Accepted Manuscript
Title: Electrodeionization: Principles, Strategies and
Applications
Author: Lucı́a Alvarado Aicheng Chen
PII:
DOI:
Reference:
S0013-4686(14)00708-7
http://dx.doi.org/doi:10.1016/j.electacta.2014.03.165
EA 22481
To appear in:
Electrochimica Acta
Received date:
Revised date:
Accepted date:
2-3-2014
28-3-2014
29-3-2014
Please cite this article as: L. Alvarado, A. Chen, Electrodeionization:
Principles, Strategies and Applications, Electrochimica Acta (2014),
http://dx.doi.org/10.1016/j.electacta.2014.03.165
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Electrodeionization: Principles, Strategies and Applications
Lucía Alvarado1, Aicheng Chen1,*
Department of Chemistry, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario,
ip
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P7B 5E, Canada
Abstract
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Electrodeionization is an ionic separation technology that initially emerged ~50 years ago. In
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an early application, it was utilized to remove metallic species from radioactive wastewater;
however, a poor understanding of its functional kinetics has slowed its development and
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applications. Steadily increasing research efforts have focused on the elucidation of detailed
operational mechanisms, thereby enabling the extension of its applications to other fields. To
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date, electrodeionization has been proven to be an excellent environmentally compatible
purification, separation and concentration method. Novel materials have been continuously
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developed toward the improvement and maturation of this technology, which may lead to
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enormous environmental and economic benefits on a global scale. This comprehensive
review examines the inception, precursor techniques and historic evolution of
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electrodeionization, as well as its underlying principles, advantages and promising
applications in wastewater treatment and water purification.
Keywords: Electrodeionization; electrodialysis; membranes; ion exchange; water
purification.
* Corresponding author. Tel.: +1 807 343-8318; fax: +1 807 346 7775
E-mail address: aicheng.chen@lakeheadu.ca (A. Chen)
1
ISE member
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Page 1 of 49
Introduction
2.
Principles
Electrodialysis
2.2
Ion Exchange
2.3
Electrodeionization
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2.1
Mechanisms
4.
Experimental set-up
5.
Development of new materials
6.
Applications
1.
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Chromium removal
6.1.2
Removal of Copper, Cadmium and Nickel
6.1.3
Cobalt removal
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6.1.1
6.2
Removal of other ions
6.3
Organic compounds separation
6.4
High purity water production
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7.
Heavy metals removal
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6.1
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3.
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1.
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Contents
Conclusions and future outlook
Introduction
Processes involving the use of ion exchange membranes are frequently employed in water
purification and wastewater treatment [1]. These strategies are known as “integrated
membrane systems”, which may be classified as pressure-driven and electrically-driven [2].
Pressure-driven processes include microfiltration (MF), ultrafiltration (UF), nanofiltration
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Page 2 of 49
(NF) and reverse osmosis (RO), whereas electrically-driven approaches encompass
electrodialysis (ED) and electrodeionization (EDI). Pressure-driven processes are typically
applied when the removal of suspended solids and bacteria are the primary goals, whereas
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electrically-driven approaches are employed when the aim is ion removal, which is achieved
through the selective control and transport of ionic species. The fundamental principle
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behind electrically-driven processes is the passage of ions through a selective barrier (ion
exchange membrane) due to a gradient or driving force (electric field). Ion exchange
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membranes play a critical role in these processes as they are responsible for accepting or
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rejecting ions in the establishment of dilute and concentrate compartments. In this way,
processes that utilize ion exchange membranes have important applications in water
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purification [3- 6], ion removal or ion concentration [7-14]. In order to obtain enhanced
selectivity for specific ions, researchers are increasingly focused on the synthesis of unique
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materials in the development of composite membranes [15-18].
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ED is the most popular technology for electrically-driven processes in industry, as it
separates undesired ions from aqueous solutions at low operational cost and with the
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advantage that it does not generate residues [19-21]. This technology, which combines the
principles of dialysis and electrolysis, was first applied for the demineralization of syrup in
1890. An initial work that described this technology in a scientific journal was published in
1930 [22]. Since its inception, the capabilities of this hybrid technology have been
demonstrated in several areas, encompassing desalination, acid and caustics production,
organic compound separation [23], radioactive wastewater treatment [24], ultrapure water
production and general ionic separation. Although ED qualifies as an excellent technology
for applications that span a broad range of processes, a specific concentration limit must be
adhered to in order to optimize energy efficiency. When ions are separated from the feed
solution, there is an inherent drawback in which a phenomenon known as concentration
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Page 3 of 49
polarization develops, whereby a high cumulative resistance within the cell is built up, which
decreases cell efficiency [25]. In order to eliminate this disadvantage, a solid conductive ion
medium has been introduced into the dilute compartment in the form of ion exchange resins.
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Thus, ED and ion exchange technologies were combined, permitting the achievement of high
quality ion separation with elevated energetic efficiency. This new hybrid system was
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subsequently named as electrodeionization (EDI).
EDI or continuous electrodeionization (CEDI) is a technology that originated in the
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late 1950’s with the aim of minimizing the phenomenon of concentration polarization,
an
present in electrodialysis systems [26]. When dilute compartments are packed with ion
exchange material, they behave as a conductor due to the presence of functional groups,
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which serve as a bridge between the ion exchange membranes. This strategy was successful
in counteracting ED concentration polarization, as was evidenced by an increase in the
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maximum ion separation efficiency from 50% to 90%. Therefore, the synergistic integration
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of ED and ion exchange, which combines the benefits of both technologies, effectively
addresses some performance issues that are associated with each technology on its own [27-
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29].
One of the earliest descriptions of electrodeionization was articulated in 1955 by
Walters et al. at the Argonne National Laboratory [30], where it was utilized as a method for
the removal of traces of radioactive elements in water. In the late 1950's an initial device was
developed by the Permutit Company, which incorporated a mixture of anionic and cationic
resins. In other work, Sammons and Watts (Harwell Atomic Energy Authority) investigated
the deionization of a saline (NaCl) solution using an EDI module and undertook to measure
and correlate the relationships between concentration values, flow rates and applied current.
Subsequent studies were conducted with solutions that contained calcium, iron, phosphates
and detergents in order to demonstrate the reliability of the process. However, various factors
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Page 4 of 49
in the cell design were not taken into account. It was not until the late 1980’s that EDI was
commercially available for the production of ultrapure water. Table 1 depicts the historical
evolution of electrodeionization technology, where it is evident that initial progress was slow.
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EDI technology evolved to encompass various applications, and in 1971 high purity water
was first obtained through the application of this system. Subsequently, researchers began to
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investigate new types of membranes and combinations with other systems such as UV. Over
the last ten years there have been additional strides made in the advancement of this
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technology, which have contributed to enabling further benefits through its use. This has
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included a clearer understanding, and as a result, an extension of its utility beyond ultrapure
water production, into the realms of analysis.
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The success of EDI for the separation of ions stems from its early applications in the
isolation of radioactive metallic species and its evolution to the production of ultrapure water.
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Presently, it is considered to be a next-generation technology for water purification and
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wastewater treatment, as well as for practical utility in the pharmaceutical, semiconductor and
energy sectors. Its ability to almost completely segregate ionic species from dilute solutions
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endows EDI with the capacity for being utilized in myriad other areas. Beyond enabling the
generation of ultrapure water, this technology may facilitate the separation of toxic metallic
ionic species that are present in industrial waste effluents, and has been the basis of additional
applications [31]. Hence, EDI is a green process that requires only electricity and proper ion
exchange materials, negating the necessity for regenerative agents or other chemicals [32]. It
is plausible that the slow advance of this technology may be attributed to a general lack of
understanding as to its functional kinetics. In order to elucidate the EDI process, in 1959
Glueckauf proposed a theoretical discussion of the technology and suggested an ion removal
model that involved two stages [33]: (i) diffusion of ions into the resin; and (ii) ionic transfer
through the resin bed. Moreover, in 1971, Matejka described a mechanism that encompassed
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Page 5 of 49
three simultaneous stages [34]: (1) ion exchange between solution and resin bed; (2) transport
of ions through a resin bed via the application of an electric field; and (3) the electrolytic
regeneration of ion exchange resin.
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Cumulatively, these works contributed to an improved understanding of the role of
ionic transport in the EDI process. In 1998, Verbeek and Neumeister proposed a model to
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predict ion exchange bed behavior via digital simulations that were based on Nernst-Plank
equations and interfacial solid-liquid equilibrium [35]. Other authors have been engaged
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with gaining a greater understanding of regeneration mechanisms [36, 37]. Meng et al.
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studied the phenomenon of resin regeneration in EDI systems, and further clarified their
kinetics by proposing the existence of four forms of interaction between ion exchange
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materials, where in situ ion exchange may play a role in the regeneration of resins. EDI has
progressed and improved to the point at which patents have been filed for the marketing of
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systems that are designed to produce ultrapure water [38-48]. Steadily increasing activities
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have been focused on the creation of applications, opportunities and improvements for this
technology, such as the use of electrodeionization reversal (EDIR) to minimize membrane
2.
2.1
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fouling [49].
Principles
Electrodialysis
Electrodialysis is a separation process that is based on the selective migration of ions in
solution through ion exchange membranes under activation of an electric field. As shown in
Fig. 1, an ED system consists of a series of anion and cation exchange membranes that are
alternately spaced between two electrodes, which results in the formation of concentrate and
dilute solution compartments. The solution is circulated through each compartment and an
electrical potential difference is applied between the two electrodes in the process. In
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Page 6 of 49
response to the presence of the electric field, cations migrate toward the cathode, whereas
anions gravitate to the anode target; however, the interposed permselective membranes act as
barriers to their migration, which allows or prevents ions from passing in accordance with
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their electrical charge. Over time, one of the compartments is stripped of ions (diluted),
while the other becomes more ionically populated (concentrated). An electrolyte solution is
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circulated through the electrode compartments, which are called electrode rinse
compartments, as the processes that take place within them are different than those occurring
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within the central compartments (those that are delimited by membranes) [50]. The reactions
that take place within these compartments are:
Cathodic reaction
2 H2O ' O2 + 4 H+ + 4 e-
Anodic reaction
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2 H2O + 2 e- ' H2 + 2 OH-
The flow of solution through the membrane during an ED process is determined by the
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applied electric potential gradient. Ion exchange membranes are permeable to ions because
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they have a matrix that is comprised of synthetic ion exchange resin. The polymer matrix of
the membrane contains fixed functional groups that are electrically charged (ions), whereas
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interchangeable mobile ions (called counterions) reside within the pore spaces. This
arrangement serves to maintain electrical balance within the matrix. In this way, there exists
an electric field that is sufficient to overcome the forces that constrain mobile ions, and the
ions that enter the pores from solution replace the loads, enabling the selective passage of
ions [51]. If the matrix is positively charged, the counterions are negative and therefore
permeable to anions. With the application of an electric field, they replace the counterions
and the fixed charges of the matrix prevent the passage of co-ions (ions with the same charge
of the fixed groups), in this case cations. Otherwise, if the polymer matrix has fixed negative
charges, the counterion is positive and therefore, the membrane is permeable to cations and
impermeable to anions [52, 53]. The flow thus partially selectively deionizes through the
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Page 7 of 49
membranes. Fig. 2 depicts the structure of the ion exchange membranes, with a description
of each of the ions that are present in each case.
When current is applied, an electric potential difference is obtained as a current
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response, which is the result of the speed at which the ions are transported within the system.
By increasing this potential difference, the current is also amplified as a result of the
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enhanced transport velocity of the ions that contact the membrane and traverse it. However,
since the rate of ion transport is much higher within the membrane in contrast to the solution,
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this increase in current attains a threshold where the concentration of ions at the
an
membrane/solution interface is degraded to the point that any subsequent increase in the
electric field results in the dissociation of water [54]. This is because the population of ions
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that is present at the membrane/solution interface is insufficient to carry an appropriate
current flow. Hence, the H+ ions and OH- products that are generated from the dissociation of
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water begin to conduct electrical current. The current at this juncture is called the limiting
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current, Ilim. Beyond this point there is an increasing resistance in the cell and the pH of the
solution is altered. This causes a decrease in system efficiency via the requirement for higher
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energy consumption, and the changes in pH may lead to the precipitation of insoluble
hydroxides on the surface of the membrane [55]. The appearance of the concentration
polarization phenomenon prevents the treatment of very dilute solutions in ED systems.
Hence, it is convenient to operate the system at 80% of Ilim in order to harness the full extent
of energy via the transport of ions.
2.2
Ion exchange
Ion exchange (IX) is the diffusive ionic redistribution that occurs between an insoluble
material that is capable of exchanging cations or anions, and a solution, which contains ionic
species. This process begins when a chemical potential gradient arises between the solution
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Page 8 of 49
and the ion exchanger. As shown in Fig. 3, the ions in the solution diffuse into the exchanger
surface and displace the mobile species within the resin, which are then retained as a result of
intermolecular attractions between fixed sites and the ion exchanger. However, in a solution
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that contains several ions of the same charge (positive or negative), introduced exchangers
exhibit an ionic preference. The causes of such selectivity are associated with ion exchanger
cr
dimensions (steric hindrance), its valence, the pore size within the matrix, as well as
electrostatic interactions between the matrix and the counterions [56].
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The kinetics of the process depends on the mobilities of the ions to be exchanged
an
within the resin, counterions and the prevailing core temperature. Thus, the process
efficiency is a function of the affinity of the ion exchange resin for a particular ion, the pH of
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the solution, the concentration of ions in solution and temperature. Once the ion exchange
resin has been saturated, the recovery of eluted ions that have been exchanged by the resin
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via an ionic solution is necessary, which exchanges mobile ions with those that are present in
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the resin. As membranes, ion exchange resins are synthetic polymers that contain a crosslinked matrix through the action of a crosslinking agent and fixed functional groups. Most
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commercial resins are based on styrene-divinylbenzene and are acrylic-based. Crosslinking
imparts mechanical stability, strength and insolubility to the polymer, which in turn
determines the swelling capacity, with swelling defined as the property that allows the
permeability of ions into the matrix and improves the accessibility of ions to the functional
groups [57].
2.3. Electrodeionization
Electrodeionization is a hybrid technology that is based on the application of dual
technologies, namely electrodialysis and ion exchange. Dynamic synergies between these
two functions serve to reduce the inherent disadvantages of each one (e.g., concentration
polarization phenomena and chemical regeneration). The combination of these capabilities
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has resulted in a technology that has the capacity for treating low ionic strength solutions
much more efficiently than the constituent processes on their own. The configuration of an
EDI cell involves cationic and anionic membranes that are alternately arranged between the
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anode and cathode to provide dilute and concentrated compartments as shown in Fig. 4.
Beds of ion exchange resins are introduced into the dilute compartments in order to provide a
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substrate for electrical activity, which prevents the occurrence of the concentration
polarization phenomenon. The applied electric field thus promotes ionic transport in two
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different compartments through the active medium (resin bed), which collects and discharges
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the ionic species, thereby initiating mass transport through the ion exchange materials
(membranes and resins) [58]. The dissociation of water occurs simultaneously inside the cell
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at sites where anionic and cationic exchange materials are in contact, thereby creating protons
and hydroxyl ions, which act to regenerate the resin in situ. The function of the ion exchange
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resin is to lower the resistance of the cell, which tends to increase as the concentration of the
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diluted solution diminishes. The ionogenic sites may acquire resin ion concentrations that are
from 1,000 to 100,000 times higher with respect to the concentration of ions in solution, and
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as a result resin bed conductivity is obtained. When an electric field is applied, it influences
the dissolution ions as well as those that are derived from solid content (mobile ions).
However, when a higher concentration of ions exists in the solid in comparison with the
dissolution ions, the contribution of ion transport through the solid is greater, relative to that
obtained from the solution toward the membrane. This is because although ion mobility
within the solid is ~20 times lower than what exists in solution, the transport rate is
determined by product mobility and concentration, giving rise to a rate that is 50 to 5,000
times higher within the solid [38].
3. Mechanisms
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In 1969, Glueckauf proposed that the mechanism of removal of ions in an EDI cell has two
stages [33]: (i) diffusion of cations to the strong cation exchanger and dissemination of the
anions to the strong anion exchanger; and (ii) ionic conduction of the solid phase to the
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border of the membranes. Because the ion concentration within the solid is very high, the
process that controls ion removal is the ion diffusion rate of the aqueous phase to the surface
cr
of the solid ion exchange, which depends of three factors: (1) surface between solid and
solution; (2) thickness of the liquid layer through which ions diffuse; and (3) concentration
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gradient between the solid and liquid phase.
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When ions are transported by diffusion into the active sites of the resin via the
application of an excess current that is higher than necessary for the movement of ions, a
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portion of the solvent (H2O) is separated into its constituents (H+ and OH-). These elements
are responsible for the regeneration of the resin-displacing ions that have been collected [59].
d
As a consequence of the applied potential difference the ions migrate to the membranes
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through the packed bed, which is responsible for the ionic transport and transfer of current
into the system, occurring as the porous-plug model proposes. This model was developed by
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Wyllie et al. in 1955 [60] and was applied to elucidate the nature of current propagation
through resin beds. Another model was proposed by Baron in 1954, which was based on the
conductivity of the resin bed, to describe the electrical behavior of an ion exchange bed using
the following statistical model [56]:
β=
κb − κ
( κ b / κ )1/ 3 ( κ − κ )
(1)
κ b = resin bed, specific conductance
κ = solid, specific conductance (discontinued phase)
κ = liquid, specific conductance (continued phase)
β = bed empty volume fraction
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However, this model takes into account a discontinuous solid and becomes inapplicable when
conductivities approach zero. When the conductivity of the liquid phase tends to zero, the
model predicts a zero electrical conductivity of the bed. However, this does not hold true as
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there exist continuous phase particles when particles are in contact with one another.
Therefore, the bed conveys conductivity and there is conduction present despite the lack
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thereof within the liquid [40]. Wyllie et al. proposed a model that more adequately described
the electrochemical properties of the resin bed (porous plug model), which explained
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geometric parameters derived from empirical evidence. This model is based on the principle
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that electrical current flows via three different routes: through the interstitial solution, through
particles and the mixture thereof, between particles and the interstitial component of the
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solution. The model is represented by three parallel conductance elements, corresponding to
the three possibilities of flow as shown in Fig. 5. Hence, the specific conductance in the resin
aκκ
d κ + eκ
(3)
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κ1 =
(2)
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κ b = κ1 + κ 2 + κ 3
d
bed is defined by three conductance elements:
κ 2 = bκ
(4)
κ 3 = cκ
(5)
κb= resin bed, specific conductance
κ1= solid and interstitial solution, specific conductance
κ2= solid, specific conductance
κ3= liquid, specific conductance
κ = interstitial solution, specific conductance
κ = resin specific conductance
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a = cross-section fraction in the conductance element: solid and interstitial solution
b = cross-section fraction in the conductance element: solid
c = cross-section fraction in the conductance element: solution (liquid)
e = fraction in the conductance element κ1 due at solution
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d = fraction in the conductance element κ1 due at solid
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The geometric parameters a, b, c, d, e are calculated from electrolyte conductivity data
illustrated in Fig. 6 [56]:
(7)
d +e =1
(9)
⎛ d κ b ⎞ = ae + c
⎜ dκ ⎟
⎝
⎠ κ =κ
(8)
(10)
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a+b+c =1
a
⎛ dκb ⎞
= +c
⎜ dκ ⎟
⎝
⎠κ =0 e
(6)
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⎛
⎞
⎜ κb ⎟ =b
⎜ κ ⎟
⎝
⎠κ =0
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derived from resin bed-specific conductance vs interstitial solution-specific conductance, as
d
Investigations of electrodeionization typically separate diffusion and migration stages in the
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study of their effects on the recovery of metal elements. For example, Dzyazko et al. [61]
studied the electrical conductivity of a strongly acidic resin loaded with chromium and
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showed a correlation between conductivity and the pressure dropSpoor et al. studied the
migration of nickel ions in an electrodeionization system, where macroporous resins were
loaded with Ni2+ ions, and observed the recovery of the ions in the system through the
application of an electric field [62]. Under this system, the flux for the ion, J, is determined
by the Nernst-Planck equation (11), taking into account only mass transport via migration
(12):
Ji = -
Di Ci dμi zi FDi Ci dφ
+ Ci u
RΤ dx
RΤ dx
J i= -
zi F Di C i dφ
RΤ
dx
(11)
(12)
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where D is the diffusion coefficient, C the concentration, μ the chemical potential, z valence,
F Faraday constant, R the ideal gases constant, T temperature in Kelvin, φ electric potential, x
the distance and u the linear velocity. The mobility (ui) of nickel ions within the resin
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according to the initial conditions of the system (13) was calculated from the following
o
o
J i = - zi F ui
dφ
dx
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relationship:
(13)
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Meanwhile, a second part of this work was completed when the researchers undertook to
explore membrane and resin resistance [63], which encompassed the study of mass transport
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within a flexible resin and the effect of an applied electric field, ΔE, knowing that (14):
(14)
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ΔEbed=Ecell – (Eanode+ Ecatode+ΔEmembranes+ΔEec)
where “ec” refers to the electrode compartments. This study revealed a linear trend between
Di F
RT
(15)
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p
ui =
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coefficient considering that:
d
the nickel ion flux and the applied electric field. It also determined the apparent diffusion
Later studies focused on concentration and potential gradients [64] in the system utilizing the
Nernst-Planck equation (11), neglecting convective mass transport. Thereby applying a
model in which diffusion coefficients, surface area, a number of ionogenic group constants,
and a uniform distribution of counter ions within the pores was considered. This study was
conducted under the same consideration that was made for transport across membranes. In
the same year, the current distribution in the resin bed of an EDI stack was determined [65].
In 2003, Mahmoud et al. worked under the same scheme of transport phenomena in the EDI
system, where diffusion and migration stages were studied separately [66]. The diffusion of
Cu2+ was investigated initially, and subsequently a current was applied in order to study the
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ionic electromigrative electric field, which revealed a current efficiency of 100%. A more
complete work on EDI was carried out in 2004 by Song et al., wherein the transport of Co2+
in continuous EDI was studied. These results indicated that the flux was constant over the
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first three hours, which is ideally when the Nernst-Planck equation may be used. Following
this time period the linear behavior observed was due mainly to the presence of the H+
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product derived from the dissociation of water, which was also transported through the resin,
and therefore modified the concentration of cobalt within the resin ( C i ) [67]. On this basis it
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was proposed that the current efficiency and ηi could be calculated for Co2+ to reflect changes
ηi =
2F( nit2 − nit1 )
Q o t2 − Q o t1
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in the resin over extended time periods (16):
(16)
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where ni is the number of moles of Co2+ in the concentrate evaluated twice and Qo is the
charge accumulated over days 1 and 2. Also determinable, are the moles of Co2+ that are
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transported to the concentrate compartment in relation to the applied load. The results
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indicated that Co2+ transport is a function of the applied load; mobility data was obtained as
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relates to the residence of cobalt ions within the resin using the porous-plug model of beds
and the Nernst-Planck equation. In the same year, studies were reported that concerned
optimal current density selection in continuous electrodeionization, Fig. 7, in which two
regions appear. The first region contains a higher resistance, which means that water splitting
takes place in the second region [68]. In 2005, Malhmoud et al. [69] studied ohmic drops in
the ion exchange bed by applying the Wyllie model to various degrees of crosslinking for the
treatment of CuSO4 solutions. Satisfactory results were found only for short-term operations.
Thus, in order to determine optimal operational conditions, the effects of applied voltage
have been investigated by a number of researchers [70, 71]. In 2008, Nikonenko et al.
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Page 15 of 49
established a semi-empirical approach to predict mass transfer parameters in ED and EDI
[72].
An additional critical phenomenon in the study and understanding of EDI pertains to
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t
its regeneration in situ. It is known that the water dissociation reaction occurs at the interface
of anionic and cationic materials, with the effect of regenerating the resin that is in close
cr
proximity to the sites where this reaction occurs, as illustrated in Fig. 8. In this way, Meng et
al. [37] proposed to explain this phenomenon by estimating the potential gradient that is
dφ F( X − Ci )
x
=
dx
ε oε r
(17)
an
F( X − Ci ) 2
x
2ε oε r
(18)
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φ=
us
developed between the interphase of ion exchange materials as:
where εο and εr are the dielectric permittivity in vacuum and relative permittivity of water at
d
the interface of material exchange, respectively, and X represents the concentration of fixed
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groups. It was considered that Ci = 0 (by the action of ion exchange), which was obtained by
Ac
ce
p
dφ/ dx = 5.45x108 V/m, constitutes a value that leads to the dissociation of water within the
interstices of the bed material from cation and anion exchange. Later, Lee et al. [73] studied
and characterized the regeneration stage via electrical impedance spectroscopy, and purposed
an electric circuit to represent the bed system in EDI and a model application as presented in
Fig. 9. Song et al. continued to study the effects of current and ionic transport in the
dissociation of water [74]. Ion mass transfer phenomena was approached through
equilibrium principles [71] and revealed the sequence of the separation of ions as: Ca2+,
Mg2+, K+ > NO-3 > Cl- > Na+, as consistent with ion exchange priority. System simulations
have been conducted since 2009 in order to generate new knowledge in regard to the
mechanisms involved, toward the emergence of predictive operational parameters for EDI in
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Page 16 of 49
dealing with specific ion species. Kurup et al. studied a multi-component electrolyte, and
worked with subroutine DNEQNF in Fortran90 to describe a steady state model of Wafer
Enhanced-EDI [74]. The simulations were validated with experimental results, which were
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in good agreement. In 2010, Lu et al. studied water dissociation during the EDI process
through a numerical simulation using COMSOL multiphysics [76], and later in the same year
cr
they reported a numerical simulation of the EDI process that was focused on the production
of ultrapure water (% removal vs current density) where the ionic concentrations at
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membrane interfaces were displayed via a computer program [77]. Another numerical model
an
was applied to weak acid conversion by DASSL (differential/algebraic system solver), adding
complete information in regard to breakthrough curves and conductivity parameters [78].
Experimental set-up
M
4.
A typical electrodeionization system consists of an EDI cell, reservoirs for each solution
d
(concentrate, dilute and electrodic rinse), pumps and a power supply. The cell consists of
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cationic and anionic membranes, which are positioned alternately between the anode and
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cathode as is shown in Fig. 10. This system was utilized by Alvarado et al. [79, 80] for the
removal of chromium, which shows single concentrate and dilute compartment and two
electrodic rinse compartments. The number of compartments and their configuration are
contingent on the objectives to be pursued. An initial step involves the design of the cell, and
in order to achieve high-quality results, appropriate ion exchange materials (resins and
membranes) should be selected. Since there are myriad materials to choose from which
possess diverse functionalities, typical resins and membranes have been summarized and
listed in Table 2 and 3. The best results are found through the use of strong exchange
materials. Recently, some exchangers as zeolite and Pozzolana have been tested focusing in
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chloride absorption, since the chloride gas generated in the positive electrode causes
corrosion in the EDI cell and damage to membranes [81].
On other hand, the system configuration should also take into account the mass
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transfer characterization of the cell, which may be defined initially as an ED system. Cell
characterization aids in the prediction of limiting current values and imparts knowledge of the
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factors that may influence mass transport within the cell. Lee et al. [82] describe how cell
characterization may be accomplished when the diffusion coefficients and transport numbers
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in the membranes are plotted vs the molar concentration of the electrolyte. When non-linear
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relationships between the Ilim/C vs molar linear velocity of the electrolyte is obtained from
different electrolyte concentrations, then plots assist with the prediction of new limiting
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current values for the cell. Additionally, the plot may indicate the values of coefficients “a”
and “b”, which relate to the cell geometry with transport numbers of the ions in the solution
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and membrane phases, as well as the hydrodynamic conditions in the system, respectively. A
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specific limiting current plot might be made with self-selecting parameters such as
configuration, flow rate, and operation temperature, and may be generated by performing a
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potential (V) sweep and registering current measurements (I). A typical graphic to Ilim
determination is shown in Fig. 11 at the intersection of two traced lines on the data tendency,
indicating the current limiting value, as well as a plot of the resistance of the solution vs I-1;
both plots were presented by Alvarado et al. [79] to illustrate the process for the removal of
hexavalent chromium.
The physical and electrochemical properties of ion exchange materials may be
evaluated in an attempt to predict and understand their behavior and the responses of the
system within which they reside. The membranes can be characterized electrically, via
impedance measurements [55, 83]; their capacities at operational temperature can be
evaluated along with resins through adsorption isotherms and within the EDI cell. The resin
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Page 18 of 49
bed may be characterized using the Wyllie model [60] to quantify the current flow into the
cell. Subsequent to these characterizations the EDI stack is loaded and the dilute
compartment is filled with wet cationic, anionic or mixed ion-exchange resin. The resin is
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wet to ensure that the final volume is correct, taking into account that the mixed bed makes
available additional regeneration sites. Finally, the EDI process is initiated in order to
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promote water splitting and the continuous regeneration of the system, by introducing an
applied energy that exceeds the limiting current value. Electrodes that have been utilized by
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different authors are summarized and listed in Table 4, in which one is used to function to the
5.
an
electrodic rinse media.
Development of new materials
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New ion exchange materials have been continuously developed in order to obtain improved
results in EDI applications. Ion exchange textiles (IETs) have been synthesized and
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characterized. In 2004, a comparative study between a commercial resin (IRN 77) and a
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textile resin was conducted, which demonstrated that an equivalent removal of cobalt was
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possible, however, a higher current efficiency was achieved when IET was employed [84].
The ion-exchanger textile has been tested and a conventional electrodialysis comparison
between EDI and ED technologies has been undertaken, where EDI current efficiencies were
found to be nearly 150% higher than ED, thereby demonstrating the success of the textile
material for this system [85, 86]. The production of high resistivity water, the desalination of
Na+ and Cl- [87], and the extraction of impurities from phosphoric acid [88] have been studied
with this class of resin with good results.
In seeking alternatives for ion exchange materials, immobilized ion-exchange
polyurethanes (IEPU) containing polyurethane foams have been synthesized by either bulk
condensation [89] or a blending method in order to obtain immobilized ion resin beads [90].
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Page 19 of 49
Effects about the permselectivity have been studied in order to increase it, and new kind of
ion exchange material developed [91]. Similarly, Dzyazko et al. have investigated a
(hydrated zirconium dioxide) xerogel ion exchange component of composite ceramic
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membranes [92], and zirconium hidrophospathe has been validated in the electromigration of
Ni2+ and Cd2+ [93- 95]. In the case of membranes, Larchet et al. proposed the use of profiled
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membranes in order to obtain a higher rate of mass transfer [96] or techniques membrane-
free, avoiding the problems associate to its use, as fouling or maintenance costs, including
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capacitive deionization [97-99].
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A number of weaknesses may be found in EDI electrochemical reactors, such as the
leakage of ions and uneven flow distributions within the dilute compartment [100]. In
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seeking to improve these deficits toward the reliable and robust development of EDI, in 2007,
Arora et al. reported the use of a “wafer in an electrodeionization” system [101]. The
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synthesis of this porous wafer involved the use of ion exchange resins that were combined
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with a binding agent. This technology was dubbed Wafer Enhanced Electrodeionization
(WE-EDI) and its development demonstrated the capacity for reducing the weaknesses of
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EDI. WE-EDI was carried out with the aim of studying variables such as porosity, capacity,
permeability and ion exchange beds. Wafer thickness and capacity were shown to have little
effect on the ability to transport. However, cation-anion resin ratios, polymer volumes (used
to bind the resins) and resin selectivity represented significant factors for improved
development [100].
Other developments involved the use of an electrochemical Faraday cage concept,
which was introduced by Dermentzis [102-104], where electronically and ionically porous
media were used in place of membranes to function as ion traps. This media collected ions
by acting as a concentrate compartment; hence these types of systems may operate without
fouling issues, as illustrated in Fig. 12.
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Page 20 of 49
6.
Applications
Since its inception, EDI has been utilized primarily in the generation of ultrapure water to
serve the energy, microelectronics, food, chemical and pharmaceutical industries. The
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product water obtained can be between 8 - 18 MΩ cm from solutions that contain from 1 to
20 ppm TDS (total dissolved solids), with an energy consumption of ~0.26 kW-h/m3. Hence,
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most EDI studies have involved the evaluation of efficacy for the removal of various ions,
such as Co2+, Cu2+, Ni2+ and NH4+, with the aim of obtaining ultrapure water. EDI is capable
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of removing all of these ions and has since found utility in a number of associated
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applications, such as the removal of wastewater resident pollutants and organic compound
purification [105]. On other hand the research about this issue involves the search of better
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operation parameters, and studies about technical and economical aspects have been
discussed by some authors [106].
Heavy metals removal
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6.1
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Electrodeionization systems have been applied for the removal of ions from various types of
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wastewater, such as mining, electroplating and nuclear processes, where the ions primarily
present are those related to chromium, copper, nickel, cobalt and others. Mining effluent is
typically treated via precipitation, which forms sludge; thus researchers are focused on
finding alternative methods. In this way, for example, electrochemical ion exchange
combined with ED has been applied for the removal of arsenic [107]. Yeon et al. studied the
removal of Fe, Co, Cr and Ag ions from synthetic solutions, attaining 99% removal [108].
Souilah et al. studied the reuse of industrial aqueous effluent water, which contained Zn, Cu,
Cd and Pb by IX, and a comparison with EDI was conducted, with the best results obtained
with EDI [109]. Thus, the success of EDI has been validated and researchers currently strive
to attain optimal operational parameters and define major application fields. For example,
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Page 21 of 49
parameters such as the influence of resin particle size distribution on the performance of the
electrodeionization process have been reported by Lu et al. [110], where it was found that a
mixed bed with a narrow size distribution improves performance.
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6.1.1 Chromium removal
Hexavalent chromium is a harmful ion that is frequently found in industrial wastewaters as
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the result of mining and electroplating processes. Therefore, various technologies have been
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applied in an attempt to remove these species of ions, and EDI has not been an exception.
The capability of EDI holds major significance insofar as the protection of the environment,
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as hexavalent chromium is highly toxic [111, 112]; and an increasing number of studies have
investigated its removal. For instance, the application of the hybrid ion exchange and
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electrodialysis system achieved an over 98% removal of Cr (VI) [113, 114]. In addition,
Bergmann et al. carried out experiments using three types of resins and conditioning to
d
remove CrO42-, where transport numbers and conductivities through the ion exchange bed
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were determined [115]. Further, Dzyazko et al. evaluated diffusion coefficients in ion
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exchange materials used during the electrodeionization of this compound [116]. A
comparison between EDI, ED and IX was also done for the removal of Cr(VI) in order to
elucidate the robustness of EDI. Results indicated that 99.8% of Cr(VI) was efficiently
removed by this technology [80]. Xing et al. noted a significant Cr(VI) removal by CEDI,
resulting in concentrations of 0.09 and 0.49 ppm, from 40 and 100 ppm Cr(VI) solutions,
respectively, over 50 h of operation, with an energy consumption of 4.1-7.3 kWh/mol Cr(VI)
[117, 118]. Most recently, Alvarado et al. reported the integration of ion exchange and
electrodeionization as a new approach for the continuous treatment of hexavalent chromium
wastewater [79]. When the mixed resin bed was completely saturated, over 98.5% of the
Cr(VI) was still continuously removed with the continuous electrodeionization (CEDI)
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Page 22 of 49
process being operated at a 10% over limiting current, which facilitates the electroregeneration of the resin in situ and the continuous removal of Cr(VI) from the dilute
compartment, while Cr(VI) is recovered in the concentrate compartment for re-use. The
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energy consumption of the developed CEDI process is very low (<0.07 kWh/m3).
6.1.2 Removal of Copper, Cadmium and Nickel
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Evaluations focused on the recovery of heavy metals from simulated dilute industrial
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wastewaters have been carried out for the recovery of copper from CuSO4, which employed
simulated rinsing waters from copper plating lines [119-123] and their models of operation
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have been studied and reported [124, 125]. The treatment of solutions with variable ions is
difficult due to the competition that is established between each ion, hence a study involving
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the priority of separation between heavy metals with the same valence was conducted, and it
was found that it follows the sequence: Pb2+>> Cd2+ > Cu 2+ ≥ Zn2+ [126].
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Dermentzis, et al. [127] studied the ion and ionic current sinks for the
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electrodeionization of simulated cadmium plating rinse waters, applied for Cd2+ removal and
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obtained ultrapure water, achieving 50 ppm to 0.1 ppm Cd2+. The properties of IX
membranes were evaluated during the EDI process, which showed that a 99% nickel ion
removal was attained with EDI membranes, while this separation efficiency was lower using
heterogeneous IX membranes.
On other hand, nickel ion removal from dilute heavy metal solutions via the
electrodeionization process has been studied extensively; examples of solutions containing 50
ppm Ni2+ have attained 99.8% removal and a concentration stream as high as 1583 ppm
during the process [128-131]. Electrostatic shielding electrodialysis / electrodeionization has
been applied to the removal of nickel. It was found that with this new system, which
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Page 23 of 49
incorporated fewer membranes that 80% removal was obtained in 35 min under 30 A/m2
current.
6.1.3 Cobalt removal
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An electrostatic shielding-based system has been employed for removal of cobalt ions. A
resulting concentration of 0.1 ppm was achieved from solutions containing 300 ppm cobalt
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[132]. In addition, the selective separation of Ni(II)/Co(II) ions from dilute aqueous solutions
us
using continuous electrodeionization in the presence of EDTA showed success in the
separation, since EDTA worked as complexing agent [133]. Li et al. removed Co2+ and Sr2+
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from a primary coolant by continuous electrodeionization packed with weak base anion
exchange resins attaining 2 ng/L for cobalt ions and 58-114 ng/L for strontium ions, which is
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much lower than the concentration achieved through the use of commercial stacks [134]. A
study on the removal of cobalt from a primary coolant by CEDI with various conducting
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spacers was undertaken [135], where the cobalt ions were removed with a stack of EDI that
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contained three types of gaps: ion exchange resin, immobilized polyurethane ion exchange
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p
resin and textile ion exchange and as a result a maximum of 99% removal was obtained
[136].
6.1.4. Removal of other ions
Ammonium ions have been removed by applying a two-step EDI process, which attained 1
ppm from a 200 ppm ammonium starting solution [137]. These ionic species removals have
been studied by other authors as well [138, 139]. In addition, silicon and boron from ultrapure
water have also been successfully attempted [140]. Nitrates are frequently found in organic
separations, and these types of ions are also present in groundwater; hence their separation
from water has been successfully achieved [141-144]. In comparison with ED, this
technology (EDI) exhibits almost complete removal (99%) [145].
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Page 24 of 49
6.2
Organic compounds separation
EDI has the capacity to work with ions and their use as ED has been applied to organic
compounds. For example, wafer-enhanced electrodeionization in a continuous fermentation
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process has been applied in the production of butyric acid with Clostridium tyrobutryricum.
The separation was shown to be efficient and operated without interruption for more than one
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month, demonstrating a 40-fold increase in concentration [146]. This technology (EDI) has
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also been shown as successful in the production of organic acids [147]; biomolecules such as
amino acids, peptides and proteins have been separated by this method as well, due to their
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amphoteric properties [148]. The extractive fermentation of L-(+)-lactic acid by Pediococcus
pensosaceus allows for various operating conditions [149]. Citric acid recovery has attained
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up to 6000 ppm in the concentrate compartment from a solution containing only 70 ppm
[150].
High purity water production
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6.3
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Because of corrosion, environmental concerns and unsafe treatment media, developments in
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EDI systems have been evolved in order to obtain high purity water [151-159]. The
electrochemical characterization of ion-exchange resin beds and the removal of cobalt from
the coolant of an energy plant have been studied by Yeon et al. [160], which showed a point a
capacity of over 4 meq/g with 97% removal. In order to obtain improved results several
technologies have been combined in order to generate new alternatives, such as RO-EDI
[161, 162], EDR and EDIR [163] and Osmosis-CEDI, which replaces RO-IX, and does not
employ chemical regeneration, as it implies the generation of a secondary wastewater.
Hence, RO-EDI is available to obtain water with a resistivity of 10-16 MΩ cm [164]. A
Reverse Osmosis-Electrodeionization-Layered Bed (RO/EDI/LB) filtration system was
utilized to obtain ultrapure water, 17 MΩ cm was attained with the treatment of 0.7 kWh/ton
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Page 25 of 49
of water [165]. Pure water production from aqueous solutions containing low hardness ion
concentrations can be treated by EDI [166] and RO-EDI processes to obtain pure water with
7.
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99.8% hardness rejection [167].
Conclusions and future outlook
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Electrodeionization has immense possibilities over an expanding range of applications, not
only for the generation of ultrapure water, but for selective separation and concentration. Its
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development was quite slow (~60 year span since its inception) due to the lack of a detailed
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understanding of its mechanisms. A solid knowledge of the fine kinetics of these systems
constitutes an important prerequisite for the further development and advancement of this
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technology. This green, environmentally compatible separation strategy has a very promising
future, with primary advantages that include residue free operation, low process cost and
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exemplary efficacy in ionic separation. The advent of improved novel materials, electrostatic
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shielding, wafer ion exchange and nanomaterials, as new electrode coatings with enhanced
activities and higher surface areas emerge, will translate to the potential for significantly
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p
increasing the efficiency of EDI systems. Further improvements will likely be seen in EDI
systems that demonstrate higher effectiveness while operating at reduced cost, due to lower
energy consumption, making them attractive for industrial scale up over a broad range of
applications worldwide.
Acknowledgements
This work was supported by a Discovery Grant from the Natural Sciences and Engineering
Research Council of Canada (NSERC). L.A. thanks to the National Council of Science and
Technology of Mexico (CONACYT) for the postdoctoral scholarship (Number 161479).
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Page 26 of 49
A.C. acknowledges NSERC and the Canadian Foundation for Innovation (CFI) for the
Canada Research Chair Award in Materials and Environmental Chemistry.
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t
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cr
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[105] P. Yu, Z. Zhu, Y. Lou, Y. Hu, S. Lu, Desalination 174 (2005) 231.
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cr
[120] X. Feng, J. Gao, Z. Wu, J. Zhejiang Univ. Sci. A. 9:9 (2008) 1283.
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te
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t
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33
Page 33 of 49
TABLES AND FIGURES
Table 1. Electrodeionization evolution
Researcher
Development
Company
1953
Kollsman
CEDI: device for acetone purification
Dutch company
1955
Walters et al.
Argonne National Lab.
1959
Glueckauf
Electrodeionization: concentration of
radioactive waste
CEDI: theory, operation and conditions
1960
Sammons and Watts
1971
Matejka
1986
Giuffrida, Jha, Ganzi.
1989
Parsi
1991
Katz, Elyanow, Sims
1992
White
1999
DiMascio, Gary, Ganzy
2004
Srinivasan, Nebojsa,
Avdalovic
Chidambaran, Devesh,
Sharma, Raina
2008
Barber
2011
Riviello
2012
Riviello
us
cr
Harwell Atomic Energy
Authority
an
M
NA
Millipore Co.
Ionics Inc.
Ionics Inc.
Millipore Co.
United States Filter Co.
Dionex Co.
Fractional deionization process
Aquatech International
Co.
d
Avijit, Gareth
NA
Water purifier and method
Apparatus and method for continuous
electrodeionization
Method and apparatus for shifting current
Method of ion chromatography wherein a
specialized electrodeionization apparatus is
used
Method of simultaneous anion and cation
suppression in a continuous
electrodeionization apparatus
Ac
ce
p
2005
Apparatus for the removal of dissolved solids
from liquids using bipolar membranes
Electrodeioization polarity reversal apparatus
and process
Electredeionization ad ultraviolet light
treatment method for purifying water
Electrodeionization apparatus and method
te
2005
Sodium salts deionization through EDI
device
Continuous Production of High-Purity Water
by Electro-deionisation
Electrodeionization apparatus
ip
t
Year
Chemitreat Pte. Ltd.
General Electric
Company
Trovion Singapore Pte.
Ltd.
Trovion Singapore Pte.
Ltd., Co.
34
Page 34 of 49
Table 2. Resins used in electrodeionization systems.
Name
Ion
Amberlite IRA-67
Amberlite IRA-400
Amberlite IRA 402Cl
Amberlite IRN 77
Amberlite IRN 78
Duolite C20
Zirconium hidrophosphate
D072
D296
SO42-
121
001*7
201*7
D354
Ca2+
CO32HCrO4-
165
165
114
Purolite C 150 PHL
Purolite A 520E
PFC100E
PFA444
Purolite A400
cr
Purolite Int. Ltd.
Purolite Int. Ltd.
Purolite Int. Ltd.
Purolite Int. Ltd.
Purolite Int. Ltd.
Ac
ce
p
te
d
M
an
4765
4766
Diaion SA10A
Diaion SK1A
Diaion SKN 1
Purolite 201
Purolite C100E
ip
t
115
115
9
9
110
110
64, 120, 123
64, 120, 123
153
153
64, 120, 123
80
95
119
119
88
86
88
Merck, Germany
Merck, Germany
Mitsubishi
Mitsubishi
Mitsubishi
Purolite Int. Ltd.
Purolite Int. Ltd.
Rohm and Haas Co.™
Rohm and Haas Co.™
Rohm and Haas Co.™
Rohm and Haas Co.™
Rohm and Haas Co.™
Rohm and Haas Co.™
Rohm and Haas Co.™
Synthesized in Lab
The chemical plant of Nankai University,
China
The chemical plant of Nankai University,
China
Tianjin Hecheng S&T Develop. Co. Ltd.
Tianjin Hecheng S&T Develop. Co. Ltd.
Zhenguang Co.
References
CrO4CrO4Ca2+, Fe2+
Cl-, SO42-, PO43Ni2+
SO42Cu2+ , Ni2+
Cu2+ , Ni2+
ClNa+
Cu2+ , Ni2+
K+
Ni2+
Cu2+
SO42PO43Cl-, Na+
Cd2+, Fe2+, HCrO4-,
Mg2+, Zn2+
Na+
ClAmino acids
Amino acids
Co2+
Cl-, F-, NO3Ca2+, Co2+, Mg2+,
Na+, Ni2+,
Cu2+
NO3Organic acids
Organic acids
Citric acid, Cl-,
HCO3-, NO3-, SO42Ca+, Co+, HCO3-,
K+, Na+, Mg2+
HCrO4 Cl-, SO4Cl-, CO32-, SO42Co2+, Na+, Ni2+
Cl-, SO4Pb2+, Cu2+, Zn2+
Ni2+
Cu2+
Lewatit MPC 64
Wolfatit SZ 30
Ceralite IR 120
Ceralite IRA 400
D072
D296
Dowex 50 WX 2%
Dowex 50 WX 4%
Dowex A550 UPW
Dowex C650 UPW
Dowex HCR-S 8%
Dowex Mac-3
Dowex MSC-1
D001
D201
AET cellulose
Bipolar IET
CET cellulose
us
Supplier
Bayer Leverkusen-Wolfen
Bayer Leverkusen-Wolfen
Central Drug House
Central Drug House
Chemical Plant of Nankai University, China
Chemical Plant of Nankai University, China
Dow Chemical Co.™
Dow Chemical Co.™
Dow Chemical Co.™
Dow Chemical Co.™
Dow Chemical Co.™
Dow Chemical Co.™
Dow Chemical Co.™
Hangzhou Resin Co.
Hangzhou Resin Co.
Institut Francais du Textile et de l`Habillement
Institut Francais du Textile et de l`Habillement
Institut Francais du Textile et de l`Habillement
Amberlite IR-120
70
70
138
138
159
71
7,132, 144, 149
118
143
101
101
144, 149
4, 31, 49, 73,
100, 103, 159
80
100, 107, 110
4, 49
68, 90, 159, 162
68, 90, 162
125
95
121
35
Page 35 of 49
Table 3. Types of membranes used in electrodeionization systems.
DuPont™
Ionics
Membranes International
Membranes International
Merck
Nafion 450
CR67
AMI-7001
CMI-7000
Packed beds of graphite powder
(ESCC1)
ADS XL 10
CDS XL 5
Heterogeneous ion Exchange
Double Flower® Heterogeneous
Ionac MA-3475
Sybron MC-3470
Neosepta ACS
Neosepta AFN
Neosepta AMH
Neosepta AMX
Ac
ce
p
Morgane-Solvay
Morgane-Solvay
Shanghai Shanghua water
treatment material Co.
SWTM
Sybron Chemicals Inc.
Sybron Chemicals Inc.
Tokuyana Soda Co. ™
Tokuyama Soda Co. ™
Tokuyama Soda Co.™
Tokuyama Soda Co. ™
References
116
104
104
ip
t
134
cr
70, 86
70, 86
49, 74
49, 74
49, 74
138
23, 92, 114, 116,117,
120, 123,124
88
7, 132
92, 95
95
103, 104, 126
us
DuPont™
an
NA
Asahi Glass
Asahi Glass
Astom Co., Japan
Astom Co., Japan
Astom Co., Japan
M
NA
Graphite powder: Ionic current
sinks (ICS)
Porous conducting bipolar
ceramic plates (EICM-IEs2)
Polimeric composite carbon plates
(EICM-Ies2)
Selemion AMV
Selemion CMV
CMX
AMX
CMX
MA-41
Nafion 117®
d
NA
Name
Composite ceramic membrane
te
Supplier
NA
144
144
121, 165
114, 117
23, 88, 142, 149
31, 49, 149
100
80
101, 153
4, 73, 79, 113, 125, 143,
162
9, 107
101
80
153
4, 73, 79, 125, 143, 162
138
138
Tokuyama Soda Co. ™
Neosepta APM
Tokuyama Soda Co. ™
C6610F.
Tokuyama Soda Co. ™
CM-1
Tokuyama Soda Co.™
Neosepta CMB
Tokuyana Soda Co. ™
Neosepta CMX
UCC Ltd.
MA-41
UCC Ltd.
MC-40
1Electrostatically shielded concentrate compartment.
2Electronically and ionically conducting media-intermediate electrodes.
36
Page 36 of 49
Table 4. Electrodes used in electrodeionization cells.
ip
t
us
C
C
C
A
A
A
C
A
A
Ac
ce
p
te
d
M
Stainless steel grids
Ti mesh
Ti plate
Ti/IrO2-SnO2-Sb2O5
Ti/IrO2-Ta2O5
Ti/Pt
Ti/Pt
Ti/RuO2
Ti mesh/TiO2, RuO2
References
86
103
103, 104, 116
116
126, 134
61, 73, 95, 115, 149
61, 73, 95
100
7, 9, 23, 26, 71, 79, 80, 100, 107,
113 , 132,141, 143, 149, 165
103, 104
114, 115
115
114, 117
80
7, 4, 49, 123, 124, 143, 132
4, 49, 120,123, 124
9, 23, 71, 119, 165
113
cr
C-Cathode/A-Anode
A, C
A
A
C
A, C
A
C
A
C
an
Electrode material
Graphite
Graphite powder
Platinized Ti
Platinized Ti
Platinized Ti grids
Pt
Pt
Stainless steel
Stainless steel
37
Page 37 of 49
+
+
+
- + +
+
+ + -
-
+
-
-
+
+
+
+
-
ip
t
ANODE
-
+
-
CATHODE
cr
+
+
-
+
+
-
us
+
-
Cationic membrane
-
+
Cationic membrane
-
Dilute, Outlet
Anionic membrane
Concentrate, Outlet
+
-
Electrodic
an
Electrodic
solution
solution
M
Feed
d
Feed
Ac
ce
p
te
Fig. 1. Representation of electrodialysis process.
38
Page 38 of 49
-
Counterion (mobile ion)
+
Co-ion
cr
-
Fixed ion
+
Counterion (mobile ion)
-
Co-ion
Ac
ce
p
te
d
Fixed ion
-
Cation Exchange Membrane
M
Anion Exchange Membrane
+
-
ip
t
+ - +
- + - +
+ + - + - + + - + ++ +
+
- +
+ - +
+ - +
- + - +
+
+
+ + - + + +
+ + + +
+
+ + - +
us
+
- + - + - +
+ - + + - +
-
an
+
+ - + - + + - + - + - + - + - + - + - +
- + - + - + - +
+
- + - +
-+
- +
+
+ - + + +
- + +
+
+ +
+
+
- +
+ - + +
+
- +
- +
+
- +
- + -
Fig. 2. Ion exchange membrane structure
39
Page 39 of 49
+ +
Counterion (mobile ion)
+
-
-
Co-ion
+
-
+
- +
-
-
+
-
-
-
-
-
-
-
+
-
-
-
-
M
an
-
- - +
- -
-
-
us
-
-
-
cr
+
ip
t
-
Fig. 3. Top: example of an anion ion exchanger, with positive charges in the matrix and its
Ac
ce
p
te
d
counterions. Bottom: example of an anion exchange process.
40
Page 40 of 49
+
Anode
CEM
AEM
Feed
an
us
cr
-
ip
t
Dilute
Concentrate
CEM
M
Feed
Cathode
Fig. 4. Representation of an electrodeionization cell. The spheres represent the ion
te
Ac
ce
p
Membrane.
d
exchange resin, AEM: Anionic Exchange Membrane, CEM: Cationic Exchange
41
Page 41 of 49
d1
e1
d2
e2
2.
Through alternating layers of particles and interstitial solution
3.
Channel of the liquid phase betwee particles
a
b
c
M
an
Through particles in contact with each another
us
d3
1.
ip
t
b)
cr
a)
Fig. 5. Porous plug model: a) Conduction routes through resin bed, b) Model representation
Ac
ce
p
te
publications.
d
of three conductance elements. Reproduction from ref. 60 with permission of ACS
42
Page 42 of 49
ip
t
cr
us
κ
an
⎛ d κ b ⎞ = ae + c
⎜ dκ ⎟
⎝
⎠κ =κ
d
M
Bed, specific conductance, Ω -1 cm-1
⎛ dκ b ⎞ = a + c
⎜ dκ ⎟
⎝
⎠κ = 0 e
Ac
ce
p
te
Interstitial solution, specific conductance, Ω -1 cm-1
Fig. 6. Plot of bed specific conductance vs interstitial solution specific conductance, to
calculate geometric parameters for the Wyllie model. Reproduced from ref. 56 with
permission of Dover.
43
Page 43 of 49
ip
t
cr
us
an
M
d
te
Ac
ce
p
Fig. 7. Current-voltage curve of the CEDI system. Reproduced from ref. 68 with permission
of Springer.
44
Page 44 of 49
+
CM
AM
CM
-
OH-
H+
us
AR
HCrO4-
an
K+
HCrO4K+
te
d
HCrO4K+
M
2H2O → O2 + 4H+ +4e-
K+
CR
2H2O + 2e- → H2 + 4OH-
HCrO4-
cr
OH-
ip
t
OH-
Ac
ce
p
Fig. 8. Water splitting sites, where “in situ” regeneration has taken place. Reproduction from
ref. 79 with permission of Elsevier.
45
Page 45 of 49
ip
t
cr
us
an
M
d
te
Ac
ce
p
Fig. 9. a) Unit of contacting ion exchange beads in a CEDI system; b) Schematic
representation of the paths that the current may take; c) Equivalent circuit for determining the
special dispersion of current. (RB is resistance of the bead, Rc is resistance of the contacting
point, Rs is resistance of the solution, Rb is resistance of the boundary layer, and C bis
capacitance of the boundary layer). Reproduction from ref. 73 with permission of Elsevier.
46
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AEM
CEM
RESIN
CEM
Stainless steel
Concentrate
compartment
Dilute
compartment
Cathodic
compartment
te
d
Anodic
compartment
M
an
us
cr
ip
t
DSA
Ac
ce
p
Fig. 10. Electrodeionization cell configuration. Reproduction from ref. 80 with permission of
Elsevier.
47
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14
ip
t
12
cr
8
Ilim
6
an
4
us
I [mA]
10
0
1
2
3
4
5
te
d
0
M
2
E [V]
Ac
ce
p
Fig. 11. Limiting current determination in a Cr (VI) removal process. Reproduction from ref.
79 with permission of Elsevier.
48
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ip
t
cr
us
an
M
d
te
Ac
ce
p
Fig. 12. Electrochemical cell applying the Faraday cage concept. Reproduction from ref.
104 with permission of Elsevier.
49
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