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Electrodeionization (EDI) is a water treatment technology that utilizes
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
, ion exchange membranes, and resin to deionize water and separate dissolved ions (impurities) from it. It differs from other water purification technologies in that it is done without the use of chemical treatments and is usually a polishing treatment to reverse osmosis (RO). There are also EDI units that are often referred to as continuous electrodeionization (CEDI) since the electric current regenerates the resin mass continuously. The CEDI technique can achieve very high purity, with a conductivity below 0.1 μS/cm. Electrodeionization (EDI) can be differentiated into three stages, so the basics of EDI reside in the simultaneity of the following processes. - Electrodialysis With a continuously applied electric voltage, both positive and negative ions are directed to the electrodes that have a load opposite. The permeable membranes of anions and cations are responsible for directing them to the concentrated area. - Ion exchange In the area between the different membranes is the resin that has different types of load. The salts present in the water adsorb onto the ion exchange resin, moving opposite to their load. Finally, they are replaced in resin exchange positions by hydroxyl anions and hydrogen cations. - Regeneration The continuous regeneration is made by hydrogen ions and hydroxyl groups. This occurs disassociation of the molecules of water that occurs in the anode and cathode when applying electric voltage (called
water splitting Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen: :2 H2O → 2 H2 + O2 Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy, base ...
).


Quality of the feed

In order to offer all its potential, EDI feedwater needs pre-treatment, usually reverse osmosis. Feed water must follow certain requirements, these parameters are fixed before to prevent damage to the equipment. Some parameters are: - The hardness of feed water must be generally < 1ppm CaCO3 - Silica content (SiO2) must be < 1ppm - CO2 must be monitored to prevent precipitation of carbonates. - TOC, which can foul resins and membranes. - Chlorine and ozone, which can oxidize resins and membranes.


History

To eliminate or minimize the
concentration polarization Concentration polarization is a term used in the scientific fields of electrochemistry and membrane science. In electrochemistry In electrochemistry, concentration polarization denotes the part of the polarization of an electrolytic cell resulting ...
phenomenon present in electrodialysis systems, electrodeionization originated in the late 1950s. In 1956, William Katz at Ionics developed one of the first Descriptions of electrodeionization and published his paper "The Present Status of Electric Membrane Demineralization" at the International Water Conference. The technology was limited in application due to the low tolerance of hardness and organics. During the 1970s and 1980s reverse osmosis became a preferred technology to
ion exchange resin An ion-exchange resin or ion-exchange polymer is a resin or polymer that acts as a medium for ion exchange. It is an insoluble matrix (or support structure) normally in the form of small (0.25–1.43 mm radius) microbeads, usually white or ye ...
for high TDS waters. As RO gained popularity, it was determined that EDI would be a suitable polishing technology. Packaged RO and EDI systems were used to displace chemically regenerated ion exchange systems. In 1986 and 1989, companies like Millipore, Ionpure, HOH Water Technologies, and Ionics Inc. developed electrodeionization devices. The initial devices were large, costly, and often unreliable. In 1995 Glegg Water Conditioning introduced E-Cell brand electrodeionization . The new technology reduced cost and improved reliability, based on a modular design standard. E-Cell was also offered to many OEMS and revolutionized the industry. Competitors like Electropure and SnowPure soon followed with modular leak-free designs. Presently, this technology is widely available from many water treatment companies, but should only be applied by experts who understand the limitations and use top-quality products.


Applications

When fed with low
total dissolved solids Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular ( colloidal sol) suspended form. TDS concentrations are often report ...
(TDS) feed (e.g., feed purified by RO), the product can reach very high purity levels (e.g., 18 megohms/cm, Resistivity / Conductivity Measurement of Purified Water). The ion exchange resins act to retain the ions, allowing these to be transported across the ion exchange membranes. The main applications of EDI technology, such as that supplied by Ionpure, E-cell, and SnowPure, are in electronics, pharmaceuticals, and power generation.


Theory

An electrode in an electrochemical cell is referred to as either an
anode 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 ...
or a
cathode 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 whi ...
, terms that were coined by Michael Faraday. The anode is defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode is the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode depending on the voltage applied to the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell. Each cell consists of an electrode and an electrolyte with ions that undergo either oxidation or reduction. An electrolyte is a substance containing free ions that behave as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions, but molten electrolytes and solid electrolytes are also possible. They are sometimes referred to in abbreviated jargon as
lytes An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
. Water is passed between an anode (positive electrode) and a cathode (negative electrode). Ion-selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity
deionized water 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 pu ...
results.


In situ regeneration

When using an excess of current that is higher than the necessary for the movement of the ions. A portion of the water will be split forming OH- and H+. This species will replace the impurity anions and cations in the resin. This process is called regeneration in situ of the resin. And because it occurs during the process itself there is no need to stop the installation and use chemicals as it happens in others techniques.


Installation scheme

The typical EDI installation has the following components: anode and cathode, anion exchange membrane, cation exchange membrane, and resin. The most simplified configuration consists of 3 compartments, to increase production the number of compartments or cells can be increased. The cations flow toward the cathode and the 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 only flow in one direction because of the membranes and the electric force, leaving the feed water free of ions (deionized water). The concentration flows (right and left of the feed flow) are rejected and they can be wasted, recycled, or used in another process. The purpose of the ion exchange resin is to maintain stable conductance of the feed water. Without the resins, the conductance will drop dramatically as the concentration of ions is decreasing. Such drop-off of conductance makes it very difficult to eliminate 100% of the ions, but using resins makes it possible.


See also

*
Electrodialysis 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 ...
*
Ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
*
Purified water 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 puri ...
* Water purification * Water treatment


References


External links


video

Continuous Electrodeionization (CEDI/EDI)
Ionpure CEDI Products
Electrodeionization Technology

EDI History

Electrodeionization Systems
Electrodeionization Systems
Advanced Electrodeionization Technology for Product Desalting
Argonne National Laboratory {{Separation processes Water treatment Ions Physical chemistry Separation processes