Electrodeionization

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Schematic diagram of an EDI module

Electrodeionization is a water treatment technology that utilizes an electrode to ionize water molecules and separate dissolved ions (impurities) from water. It differs from other water purification technologies in that it is done without the use of chemical treatments and is usually a tertiary treatment to reverse osmosis (RO). There is also EDI units that use a small bed with ion-exchange resin to enhance the deionization further, this is often referred to as Continuous electrodeionization (CEDI) since the electric current regenerates the resin mass continuously. CEDI technique can achieve very high purity, with conductivity being below 0.1uS/cm.

Applications[edit]

When fed with low TDS feed (e.g., feed purified by RO), the product can reach very high purity levels (e.g., 18 Megohms/cm[1]). The ion exchange resins act to retain the ions, allowing these to be transported across the ion exchange membranes. The main usage of EDI technology such as that supplied by Ionpure and SnowPure are in electronics, pharmaceutical, and power generation applications.

One important aspect in the water treatment application is that water to the EDI needs to be free from CO2, since this in its dissolved form will put unnecessary strain on the EDI unit and will reduce performance.

Theory[edit]

An electrode in an electrochemical cell is referred to as either an anode or a cathode, 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 as 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 behaves 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.

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 results.

See also[edit]

References[edit]

  1. ^ [1], Resistivity / Conductivity Measurement of Purified Water, Lab Manager Magazine