Electrodeionization

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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. [1]

Contents

Electrodeionization can be used to produce high purity water, reaching electrical resistivity values as high as 18.2 MΩ/cm.

Electrodeionization (EDI) integrates three distinct processes:

  1. Electrolysis: A continuous DC current directs positive and negative ions toward electrodes with opposing electrical charges. The electrical potential draws anions and cations from diluting chambers, through cation or anion exchange membranes, into concentrating chambers.
  2. Ion exchange: An ion exchange resin fills the diluting chambers. As water flows through the resin bed, cations and anions become affixed to resin sites.
  3. Electrochemical regeneration: Unlike chemically regenerated mixed beds, EDI accomplishes regeneration through water splitting induced by the continuous electric current. Water splits from H2O into H+ and OH- to effectively regenerate the resin without the need for external chemical additives.

EDI is sometimes labeled "continuous electrodeionization" (CEDI) because the electric current continually regenerates the ion exchange resin mass.

Quality of the feed

To maximize the purity of product water, EDI feedwater needs pre-treatment, usually done via reverse osmosis. When fed with feedwater that is low in total dissolved solids (e.g., purified by RO), the product can reach very high purity levels. The contents of the feedwater must be kept within certain parameters to prevent damage to the EDI instrument.

Common feedwater quality concerns are:

History

Electrodeionization was developed in the early 1950s to eliminate or minimize the concentration polarization phenomenon present in electrolysis systems of the time. A patent on the technology was filed in 1953, and subsequent publications popularized the technology. [2]

The technology was limited in application because of the low tolerance of total dissolved solids, hardness and organics. During the 1970s and 1980s, reverse osmosis became a preferred technology to ion exchange resin for high TDS waters. As RO gained popularity, EDI emerged as a suitable polishing technology. Packaged RO and EDI systems began to displace chemically regenerated ion exchange systems.

In 1986 and 1989, several companies developed new EDI devices. The initial devices were large, costly, and often unreliable. However, in the 1990s, smaller and less costly modular designs were introduced. Nonetheless, these designs and their contemporary descendants still face limitations such as cost and limited operational envelope. [3] [4]

Applications

In the electronics industry, deionized water is used to rinse components during manufacturing. This is necessary to avoid potential short circuits that could destroy electronic chips. As electronic chips are very small, there is little free space between component elements and unwanted electricity may conduct across components via even a small number of ions, causing a short circuit. Using deionized water to clean the components helps minimize the ions on their surfaces and thus minimizes short circuits.

In the pharmaceutical industry, the presence of unwanted ions in water used in drug development can lead to unwanted side reactions and introduce harmful impurities.

In power generation, the presence of ions in boiler feedwater can lead to the buildup of solids or the degradation of boiler walls, both of which can lower boiler efficiency and present safety hazards.

Due to the large financial and safety concerns present in these three industries, their economic demand for highly pure water provides the bulk of the demand for EDI devices and development.

Electrodeionization systems have also been applied to the removal of heavy metals from different types of wastewater from mining, electroplating, and nuclear processes. The primary ions removed in these processes are chromium, copper, cobalt, and caesium, though EDI sees use in the removal of others as well. [5]

Theory

The electrodes in an electrochemical cell are each classified as either an anode or a cathode. An anode is an electrode at which electrons leave the cell and oxidation occurs, while a cathode is an electrode at which electrons enter the cell and reduction occurs. Each electrode may become either an anode or a cathode depending on the voltage applied to the cell.

Each deionization cell consists of an electrode and an electrolyte with ions that undergo either oxidation or reduction. Because they commonly consist of ions in solution, the electrolytes are often known as "ionic solutions", but molten and solid electrolytes are also possible.

Water passes between an anode and a cathode. Ion-selective membranes allow positive ions to separate from the water toward the negative electrode and negative ions toward the positive electrode. As a result, the ions cannot escape the cell and deionized water is produced. [4]

When using a current that is higher than necessary for the movement of the ions, a portion of the incident water will be split, forming hydroxide (OH-) anions and hydrogen (H+) cations. These species will replace the impurity anions and cations in the resin. This process is called "in situ regeneration" of the resin. Because this replacement occurs alongside the deionization process it allows for continuous purification, as opposed to deionization techniques that require a pause in operation to chemically regenerate ion exchange resins. [6]

The purpose of the ion exchange resin is to maintain a stable conductance across the feedwater. Without the resin, ions could be removed initially, but the conductance would drop dramatically as the concentration of ions decreases. With lower conductance, the electrodes would become less able to efficiently direct the flow of electrons across the cell, whereas with the addition of resin and thus a steady conductance, electron flow remains steady and ensures a steady rate of ion removal. With a resin, therefore, the final remaining ion concentrations in the processed water can be lower by orders of magnitude. [5]

Installation scheme

Electrodeionization installation scheme Electrodeionization scheme.jpg
Electrodeionization installation scheme

The typical EDI installation has the following components: electrodes, anion exchange membranes, cation exchange membranes, and resin. The simplest configurations comprise three compartments. To increase production intensity or efficiency, the number of compartments or cells can be increased as desired.

Once the system is installed and feedwater begins to flow through it, cations flow toward the cathode and anions flow toward the anode. Only anions can go through the anion exchange membrane, and only cations can go through the cation exchange membrane. This configuration allows anions and cations to flow in only one direction because of the selectivity of the membranes and the electrical forces, rendering the feedwater relatively free of ions. It also allows for the separate collection of cation and anion concentration flows, creating the opportunity for more selective waste disposal, recycling, or reuse; this is especially useful in the removal of heavy metal cations.

See also

Related Research Articles

<span class="mw-page-title-main">Anode</span> Electrode through which conventional current flows into a polarized electrical device

An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ACID, for "anode current into device". The direction of conventional current in a circuit is opposite to the direction of electron flow, so electrons flow from the anode of a galvanic cell, into an outside or external circuit connected to the cell. For example, the end of a household battery marked with a "+" is the cathode.

<span class="mw-page-title-main">Cathode</span> Electrode where reduction takes place

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit. For example, the end of a household battery marked with a + (plus) is the cathode.

<span class="mw-page-title-main">Electrolysis</span> Technique in chemistry and manufacturing

In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential. The word "lysis" means to separate or break, so in terms, electrolysis would mean "breakdown via electricity."

<span class="mw-page-title-main">Redox</span> Chemical reaction in which oxidation states of atoms are changed

Redox is a type of chemical reaction in which the oxidation states of a reactant change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a decrease in the oxidation state.

<span class="mw-page-title-main">Semipermeable membrane</span> Membrane which will allow certain molecules or ions to pass through it by diffusion

Semipermeable membrane is a type of biological or synthetic, polymeric membrane that allows certain molecules or ions to pass through it by osmosis. The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the permeability. Many natural and synthetic materials which are rather thick are also semipermeable. One example of this is the thin film on the inside of an egg.

<span class="mw-page-title-main">Galvanic cell</span> Electrochemical device

A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous oxidation–reduction reactions. A common apparatus generally consists of two different metals, each immersed in separate beakers containing their respective metal ions in solution that are connected by a salt bridge or separated by a porous membrane.

<span class="mw-page-title-main">Electrolytic cell</span> Cell that uses electrical energy to drive a non-spontaneous redox reaction

An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would otherwise not occur. The external energy source is a voltage applied between the cell's two electrodes; an anode and a cathode, which are immersed in an electrolyte solution. This is in contrast to a galvanic cell, which itself is a source of electrical energy and the foundation of a battery. The net reaction taking place in a galvanic cell is a spontaneous reaction, i.e., the Gibbs free energy remains -ve, while the net reaction taking place in an electrolytic cell is the reverse of this spontaneous reaction, i.e., the Gibbs free energy is +ve.

<span class="mw-page-title-main">Purified water</span> Water treated to remove all impurities

Purified water is water that has been mechanically filtered or processed to remove impurities and make it suitable for use. Distilled water was, formerly, the most common form of purified water, but, in recent years, water is more frequently purified by other processes including capacitive deionization, reverse osmosis, carbon filtering, microfiltration, ultrafiltration, ultraviolet oxidation, or electrodeionization. Combinations of a number of these processes have come into use to produce ultrapure water of such high purity that its trace contaminants are measured in parts per billion (ppb) or parts per trillion (ppt).

<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.

Electrocoagulation (EC) is a technique used for wastewater treatment, wash water treatment, industrially processed water, and medical treatment. Electrocoagulation has become a rapidly growing area of wastewater treatment due to its ability to remove contaminants that are generally more difficult to remove by filtration or chemical treatment systems, such as emulsified oil, total petroleum hydrocarbons, refractory organics, suspended solids, and heavy metals. There are many brands of electrocoagulation devices available, and they can range in complexity from a simple anode and cathode to much more complex devices with control over electrode potentials, passivation, anode consumption, cell REDOX potentials as well as the introduction of ultrasonic sound, ultraviolet light and a range of gases and reactants to achieve so-called Advanced Oxidation Processes for refractory or recalcitrant organic substances.

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

Electrolysis of water is using electricity to split water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, but must be kept apart from the oxygen as the mixture would be extremely explosive. Separately pressurised into convenient 'tanks' or 'gas bottles', hydrogen can be used for oxyhydrogen welding and other applications, as the hydrogen / oxygen flame can reach approximately 2,800°C.

<span class="mw-page-title-main">Electrodialysis</span> Applied electric potential transport of salt ions.

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 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.

Chlorine gas can be produced by extracting from natural materials, including the electrolysis of a sodium chloride solution (brine) and other ways.

<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.

Ultrapure water (UPW), high-purity water or highly purified water (HPW) is water that has been purified to uncommonly stringent specifications. Ultrapure water is a term commonly used in manufacturing to emphasize the fact that the water is treated to the highest levels of purity for all contaminant types, including: organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile; reactive, and inert; hydrophilic and hydrophobic; and dissolved gases.

<span class="mw-page-title-main">Alkaline anion-exchange membrane fuel cell</span>

An alkaline anion-exchange membrane fuel cell (AAEMFC), also known as anion-exchange membrane fuel cells (AEMFCs), alkaline membrane fuel cells (AMFCs), hydroxide-exchange membrane fuel cells (HEMFCs), or solid alkaline fuel cells (SAFCs) is a type of alkaline fuel cell that uses an anion-exchange membrane to separate the anode and cathode compartments.

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

A mixed oxidant solution (MOS) is a type of disinfectant that has many uses including disinfecting, sterilizing, and eliminating pathogenic microorganisms in water. An MOS may have advantages such as a higher disinfecting power, stable residual chlorine in water, elimination of biofilm, and safety. The main components of an MOS are chlorine and its derivatives, which are produced by electrolysis of sodium chloride. It may also contain high amounts of hydroxy radicals, chlorine dioxide, dissolved ozone, hydrogen peroxide and oxygen from which the name "mixed oxidant" is derived.

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. ↑ Arar, Özgür; Yüksel, Ümran; Kabay, Nalan; Yüksel, Mithat (2014-06-02). "Various applications of electrodeionization (EDI) method for water treatment—A short review". Desalination. Special Issue: Electromembrane Processes for Desalination. 342: 16–22. doi:10.1016/j.desal.2014.01.028. ISSN   0011-9164.
  2. ↑ Kollsman, Paul (1953-10-23). Method of and apparatus for treating ionic fluids by dialysis. United States Patent Office.
  3. ↑ "Fundamentals of Electrodeionization (EDI) Technology". WCP Online. 2007-03-10. Retrieved 2022-08-05.
  4. 1 2 Rathi, B. Senthil; Kumar, P. Senthil (July 2020). "Electrodeionization theory, mechanism, and environmental applications. A review". Environmental Chemistry Letters. 18 (4): 1209–1227. doi:10.1007/s10311-020-01006-9. ISSN   1610-3653. S2CID   216031814.
  5. 1 2 Wardani, Anita Kusuma; Hakim, Ahmad Nurul; Khoiruddin, null; Wenten, I. Gede (June 2017). "Combined ultrafiltration-electrodeionization technique for production of high purity water". Water Science and Technology. 75 (12): 2891–2899. doi: 10.2166/wst.2017.173 . ISSN   0273-1223. PMID   28659529.
  6. ↑ Alvarado, Lucía; Chen, Aicheng (2014-06-20). "Electrodeionization: Principles, Strategies and Applications". Electrochimica Acta. 132: 583–597. doi:10.1016/j.electacta.2014.03.165. ISSN   0013-4686.
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