removal of hardness ions
TRANSCRIPT
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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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J.-S. Park et al. / Desalination 202 (2006) 18 5
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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6 J.-S. Park et al. / Desalination 202 (2006) 18
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 com- partment 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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J.-S. Park et al. / Desalination 202 (2006) 18 7
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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