Electrodialysis

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Water desalination
Methods
Edprinc.jpg

Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis cell. The cell consists of a feed (dilute) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes. In almost all practical electrodialysis processes, multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation-exchange membranes forming the multiple electrodialysis cells. Electrodialysis processes are different from distillation techniques and other membrane based processes (such as reverse osmosis (RO)) in that dissolved species are moved away from the feed stream, whereas other processes move away the water from the remaining substances. Because the quantity of dissolved species in the feed stream is far less than that of the fluid, electrodialysis offers the practical advantage of much higher feed recovery in many applications. [1] [2] [3] [4] [5] [6] [7]

Contents

Method

In an electrodialysis stack, the dilute (D) feed stream, brine or concentrate (C) stream, and electrode (E) stream are allowed to flow through the appropriate cell compartments formed by the ion-exchange membranes. Under the influence of an electrical potential difference, the negatively charged ions (e.g., chloride) in the dilute stream migrate toward the positively charged anode. These ions pass through the positively charged anion-exchange membrane, but are prevented from further migration toward the anode by the negatively charged cation-exchange membrane and therefore stay in the C stream, which becomes concentrated with the anions. The positively charged species (e.g., sodium) in the D stream migrate toward the negatively charged cathode and pass through the negatively charged cation-exchange membrane. These cations also stay in the C stream, prevented from further migration toward the cathode by the positively charged anion-exchange membrane. [8] As a result of the anion and cation migration, electric current flows between the cathode and anode. Only an equal number of anion and cation charge equivalents are transferred from the D stream into the C stream and so the charge balance is maintained in each stream. The overall result of the electrodialysis process is an ion concentration increase in the concentrate stream with a depletion of ions in the dilute solution feed stream.

The E stream is the electrode stream that flows past each electrode in the stack. This stream may consist of the same composition as the feed stream (e.g., sodium chloride) or may be a separate solution containing a different species (e.g., sodium sulfate). [5] The E stream is usually employed to prevent the reduction and/or oxidation of salt ions from the feed into the electrode plates. Depending on the stack configuration, anions and cations from the electrode stream may be transported into the C stream, or anions and cations from the D stream may be transported into the E stream. In each case, this transport is necessary to carry current across the stack and maintain electrically neutral stack solutions.

Anode and cathode reactions

Reactions take place at each electrode. At the cathode, [3]

2e− + 2 H2O → H2 (g) + 2 OH−

while at the anode, [3]

H2O → 2 H+ + ½ O2 (g) + 2e− or 2 Cl− → Cl2 (g) + 2e−

Small amounts of hydrogen gas are generated at the cathode and small amounts of either oxygen or chlorine gas (depending on composition of the E stream and end ion-exchange membrane arrangement) at the anode. These gases are typically subsequently dissipated as the E stream effluent from each electrode compartment is combined to maintain a neutral pH and discharged or re-circulated to a separate E tank. However, some (e.g.,) have proposed collection of hydrogen gas for use in energy production.

Efficiency

Current efficiency is a measure of how effective ions are transported across the ion-exchange membranes for a given applied current. Typically current efficiencies >80% are desirable in commercial stacks to minimize energy operating costs. Low current efficiencies indicate water splitting in the diluate or concentrate streams, shunt currents between the electrodes, or back-diffusion of ions from the concentrate to the diluate could be occurring.

Current efficiency is calculated according to: [9]

where

= current utilization efficiency
= charge of the ion
= Faraday constant, 96,485 Amp-s/mol
= dilute flow rate, L/s
= dilute ED cell inlet concentration, mol/L
= dilute ED cell outlet concentration, mol/L
= number of cell pairs
= current, Amps.

Current efficiency is generally a function of feed concentration. [10]

As electrodialysis works by transporting salt ions from the diluted channels to the concentrated channels, energy consumption greatly increases as the feed salt concentration increases. Seawater desalination is usually more energy efficient by reverse osmosis than electrodialysis.[ citation needed ] However, for water streams with lower salt concentration electrodialysis may be the most energy efficient process. Additionally, water streams with very high salt concentrations, that cannot be separated by reverse osmosis, can be concentrated by electrodialysis up to concentrations near to saturation. This is very useful for Zero Liquid Discharge treatments, providing a reduction in energy consumption compared to evaporation.

Applications

In application, electrodialysis systems can be operated as continuous production or batch production processes. In a continuous process, feed is passed through a sufficient number of stacks placed in series to produce the final desired product quality. In batch processes, the diluate and/or concentrate streams are re-circulated through the electrodialysis systems until the final product or concentrate quality is achieved.

Electrodialysis is usually applied to deionization of aqueous solutions. However, desalting of sparingly conductive aqueous organic and organic solutions is also possible. Some applications of electrodialysis include: [2] [4] [5]

The major application of electrodialysis has historically been the desalination of brackish water or seawater as an alternative to RO for potable water production and seawater concentration for salt production (primarily in Japan). [4] In normal potable water production without the requirement of high recoveries, reverse osmosis is generally believed to be more cost-effective when total dissolved solids (TDS) are 3,000 parts per million (ppm) or greater, while electrodialysis is more cost-effective for TDS feed concentrations less than 3,000 ppm or when high recoveries of the feed are required.[ citation needed ]

Another important application for electrodialysis is the production of pure water and ultrapure water by electrodeionization (EDI). In EDI, the purifying compartments and sometimes the concentrating compartments of the electrodialysis stack are filled with ion-exchange resin. When fed with low TDS feed (e.g., feed purified by RO), the product can reach very high purity levels (e.g., 18 MΩ-cm). The ion-exchange resins act to retain the ions, allowing these to be transported across the ion-exchange membranes. The main usage of EDI systems are in electronics, pharmaceutical, power generation, and cooling tower applications.

Electrodialysis can adapt to intermittent energy input and voltage variations, so it can be easily coupled to renewable electricity sources [14]

Selective electrodialysis

Selective electrodialysis uses ion selective exchange membranes to concentrate only some ions, whereas other species remain in the diluted channel. Selective electrodialysis is usually done by employing monovalent anion and/or cation exchange membranes, that only allows migration of monovalent anion or cations, respectively. This is useful when only monovalent ions are required to be separated, reducing electricity consumption and desalination time. [15] For example, this is useful for irrigation water. Monovalent cations usually are particularly (Na+, Cl-) harmful for crops, whereas most divalent ions (Ca+2, Mg+2, SO4-2) are beneficial nutrients for plants. Therefore, monovalent selective electrodialysis may provide water with an ideal composition for agriculture, reducing the necessity for minerals fertilization. [16]

Limitations

Electrodialysis has inherent limitations, working best at removing low molecular weight ionic components from a feed stream. Non-charged, higher molecular weight, and less mobile ionic species will not typically be significantly removed. Also, in contrast to RO, electrodialysis becomes less economical when extremely low salt concentrations in the product are required and with sparingly conductive feeds: current density becomes limited and current utilization efficiency typically decreases as the feed salt concentration becomes lower, and with fewer ions in solution to carry current, both ion transport and energy efficiency greatly declines. Consequently, comparatively large membrane areas are required to satisfy capacity requirements for low concentration (and sparingly conductive) feed solutions. Innovative systems overcoming the inherent limitations of electrodialysis (and RO) are available; these integrated systems work synergistically, with each sub-system operating in its optimal range, providing the least overall operating and capital costs for a particular application. [17]

As with RO, electrodialysis systems may require feed pretreatment to remove species that coat, precipitate onto, or otherwise "foul" the surface of the ion-exchange membranes. This fouling decreases the efficiency of the electrodialysis system. Species of concern include calcium and magnesium hardness, suspended solids, silica, and organic compounds. Water softening can be used to remove hardness, and micrometre or multimedia filtration can be used to remove suspended solids. Hardness in particular is a concern since scaling can build up on the membranes. However, electrodialysis can support higher concentrations of those foulants than reverse osmosis. In addition, electrodialysis membranes, as they have rectangular shape, can be removed from the stack and cleaned, whereas reverse osmosis membranes can't be cleaned that way because of their spiral geometry. Various chemicals are also available to help prevent scaling. Also, electrodialysis reversal systems seek to minimize scaling by periodically reversing the flows of diluate and concentrate and polarity of the electrodes.

See also

Related Research Articles

<span class="mw-page-title-main">Brine</span> Concentrated solution of salt in water

Brine is water with a high-concentration solution of salt. In diverse contexts, brine may refer to the salt solutions ranging from about 3.5% up to about 26%. Brine forms naturally due to evaporation of ground saline water but it is also generated in the mining of sodium chloride. Brine is used for food processing and cooking, for de-icing of roads and other structures, and in a number of technological processes. It is also a by-product of many industrial processes, such as desalination, so it requires wastewater treatment for proper disposal or further utilization.

<span class="mw-page-title-main">Desalination</span> Removal of salts from water

Desalination is a process that takes away mineral components from saline water. More generally, desalination is the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture. Saltwater is desalinated to produce water suitable for human consumption or irrigation. The by-product of the desalination process is brine. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few rainfall-independent water resources.

<span class="mw-page-title-main">Dialysis (chemistry)</span> Process of separating molecules

In chemistry, dialysis is the process of separating molecules in solution by the difference in their rates of diffusion through a semipermeable membrane, such as dialysis tubing.

<span class="mw-page-title-main">Forward osmosis</span> Water purification process

Forward osmosis (FO) is an osmotic process that, like reverse osmosis (RO), uses a semi-permeable membrane to effect separation of water from dissolved solutes. The driving force for this separation is an osmotic pressure gradient, such that a "draw" solution of high concentration, is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes. In contrast, the reverse osmosis process uses hydraulic pressure as the driving force for separation, which serves to counteract the osmotic pressure gradient that would otherwise favor water flux from the permeate to the feed. Hence significantly more energy is required for reverse osmosis compared to forward osmosis.

<span class="mw-page-title-main">Ion exchange</span> Exchange of ions between an electrolyte solution and a solid

Ion exchange is a reversible interchange of one species of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid. Ion exchange is used in softening or demineralizing of water, purification of chemicals, and separation of substances.

Electrodialysis reversal (EDR) is an electrodialysis reversal water desalination membrane process that has been commercially used since the early 1960s. An electric current migrates dissolved salt ions, including fluorides, nitrates and sulfates, through an electrodialysis stack consisting of alternating layers of cationic and anionic ion exchange membranes. Periodically, the direction of ion flow is reversed by reversing the polarity of the applied electric current.

Solar desalination is a desalination technique powered by solar energy. The two common methods are direct (thermal) and indirect (photovoltaic).

<span class="mw-page-title-main">Electrolysis of water</span> Electricity-induced chemical reaction

Électrolysis of

<span class="mw-page-title-main">Osmotic power</span> Energy available from the difference in the salt concentration between seawater and river water

Osmotic power, salinity gradient power or blue energy is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis (RED) and pressure retarded osmosis (PRO). Both processes rely on osmosis with membranes. The key waste product is brackish water. This byproduct is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water.

Nanofiltration is a membrane filtration process that uses nanometer sized pores through which particles smaller than about 1–10 nanometers pass through the membrane. Nanofiltration membranes have pore sizes of about 1–10 nanometers, smaller than those used in microfiltration and ultrafiltration, but a slightly bigger than those in reverse osmosis. Membranes used are predominantly polymer thin films. It is used to soften, disinfect, and remove impurities from water, and to purify or separate chemicals such as pharmaceuticals.

Reverse electrodialysis (RED) is the salinity gradient energy retrieved from the difference in the salt concentration between seawater and river water. A method of utilizing the energy produced by this process by means of a heat engine was invented by Prof. Sidney Loeb in 1977 at the Ben-Gurion University of the Negev. --United States Patent US4171409

<span class="mw-page-title-main">Pressure exchanger</span> Device for exchanging pressure between two fluids

A pressure exchanger transfers pressure energy from a high pressure fluid stream to a low pressure fluid stream. Many industrial processes operate at elevated pressures and have high pressure waste streams. One way of providing a high pressure fluid to such a process is to transfer the waste pressure to a low pressure stream using a pressure exchanger.

Reverse osmosis (RO) is a water purification process that uses a semi-permeable membrane to separate water molecules from other substances. RO applies pressure to overcome osmotic pressure that favors even distributions. RO can remove dissolved or suspended chemical species as well as biological substances, and is used in industrial processes and the production of potable water. RO retains the solute on the pressurized side of the membrane and the purified solvent passes to the other side. It relies on the relative sizes of the various molecules to decide what passes through. "Selective" membranes reject large molecules, while accepting smaller molecules.

<span class="mw-page-title-main">Pressure-retarded osmosis</span>

Pressure retarded osmosis (PRO) is a technique to separate a solvent from a solution that is more concentrated and also pressurized. A semipermeable membrane allows the solvent to pass to the concentrated solution side by osmosis. The technique can be used to generate power from the salinity gradient energy resulting from the difference in the salt concentration between sea and river water.

<span class="mw-page-title-main">Capacitive deionization</span>

Capacitive deionization (CDI) is a technology to deionize water by applying an electrical potential difference over two electrodes, which are often made of porous carbon. In other words, CDI is an electro-sorption method using a combination of a sorption media and an electrical field to separate ions and charged particles. Anions, ions with a negative charge, are removed from the water and are stored in the positively polarized electrode. Likewise, cations are stored in the cathode, which is the negatively polarized electrode.

<span class="mw-page-title-main">Membrane</span> Thin, film-like structure separating two fluids, acting as a selective barrier

A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Biological membranes include cell membranes ; nuclear membranes, which cover a cell nucleus; and tissue membranes, such as mucosae and serosae. Synthetic membranes are made by humans for use in laboratories and industry.

Electrodeionization (EDI) is a water treatment technology that utilizes DC power, ion exchange membranes, and ion exchange resin to deionize water. EDI is typically employed as a polishing treatment following reverse osmosis (RO), and is used in the production of ultrapure water. It differs from other RO polishing methods, like chemically regenerated mixed beds, by operating continuously without chemical regeneration.

Concentration polarization is a term used in the scientific fields of electrochemistry and membrane science.

An ion-exchange membrane is a semi-permeable membrane that transports certain dissolved ions, while blocking other ions or neutral molecules.

A microbial desalination cell (MDC) is a biological electrochemical system that implements the use of electro-active bacteria to power desalination of water in situ, resourcing the natural anode and cathode gradient of the electro-active bacteria and thus creating an internal supercapacitor. Available water supply has become a worldwide endemic as only .3% of the earth's water supply is usable for human consumption, while over 99% is sequestered by oceans, glaciers, brackish waters, and biomass. Current applications in electrocoagulation, such as microbial desalination cells, are able to desalinate and sterilize formerly unavailable water to render it suitable for safe water supply. Microbial desalination cells stem from microbial fuel cells, deviating by no longer requiring the use of a mediator and instead relying on the charged components of the internal sludge to power the desalination process. Microbial desalination cells therefore do not require additional bacteria to mediate the catabolism of the substrate during biofilm oxidation on the anodic side of the capacitor. MDCs and other bio-electrical systems are favored over reverse osmosis, nanofiltration and other desalination systems due to lower costs, energy and environmental impacts associated with bio-electrical systems.

References

  1. ↑ Davis, T.A., "Electrodialysis", in Handbook of Industrial Membrane Technology, M.C. Porter, ed., Noyes Publications, New Jersey (1990)
  2. 1 2 Strathmann, H., "Electrodialysis", in Membrane Handbook, W.S.W. Ho and K.K. Sirkar, eds., Van Nostrand Reinhold, New York (1992)
  3. 1 2 3 Mulder, M., Basic Principles of Membrane Technology, Kluwer, Dordrecht (1996)
  4. 1 2 3 Sata, T., Ion Exchange Membranes: Preparation, Characterization, Modification and Application, Royal Society of Chemistry, London (2004)
  5. 1 2 3 Strathmann, H., Ion-Exchange Membrane Separation Processes, Elsevier, New York (2004)
  6. ↑ "ED - Turning Seawater into Drinking Water". Archived from the original on 2007-02-03. Retrieved 2007-01-17.
  7. ↑ Panagopoulos, Argyris; Haralambous, Katherine-Joanne; Loizidou, Maria (2019-11-25). "Desalination brine disposal methods and treatment technologies - A review". Science of the Total Environment. 693: 133545. Bibcode:2019ScTEn.693m3545P. doi:10.1016/j.scitotenv.2019.07.351. ISSN   0048-9697. PMID   31374511.
  8. ↑ AWWA, Electrodialysis and Electrodialysis Reversal, American Water Works Association, Denver (1995)
  9. ↑ Shaffer, L., and Mintz, M., "Electrodialysis" in Principles of Desalination, Spiegler, K., and Laird, A., eds., 2nd Ed., Academic Press, New York (1980)
  10. ↑ Current Utilization Efficiency
  11. ↑ Lukin, A; Marayeva, O; Vysotskaya, Y (2020-01-01). "Using electrodialysis technology for regeneration of hydrochloric acid from sour pectin extracts". IOP Conference Series: Earth and Environmental Science. 422 (1): 012080. doi: 10.1088/1755-1315/422/1/012080 . ISSN   1755-1307.
  12. ↑ Devda, Viralkunvar; Chaudhary, Kashika; Varjani, Sunita; Pathak, Bhawana; Patel, Anil Kumar; Singhania, Reeta Rani; Taherzadeh, Mohammad J.; Ngo, Huu Hao; Wong, Jonathan W. C.; Guo, Wenshan; Chaturvedi, Preeti (2021-01-01). "Recovery of resources from industrial wastewater employing electrochemical technologies: status, advancements and perspectives". Bioengineered. 12 (1): 4697–4718. doi:10.1080/21655979.2021.1946631. ISSN   2165-5979. PMC   8806852 . PMID   34334104.
  13. ↑ "Electrodialysis-based zero liquid discharge in industrial wastewater treatment". iwaponline.com. doi:10.2166/wst.2019.161 . Retrieved 2023-02-13.
  14. ↑ Fernandez-Gonzalez, C.; Dominguez-Ramos, A.; Ibañez, R.; Irabien, A. (2015-07-01). "Sustainability assessment of electrodialysis powered by photovoltaic solar energy for freshwater production". Renewable and Sustainable Energy Reviews. 47: 604–615. doi:10.1016/j.rser.2015.03.018. ISSN   1364-0321.
  15. ↑ "Model SED - Selective Electrodialysis Technology".
  16. ↑ "Novel Applications of Monovalent Selective Electrodialysis for the Treatment of Irrigation Waters". Prof. Lienhard. 2021.
  17. ↑ Inamuddin (2017-06-01). Applications of Adsorption and Ion Exchange Chromatography in Waste Water Treatment. Materials Research Forum LLC. ISBN   9781945291333.

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