Principle of ionexchange membrane processes

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The processes that utilize ion-exchange membranes as a key component can conveniently be divided into three types: (1) electrodeionization processes, (2) electrosynthesis processes, and (3) electromembrane energy conversion processes. In the first type of processes, an electrical potential gradient is used to remove charged components such as dissociated salts from a solution. In the second type of processes, the transport of ions is combined with an electrochemical reaction producing certain chemicals such as bases and chlorine from the corresponding salts. The third type of processes involves the conversion of chemical into electrical energy as, for example, in fuel cells.

1.2.1 Ion-exchange membranes deionization processes

In ion-exchange membrane deionization processes such as electrodialysis, diffusion, and Donnan dialysis, or electrodeionization and capacitive deionization low-molecular-weight ions are removed from a feed solution through ion-exchange membranes and concentrated under the driving force of an electrochemical gradient.

Electrodialysis. The principle of electrodialysis is illustrated in Fig. 2, which shows a schematic diagram of an electrodialysis cell arrangement consisting of a series of anion- and cation-exchange membranes arranged in an alternating pattern between an anode and a cathode to form individual cells. If an ionic solution such as an aqueous salt solution is pumped through these cells and an electrical potential is established between the anode and cathode, the positively charged cations migrate toward the cathode and the negatively charged anions toward the anode. The cations pass through the negatively charged cation-exchange membrane but are retained by the positively charged anion-exchange membrane. Likewise, the negatively charged anions pass through the anion-exchange membrane, and are retained by the cation-exchange membrane. The overall result is an increase

Concentrate

Anode

Electrode rinse

Concentrate

Anode

Electrode rinse

Cathode b©

Feed

Repeating unit

Figure 2 Schematic diagram illustrating the principle of desalination by electro-dialysis in a stack with cation- and anion-exchange membranes in alternating series between two electrodes.

Cathode b©

Electrode rinse

Feed

Repeating unit

Figure 2 Schematic diagram illustrating the principle of desalination by electro-dialysis in a stack with cation- and anion-exchange membranes in alternating series between two electrodes.

in the ion concentration in alternate compartments, while the other compartments simultaneously become depleted of ions. The depleted solution is referred to as the diluate and the concentrated solution as the brine. The space between two contiguous membranes occupied by the diluate and the brine and the two contiguous anion- and cation-exchange membranes make up a cell pair, which is a repeating unit in a so-called electrodialysis stack, which may have a few hundreds cell pairs between two electrodes [4].

Electrodialysis is used mainly today for desalination of brackish water and demineralization of solution in the food and drug industry as well as in the concentration of salts from seawater.

Electrodialysis with bipolar membranes. The conventional electrodialysis can be combined with bipolar membranes and utilized to produce acids and bases from the corresponding salts. In this process monopolar cation- and anion-exchange membranes are installed together with bipolar membranes in alternating series in an electrodialysis stack as illustrated in Fig. 3. A bipolar membrane consists of a laminate of a cation- and an anion-exchange layer. If an electrical potential difference is established across the membrane, charged species are removed from the interphase between the two ion-exchange layers. When this interphase contains only water, the transport of electrical charges is accomplished by protons and hydroxide ions, which are

Acid

Base

Acid

Base

Salt

Figure 3 Schematic drawing illustrating the principle of electrodialytic production of acids and bases from the corresponding salts with bipolar membranes.

produced continuously in the bipolar membrane by water dissociation due to the driving force of an electrical potential gradient. The H+ and OH~ ions, removed from the interphase of the bipolar membrane form with the salt ions of the feed solution an acid and a base in the two compartments between the two monopolar and the bipolar membranes. A cation-exchange, an anion-exchange, and a bipolar membrane form a repeating unit in the stack between two electrodes. Thus, a repeating unit is composed of three separate flow streams, that is, the salt containing feed solution flow stream and two product solution flow streams containing an acid and a base.

The utilization of electrodialysis with bipolar membranes is economically very attractive and has a multitude of interesting potential applications [5].

Continuous electrodeionization. Continuous electrodeionization is very similar to conventional electrodialysis. However, the cell of the diluate flow stream is filled with a mixed-bed ion-exchange resin. The principle of the process is illustrated in Fig. 4.

Feed

Cl"

Feed

Cl"

Concentrate m it

Diluate Concentrate

----Repeating unit

O Anion-exchange O Cation-exchange

Figure 4 Schematic drawing illustrating the principle of the continuous electro-deionization process.

The mixed-bed ion-exchange resin in the diluate cell of an electro-dialysis stack binds the ions of a feed solution. Due to an applied electrical field, the ions migrate through the ion-exchange bed toward the adjacent concentrate cells. The ion-exchange resin increases the conductivity in the diluate cell substantially, and at very low salt concentrations in the feed solution water is dissociated at the contact point of the cation- and anion-exchange resin beads generating protons and hydroxide ions, which further replace the salt ions in the resins. The result is completely deionized water as a product. Compared to the deionization by a conventional mixed-bed ion-exchange resin, continuous electrodeionization has several advantages since no chemicals are needed for the regeneration of the ion-exchange resins, which is time-consuming, labor-intensive, and generates a salt containing wastewater. Fig. 4 shows just one concept of an electro-deionization stack. In practicable applications, various stack concepts based on separate ion-exchange beds or bipolar membranes are used [6].

Diffusion dialysis. Diffusion dialysis is used mainly today to recover acids or bases from a mixture with salt ions. Its principle is illustrated in Fig. 5, which shows a schematic diagram of a typical diffusion dialysis cell arrangement consisting of a series of anion-exchange membranes arranged in parallel to form individual cells. If a feed solution containing a salt in a mixture with an acid is separated by an anion-exchange membrane from a

Acid

Water

Salt + Acid

Acid

Water

Figure 5 Schematic drawing illustrating the principle of diffusion dialysis used to recover an acid from a mixture with salt in a stack of anion-exchange membranes only.

compartment containing pure water as so-called stripping solution anions will diffuse from the feed solution through the ion-exchange membrane into the stripping solution due to a concentration difference, while the salt cations will be retained by the membrane. The protons, however, can pass the anion-exchange membrane in spite of their positive charge. Thus, the acid will be removed from the salt solution. Correspondingly, a base can be removed from mixtures with salts if cation-exchange membranes are used.

Diffusion dialysis is used to recover acids from pickling solutions in the metal surface treating industry [7]. However, its commercial relevance is still rather limited because ofcosts. Since the diffusion through the relatively thick ion-exchange membranes is a rather slow process, large membrane areas are required to remove a significant amount of ions from a feed solution, resulting in high investment costs for a given capacity plant.

Donnan dialysis. The principle of Donnan dialysis is shown in Fig. 6. Only cation- or anion-exchange membranes are installed in a stack. The driving force for the transport of ions is their concentration difference in the two phases separated by the membranes [8]. A typical application of Donnan dialysis is the removal of divalent ions such as Ca2+ from a feed stream by the exchange for monovalent ions such as Na+ in water softening

Repeating cell unit Product Na2SO4

Repeating cell unit Product Na2SO4

Stripping NaCl Feed CaSO4 Stripping NaCl

Figure 6 Schematic drawing illustrating the principle of the Donnan dialysis water softening process by the exchange of Na+ and Ca2+ ions in a stack with cation-exchange membranes only.

Stripping NaCl Feed CaSO4 Stripping NaCl

Figure 6 Schematic drawing illustrating the principle of the Donnan dialysis water softening process by the exchange of Na+ and Ca2+ ions in a stack with cation-exchange membranes only.

as illustrated in Fig. 6, which shows a feed solution containing CaCl2 in relatively low concentration and a stripping solution containing NaCl in relatively high concentration flowing through alternating cells of a stack of cation-exchange membranes. Because of the concentration difference in the feed and the stripping solution Na+ ions diffuse from the stripping solution through the cation-exchange membrane into the feed solution. Since the Cl_ ions cannot permeate the negatively charged cation-exchange membrane, an electrical potential is generated between the two solutions, which acts as driving force for the transport of Ca2+ ions from the feed to the stripping solution. Because of the required electroneutrality the identical charges are exchanged between the two solutions, that is, for two Na+ ions diffusing from the stripping into the feed solution one Ca + ion is removed from the feed solution if the membrane is completely impermeable for Cl ions. The ion transport in Donnan dialysis is referred to as countercurrent transport.

In addition to water softening, there are several other interesting applications in wastewater treatment, but up to today, there is very little large-scale commercial use of Donnan dialysis.

Capacitive deionization. Capacitive deionization is an electrosorption process that can be used to remove ions from an aqueous solution by charge separation. The process is similar to conventional electrodialysis. But it also differs in a number of ways from electrodialysis as well. The main difference is that in capacitive deionization ions are removed from a solution without an oxidation/reduction reaction and the electrode compartments participate directly in the deionization and ion concentration process, that is, the anolyte and catholyte are contained within the porous electrodes and electrons are not transmuted by oxidation/reduction reactions but by electrostatic adsorption [9]. A cell of a capacitive deionization unit consists of two electrodes made out of activated carbon separated by a spacer that acts as a flow channel for an ion containing solution as illustrated in Fig. 7. The system resembles a ''flow-through capacitor.'' If an electrical potential is applied between the electrodes, ions are removed from the solution and adsorbed at the surface of the charged electrodes. When the carbon electrodes are saturated with the charges, that is, the ions are released from the electrodes by reversing the potential, that is, the cathode becomes the anode and vice versa the anode becomes the cathode. Thus, capacitive deionization is a two-step process. In a first step as shown in Fig. 7a, ions are removed from a feed solution by electrosorption and migration in the feed solution under an electrical potential driving force, resulting in deionized

Figure 7 Schematic diagrams illustrating the capacitive deionization process: (a) sorption of ions from a feed solution at the porous carbon electrodes producing deionized product water and (b) desorption of ions from the porous carbon electrodes into the feed solution due to a change of polarity-producing concentrated brine.

Figure 7 Schematic diagrams illustrating the capacitive deionization process: (a) sorption of ions from a feed solution at the porous carbon electrodes producing deionized product water and (b) desorption of ions from the porous carbon electrodes into the feed solution due to a change of polarity-producing concentrated brine.

product water. In a second step as shown in Fig. 7b, the adsorbed ions are released from the carbon electrodes and transported back into the feed solution by reversing the polarity-producing concentrated brine.

A key component in this process is the carbon electrode. Since the number of ions adsorbed at the electrodes is directly proportional to the available surface area the specific surface area, that is, the surface area per unit weight of the electrodes should be as high as possible. Activated carbon, carbon nanotubes, and especially carbon aerogels are the most promising materials. Their specific surface area is up to 1100m2g~1. Another parameter that determines the energy consumption required to transport the ions from the feed solution to the electrodes is determined by the number of ions removed from the feed solution, that is, the concentration difference between the feed solution and the product and the applied voltage between the electrodes. Therefore, the resistance of the feed solution should be as low as possible. To avoid electrode reaction, which results in water dissociation and the production of hydrogen and oxygen or chlorine, the voltage drop at the electrodes should not exceed a certain value given by the water dissociation potential. Therefore, capacitive deionization cells are operated at a voltage drop of between 0.8 and 1.5 V. The deionization of a given feed solution and the regeneration of the capacitor is a function of time. The efficiency of the capacitive deionization is impaired by incomplete sorption and desorption of ions at the corresponding electrode especially at high ion concentrations due to the electrode pore solution concentration. Dissolved counterions in the pore solution are adsorbed on the electrode surface while the co-ions are expelled. Thus, counterions occupy capacitance within the electrode, which then is unavailable for the removal of ions from the feed solution. Co-ions expelled from the electrodes enter into the feedwater stream and increase the ion concentration in the purified product water in the deionization step. During the regeneration step, ions are desorbed from the electrodes and transported into the feed solution, increasing its concentration above its original value. However, when the voltage is reversed, ions are simultaneously adsorbed and repelled. This affects the upper limit of the concentration of the regeneration stream and reduces the ionic efficiency of capacitive deionization. The effect of the pore solution transport can be reduced significantly by placing a charged barrier between the feed solution and the electrodes as illustrated in Fig. 8, which shows schematically the ion transport in membrane-capacitive deionization.

During the deionization step, anions are prevented to diffuse into the product water by a cation-exchange membrane on the cathode- and an anion-exchange membrane on the anode as shown in Fig. 8a. In the

Product

Cathode

Brine

Anode

Feed

Anode

Cathode

Feed

Cation-exchange membrane Anion-exchange membrane

Figure 8 Schematic diagrams illustrating the capacitive deionization process with ion-exchange membranes between the feed solution and the porous carbon electrodes (a) shows adsorption, that is, the deionization step and (b) shows the desorption of ions due to a change of polarity-producing concentrated brine, that is, the regeneration step.

regeneration step under reverse polarity condition, the cation-exchange membrane prevents now the transport of anions toward the anode and the anion-exchange membrane the transport of cations toward the cathode as shown in Fig. 8b and thus avoids ion adsorption at the electrodes during the regeneration step. The consequence of introducing an ion-exchange membrane between the feed solution and the electrode is that more ions are adsorbed during the deionization step and more ions are desorbed and released during the regeneration step than in a capacitive deionization process without ion-exchange membranes between feed solution and electrodes. In an industrial-size capacitive deionization unit, a cathode, a feedflow channel, two ion-exchange membranes, and an anode are stacked as repeating units between two endplates.

1.2.2 Ion-exchange membranes in electrochemical synthesis and power generation

Ion-exchange membranes are also used in electrochemical synthesis of certain organic compounds and chemicals such as chlorine and caustic soda, or oxygen and hydrogen. They are also used in energy conversion systems, such as fuel cells and reverse electrodialysis systems. The applications are not subjects of this discussion, which is concentrated on water treatment only.

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