained by the same water dissoci-

ation between cation exchange membrane and

anion exchange resins as shown in Fig. 9.

Fig. 10 shows the removal rate and current

efficiency for the operation of EDIR. Removal

rates of EDIR were similar to the EDR process

while scale formation was avoided. The total

power consumption was calculated to examine

the cost effectiveness of each process using the

following Eq. (3):

(3)

where VDC (volt) and ADC (ampere/s) are the

operating voltage and current with a DC power

supply, respectively.

Calculated results indicated that the power

consumption of the EDR process was 21.3

Wh/L product waterand the EDIR was 15.1 Wh/

Lproduct water. Although the removal rate of EDIR wascomparable to EDR, the EDIR showed a reduced

power consumption due to the conductivity

increase by ion exchange resin. Therefore, an

EDIR system is more economically competitive

process compared to EDR process for hardness

removal of tap water.

4. Conclusions

This study showed the performances of EDRand EDIR systems as an alternative process of

ion exchange for removal of hardness ions in

tap water. Operation of an EDR system for the

removal of hardness material could overcome

the scaling problem of ED on the cation

exchange membrane surface. The high removal

efficiency and high recovery rate of EDR and

EDIR process showed the feasibility as a

softening process. EDIR operation showed the

reduced potential drop compared to the EDR

system due to the increased conductivity in the

system. The EDIR operation showed cost

effectiveness by reducing power consumption.

However, pH and conductivity varied in an

AEM CEM AEM AEM CEM AEM

D C DC

POLARITY

REVERSAL

OH OHH+

H+

: Cation exchange resin (IR 120 Na)

: Anion exchange resin (IR 402 Cl)

Fig. 9. Water dissociation mechanism of EDIR process.

P W I ( / )( / min) (min)

=

VDC ADC

Flow rate L operating time

i

i

Operation time (h)0

Currentefficiencyorremovalrate(%)

0

20

40

60

80

100

Current efficiencyRemoval rate of Mg(II)Removal rate of Ca(II)Removal rate of Na(II)

4 8 12 16 20 24

Fig. 10. Current efficiency of EDIR process.


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EDIR system due to the water dissociation in a

diluate compartment, which should be further

studied.

Acknowledgements

This work was supported by the grant (4-1-2)

from the Sustainable Water Resource Research

Center of the 21st Century Frontier R&D Program

through the Water Reuse Technology Center at

Gwangju Institute of Science and Technology.

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removal of hardness ions

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